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First X-Ray Observations of the Young Pulsar J1357-6429
The first short Chandra and XMM-Newton observations of the young andenergetic pulsar J1357-6429 provided indications of a tail-like pulsarwind nebula associated with this object as well as pulsations of itsX-ray flux with a pulsed fraction pf>~50% and a thermalcomponent dominating at energies E<~2 keV. The putative nebula isvery compact in size and might be interpreted as evidence for a pulsarjet. The thermal radiation is most plausibly emitted from the entireneutron star surface of a 10 km radius and a 1.0+/-0.1 MK temperature,covered with a hydrogen atmosphere. At higher energies, the pulsar'semission is of a nonthermal origin, with a power-law spectrum of aphoton index Γ=1.1+/-0.2. This makes the properties of PSRJ1357-6429 very similar to those of the young pulsars J1119-6127 andVela with detected thermal radiation.
| How to blow up a star.
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| Very High Energy Gamma-Ray Radiation from the Stellar Mass Black Hole Binary Cygnus X-1
We report on the results from the observations in the very high energyband (VHE; Eγ>=100 GeV) of the black hole X-raybinary (BHXB) Cygnus X-1. The observations were performed with the MAGICtelescope, for a total of 40 hr during 26 nights, spanning the periodbetween 2006 June and November. Searches for steady γ-ray signalsyielded no positive result, and upper limits to the integral fluxranging between 1% and 2% of the Crab Nebula flux, depending on theenergy, have been established. We also analyzed each observation nightindependently, obtaining evidence of γ-ray signals at the 4.0σ significance level (3.2 σ after trial correction) for 154minutes of effective on-time (EOT) on September 24 between 20:58 and23:41 UTC, coinciding with an X-ray flare seen by RXTE, Swift, andINTEGRAL. A search for faster-varying signals within a night resulted inan excess with a significance of 4.9 σ (4.1 σ after trialcorrection) for 79 minutes EOT between 22:17 and 23:41 UTC. The measuredexcess is compatible with a pointlike source at the position of CygnusX-1 and excludes the nearby radio nebula powered by its relativisticjet. The differential energy spectrum is well fitted by an unbrokenpower law described asdN/(dAdtdE)=(2.3+/-0.6)×10-12(E/1TeV)-3.2+/-0.6. This is the first experimental evidence ofVHE emission from a stellar mass black hole and therefore from aconfirmed accreting X-ray binary.
| Variable Very-High-Energy Gamma-Ray Emission from the Microquasar LS I +61 303
Microquasars are binary star systems with relativistic radio-emittingjets. They are potential sources of cosmic rays and can be used toelucidate the physics of relativistic jets. We report the detection ofvariable gamma-ray emission above 100 gigaelectron volts from themicroquasar LS I +61 303. Six orbital cycles were recorded. Severaldetections occur at a similar orbital phase, which suggests that theemission is periodic. The strongest gamma-ray emission is not observedwhen the two stars are closest to one another, implying a strong orbitalmodulation of the emission or absorption processes.
| Detection of Crab Giant Pulses Using the Mileura Widefield Array Low Frequency Demonstrator Field Prototype System
We report on the detection of giant pulses from the Crab Nebula pulsarat a frequency of 200 MHz using the field deployment system designed forthe Mileura Widefield Array's Low Frequency Demonstrator (MWA-LFD). Ourobservations are among the first high-quality detections at such lowfrequencies. The measured pulse shapes are deconvolved for interstellarpulse broadening, yielding a pulse-broadening time of 670+/-100 μs,and the implied strength of scattering (scattering measure) is thelowest that is estimated toward the Crab Nebula from observations madeso far. The sensitivity of the system is largely dictated by the skybackground, and our simple equipment is capable of detecting pulses thatare brighter than ~9 kJy in amplitude. The brightest giant pulsedetected in our data has a peak amplitude of ~50 kJy, and the impliedbrightness temperature is 1031.6 K. We discuss the giantpulse detection prospects with the full MWA-LFD system. With asensitivity over 2 orders of magnitude larger than the prototypeequipment, the full system will be capable of detecting such brightgiant pulses out to a wide range of Galactic distances; from ~15 to ~30kpc depending on the frequency. The MWA-LFD will thus be a highlypromising instrument for the studies of giant pulses and other fastradio transients at low frequencies.
| CHANDRA : taking the Universe's X-ray.
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| A Spitzer Space Telescope Study of SN 2003gd: Still No Direct Evidence that Core-Collapse Supernovae are Major Dust Factories
We present a new, detailed analysis of late-time mid-infraredobservations of the Type II-P supernova (SN) 2003gd. At about 16 monthsafter the explosion, the mid-IR flux is consistent with emission from4×10-5 Msolar of newly condensed dust in theejecta. At 22 months emission from pointlike sources close to the SNposition was detected at 8 and 24 μm. By 42 months the 24 μm fluxhad faded. Considerations of luminosity and source size rule out theejecta of SN 2003gd as the main origin of the emission at 22 months. Apossible alternative explanation for the emission at this later epoch isan IR echo from preexisting circumstellar or interstellar dust. Weconclude that, contrary to the claim of Sugerman and coworkers, themid-IR emission from SN 2003gd does not support the presence of 0.02Msolar of newly formed dust in the ejecta. There is, as yet,no direct evidence that core-collapse supernovae are major dustfactories.
| Astronomical Observations of Galactic Radiosources with ESCORT: an small radiotelescope
The aim of this work was to measure the radio flux density at 3.7 GHz ofthermal (HII regions) and nonthermal emissions (synchrotron radiation)of Galactic radio sources. The observations were made through ESCORT,the 2 meter diameter antenna of the radioastronomy center at Universidadde Oriente (UDO), situated in Ciudad Bolívar, Venezuela. Theantenna temperature and system temperature were calibrated by using thetransit time and the flux density of the sun as reference. In adition,the HPBW and effective aperture of ESCORT were determined.
| Search for Electron-Positron Annihilation Radiation from the Jet in 3C 120
We report an attempt to detect the electron-positron annihilation linefrom the radio galaxy 3C 120, in which the jet interacts strongly withinterstellar clouds. Such interactions should cause most of the jetplasma to mix with the gas in the clouds. This will thermalize themajority of any positrons in the jet, leading to continuous annihilationwith ambient electrons. We derive the number density of the combinedelectron-positron population in the core and compact knots in the jet of3C 120 using ultrahigh-resolution observations with the Globalmillimeter-VLBI Array at 86 GHz and the Very Long Baseline Array at 43GHz, along with the millimeter-wave continuum spectrum and acomputational code that maps the synchrotron intensity of a model jet.If the jet contains a pure pair plasma, the production rate of positronsrequired to produce this density plus the efficiency of eventualannihilation predict the emission of a narrow spectral line at a restenergy of 511 keV, or 495 keV in the observer's frame. Our spectralobservations with the SPI instrument on INTEGRAL failed to detect theline. The upper limit, which is 30% lower than our rather uncertainprediction, does not significantly constrain the positron-to-protonratio in the jet of 3C 120. However, our procedure provides a robustmethod for determining the flux of electrons and positrons in a jet thatwill be useful when more sensitive soft γ-ray spectrometers andmillimeter-wave VLBI arrays become available.
| Astronomie gamma : le ciel revele aux tres hautes energies.
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| Occultation of X-rays from Scorpius X-1 by small trans-neptunian objects
Since the discovery of the trans-neptunian objects (TNOs) in 1992,nearly one thousand new members have been added to our Solar System,several of which are as big as-or even larger than-Pluto. The propertiesof the population of TNOs, such as the size distribution and the totalnumber, are valuable information for understanding the formation of theSolar System, but direct observation is only possible for larger objectswith diameters above several tens of kilometres. Smaller objects, whichare expected to be more abundant, might be found when they occultbackground stars, but hitherto there have been no definite detections.Here we report the discovery of such occultation events at millisecondtimescales in the X-ray light curve of Scorpius X-1. The estimated sizesof these occulting TNOs are <=100m. Their abundance is in line withan extrapolation of the distribution of sizes of larger TNOs.
| The Suzaku Observation of the Nucleus of the Radio-loud Active Galaxy Centaurus A: Constraints on Abundances of the Accreting Material
A Suzaku observation of the nucleus of the radio-loud AGN Centaurus A in2005 has yielded a broadband spectrum spanning 0.3-250 keV. The netexposure times after screening were 70 ks per X-ray Imaging Spectrometer(XIS) camera, 60.8 ks for the Hard X-ray Detector (HXD) PIN, and 17.1 ksfor the HXD GSO. The hard X-rays are fit by two power laws of the sameslope, absorbed by columns of 1.5 and 7×1023cm-2, respectively. The spectrum is consistent with previoussuggestions that the power-law components are X-ray emission from thesubparsec VLBI jet and from Bondi accretion at the core, but it is alsoconsistent with a partial-covering interpretation. The soft band isdominated by thermal emission from the diffuse plasma and is fit well bya two-temperature VAPEC model, plus a third power-law component toaccount for scattered nuclear emission, jet emission, and emission fromX-ray binaries and other point sources. Narrow fluorescent emissionlines from Fe, Si, S, Ar, Ca, and Ni are detected. The Fe Kα linewidth yields a 200 lt-day lower limit on the distance from the blackhole to the line-emitting gas. Fe, Ca, and S K-shell absorption edgesare detected. Elemental abundances are constrained via absorption edgedepths and strengths of the fluorescent and diffuse plasma emissionlines. The high metallicity ([Fe/H]=+0.1) of the circumnuclear materialsuggests that it could not have originated in the relatively metal-poorouter halo unless enrichment by local star formation has occurred.Relative abundances are consistent with enrichment from Type II and Iasupernovae.
| TeV Gamma-Ray Sources from a Survey of the Galactic Plane with Milagro
A survey of Galactic gamma-ray sources at a median energy of ~20 TeV hasbeen performed using the Milagro Gamma-Ray Observatory. Eight candidatesources of TeV emission are detected with pretrial significances >4.5σ in the region of Galactic longitude l ∈ [30°, 220°]and latitude b ∈ [-10°, 10°]. Four of these sources,including the Crab Nebula and the recently published MGRO J2019+37, areobserved with significances >4 σ after accounting for thetrials. All four of these sources are also coincident with EGRETsources. Two of the lower significance sources are coincident with EGRETsources, and one of these sources is Geminga. The other two candidatesare in the Cygnus region of the Galaxy. Several of the sources appear tobe spatially extended. The fluxes of the sources at 20 TeV range from~25% of the Crab flux to nearly as bright as the Crab.
| Transient radio bursts from rotating neutron stars
The radio sky is relatively unexplored for transient signals, althoughthe potential of radio-transient searches is high. This was demonstratedrecently by the discovery of a previously unknown type of source,varying on timescales of minutes to hours. Here we report a search forradio sources that vary on much shorter timescales. We found elevenobjects characterized by single, dispersed bursts having durationsbetween 2 and 30ms. The average time intervals between bursts range from4min to 3h with radio emission typically detectable for <1s per day.From an analysis of the burst arrival times, we have identifiedperiodicities in the range 0.4-7s for ten of the eleven sources,suggesting origins in rotating neutron stars. Despite the small numberof sources detected at present, their ephemeral nature implies a totalGalactic population significantly exceeding that of the regularlypulsing radio pulsars. Five of the ten sources have periods >4s, andthe rate of change of the pulse period has been measured for three ofthem; for one source, we have inferred a high magnetic field strength of5 × 1013G. This suggests that the new population isrelated to other classes of isolated neutron stars observed at X-ray andγ-ray wavelengths.
| Discovery of Very High Energy Gamma Radiation from IC 443 with the MAGIC Telescope
We report the detection of a new source of very high energy (VHE;Eγ>=100 GeV) γ-ray emission located close tothe Galactic plane, MAGIC J0616+225, which is spatially coincident withsupernova remnant IC 443. The observations were carried out with theMAGIC telescope in the periods 2005 December-2006 January and 2006December-2007 January. Here we present results from this source, leadingto a VHE γ-ray signal with a statistical significance of 5.7σ in the 2006/2007 data and a measured differential γ-rayflux consistent with a power law, described asdNγ/(dAdtdE)=(1.0+/-0.2)×10-11(E/0.4TeV)-3.1+/-0.3 cm-2 s-1TeV-1. We briefly discuss the observational technique usedand the procedure implemented for the data analysis. The results areplaced in the context of the multiwavelength emission and the molecularenvironment found in the region of IC 443.
| Strong Langmuir turbulence in a pulsar emission region: statistical analysis
We test a turbulent plasma emission model previously proposed tointerpret micropulses in individual pulses received from a certainnumber of pulsars. In this model, we assumed that pulsar radio emissionscan be related to the development of a strong Langmuir turbulence in apulsar emission region. As shown, the elements of such a turbulence inthis region are stable Langmuir solitons which have electrostatic andelectromagnetic components at the same time. Consequently, they radiateelectromagnetic waves likely to reach an observer. On this basis, weassumed that such a strong Langmuir turbulence would give rise to alattice of stable Langmuir structures, regularly spaced, all with thesame amplitude, width, position and velocity along the open magneticfield lines of the pulsar magnetosphere. Properties of such radiatingLangmuir structures were associated with micropulses observed in radiopulses.Actually, such Langmuir structures in strongly turbulent pulsar plasmasshould have random amplitudes, positions and velocities. Consideringthese as elementary structures in strongly turbulent pulsar plasmas, wepropose a statistical theory for such plasmas. We start with theelementary soliton-like solutions of a non-linear Schrödingerequation, describing the strong turbulence in terms of a set of randomsolitons. The initial amplitudes, positions and velocities of thesesolitons are themselves random variables whose distributions act as freeparameters for our statistical description. Assuming an ensemble ofinitial conditions, we are able to determine the mean number ofsolitons, the dynamical form factor, the resulting energy spectrum, theassociated intensity and the intensity distribution. The energy spectrumqualitatively agrees with some of the observed pulsar spectra. Thevariation with time of the intensity distribution shows a realisticbehaviour in a large part of the domain of the intensities.
| Suzaku Observations of Active Galactic Nuclei Detected in the Swift BAT Survey: Discovery of a ``New Type'' of Buried Supermassive Black Holes
We present the Suzaku broadband observations of two AGNs detected by theSwift BAT hard X-ray (>15 keV) survey that did not have previousX-ray data, SWIFT J0601.9-8636 and SWIFT J0138.6-4001. The Suzakuspectra reveal in both objects a heavily absorbed power-law componentwith a column density ofNH~=1023.5-1024 cm-2 thatdominates above 10 keV and an intense reflection component with a solidangle >~2π from a cold, optically thick medium. We find that theseAGNs have an extremely small fraction of scattered light from thenucleus, <~0.5% with respect to the intrinsic power-law component.This indicates that they are buried in a very geometrically thick toruswith a small opening angle and/or have an unusually small amount of gasresponsible for scattering. In the former case, the geometry of SWIFTJ0601.9-8636 should be nearly face-on as inferred from the smallabsorption for the reflection component. The discovery of two suchobjects in this small sample implies that there must be a significantnumber of yet unrecognized, very Compton thick AGNs viewed at largerinclination angles in the local universe, which are difficult to detecteven in the currently most sensitive optical or hard X-ray surveys.
| Fragmentation of star-forming clouds enriched with the first dust
The thermal and fragmentation properties of star forming clouds haveimportant consequences on the corresponding characteristic stellar mass.The initial composition of the gas within these clouds is a record ofthe nucleosynthetic products of previous stellar generations. In thispaper, we present a model for the evolution of star forming cloudsenriched by metals and dust from the first supernovae (SNe), resultingfrom the explosions of metal-free progenitors with masses in the range12-30Msolar and 140-260Msolar. Using aself-consistent approach, we show that: (i) metals depleted on to dustgrains play a fundamental role, enabling fragmentation to solar orsubsolar mass scales already at metallicities Zcr =10-6Zsolar (ii) even at metallicities as high as10-2Zsolar, metals diffused in the gas phase leadto fragment mass scales which are >~100Msolar (iii) Catoms are strongly depleted on to amorphous carbon grains and COmolecules so that CII plays a minor role in gas cooling, leaving OI asthe main gas-phase cooling agent in low-metallicity clouds. Theseconclusions hold independently of the assumed SN progenitors and suggestthat the onset of low-mass star formation is conditioned to the presenceof dust in the parent clouds.
| An Exceptional Very High Energy Gamma-Ray Flare of PKS 2155-304
The high-frequency peaked BL Lac PKS 2155-304 at redshift z=0.116 is awell-known VHE (>100 GeV) γ-ray emitter. Since 2002 its VHEflux has been monitored using the H.E.S.S. stereoscopic array of imagingatmospheric Cerenkov telescopes in Namibia. During the 2006 July darkperiod, the average VHE flux was measured to be more than 10 timestypical values observed from the object. This article focuses solely onan extreme γ-ray outburst detected in the early hours of 2006 July28 (MJD 53,944). The average flux observed during this outburst isI(>200 GeV)=(1.72+/-0.05stat+/-0.34syst)×10-9cm-2 s-1, corresponding to ~7 times the flux,I(>200 GeV), observed from the Crab Nebula. Peak fluxes are measuredwith 1 minute timescale resolution at more than twice this averagevalue. Variability is seen up to ~600 s in the Fourier power spectrum,and well-resolved bursts varying on timescales of ~200 s are observed.There are no strong indications for spectral variability within thedata. Assuming the emission region has a size comparable to theSchwarzschild radius of a ~109 Msolar black hole,Doppler factors greater than 100 are required to accommodate theobserved variability timescales.
| Simultaneous X-ray/optical observations of GX9+9 (4U1728-16)
We report on the results of the first simultaneous X-ray (RXTE) andoptical [South African Astronomical Observatory (SAAO)] observations ofthe luminous low-mass X-ray binary (LMXB) GX9+9 in 1999 August. Thehigh-speed optical photometry revealed an orbital period of 4.1958 h andconfirmed previous observations, but with greater precision. No X-raymodulation was found at the orbital period. On shorter time-scales, apossible 1.4-h variability was found in the optical light curves whichmight be related to the MHz quasi-periodic oscillations seen in otherLMXBs. We do not find any significant X-ray/optical correlation in thelight curves. In X-rays, the colour-colour and hardness-intensitydiagrams indicate that the source shows characteristics of an atollsource in the upper banana state, with a correlation between intensityand spectral hardness. Time-resolved X-ray spectroscopy suggests thattwo-component spectral models give a reasonable fit to the X-rayemission. Such models consist of a blackbody component which can beinterpreted as the emission from an optically thick accretion disc or anoptically thick boundary layer, and a hard Comptonized component for anextended corona.
| Statistical properties of giant pulses from the Crab pulsar
Aims.We have studied the statistics of giant pulses from the Crab pulsarfor the first time with particular reference to their widths. Methods:We have analyzed data collected during 3.5 h of observations conductedwith the Westerbork Synthesis Radio Telescope operated in a tied-arraymode at a frequency of 1200 MHz. The PuMa pulsar backend providedvoltage recording of X and Y linear polarization states in two conjugate10 MHz bands. We restricted the time resolution to 4 μs to match thescattering on the interstellar inhomogeneities. Results: In total about18000 giant pulses (GP) were detected in full intensity with a thresholdlevel of 6σ. We analyzed cumulative probability distribution (CPD)of giant pulse energies for groups of GPs with different effectivewidths in the range 4 to 65 μs. The CPDs were found to manifestnotable differences for the different GP width groups. The slope of apower-law fit to the high-energy portion of the CPD evolves from -1.7 to-3.2 when going from the shortest to the longest GPs. There are breaksin the CPD power-law fits indicating flattening at low energies withindices varying from -1.0 to -1.9 for the short and long GPs,respectively. The GPs with a stronger peak flux density were found to beof shorter duration. We compare our results with previously publisheddata and discuss the importance of these peculiarities in thestatistical properties of GPs for the theoretical understanding of theemission mechanism responsible for GP generation.
| A 0535+26: back in business
In 2005 May/June, after 10 yr of inactivity, the Be/X-ray binary systemA 0535+26 underwent a major X-ray outburst. In this paper, data arepresented from 10 yr of optical, infrared and X-ray monitoring showingthe behaviour of the system during the quiescent epoch and the lead upto the new outburst. The results show the system going through a periodwhen the Be star in the system had a minimal circumstellar disc and thena dramatic disc recovery leading, presumably, to the latest flare up ofX-ray emission. The data are interpreted in terms of the state of thedisc and its interaction with the neutron star companion.
| The Radio Emission, X-Ray Emission, and Hydrodynamics of G328.4+0.2: A Comprehensive Analysis of a Luminous Pulsar Wind Nebula, Its Neutron Star, and the Progenitor Supernova Explosion
We present new observational and modeling results obtained for theGalactic nonthermal radio source G328.4+0.2. Using X-ray data obtainedby XMM-Newton, we confirm that its X-ray emission is heavily absorbed,has a spectrum best fitted by a power-law model of photon indexΓ=2 with no evidence for a thermal component, comes from a regionsignificantly smaller than the radio emission, and that the X-ray andradio emissions are significantly offset from each other. We alsopresent the results of a new high-resolution (7") 1.4 GHz image ofG328.4+0.2 obtained using ATCA and a deep search for radio pulsationsusing the Parkes radio telescope. By comparing this 1.4 GHz image with asimilar resolution image at 4.8 GHz, we find that the radio emission hasa flat spectrum (α~0 Sν~να).Additionally, we are able to limit the pseudoluminosity of any pulsar toL1400≡S1400400d2<~30 mJykpc2 for the central radio pulsar, assuming a distance of 17kpc. In light of these observational results, we test whether G328.4+0.2is a pulsar wind nebula or an SNR that contains a large pulsar windnebula using a simple hydrodynamic model for the evolution of a pulsarwind nebula inside an SNR. As a result of this analysis, we concludethat G328.4+0.2 is a young (<~10,000 years old) pulsar wind nebulaformed by a low magnetic field (<~1012 G) neutron starborn spinning rapidly (<~10 ms) expanding into an undetected SNRformed by an energetic (>~1051 ergs), low ejecta mass(Mej<~5 Msolar) supernova explosion thatoccurred in a low-density (n~0.03 cm-3) environment.
| Structure in the radio counterpart to the 2004 December 27 giant flare from SGR 1806-20
On 2004 December 27, the magnetar SGR 1806-20 underwent an enormousoutburst resulting in the formation of an expanding, moving, and fadingradio source. We report observations of this radio source with theMulti-Element Radio-Linked Interferometer Network and the Very LongBaseline Array. The observations confirm the elongation and expansionalready reported based on observations at lower angular resolutions, butsuggest that at early epochs the structure is not consistent with thevery simplest models such as a smooth flux distribution. In particular,there appears to be significant structure on small angular scales, with~10 per cent of the radio flux arising on angular scales <=100milliarcsec. This structure may correspond to localized sites ofparticle acceleration during the early phases of expansion andinteraction with the ambient medium.
| Modelling of isolated radio pulsars and magnetars on the fossil field hypothesis
We explore the hypothesis that the magnetic fields of neutron stars areof fossil origin. For parametrized models of the distribution ofmagnetic flux on the main sequence and of the birth spin period of theneutron stars, we calculate the expected properties of isolated radiopulsars in the Galaxy using as our starting point the initial massfunction and star formation rate as a function of Galactocentric radius.We then use the 1374-MHz Parkes Multi-Beam Survey of isolated radiopulsars to constrain the parameters in our model and to deduce therequired distribution of magnetic fields on the main sequence. We findagreement with observations for a model with a star formation rate thatcorresponds to a supernova rate of 2 per century in the Galaxy fromstars with masses in the range 8-45Msolar and predict 447000active pulsars in the Galaxy with luminosities greater than 0.19 mJykpc2. The progenitor OB stars have a field distribution whichpeaks at ~46 G with ~8 per cent of stars having fields in excess of 1000G. The higher-field progenitors yield a population of 24 neutron starswith fields in excess of 1014 G, periods ranging from 5 to 12s, and ages of up to 100000 yr, which we identify as the dominantcomponent of the magnetars. We also predict that high-field neutronstars (logB > 13.5) originate preferentially from higher-massprogenitors and have a mean mass of 1.6Msolar, which issignificantly above the mean mass of 1.4Msolar calculated forthe overall population of radio pulsars.
| X-Ray Timing Observations of PSR J1930+1852 in the Crab-like SNR G54.1+0.3
We present new X-ray timing and spectral observations of PSR J1930+1852,the young energetic pulsar at the center of the nonthermal supernovaremnant G54.1+0.3. Using data obtained with the Rossi X-Ray TimingExplorer (RXTE) and Chandra X-ray observatories we have derived anupdated timing ephemeris of the 136 ms pulsar spanning 6 years. Duringthis interval, however, the period evolution shows significantvariability from the best-fit constant spin-down rate ofP˙=7.5112(6)×10-13 s s-1, suggestingstrong timing noise and/or glitch activity. The X-ray emission is highlypulsed (71%+/-5% modulation) and is characterized by an asymmetric,broad profile (~70% duty cycle) that is nearly twice the radio width.The spectrum of the pulsed emission is well fitted with an absorbedpower law of photon index Γ=1.2+/-0.2 this is marginally harderthan that of the unpulsed component. The total 2-10 keV flux of thepulsar is 1.7×10-12 ergs cm-2s-1. These results confirm PSR J1930+1852 as a typicalCrab-like pulsar.
| On the origin and excitation of the extended nebula surrounding NGC1275
We use line-of-sight velocity information on the filamentaryemission-line nebula of NGC1275 to infer a dynamical model of thenebula's flow through the surrounding intracluster gas. We detectoutflowing gas and flow patterns that match simulations of buoyantlyrising bubbles from which we deduce that some of the nebula filamentshave been drawn out of NGC1275. We find a radial gradient of the ratio[NII]λ6584/Hα which may be due to a variation inmetallicity, interactions with the surrounding intracluster medium or ahardening of the excitation mechanism. We find no preferred spatialcorrelation of stellar clusters within the filaments and there is anotable lack of [OIII]λ5007 emission, therefore it is unlikelythat the filaments are ionized by stellar ultraviolet.
| Observations of Markarian 421 with the MAGIC Telescope
The MAGIC telescope took data of very high energy γ-ray emissionfrom the blazar Markarian 421 (Mrk 421) between 2004 November and 2005April. We present a combined analysis of data samples recorded underdifferent observational conditions, down to γ-ray energies of 100GeV. The flux was found to vary between 0.5 and 2 crab (integrated above200 GeV), considered a low state when compared to known data. Althoughthe flux varied day by day, no short-term variability was observed,although there is some indication that not all nights show an equallyquiescent state. The results at higher energies were found to beconsistent with previous observations. A clear correlation is observedbetween γ-ray and X-ray fluxes, whereas no significant correlationbetween γ-ray and optical data is seen. The spectral energydistribution between 100 GeV and 3 TeV shows a clear deviation from apower law, more clearly and at lower flux than previous observations athigher energies. The deviation persists after correcting for the effectof attenuation by the extragalactic background light, and most likely issource-inherent. There is a rather clear indication of an inverseCompton peak around 100 GeV. The spectral energy distribution of Mrk 421can be fitted by a one-zone synchrotron self-Compton model, suggestingonce again a leptonic origin of the very high energy γ-rayemission from this blazar.
| Simulations of the axisymmetric magnetospheres of neutron stars
In this paper we present the results of time-dependent simulations ofthe dipolar axisymmetric magnetospheres of neutron stars carried outwithin the frameworks of both relativistic magnetohydrodynamics (MHD)and resistive force-free electrodynamics. The results of force-freesimulations reveal the inability of our numerical method to accommodatethe equatorial current sheets of pulsar magnetospheres, and raise aquestion mark about the robustness of this approach. On the other hand,the MHD approach allows us to make significant progress. We start with anon-rotating magnetically dominated dipolar magnetosphere and follow itsevolution as the stellar rotation is switched on. We find that thetime-dependent solution gradually approaches a steady state that is veryclose to the stationary solution of the pulsar equation found in 1999 byContopoulos, Kazanas & Fendt. This result suggests that otherstationary solutions that have the Y-point located well inside the lightcylinder are unstable. The role of particle inertia and pressure on thestructure and dynamics of MHD magnetospheres is studied in detail, aswell as the potential implications of dissipative processes in theequatorial current sheet. We argue that pulsars may have differentiallyrotating magnetospheres which develop noticeable structuraloscillations, and that this may help to explain the nature of thesubpulse phenomena.
| A method to measure the mirror reflectivity of a prime focus telescope
We have developed a method to measure the mirror reflectivity oftelescopes. While it is relatively easy to measure the localreflectivity of the mirror material, it is not so straightforward tomeasure the amount of light that it focuses in a spot of a givendiameter. Our method is based on the use of a CCD camera that is fixedon the mirror dish structure and observes simultaneously part of thetelescope’s focal plane and the sky region around its opticalaxis. A white diffuse reflecting disk of known reflectivity is fixed inthe telescopes focal plane. During a typical reflectivity measurementthe telescope is directed to a selected star. The CCD camera can see twoimages of the selected star, one directly and another one as a spotfocused by the mirror on the white disk. The ratio of the reflectedstarlight integrated by the CCD from the white disk to the directlymeasured one provides a precise result of the product of (mirror area× mirror reflectivity).
| A two-dimensional electrodynamical outer gap model for γ-ray pulsars: γ-ray spectrum
A two-dimensional electrodynamical model is used to study particleacceleration in the outer magnetosphere of a pulsar. The chargedepletion from the Goldreich-Julian charge density causes a largeelectric field along the magnetic field lines. The charge particles areaccelerated by the electric field and emit γ-rays via thecurvature process. Some of the emitted γ-rays may collide withX-ray photons to make new pairs, which are accelerated again on thedifferent field lines and emit γ-rays. We simulate the paircreation cascade in the meridional plane using the pair creationmean-free path, in which the X-ray photon number density is proportionalto the inverse square of the radial distance. With the space chargedensity determined by the pair creation simulation, we solve theelectric structure of the outer gap in the meridional plane andcalculate the curvature spectrum.We investigate in detail the relation between the spectrum and totalcurrent, which is carried by the particles produced in the gap and/orinjected at the boundaries of the gap. We demonstrate that the hardnessof the spectrum is strongly controlled by the current carriers.Especially, the spectrum sharply softens if we assume a larger particleinjection at the outer boundary of the outer gap. This is because themean-free path of the pair creation of the inwardly propagatingγ-ray photons is much shorter than the light radius, so many pairsare produced in the gap to quench the outer gap.Because the two-dimensional model can link both gap width along themagnetic field line and trans-field thickness with the spectral cut-offenergy and flux, we can diagnose both the current through the gap andthe inclination angle between the rotational and magnetic axes. We applythe theory to the Vela pulsar. By comparing the results with theEnergetic Gamma Ray Experiment Telescope (EGRET) data, we rule out anycases that have a large particle injection at the outer boundary. Wealso suggest the inclination angle of αinc >=65°. The present model predicts the outer gap starting from near theconventional null charge surface for the Vela pulsar.
| LS I +61 303 as a potential neutrino source on the light of magic results
Very high energy γ-rays have recently been detected from themicroquasar LS I +61 303 using the MAGIC telescope. A phenomenologicalstudy on the concomitant neutrinos that would be radiated if theγ-ray emission is hadronic in origin is herein presented. Neutrinooscillations are considered, and the expected number of events in akm-scale detector such as ICECUBE is computed under differentassumptions including orbital periodicity and modulation, as well asdifferent precision in the modeling of the detector. We argue that theupper limits already imposed on the neutrino emission of LS I +61 303using AMANDA-II and the forthcoming measurements by ICECUBE maysignificantly constrain in an independent and unbiased way theγ-ray to neutrino flux ratio, and thus the possibility of ahadronic origin of the γ-rays. The viability of hadronic modelsbased on wind jet interactions in the LS +61 303 system after MAGICmeasurements is discussed.
| Observational constraints on the radio and γ-ray emission regions of PSR B1055-52
Observational constraints on the radio and γ-ray emission regionsof PSR B1055-52 seen through our line of sight are presented byanalysing the position angle curves of radio linear polarization andfitting the observed pulse widths of radio and γ-ray pulses andthe phase offsets between them. Aberration, retardation and magneticfield sweep back effects that can cause additional phase offset betweenthe emissions from different locations are taken into account. Thefollowing conclusions are obtained. (i) The radio main pulse andγ-ray pulses are emitted from the same pole, while the radiointerpulse is emitted from the opposite pole. (ii) The interpulseemission region locates on the open field lines much closer to themagnetic axis than those of the main pulse, and the emission altitudesare higher than those of the main pulse. (iii) At each pole, there areprobably two groups of field lines where radio emission is generated, ofwhich the outer one consists of open field lines near (or including) thelast open field lines and the inner one consists of open field linesfrom very near the magnetic pole to approximately the midway between themagnetic axis and the last open field lines. (iv) The γ-ray pulsecomes from inner open field lines rather than from the last open fieldlines, and the emission altitudes are beyond the null charge surface.
| The subpulse modulation properties of pulsars at 92 cm and the frequency dependence of subpulse modulation
Context: A large sample of pulsars has been observed to study theirsubpulse modulation at an observing wavelength (when achievable) of both21 and 92 cm using the Westerbork Synthesis Radio Telescope. In thispaper we present the 92-cm data and a comparison is made with thealready published 21-cm results. Aims: The main goals are to determinewhat fraction of the pulsars have drifting subpulses, whether thosepulsars share some physical properties and to find out if subpulsemodulation properties are frequency dependent. Methods: We analysed191 pulsars at 92 cm searching for subpulse modulation using fluctuationspectra. The sample of pulsars is as unbiased as possible towards anyparticular pulsar characteristics. Results: For 15 pulsars driftingsubpulses are discovered for the first time and 26 of the new driftersfound in the 21-cm data are confirmed. We discovered nulling for 8sources and 8 pulsars are found to intermittently emit single pulsesthat have pulse energies similar to giant pulses. Another pulsar wasshown to exhibit a subpulse phase step. It is estimated that at leasthalf of the total population of pulsars have drifting subpulses whenobservations with a high enough signal-to-noise ratio would beavailable. It could well be that the drifting subpulse mechanism is anintrinsic property of the emission mechanism itself, although for somepulsars it is difficult or impossible to detect. Drifting subpulses arein general found at both frequencies, although the chance of detectingdrifting subpulses is possibly slightly higher at 92 cm. It appears thatthe youngest pulsars have the most disordered subpulses and thesubpulses become more and more organized into drifting subpulses as thepulsar ages. The modulation indices measured at the two frequencies areclearly correlated, although at 92 cm they are on average possiblyhigher. At 92 cm the modulation index appears to be correlated with thecharacteristic age of the pulsar and the complexity parameters aspredicted by three different emission models. The correlations with themodulation indices are argued to be consistent with the picture in whichthe radio emission can be divided in a drifting subpulse signal plus aquasi-steady signal which becomes, on average, stronger at highobserving frequencies. The measured values of P3 at the twofrequencies are highly correlated, but there is no evidence for acorrelation with other pulsar parameters.Appendix A is only available in electronic form at http://www.aanda.org.Table [see full textsee full text] is also available in electronic format the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/469/607
| A search for 22-GHz H2O masers in supernova remnants
We have examined 18 known supernova remnants for evidence of the22.235-GHz water maser spectral line, using the 20-m telescope at theOnsala observatory in Sweden. No evidence of such emission wasdiscovered above the background noise in any of the observations. Wehave developed a radiative transfer water maser model for the shockedregions of supernova remnants in order to determine why this emissionwas not detected. The model indicates an insignificant amount of22.235-GHz gain; the gain lengths are far too short to allow theformation of detectable water masers from these objects. We predict thatonly in unusual environments can 22-GHz water masers be detected aroundsupernova remnants.
| A starquake model for the Vela pulsar
The measured values of the glitch healing parameter, Q, of the Velapulsar are found to be inconsistent with the starquake mechanism forglitch generation in various neutron star (NS) models, based upon theparametrized equations of state (EOSs) of dense nuclear matter. Suchmodels correspond to an unrealistic mass range <=0.5Msolarfor the pulsar, if the observational constraints of the fractionalmoment of inertia of the core component(Icore/Itotal<= 0.2) which is called the glitchhealing parameter, Q, according to the starquake model, are imposed onthese models. However, we show that these observational constraintsyield a realistic mass range for NS models, corresponding to a coregiven by the stiffest equation of state, dP/dE= 1 (in geometrizedunits), and the envelope is characterized by the well-known EOS of anadiabatic polytrope (d lnP/d lnρ=Γ1), if thecontinuity of the adiabatic speed of sound , together with the pressure(P), the energy density (E) and the two metric parameters (ν andλ), is assured at the core-envelope boundary of the models andthis boundary is worked out on the basis of the `compatibilitycriterion' for hydrostatic equilibrium. The models yield a stablesequence of NS masses in the range 1.758 <=M<=2.2Msolar, corresponding to the glitch healing parameterrange 0 <=Q<= 0.197, for a choice of the `transitiondensity'Eb= 1.342 × 1015gcm-3 atthe core-envelope boundary. The maximum stable value of2.2Msolar in this sequence, in fact, corresponds to thelowest possible upper bound on NS masses calculated in the literature,on the basis of modern EOSs for NS matter. The models yield the surfaceredshift zR~= 0.6913 and mass M~= 2.153Msolar forthe `central' weighted mean value, Q= 0.12 +/- 0.07, of the glitchhealing parameter of the Vela pulsar. This value of mass can increaseslightly up to M~= 2.196Msolar, whereas the surface redshiftcan increase up to the value zR~= 0.7568[which represents anultracompact object (zR>= 0.73)], if the observationalconstraint of the upper weighted mean value of Q~= 0.19 is imposed onthese models. However, for the lower weighted mean value of Q~= 0.05,the mass and surface redshift can decrease to the values ofzR~= 0.6066 and M~= 2.052Msolar respectively.These results set the lower bound on the energy of gravitationallyredshifted radiation in the rather narrow range of 0.291-0.302 MeV. Theobservation of the lower bound on the energy of a γ-ray pulse atabout 0.30 MeV from the Vela pulsar in 1984 is in excellent agreementwith this result, provided that this energy can be interpreted as theenergy of gravitationally redshifted electron-positron annihilationradiation from the surface of the star.
| Hard X-ray emission of the Earth's atmosphere: Monte Carlo simulations
We perform Monte Carlo simulations of cosmic ray-induced hard X-rayradiation from the Earth's atmosphere. We find that the shape of thespectrum emergent from the atmosphere in the energy range 25-300 keV ismainly determined by Compton scatterings and photoabsorption, and isalmost insensitive to the incident cosmic ray spectrum. We provide afitting formula for the hard X-ray surface brightness of the atmosphereas would be measured by a satellite-borne instrument, as a function ofenergy, solar modulation level, geomagnetic cut-off rigidity and zenithangle. A recent measurement by the INTEGRAL observatory of theatmospheric hard X-ray flux during the occultation of the cosmic X-raybackground by the Earth agrees with our prediction within 10 per cent.This suggests that Earth observations could be used for in-orbitcalibration of future hard X-ray telescopes. We also demonstrate thatthe hard X-ray spectra generated by cosmic rays in the crusts of theMoon, Mars and Mercury should be significantly different from thatemitted by the Earth's atmosphere.
| GCRT J1745-3009: a precessing radio pulsar?
A unique transient bursting radio source, GCRT J1745-3009, has beendiscovered near the direction of the Galactic Centre. The explanation ofthis phenomenon is still an open question, although some efforts tounderstand its nature have been made. This Letter shows that most of theobserved features can be reproduced by our proposed precessing pulsarmodel. It is found that the precession angle of the pulsar should belarger (>~15°) than that of previously known precessing pulsars,which have a precession angle <~10°, if the beam width of thepulsar is larger than 10°. The pulsar could be a nulling (or evenextremely nulling) radio pulsars to account for the transient nature ofthe source. This model can be confirmed if a pulsar is detected at theposition of the source. The pulsar could hardly be a normal neutron star(but could probably be a solid quark star) if the spin period of thepulsar is detected to be >~10ms in the future.
| Three-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Polarization Analysis
The Wilkinson Microwave Anisotropy Probe (WMAP) has mapped the entiresky in five frequency bands between 23 and 94 GHz withpolarization-sensitive radiometers. We present 3 year full-sky maps ofthe polarization and analyze them for foreground emission andcosmological implications. These observations open up a new window forunderstanding how the universe began and help set a foundation forfuture observations. WMAP observes significant levels of polarizedforeground emission due to both Galactic synchrotron radiation andthermal dust emission. Synchrotron radiation is the dominant signal atl<50 and ν<~40 GHz, while thermal dust emission is evident at94 GHz. The least contaminated channel is at 61 GHz. We present a modelof polarized foreground emission that captures the large angular scalecharacteristics of the microwave sky. After applying a Galactic maskthat cuts 25.7% of the sky, we show that the high Galactic latitude rmspolarized foreground emission, averaged over l=4-6, ranges from ~5 μKat 22 GHz to <~0.6 μK at 61 GHz. By comparison, the levels ofintrinsic CMB polarization for a ΛCDM model with an optical depthof τ=0.09 and assumed tensor-to-scalar ratio r=0.3 are ~0.3 μKfor E-mode polarization and ~0.1 μK for B-mode polarization. Toanalyze the maps for CMB polarization at l<16, we subtract a model ofthe foreground emission that is based primarily on a scaling WMAP's 23GHz map. In the foreground-corrected maps, we detectl(l+1)CEEl=<2-6>/2π=0.086+/-0.029(μK)2. This is interpreted as the result of rescatteringof the CMB by free electrons released during reionization atzr=10.9+2.7-2.3 for a model withinstantaneous reionization. By computing the likelihood of just the EEdata as a function of τ we find τ=0.10+/-0.03. When the same EEdata are used in the full six-parameter fit to all WMAP data (TT, TE,EE), we find τ=0.09+/-0.03. Marginalization over the foregroundsubtraction affects this value by δτ<0.01. We see noevidence for B modes, limiting them tol(l+1)CBBl=<2-6>/2π=-0.04+/-0.03(μK)2. We perform a template fit to the E-mode and B-modedata with an approximate model for the tensor scalar ratio. We find thatthe limit from the polarization signals alone is r<2.2 (95% CL),where r is evaluated at k=0.002 Mpc-1. This corresponds to alimit on the cosmic density of gravitational waves ofΩGWh2<5×10-12. From thefull WMAP analysis, we find r<0.55 (95% CL) corresponding to a limitof ΩGWh2<1×10-12 (95%CL). The limit on r is approaching the upper bound of predictions forsome of the simplest models of inflation, r~0.3.
| Minimal models of cooling neutron stars with accreted envelopes
We study the `minimal' cooling scenario of superfluid neutron stars withnucleon cores, where the direct Urca process is forbidden and enhancedcooling is produced by neutrino emission due to the Cooper pairing ofneutrons. Extending our recent previous work, we include the effects ofsurface accreted envelopes of light elements. We employ thephenomenological density-dependent critical temperaturesTcp(ρ) and Tcnt(ρ) of singlet-state protonand triplet-state neutron pairing in a stellar core, as well as thecritical temperature Tcns(ρ) of singlet-state neutronpairing in a stellar crust. We show that the presence of accretedenvelopes simplifies the interpretation of observations of thermalradiation from isolated neutron stars in the scenario of our recentprevious work and widens the class of models for nucleon superfluidityin neutron star interiors consistent with the observations.
| High-Energy Emission from Pulsar Outer Magnetospheres
We investigate particle accelerators in rotating neutron-starmagnetospheres by simultaneously solving the Poisson equation for theelectrostatic potential together with the Boltzmann equations forelectrons, positrons, and photons on the poloidal plane. Applying thescheme to the three pulsars, Crab, Vela, and PSR B1951+32, wedemonstrate that the observed phase-averaged spectra are basicallyreproduced from infrared to very high energies. It is found that theVela's spectrum in 10-50 GeV is sensitive to the three-dimensionalmagnetic field configuration near the light cylinder; thus, a carefulargument is required to discriminate the inner-gap and outer-gapemissions using a gamma-ray telescope like GLAST. It is also found thatPSR B1951+32 has a large inverse-Compton flux in TeV energies, which canbe detected by ground-based air Cerenkov telescopes as a pulsedemission.
| Precession of the isolated neutron star PSR B1828-11
Stairs, Lyne & Shemar have found that the arrival-time residualsfrom PSR B1828-11 vary periodically with a period ~500 d. This behaviourcan be accounted for by precession of the radio pulsar, aninterpretation that is reinforced by the detection of variations in itspulse profile on the same time-scale. Here, we model the periodresiduals from PSR B1828-11 in terms of precession of a triaxial rigidbody. We include two contributions to the residuals: (i) the geometriceffect, which arises because the times at which the pulsar emission beampoints towards the observer varies with precession phase; and (ii) thespin-down contribution, which arises from any dependence of thespin-down torque acting on the pulsar on the angle between its spin andmagnetic axes. We use the data to probe numerous properties of thepulsar, most notably its shape, and the dependence of its spin-downtorque on , for which we assume the sum of a spin-aligned component(with a weight 1 -a) and a dipolar component perpendicular to themagnetic beam axis (weight a), rather than the vacuum dipole torque (a=1). We find that a variety of shapes are consistent with the residuals,with a slight statistical preference for a prolate star. Moreover, arange of torque possibilities fit the data equally well, with no strongpreference for the vacuum model. In the case of a prolate star, we findevidence for an angle-dependent spin-down torque. Our results show thatthe combination of geometrical and spin-down effects associated withprecession can account for the principal features of the timingbehaviour of PSR B1828-11, without fine tuning of the parameters.
| The X-Ray Structure and Spectrum of the Pulsar Wind Nebula Surrounding PSR B0540-69.3
We present the first spatially resolved spectral study of anextragalactic pulsar wind nebula (PWN), that surrounding the 50 mspulsar B0540-69.3 in the Large Magellanic Cloud, using the ACISinstrument on board the Chandra X-ray Observatory. The spectralproperties of the PWN change at a radius of ~5", corresponding to ashell detected optically in [O III]. Within this radius, the spectra arewell characterized by a simple power law, whose spectral index variesstrongly with radius, so that the effective size of the nebula decreaseswith increasing photon energy. These X-ray measurements support the ideathat the [O III] shell represents an outer skin surrounding the windnebula, analogous to that seen in the Crab Nebula. We use thesize-energy relation and radial variability of the X-ray spectral indexto estimate PWN parameters, including the radial dependence of themagnetic field, and the average ratio of electromagnetic to particleflux.
| Unusual Strong Hard Flare or Burst in 1A 1246-588
Observations with the RXTE All-Sky Monitor revealed a fast flare or unusual burst in the X-ray source 1A 1246-588 (RA 192.414, Dec -59.088;J2000) in four consecutive dwells on 2006 May 26. Each dwell was 90 slong with a 6 s interval between dwells to rotate the camera assembly.The X-ray flare began at 2006 May 26 18:57:34 (UTC; MJD 53881.78998)and lasted at least 340 s. The average source strength in each of thefour dwells was 103 +- 4, 144 +- 4, 73 +- 3, and 39 +- 2 ASM counts/sin the 1.5-12 keV band (corresponding to 1.37 +- 0.05, 1.91 +- 0.06,0.97 +- 0.03, and 0.52 +- 0.02 Crab).
| Observation of Very High Energy γ-Rays from the AGN 1ES 2344+514 in a Low Emission State with the MAGIC Telescope
The MAGIC collaboration has observed very high energy gamma-ray emissionfrom the AGN 1ES 2344+514. A gamma-ray signal corresponding to an 11σ excess and an integral flux of(2.38+/-0.30stat+/-0.70syst)×10-11cm-2 s-1 above 200 GeV has been obtained from 23.1hr of data taking between 2005 August 3 and 2006 January 1. The dataconfirm the previously detected gamma-ray emission from this objectduring a flare seen by the Whipple collaboration in 1995 and theevidence (below 5 σ significance level) from long-termobservations conducted by the Whipple and HEGRA groups. The MAGICobservations show a relatively steep differential photon spectrum thatcan be described by a power law with a photon index ofα=-2.95+/-0.12stat+/-0.2syst between 140 GeVand 5.4 TeV. The observations reveal a low-flux state, about 6 timesbelow the 1995 flare seen by Whipple and comparable with the previousWhipple and HEGRA long-term measurements. During the MAGIC observationsno significant time variability was observed.
| An Engineer Becomes an Astronomer
The early days of radio astronomy in Australia are revisited. Theevolution of ideas and the way they led to various instrumentaldevelopments and some of the results of these developments arepresented. Besides these personal reminiscences, an indication of thepolitical background that sometimes influenced developments is givenand, as a coda, an account of a different approach to relativity throughthe so-called twin paradox.
| The Energy Spectrum of the Blazar Markarian 421 above 130 GeV
Markarian 421 (Mrk 421) was the first blazar detected at gamma-rayenergies above 300 GeV, and it remains one of only twelve TeV blazarsdetected to date. TeV gamma-ray measurements of its flaring activity andspectral variability have placed constraints on models of thehigh-energy emission from blazars. However, observations between 50 and300 GeV are rare, and the high-energy peak of the spectral energydistribution (SED), predicted to be in this range, has never beendirectly detected. We present a detection of Mrk 421 above 100 GeV asmade by the Solar Tower Atmospheric Cherenkov Effect Experiment (STACEE)during a multiwavelength campaign in early 2004. STACEE is aground-based atmospheric Cherenkov telescope using the wave-frontsampling technique to detect gamma rays at lower energies than achievedby most imaging Cherenkov telescopes. We also outline a method forreconstructing gamma-ray energies using a solar heliostat telescope.This technique was applied to the 2004 data, and we present thedifferential energy spectrum of Mrk 421 above 130 GeV. Assuming adifferential photon flux dN/dE~E-α, we measure aspectral index α=2.1+/-0.2stat+0.2-0.1sys. Finally, we discuss the STACEEspectrum in the context of the multiwavelength results from the sameepoch.
| Far Ultraviolet Spectroscopic Explorer Survey of Magellanic Cloud Supernova Remnants
We report the progress to date from an ongoing unbiased ultravioletsurvey of supernova remnants in the Magellanic Clouds using the FarUltraviolet Spectroscopic Explorer (FUSE) satellite. Earlier work withFUSE and other instruments has indicated that optical and/or X-raycharacteristics of supernova remnants are not always good predictors oftheir brightness in the ultraviolet. This survey is obtaining spectra ofa random large sample of Magellanic Cloud supernova remnants with abroad range of radio, optical, and X-ray properties. We proposed 39objects in the Large Magellanic Cloud and 11 objects from the SmallMagellanic Cloud, with a standard request of 10 ks per object using theFUSE 30" square aperture. To date, 39 objects have been observed in thesurvey (38 in the LMC and 1 in the SMC) and 15 have been detected, adetection rate of nearly 40%. Our survey has nearly tripled the numberof UV-detected SNRs in the Magellanic Clouds (from 8 to 22). Because ofthe diffuse source sensitivity of FUSE, upper limits on nondetectedobjects are quite sensitive in many cases, dependent on night observingfraction and whether stellar light contamination plays a role for agiven object. Estimated total luminosities in O VI, based simply onscaling the flux at the observed positions to an entire object, span abroad range from considerably brighter to many times fainter than theinferred soft X-ray luminosities, indicating that O VI can be animportant and largely unrecognized coolant in certain objects. Wecompare the optical and X-ray properties of the detected and nondetectedobjects but do not find a simple indicator for ultravioletdetectability. Nondetections may be due to clumpiness of the emission,high foreground extinction, slow shocks whose emission gets attenuatedby the Magellanic interstellar medium, or a combination of theseeffects. The characteristics of individual detected supernova remnantsare summarized in an Appendix.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by the Johns HopkinsUniversity under NASA contract NAS5-32985.
| The Spin Periods and Rotational Profiles of Neutron Stars at Birth
We present results from an extensive set of one- and two-dimensionalradiation-hydrodynamic simulations of the supernova core-collapse,bounce, and postbounce phases and focus on the proto-neutron star (PNS)spin periods and rotational profiles as a function of initial iron coreangular velocity, degree of differential rotation, and progenitor mass.For the models considered, we find a roughly linear mapping betweeninitial iron core rotation rate and PNS spin. The results indicate thatthe magnitude of the precollapse iron core angular velocities is thesingle most important factor in determining the PNS spin. Differences inprogenitor mass and degree of differential rotation lead only to smallvariations in the PNS rotational period and profile. Based on ourcalculated PNS spins at ~200-300 ms after bounce and assuming angularmomentum conservation, we estimate final neutron star rotation periods.We find periods of 1 ms and shorter for initial central iron coreperiods of <~10 s. This is appreciably shorter than what previousstudies have predicted and is in disagreement with current observationaldata from pulsar astronomy. After considering possible spin-downmechanisms that could lead to longer periods, we conclude that there isno mechanism that can robustly spin down a neutron star from ~1 msperiods to the ``injection'' periods of tens to hundreds of millisecondsobserved for young pulsars. Our results indicate that, given currentknowledge of the limitations of neutron star spin-down mechanisms,precollapse iron cores must rotate with periods of around 50-100 s toform neutron stars with periods generically near those inferred for theradio pulsar population.
| Observations of TeV γ-rays from Mrk 421 during December 2005 to April 2006 with the TACTIC telescope
The TACTIC γ-ray telescope has observed Mrk 421 on 66 clear nightsfrom December 07, 2005 to April 30, 2006, totalling ˜202 h ofon-source observations. Here, we report the detection of flaringactivity from the source at ⩾1 TeV energy and the time-averageddifferential γ-ray spectrum in the energy range 1 11 TeV for thedata taken between December 27, 2005 and February 07, 2006 when thesource was in a relatively higher state as compared to the rest of theobservation period. Analysis of this data spell, comprising ˜97 hreveals the presence of a ˜12.0σ γ-ray signal withdaily flux of >1 Crab unit on several days. A pure power law spectrumwith exponent ‑3.11 ± 0.11 as well as a power law spectrumwith an exponential cutoff (Γ = ‑2.51 ± 0.26 andE0 = (4.7 ± 2.1) TeV) are found to provide reasonablefits to the inferred differential spectrum within statisticaluncertainties. We believe that the TeV light curve presented here, fornearly 5 months of extensive coverage, as well as the spectralinformation at γ-ray energies of >5 TeV provide a useful inputfor other groups working in the field of γ-ray astronomy.
| The search for the elusive Zeeman effect in H {\large I}
% The paper describes the early endeavours by radioastronomers to detectthe weak signature of the Zeeman effect in interstellar neutral hydrogenclouds in an effort to measure the Galactic magnetic field strength. Thesearch is set in the context of the neutral hydrogen programme atJodrell Bank.
| Calibration of the Rossi X-Ray Timing Explorer Proportional Counter Array
We present the calibration and background model for the ProportionalCounter Array on board the Rossi X-Ray Timing Explorer. The energycalibration is systematics-limited below 10 keV, with deviations from apower-law fit to the Crab Nebula plus pulsar of less than 1%. Unmodeledvariations in the instrumental background amount to less than 2% of theobserved background below 10 keV and less than 1% between 10 and 20 keV.Individual photon arrival times are accurate to 4.4 μs at all timesduring the mission and to 2.5 μs after 1997 April 29. The peakpointing direction of the five collimators is known to a precision of afew arcseconds.
| X-Ray Spectral Variability of TeV Blazars during Rapid Flares
The spectral energy distribution (SED) of TeV blazars peaks both at keVand TeV energies. The X-ray emission is generally believed to originatein the synchrotron emission from relativistic electrons (and positrons)in the jet of these sources, while the origin of the gamma-ray emissionis still being debated. We report results from a systematic study ofX-ray spectral variability of Mrk 421 and Mrk 501 during individualflares that last for several days, making use of some of thehigh-quality data that have recently become available. The X-ray spectraof the two sources fall on the opposite sides of the synchrotron peak oftheir respective SEDs, so they together may offer additional insightsinto the physical origin of X-ray variability. We modeled each of thetime-resolved X-ray spectra over a single flare by adopting ahomogeneous spatial distribution and an instantaneous power-law spectraldistribution for the emitting particles. We focused on the variation offour key parameters: particle spectral index, maximum Lorentz factor,energy density, and magnetic field. While there is considerabledegeneracy in the fits, we show that, in order to account for the X-rayspectral variability observed in Mrk 421, at least three of theparameters are required to vary in most cases, with the spectral indexbeing one of them. The observations of Mrk 501 support the conclusion,although the quality of the data is not as good. We discuss theimplications of the results.
| Detection of the polarized radio emission from the Crab Nebula
% The radio emission that was observed in earliest times could not beinterpreted in terms of thermal emission process. The suggestion thatsynchrotron radiation was observed required the confirmation by thedetection of linear polarization. We have made such a detection in 1957which we describe in the present article.
| On the Nature of Part-Time Radio Pulsars
The recent discovery of rotating radio transients and thequasi-periodicity of pulsar activity in the radio pulsar PSR B1931+24has challenged the conventional theory of radio pulsar emission. Here wesuggest that these phenomena could be due to the interaction between theneutron star magnetosphere and the surrounding debris disk. The patternof pulsar emission depends on whether the disk can penetrate the lightcylinder and efficiently quench the processes of particle production andacceleration inside the magnetospheric gap. A precessing disk maynaturally account for the switch-on/off behavior in PSR B1931+24.
| Early Cambridge radio astronomy
% Radio astronomy started in Cambridge immediately after the hostilitiesof the World War II have ceased. Martin Ryle was the inspiring leader ofa small group that started to develop interferometry techniques at theCavendish Laboratory. From this development came the numerous Cambridgeradio source surveys culminating in the Nobel prize awarded to MartinRyle for invention of aperture synthesis. The history of this earlydevelopment is the subject of the present paper.
| The beginnings of decameter radio astronomy: pioneering works of Semen Ya. Braude and his followers in Ukraine
% S.Ya. Braude (1911-2003) was the well-known radio astronomer, one ofthe founders of low-frequency astronomical research in the world, inparticular in the former Soviet Union. He began to work in this field ofscience in 1957, in Kharkiv city (Ukraine), from the design andmanufacturing small decameter interferometer ID-1 and ID-2. Since thattime Braude and his team have developed more sophisticated radiodecameter telescopes as UTR-1 and UTR-2 (the largest in the world tillnow) as well as the first decameter VLBI network URAN. They haveobtained some important pioneering results about low-frequency radioemission of objects in our Solar system, Galaxy and Metagalaxy by meansof these telescopes. In this paper the key events of early history ofdecameter radio astronomy research in the former USSR are mentioned withemphasizing the role of S. Braude. For the period of 1957-1962, thequotations of Braude's Personal Diary (2003) are first laying open tothe public. The most important results obtained by S.Ya. Braude and hisfollowers as well as perspectives of decameter radio astronomy inUkraine and in the world are highlighted briefly.
| A Further Study of the Luminosity-dependent Cyclotron Resonance Energies of the Binary X-Ray Pulsar 4U 0115+63 with the Rossi X-Ray Timing Explorer
We report on the RXTE observations of the binary X-ray pulsar 4U0115+63, covering an outburst in 1999 March-April with 44 pointings. The3-30 keV PCA spectra and the 15-50 keV HEXTE spectra were analyzedjointly for cyclotron resonance features. When the 3-50 keV luminosityat an assumed distance of 7 kpc was in the range(5-13)×1037 ergs s-1, harmonic doublecyclotron features were observed in absorption at ~11 and ~22 keV, aswas measured previously during typical outbursts. As the luminositydecreased below ~5×1037 ergs s-1, the secondresonance disappeared, and the fundamental resonance energy graduallyincreased, up to ~16 keV at 0.16×1037 ergss-1. These results reconfirm the report by Mihara et al.using Ginga, who observed a single absorption at ~16 keV in a minor(~1037 ergs s-1) outburst of this object. Theluminosity-dependent cyclotron resonance energy might possibly beunderstood as a result of a decrease in the accretion column height, inresponse to a decrease in the mass accretion rate.
| Attempts by a theorist to work with Martin Ryle in the Cavendish, 1953-1955
This article gives story of interferometer with independent elements(Very Long Baseline Interferometer) in Russia. At the end of February1962 the author discussed with G.Ya. Gus'kov, DSN Station, Evpatorija anew type of radio interferometer and proposed an experiment between twoDSN stations. In September 1962 he reported the new method and proposeda VLBI experiment at seminar of Radio Astronomical Laboratory, Pushino,and then at a seminar of Astronomical Institute GAISH which recommendedto take out a Patent. In December GAISH sent documents to the PatentBureau. In summer 1963 the author discussed with B. Lovell in Evpatorijathe VLBI method of and we signed memorandum an Ev-JB experiment atěc{\lambda}bda=32 cm. In December 1963 the Patent Bureaupermitted publication, and the paper was sent to Radiofizika. ReallyVLBI in the USSR began with the proposal of M. Cohen and K. Kellermann,February 1968, to do an experiment between 22-m antenna Pushino and 43-mGreen Bank.
| Spitzer MIPS Limits on Asteroidal Dust in the Pulsar Planetary System PSR B1257+12
With the MIPS camera on Spitzer, we have searched for far-infraredemission from dust in the planetary system orbiting pulsar PSR B1257+12.With accuracies of 0.05 mJy at 24 μm and 1.5 mJy at 70 μm,photometric measurements find no evidence for emission at thesewavelengths. These observations place new upper limits on the luminosityof dust with temperatures between 20 and 1000 K. They are particularlysensitive to dust temperatures of 100-200 K, for which they limit thedust luminosity to below 3×10-5 of the pulsar'sspin-down luminosity, 3 orders of magnitude better than previous limits.Despite these improved constraints on dust emission, an asteroid beltsimilar to the solar system's cannot be ruled out.
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