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On the Apparent Lack of Be X-Ray Binaries with Black Holes
In our Galaxy there are 64 Be X-ray binaries known to date. Out ofthese, 42 host a neutron star (NS), and for the remainder the nature ofthe companion is unknown. None, so far, are known to host a black hole(BH). There seems to be no apparent mechanism that would preventformation or detection of Be stars with BHs. This disparity is referredto as a missing Be-BH X-ray binary problem. We point out that currentevolutionary scenarios that lead to the formation of Be X-ray binariespredict that the ratio of binaries with NSs to the ones with BHs israther high, F NStoBH ~ 10-50, with the more likely formationmodels providing the values at the high end. The ratio is a naturaloutcome of (1) the stellar initial mass function that produces more NSsthan BHs and (2) common envelope evolution (i.e., a major mechanisminvolved in the formation of interacting binaries) that naturallyselects progenitors of Be X-ray binaries with NSs (binaries withcomparable mass components have more likely survival probabilities) overones with BHs (which are much more likely to be common envelopemergers). A comparison of this ratio (i.e., F NStoBH ~ 30)with the number of confirmed Be-NS X-ray binaries (42) indicates thatthe expected number of Be-BH X-ray binaries is of the order of only~0-2. This is entirely consistent with the observed Galactic sample.
| A systematic study of variability among OB-stars based on HIPPARCOS photometry
Context: Variability is a key factor for understanding the nature of themost massive stars, the OB stars. Such stars lie closest to the unstableupper limit of star formation. Aims: In terms of statistics, thedata from the HIPPARCOS satellite are unique because of time coverageand uniformity. They are ideal to study variability in this large,uniform sample of OB stars. Methods: We used statisticaltechniques to determine an independant threshold of variabilitycorresponding to our sample of OB stars, and then applied an automaticalgorithm to search for periods in the data of stars that are locatedabove this threshold. We separated the sample stars into 4 maincategories of variability: 3 intrinsic and 1 extrinsic. The intrinsiccategories are: OB main sequence stars (~2/3 of the sample), OBe stars(~10%) and OB Supergiant stars (~1/4).The extrinsic category refers toeclipsing binaries. Results: We classified about 30% of the wholesample as variable, although the fraction depends on magnitude level dueto instrumental limitations. OBe stars tend to be much more variable(≈80%) than the average sample star, while OBMS stars are belowaverage and OBSG stars are average. Types of variables include αCyg, β Cep, slowly pulsating stars and other types from the generalcatalog of variable stars. As for eclipsing binaries, there arerelatively more contact than detached systems among the OBMS and OBestars, and about equal numbers among OBSG stars.
| Spectral distribution of Be/X-ray binaries in the Small Magellanic Cloud
The spectral distributions of Be/X-ray binaries in the Large MagellanicCloud and Galaxy have been shown to differ significantly from thedistribution of isolated Be stars in the Galaxy. Population synthesismodels can explain this difference in spectral distributions throughsubstantial angular momentum loss from the binary system. In this work,we explore the spectral distribution of Be/X-ray binaries in the SmallMagellanic Cloud (SMC) using high signal-to-noise ratio spectroscopy ofa sample of 37 optical counterparts to known X-ray pulsars. Our resultsshow that the spectral distribution of Be/X-ray binaries in the SMC isconsistent with that of the Galaxy, despite the lower metallicityenvironment of the SMC. This may indicate that, although the metallicityof the SMC is conducive to the formation of a large number of high-massX-ray binaries, the spectral distribution of these systems is likely tobe most strongly influenced by angular momentum losses during binaryevolution, which are not particularly dependent on the localmetallicity.Based on astronomical observations at the European Southern ObservatoryLa Silla in programmes 077.D-0415 and 079.D-0371.E-mail: vanessa@soton.ac.uk (VAM)
| A Search for New Galactic Magnetars in Archival Chandra and XMM-Newton Observations
We present constraints on the number of Galactic magnetars, which wehave established by searching for sources with periodic variability in506 archival Chandra observations and 441 archival XMM-Newtonobservations of the Galactic plane (|b|<5deg). Our searchrevealed four sources with periodic variability on timescales of200-5000 s, all of which are probably accreting white dwarfs. Weidentify 7 of 12 known Galactic magnetars, but find no new examples withperiods between 5 and 20 s. We convert this nondetection into limits onthe total number of Galactic magnetars by computing the fraction of theyoung Galactic stellar population that our survey covered. We find thateasily detectable magnetars, modeled after persistent anomalous X-raypulsars (e.g., with LX=1035 ergs s-1[0.5-10.0 keV] and Arms=12%), could have been identified in~5% of the Galactic spiral arms by mass. If we assume that threepreviously known examples randomly fall within our survey, then thereare 59+92-32 in the Galaxy. Barely detectablemagnetars (LX=3×1033 ergs s-1 andArms=15%) could have been identified throughout ~0.4% of thespiral arms. The lack of new examples implies that <540 exist in theGalaxy (90% confidence). Similar constraints are found by consideringthe detectability of transient magnetars in outburst. For assumedlifetimes of 104 yr, the birth rate of magnetars is between0.003 and 0.06 yr-1. Therefore, the birth rate of magnetarsis at least 10% of that for normal radio pulsars, and could exceed thatvalue, unless transient magnetars are active for >~105 yr.
| INTEGRAL observation of the accreting pulsar 1E1145.1-6141
Aims: We analyze 1050 ks of INTEGRAL data of the high mass X-ray binarypulsar 1E 1145.1-6141 to study its properties over a long time baseline,from June 2003 to June 2004, with wide spectral coverage. Methods: We study three high luminosity episodes, two of them at thesystem apoastron, three brightening with lower intensity, two at theperiastron, and one extended period of intermediate luminosity spanningone orbital cycle. We perform a timing analysis to determine the pulseperiod and pulse profiles at different energy ranges. We also analyzethe broad band phase averaged spectrum of different luminosity statesand perform phase resolved spectroscopy for the first flare. Results: From the timing analysis, we find a pulse period of ~297 saround MJD 53 000 with a significant scatter around the mean value. Fromthe spectral analysis we find that the source emission can be describedby an absorbed bremsstrahlung model in which the electron temperaturevaries between ~25 and ~37 keV, without any correlation with luminosity,and the intrinsic absorbing column is constantly of the order of1023 cm-2. Phase resolved spectral analysisreveals a different temperature of the plasma in the ascending anddescending edges of the pulse during the first flare. This justifies thepulse maximum shift by ~0.4 phase units between 20 and 100 keV observedin the pulse profiles. Conclusions: The comparison with theprevious period measurements reveals that the source is currentlyspinning down, in contrast to the long term secular trend observed sofar, indicating that at least a temporary accretion disk is formed. Thestudy of the spectral property variations with respect to time and spinphase suggests the presence of two emitting components at differenttemperatures whose relative intensity varies with time.
| Taking the Measure of the Universe: Precision Astrometry with SIM PlanetQuest
Precision astrometry at microarcsecond accuracy has applications for awide range of astrophysical problems. This paper is a study of thescience questions that can be addressed using an instrument withflexible scheduling that delivers parallaxes at about 4 μas ontargets as faint as V=20, and differential accuracy of 0.6 μas onbright targets. The science topics are drawn primarily from the team keyprojects, selected in 2000, for the Space Interferometry MissionPlanetQuest (SIM PlanetQuest). We use the capabilities of this missionto illustrate the importance of the next level of astrometric precisionin modern astrophysics. SIM PlanetQuest is currently in the detaileddesign phase, having completed in 2005 all of the enabling technologiesneeded for the flight instrument. It will be the first space-basedlong-baseline Michelson interferometer designed for precisionastrometry. SIM PlanetQuest will contribute strongly to manyastronomical fields, including stellar and galactic astrophysics,planetary systems around nearby stars, and the study of quasar and AGNnuclei. Using differential astrometry SIM PlanetQuest will search forplanets with masses as small as Earth orbiting in the "habitable zone"around the nearest stars, and could discover many dozen if Earth-likeplanets are common. It will characterize the multiple-planet systemsthat are now known to exist, and it will be able to search forterrestrial planets around all of the candidate target stars in theTerrestrial Planet Finder and Darwin mission lists. It will be capableof detecting planets around young stars, thereby providing insights intohow planetary systems are born and how they evolve with time. Precisionastrometry allows the measurement of accurate dynamical masses for starsin binary systems. SIM PlanetQuest will observe significant numbers ofvery high- and low-mass stars, providing stellar masses to 1%, theaccuracy needed to challenge physical models. Using precisionproper-motion measurements, SIM PlanetQuest will probe the Galactic massdistribution, and, through studies of tidal tails, the formation andevolution of the Galactic halo. SIM PlanetQuest will contribute tocosmology through improved accuracy of the Hubble constant. Withrepeated astrometric measurements of the nuclei of active galaxies, SIMPlanetQuest will probe the dynamics of accretion disks aroundsupermassive black holes, and the relativistic jets that emerge fromthem.
| INTEGRAL/IBIS all-sky survey in hard X-rays
We present results of an all-sky hard X-ray survey based on almost fouryears of observations with the IBIS telescope onboard the INTEGRALobservatory. The dead time-corrected exposure of the survey is ~33 Ms.Approximately 12% and 80% of the sky has been covered to limiting fluxeslower than 1 and 5 mCrab, respectively. Our catalog of detected sourcesincludes 403 objects, 316 of which exceed a 5σ detection thresholdon the time-averaged map of the sky, and the rest were detected invarious subsamples of exposures. Among the identified sources, 219 areGalactic (90 low-mass X-ray binaries, 76 high-mass X-ray binaries, 21cataclysmic variables, 6 coronally active stars, and other types) and137 are extragalactic, including 130 active galactic nuclei (AGNs) and 3galaxy clusters. We derived number-flux functions of AGNs and Galacticsources. The log N-log S relation of non-blazar AGNs is based on 68sources located at Galactic latitudes |b| > 5°, where the surveyis characterized by high identification completeness, with fluxes higherthan S_lim = 1.1 × 10-11 erg s-1cm-2(~0.8 mCrab) in the 17-60 keV energy band. The cumulativeAGN number-flux function can be described by a power law with a slope of1.62 ± 0.15 and normalization of (5.7±0.7) ×10-3 sources per deg2 at fluxes > 1.43 ×10-11 erg s-1 cm-2(>1 mCrab). ThoseAGNs with fluxes higher than S_lim make up ~1% of the cosmic X-raybackground at 17-60 keV. We present evidence of strong inhomogeneity inthe spatial distribution of nearby (⪉70 Mpc) AGNs, which reflectsthe large-scale structure in the local Universe.Based on observations with INTEGRAL, an ESA project with the instrumentsand science data center funded by ESA member states (especially the PIcountries: Denmark, France, Germany, Italy, Switzerland, Spain), CzechRepublic, and Poland, and with the participation of Russia and the USA.Table 1 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/475/775
| HeI doubled emission lines from A0535+26 ? HDE 245770. A possible interpretation
We report results of the spectroscopy for V725 Tau (HDE 245770, BD+26°883), commonly known as Flavia's star, which is the opticalcounterpart of the X-ray pulsar A0535+26, carried out at the 1.5 mLoiano telescope during one run of observations of this star spread overa period of years. The HeI emission lines clearly show doubling, whichis good evidence for the presence of a disc. In this paper we criticallydiscuss the possibility that this disc is a temporary accretion discaround the neutron star, a view that contrasts to the usualinterpretation, which considers that this sort of doubling in the HeIemission lines is due to a disc formed by gas expelled from the Be star.In the former case the outer radius of the accretion disc could rangefrom 5.9 × 1010 cm to 2.1 × 1011 cm,taking the most probable range into account for the orbital inclinationof the binary system, from 35° to 39°. The presence of such atemporary accretion disc around the neutron star was predicted byGiovannelli & Zió?kowski (1990).
| Radiative Column and Light Curve of X-Ray Binary Pulsars
We examine the published light curves (LCs) of 117 X-ray binary pulsars,while focusing on the dependence of their light curves on the observedenergy bands. It is found that the energy dependence of the LCs appearsonly when the X-ray luminosity is larger than ˜ 5 ×1036 erg s-1. Assuming that the behavior of lightcurve is related to the radiative accretion column on the neutron-starsurface, this energy threshold can be considered as observational proofof the accretion column formation proposed by Basko and Sunyaev. Once wecan grasp the existence of a radiative column, we can also obtainseveral useful pieces of information about the neutron-star properties.As an instance, we performed a statistical analysis of the orientationangle of the magnetic axis, and we found that the inclination angle ofthe magnetic axis should be small in order to explain the observedstatistics.
| Measurement of Dust Optical Properties in the Coalsack Nebula
We have used FUSE and Voyager observations of dust-scattered starlightin the neighborhood of the Coalsack Nebula to derive the opticalconstants of the dust grains. The albedo is consistent with a value of0.28+/-0.04, and the phase function asymmetry factor with a value of0.61+/-0.07, throughout the spectral range from 900-1200 Å, inagreement with previous determinations as well as theoreticalpredictions. We have now observed two regions (Ophiuchus and Coalsack)with intense diffuse background radiation and in both cases have foundthat the emission is due to light from nearby hot stars scattered by arelatively thin foreground cloud, with negligible contribution from thebackground molecular cloud.
| On the neutron star-disc interaction in Be/X-ray binaries
We have investigated the long-term X-ray variability, defined as theroot-mean-square (rms) of the All Sky Monitor Rossi X-ray TimingExplorer (ASM RXTE) light curves, of a set of galactic Be/X-ray binariesand searched for correlations with system parameters, such as the spinperiod of the neutron star and the orbital period and eccentricity ofthe binary. We find that systems with larger rms are those harbouringfast-rotating neutron stars, low eccentric and narrow orbits. Theserelationships can be explained as the result of the truncation of thecircumstellar disc. We also present an updated version of the Hαequivalent width-orbital period diagram, including sources in the SmallMagellanic Cloud (SMC). This diagram provides strong observationalevidence of the interaction of neutron star with the circumstellarenvelope of its massive companion.
| The Third IBIS/ISGRI Soft Gamma-Ray Survey Catalog
In this paper we report on the third soft gamma-ray source catalogobtained with the IBIS/ISGRI gamma-ray imager on board the INTEGRALsatellite. The scientific data set is based on more than 40 Ms ofhigh-quality observations performed during the first 3.5 yr of CoreProgram and public IBIS/ISGRI observations. Compared to previousIBIS/ISGRI surveys, this catalog includes a substantially increasedcoverage of extragalactic fields, and comprises more than 400high-energy sources detected in the energy range 17-100 keV, includingboth transients and faint persistent objects that can only be revealedwith longer exposure times.Based on observations with INTEGRAL, an ESA project with instruments andscience data center funded by ESA member states (especially the PIcountries: Denmark, France, Germany, Italy, Switzerland, Spain), CzechRepublic, and Poland, and with the participation of Russia and the USA.
| A description of sources detected by INTEGRAL during the first 4 years of observations
Context: In its first 4 years of observing the sky above 20 keV,INTEGRAL-ISGRI has detected 500 sources, around half of which are new orunknown at these energies. Follow-up observations at other wavelengthsrevealed that some of these sources feature unusually large columndensities, long pulsations, and other interesting characteristics. Aims: We investigate where new and previously-known sources detected byISGRI fit in the parameter space of high-energy objects, and we use theparameters to test correlations expected from theoretical predictions.For example, the influence of the local absorbing matter on periodicmodulations is studied for Galactic High-Mass X-ray Binaries (HMXBs)with OB supergiant and Be companions. We examine the spatialdistribution of different types of sources in the Milky Way usingvarious projections of the Galactic plane, in order to highlightsignatures of stellar evolution and to speculate on the origin of thegroup of sources whose classifications are still uncertain. Methods:Parameters that are available in the literature, such as positions,photoelectric absorption (NH), spin and orbital periods, anddistances or redshifts, were collected for all sources detected byISGRI. These values and their references are provided online. Results:ISGRI has detected similar numbers of X-ray Binaries and Active GalacticNuclei (AGN). The former group contains new members of the class ofHMXBs with supergiant stellar companions. Usually, this type of objectpresents strong intrinsic absorption which leads to a peak emission inan energy range that ISGRI is ideally suited to detect. Thanks to theseadditional systems, we are able to show that HMXBs are generallysegregated in plots of intrinsic NH versus the orbital periodof the system and versus the spin period of the pulsar, based on whetherthe companion is a Be or an OB supergiant star. We also find a tentativebut expected anti-correlation between NH and the orbitalperiod, and a possible and unexpected correlation between theNH and the spin period. While only a handful of new Low-MassX-ray Binaries (LMXBs) have been discovered, there are many sources thatremain unclassified and they appear to follow a spatial distributiontypical of Galactic sources (especially LMXBs) rather than extragalacticsources.
| Pulsations Detected from Accreting High-Mass X-Ray Binaries at Low Luminosities
We report the detection of pulsations from two accreting transienthigh-mass X-ray binaries (HMXBs) at low X-ray luminosities(~1-4×1034 ergs s-1). While one (4U1145-619) has been detected pulsing previously at a lower luminosity,the second (1A 1118-615) has not, making it the third such transientHMXB from which pulsations are detected at low luminosity. The pulsarsexhibit broadband rms variability (22.3%+/-0.1% and 9.2%+/-0.1%,respectively). While the present observations do not permit measurementof a spin frequency derivative, spin-down is implied in comparison withprevious Compton Gamma Ray Observatory/BATSE observations. If spin-downoccurred at the presently observed X-ray luminosity of 4U 1145-619, therequired mass accretion rate is insufficient to produce the observedX-ray luminosity from a shock at the magnetosphere, and only a fractionof the mass accretion makes its way to the neutron star surface, withmost of the mass leaving the system, carrying away angular momentum toaffect spin-down.
| Detection of Additional Periodicities in RXTE ASM Light Curves
We have detected periodicities in the Rossi X-ray Timing Explorer (RXTE)All-sky Monitor (ASM) X-ray light curves beyond those reported by Wenet al. (2006, ApJS, 163, 372). We analyzed data accumulated over nearly10 years (1996 March through 2005 November) from some 450 sources.Several weighting and filtering techniques not used by Wen et al. wereapplied previous to the Fourier analysis in order to reduce noiselevels.
| An Extended FUSE Survey of Diffuse O VI Emission in the Interstellar Medium
We present a survey of diffuse O VI emission in the interstellar medium(ISM) obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE).Spanning 5.5 yr of FUSE observations, from launch through 2004 December,our data set consists of 2925 exposures along 183 sight lines, includingall of those with previously published O VI detections. The data wereprocessed using an implementation of CalFUSE version 3.1 modified tooptimize the signal-to-noise ratio and velocity scale of spectra from anaperture-filling source. Of our 183 sight lines, 73 show O VIλ1032 emission, 29 at >3 σ significance. Six of the 3σ features have velocities |vLSR|>120 kms-1, while the others have |vLSR|<=50 kms-1. Measured intensities range from 1800 to 9100 LU (lineunit; 1 photon cm-2 s-1 sr-1), with amedian of 3300 LU. Combining our results with published O VI absorptiondata, we find that an O VI-bearing interface in the local ISM yields anelectron density ne=0.2-0.3 cm-3 and a path lengthof 0.1 pc, while O VI-emitting regions associated with high-velocityclouds in the Galactic halo have densities an order of magnitude lowerand path lengths 2 orders of magnitude longer. Although the O VIintensities along these sight lines are similar, the emission isproduced by gas with very different properties.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by Johns HopkinsUniversity under NASA contract NAS5-32985.
| A Systematic Search for Periodicities in RXTE ASM Data
We present the results of a systematic search in 8.5 yr of Rossi X-RayTiming Explorer All-Sky Monitor data for evidence of periodicities. Thesearch was conducted by application of the Lomb-Scargle periodogram tothe light curves of each of 458 actually or potentially detected sourcesin each of four energy bands (1.5-3, 3-5, 5-12, and 1.5-12 keV). Awhitening technique was applied to the periodograms before evaluation ofthe statistical significance of the powers. We discuss individualdetections with a focus on relatively new findings.
| The Second IBIS/ISGRI Soft Gamma-Ray Survey Catalog
In this paper we report the second soft gamma-ray source catalogobtained with the IBIS/ISGRI gamma-ray imager on board the INTEGRALsatellite. The scientific data set is based on more than 10 Ms ofhigh-quality observations performed during the first 2 years of CoreProgram and public IBIS/ISGRI observations, and covers ~50% of the wholesky. The main aim of the first survey was to scan systematically, forthe first time at energies above 20 keV, the whole Galactic plane toachieve a limiting sensitivity of ~1 mcrab in the central radian. Thetarget of the second year of the INTEGRAL mission lifetime was to expandas much as possible our knowledge of the soft gamma-ray sky, with thesame limiting sensitivity, to at least 50% of the whole sky, mainly byincluding a substantial coverage of extragalactic fields. This catalogcomprises more than 200 high-energy sources detected in the energy range20-100 keV, including new transients not active during the first year ofoperation, faint persistent objects revealed with longer exposure time,and several Galactic and extragalactic sources in sky regions notobserved in the first survey. The mean position error for all thesources detected with significance above 10 σ is ~40", enough toidentify most of them with a known X-ray counterpart and to unveil thenature of most of the strongly absorbed ones, even though they are verydifficult to detect in X-rays.Based on observations with INTEGRAL, an ESA project with instruments andscience data center funded by ESA member states (especially the PIcountries: Denmark, France, Germany, Italy, Switzerland, Spain), CzechRepublic, and Poland, and with the participation of Russia and the USA.
| Hard Spectra of X-ray Pulsars from INTEGRAL Data
We present spectra for 34 accretion-powers X-ray pulsars and onemillisecond pulsar that were within the field of view of the INTEGRALobservatory over two years (December 2002 January 2005) of its in-orbitoperation and that were detected by its instruments at a statisticallysignificant level (> 8σ in the energy range 18 60 keV). Thereare seven recently discovered objects of this class among the pulsarsstudied: 2RXP J130159.6-635806, IGR/AX J16320-4751, IGR J16358-4726, AXJ163904-4642, IGR J16465-4507, SAX/IGR J18027-2017, and AX J1841.0-0535.We have also obtained hard X-ray (>20 keV) spectra for theaccretion-powered pulsars RX J0146.9+6121, AX J1820.5-1434, and AXJ1841.0-0535 for the first time. We analyze the evolution of spectralparameters as a function of the intensity of the sources and comparethese with the results of previous studies.
| Chandra Localization of XTE J1906+090 and Discovery of its Optical and Infrared Counterparts
We present the Chandra identification and localization of the transientX-ray source XTE J1906+090 and the discovery of its optical and infraredcounterparts. Our analysis of archival Chandra ACIS-I observations ofthe field found the source approximately 8' away from the positiondetermined earlier with the RXTE PCA. We have confirmed the sourceidentification with timing analysis of the X-ray data, which detectedthe source spin period of 89.6 s. The best Chandra position for thesource is R.A.=19h04m47.491s,decl.=+09deg02'41.40". Subsequently, we performedoptical observations of the field around the new location and discovereda coincident optical source with R-band magnitude of 18.7. A search inthe Two Micron All Sky Survey catalog revealed an infrared point sourcewith J=15.2, H=14.2, and K=13.5, whose location is also coincident withour Chandra and optical positions. Our results add fresh evidence for aBe/X-ray transient nature for XTE J1906+090.
| New Estimates of the Solar-Neighborhood Massive Star Birthrate and the Galactic Supernova Rate
The birthrate of stars of masses >=10 Msolar is estimatedfrom a sample of just over 400 O3-B2 dwarfs within 1.5 kpc of the Sunand the result extrapolated to estimate the Galactic supernova ratecontributed by such stars. The solar-neighborhood Galactic-plane massivestar birthrate is estimated at ~176 stars kpc-3Myr-1. On the basis of a model in which the Galactic stellardensity distribution comprises a ``disk+central hole'' like that of thedust infrared emission (as proposed by Drimmel and Spergel), theGalactic supernova rate is estimated at probably not less than ~1 normore than ~2 per century and the number of O3-B2 dwarfs within the solarcircle at ~200,000.
| Long-term optical/IR variability of the Be/X-ray binary LS V +44 17/RX J0440.9+4431
We present the first long-term study of the optical counterpart to theX-ray pulsar RX J0440.9+4431/LS V +4417. The data consist of optical spectroscopic and infraredphotometric observations taken during the period 1995-2005. The infraredobservations are the first published for this source. The results of ourphotometric and spectroscopic analysis show that RXJ0440.9+4431/LS V +44 17 contains amoderately reddened, E(B-V)=0.65±0.05, B0.2V star located atabout 3.3 kpc. The Hα line consistently shows a double-peakprofile varying from symmetric shape to completely distorted on one side(V/R phases). A correlation between the equivalent width of the Hαline and the infrared magnitudes is seen: as the EW(Hα) decreasesthe IR magnitudes become fainter. This long-term optical/IR variabilityis attributed to structural changes in the Be star's circumstellar disc.The observations include a recent decline in the circumstellar disc andsubsequent recovery. We have witnessed the cessation of a globaloscillation due to the decline of the circumstellar disc. If the presentdisc growth rate continues we predict the onset of another episode ofV/R variability by the end of 2006. We have investigated the typicaltime scales for disc variability of various Be/X-ray binaries and founda correlation with the orbital period. This correlation is hard toestablish due to the difficulty in defining the exact duration of thevarious activity states, but it is seen both in the duration of the discgrowth/dissipation phase and the value of the Hα equivalent widthprior to the appearance of asymmetric profiles. These relationshipsprovide further evidence for the interaction of the neutron star withthe circumstellar disc of the Be star's companion and confirms the needof a fully developed disc for the V/R variability to be observed.
| On the feasibility of detection of neutron star companions to OB runaways using Gaia astrometry
For an illustrative sample of classical OB runaway stars, we examine thecapability of the upcoming Gaia satellite to detect compact companionsby the use of astrometric techniques. For the OB runaway stars in oursample, we estimate initial system parameters and consider the modifyingevolutionary effects of mass transfer and supernova explosion of theprimary. The possible system configurations that follow from this, andthe expected Gaia accuracy, determine the likelihood of detecting amovement of the photocentre due to an unseen companion. As the size ofthe natal kick imparted to the core of the exploding star is increasedthe overall probability of detecting a neutron star companion decreasesas more systems become disrupted. The overall detection probabilitiesfor our illustrative sample range from 2% to 27%, which imply thatwithin a distance of approximately 5 kpc from the Sun around 48detections of compact companions to runaway stars can be expected. Forcomparison, around 15% of High Mass X-ray Binaries would exhibit wobblesdetectable with Gaia.
| IGR J11435-6109: Faded, but Variable - Possibly ~ 166 s Pulsations
On Dec. 4, 2004 at 03:42 UT RXTE observed the source IGR J11435-6109that was recently discovered with INTEGRAL's IBIS/ISGRI. (ATEL #350).The pointing position was offset to insure no contribution in the PCAfrom nearby 4U 1145-619 and 1E1145.1-6141. An average net count rate of1.7/s was observed over 896 s. Variations of about 50% were notable,which could be due to a noisy pulsation.
| Another X-ray brightening of H 1743-322 = IGR J1746-3213.
Not Available
| New X-ray transient IGR J11435-6109 discovered with INTEGRAL
A new hard X-ray transient, IGR J11435-6109, was discovered with theIBIS/ISGRI telescope on board INTEGRAL on Nov. 23, 2004, duringobservations (3 successive pointings at 12:30-14:20 UTC) dedicated tothe Galactic Plane Scan. The source was located 1 deg off the knownX-ray pulsar twins, 4U1145-619 and 1E1145.1-6141, at the position, R.A.=11h43m52s, Dec.= -61d09m00s (equinox 2000.0, error radius 2.5'),generally consistent with that of the faint ROSAT source 1RXSJ114358.1-610736.
| Observations of the Diffuse Far-Ultraviolet Background with the Far Ultraviolet Spectroscopic Explorer
We have used observations taken under the Far Ultraviolet SpectroscopicExplorer (FUSE) S405/505 channel realignment program to explore thediffuse far-ultraviolet (FUV; 1000-1200 Å) radiation field. Of the71 independent locations in that program, we have observed a diffusesignal in 32, ranging in brightness from 1600 to a maximum of2.9×105 photons cm-2 sr-1s-1 Å-1 in Orion. The FUSE data confirm thatthe diffuse FUV sky is patchy with regions of intense emission, usuallynear bright stars, but also with dark regions, even at low Galacticlatitudes. We find a weak correlation between the FUV flux and the 100μm ratio but with wide variations, perhaps due to differences in thelocal radiation field.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 Burst and Transient Source Experiment (BATSE) Earth Occultation Catalog of Low-Energy Gamma-Ray Sources
The Burst and Transient Source Experiment (BATSE), aboard the ComptonGamma Ray Observatory (CGRO), provided a record of the low-energygamma-ray sky (~20-1000 keV) between 1991 April and 2000 May (9.1 yr).BATSE monitored the high-energy sky using the Earth occultationtechnique (EOT) for point sources whose emission extended for times onthe order of the CGRO orbital period (~92 min) or greater. Using the EOTto extract flux information, a catalog of sources using data from theBATSE Large Area Detectors has been prepared. The first part of thecatalog consists of results from the all-sky monitoring of 58 sources,mostly Galactic, with intrinsic variability on timescales of hours toyears. For these sources, we have included tables of flux and spectraldata, and outburst times for transients. Light curves (or fluxhistories) have been placed on the World Wide Web.We then performed a deep-sampling of these 58 objects, plus a selectionof 121 more objects, combining data from the entire 9.1 yr BATSE dataset. Source types considered were primarily accreting binaries, but asmall number of representative active galaxies, X-ray-emitting stars,and supernova remnants were also included. The sample represents acompilation of sources monitored and/or discovered with BATSE and otherhigh-energy instruments between 1991 and 2000, known sources taken fromthe HEAO 1 A-4 and Macomb & Gehrels catalogs. The deep sampleresults include definite detections of 83 objects and possibledetections of 36 additional objects. The definite detections spannedthree classes of sources: accreting black hole and neutron starbinaries, active galaxies, and supernova remnants. The average fluxesmeasured for the fourth class, the X-ray emitting stars, were below theconfidence limit for definite detection.Flux data for the deep sample are presented in four energy bands: 20-40,40-70, 70-160, and 160-430 keV. The limiting average flux level (9.1 yr)for the sample varies from 3.5 to 20 mcrab (5 σ) between 20 and430 keV, depending on systematic error, which in turn is primarilydependent on the sky location.To strengthen the credibility of detection of weaker sources (~5-25mcrab), we generated Earth occultation images, searched for periodicbehavior using FFT and epoch folding methods, and critically evaluatedthe energy-dependent emission in the four flux bands. The deep sampleresults are intended for guidance in performing future all-sky surveysor pointed observations in the hard X-ray and low-energy gamma-ray band,as well as more detailed studies with the BATSE EOT.
| Origin of the Soft Excess in X-Ray Pulsars
The spectra of many X-ray pulsars show, in addition to a power law, alow-energy component that has often been modeled as a blackbody withkTBB~0.1 keV. However, the physical origin of this softexcess has remained a mystery. We examine a sample of well-studiedbright X-ray pulsars, which have been observed using ROSAT, ASCA, Ginga,RXTE, BeppoSAX, Chandra, and XMM-Newton. In particular, we consider theMagellanic Cloud pulsars SMC X-1, LMC X-4, XTE J0111.2-7317, and RXJ0059.2-7138 and the Galactic sources Her X-1, 4U 1626-67, Cen X-3, andVela X-1. We show that the soft excess is a very common if notubiquitous feature intrinsic to X-ray pulsars. We evaluate severalpossible mechanisms for the soft emission, using theoretical arguments,as well as observational clues, such as spectral shapes, eclipses,pulsations of the soft component, and superorbital modulation of thesource flux. We find that reprocessing of hard X-rays from the neutronstar by the inner region of the accretion disk is the only process thatcan explain the soft excess in all the pulsars withLX>~1038 ergs s-1. Other mechanisms,such as emission from diffuse gas in the system, are important in lessluminous objects.
| The Effects of Binary Evolution on the Dynamics of Core Collapse and Neutron Star Kicks
We systematically examine how the presence in a binary affects the finalcore structure of a massive star and its consequences for the subsequentsupernova explosion. Interactions with a companion star may change thefinal rate of rotation, the size of the helium core, the strength ofcarbon burning, and the final iron core mass. Stars with initial masseslarger than ~11 Msolar that experience core collapse willgenerally have smaller iron cores at the point of explosion if they losttheir envelopes through a binary interaction during or soon after corehydrogen burning. Stars below ~11 Msolar, on the other hand,can end up with larger helium and metal cores if they have a closecompanion, since the second dredge-up phase that reduces the helium coremass dramatically in single stars does not occur once the hydrogenenvelope is lost. We find that the initially more massive stars inbinary systems with masses in the range 8-11 Msolar arelikely to undergo an electron-capture supernova, while single stars inthe same mass range would end as ONeMg white dwarfs. We suggest that thecore collapse in an electron-capture supernova (and possibly in the caseof relatively small iron cores) leads to a prompt or fast explosionrather than a very slow, delayed neutrino-driven explosion and that thisnaturally produces neutron stars with low-velocity kicks. This leads toa dichotomous distribution of neutron star kicks, as inferredpreviously, where neutron stars in relatively close binaries attain lowkick velocities. We illustrate the consequences of such a dichotomouskick scenario using binary population synthesis simulations and discussits implications. This scenario has also important consequences for theminimum initial mass of a massive star that becomes a neutron star. Forsingle stars the critical mass may be as high as 10-12Msolar, while for close binaries it may be as low as 6-8Msolar. These critical masses depend on the treatment ofconvection, the amount of convective overshooting, and the metallicityof the star, and will generally be lower for larger amounts ofconvective overshooting and lower metallicity.
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Membre dels grups següents:
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Dades d'Observació i Astrometria
| Constel·lació: | Centaurus |
| Ascensió Recta: | 11h48m00.02s |
| Declinació: | -62°12'24.9" |
| Magnitud Aparent: | 8.936 |
| Distancia: | 505.051 parsecs |
| Moviment propi RA: | -8 |
| Moviment propi Dec: | 2.4 |
| B-T magnitude: | 9.161 |
| V-T magnitude: | 8.955 |
Catàlegs i designacions:
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