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Публикации по объекту
Testing the beamed inverse-Compton model for jet X-ray emission: velocity structure and deceleration
By considering a small sample of core-dominated radio-loud quasars withX-ray jets, I show, as has been argued previously by others, that theobservations require bulk jet deceleration if all of the X-ray emissionis to be explained using the widely adopted beamed inverse-Comptonmodel, and argue that jets even in these powerful objects must havevelocity structure in order to reconcile their radio and X-rayproperties. I then argue that the deceleration model has several seriousweaknesses, and discuss the viability of alternative models for thedecline in X-ray/radio ratio as a function of position. Althoughinverse-Compton scattering from the jets is a required process and mustcome to dominate at high redshifts, adopting an alternative model forthe X-ray emission of some nearby, well-studied objects can greatlyalleviate some of the problems posed by these observations for thebeamed inverse-Compton model.
| High-redshift Faranoff-Riley type II radio galaxies: X-ray properties of the cores
We present an extensive X-ray spectral analysis of the cores of 19Faranoff-Riley type II sources in the redshift range 0.5 < z < 1.0which were selected to be matched in isotropic radio power. The sampleconsists of 10 radio galaxies (RGs) and nine quasars. We compare ourresults with the expectations from a unification model that ascribes thedifference between these two types of sources to the viewing angle tothe line of sight, beaming and the presence of a dust and gas torus. Wefind that the spectrum of all the quasars can be fitted with a singlepower law, and that the spectral index flattens with decreasing angle tothe line of sight. We interpret this as the effect of increasinglydominant inverse Compton X-ray emission, beamed such that the jetemission outshines other core components. For up to 70 per cent of theRGs we detect intrinsic absorption; their core spectra are best fittedwith an unabsorbed steep power law of average spectral index Γ=2.1 and an absorbed power law of spectral index Γ= 1.6, which isflatter than that observed for radio-quiet quasars (RQQs). We furtherconclude that the presence of a jet affects the spectral properties ofabsorbed nuclear emission in active galactic nuclei. In RGs, anysteep-spectrum component of nuclear X-ray emission, similar to that seenin RQQs, must be masked by a jet or by jet-related emission.
| A Multiwavelength Study of the Jets in FR-I Radio Galaxies: I. Data and Analysis
We compile a sample of 11 Fanaroff-Riley type I Radio Galaxies (FR-IRGs) with multi-wavelength observations to address the dynamic behaviorof jets in these objects. Optical images acquired by the Hubble SpaceTelescope (HST) are carefully analyzed. The method and reductionprocedure are described in detail. Unresolved optical cores emerge afterhaving properly removed starlight from the host galaxies in eight of theFR-I RGs, of which five are new identifications. Broad band spectralproperties of these newly identified compact cores are compared withthat previously found in FR-I RGs, as well as the low-energy-peaked BLLac objects. The similarity between them argues for the same non-thermalsynchrotron origin. Well-resolved optical jets with knotty morphologiesare found in three FR-I RGs in our sample, namely 3C 15, 3C 66B and B20755+37. The optical counterparts to the inner radio/X-ray jets areidentified and a clear one-to-one correspondence between the optical,radio and X-ray knots is found. The structure and information on theoptical jets are discussed. Physical parameters such as the knotsposition, flux and size are also presented. Detailed comparison betweenthe multi-wavelength data and radiative and dynamic models of jet willbe made in a forthcoming paper.
| The Warped Nuclear Disk of Radio Galaxy 3C 449
Among radio galaxies containing nuclear dust disks, the bipolar jet axisis generally observed to be perpendicular to the disk major axis. The FRI radio source 3C 449 is an outlier to this statistical majority, as itpossesses a nearly parallel jet/disk orientation on the sky. We examinethe 600 pc dusty disk in this galaxy with images from the Hubble SpaceTelescope. We find that a 1.6 μm/0.7 μm color map of the diskexhibits a twist in its isocolor contours (isochromes). We model thecolor map by integrating galactic starlight through an absorptive diskand find that the anomalous twist in the isochromes can be reproduced inthe model with a vertically thin, warped disk. The model predicts thatthe disk is nearly perpendicular to the jet axis within 100 pc of thenucleus. We discuss physical mechanisms capable of causing such a warp.We show that precessional models or a torque on the disk arising from apossible binary black hole in the AGN causes precession on a timescalethat is too long to account for the predicted disk morphology. However,we estimate that the pressure in the X-ray-emitting interstellar mediumis large enough to perturb the disk, and we argue that jet-drivenanisotropy in the excited ISM may be the cause of the warp. In this way,the warped disk in 3C 449 may be a new manifestation of feedback from anactive galactic nucleus.
| Chandra and XMM-Newton Observations of a Sample of Low-Redshift FR I and FR II Radio Galaxy Nuclei
We present spectral results from Chandra and XMM-Newton observations ofa sample of 22 low-redshift (z<0.1) radio galaxies and considerwhether the core emission originates from the base of a relativisticjet, or an accretion flow, or contains contributions from both. We findcorrelations between the unabsorbed X-ray, radio, and optical fluxes andluminosities of FR I-type radio-galaxy cores, implying a common originin the form of a jet. On the other hand, we find that the X-ray spectraof FR II-type radio galaxy cores are dominated by absorbed emission,with NH>~1023 atoms cm-2, which islikely to originate in an accretion flow. We discuss several models thatmay account for the different nuclear properties of FR I- and FR II-typecores and also demonstrate that both heavily obscured, accretion-relatedand unobscured, jet-related components may be present in all radiogalaxy nuclei. Any absorbed, accretion-related components in FR I-typegalaxies have low radiative efficiencies.
| Type I Ultraluminous Infrared Galaxies: Transition Stage from ULIRGs to QSOs
We examine whether the ultraluminous infrared galaxies that contain atype 1 Seyfert nucleus (a type I ULIRG) are in the transition stage fromULIRGs to quasi-stellar objects (QSOs). To investigate this issue, wecompare the black hole (BH) mass, the bulge luminosity, and thefar-infrared luminosity among type I ULIRGs, QSOs, and ellipticalgalaxies. As a result, we find the following results: (1) The type IULIRGs have systematically smaller BH masses in spite of the comparablebulge luminosity relative to QSOs and elliptical galaxies. (2) Thefar-infrared luminosity of most type I ULIRGs is larger than theEddington luminosity. We show that the above results do not changesignificantly for three type I ULIRGs for which we can estimate thevisual extinction from the column density. Also, for all eight type IULIRGs, we investigate the effect of uncertainties of BH massmeasurements and our sample bias to make sure that our results are notaltered even if we consider the above two effects. In addition, Anabukirecently revealed that their X-ray properties are similar to those ofthe narrow-line Seyfert 1 galaxies. These would indicate that activegalactic nuclei (AGNs) with a high mass accretion rate exist in type IULIRGs. On the basis of all of these findings, we conclude that it wouldbe a natural interpretation that type I ULIRGs are the early phase of BHgrowth, namely, the missing link between ULIRGs and QSOs. Moreover, bycomparing our results with a theoretical model of a coevolution scenarioof a QSO BH and a galactic bulge, we show clearly that this explanationcould be valid.
| Understanding the Nuclear Gas Dispersion in Early-Type Galaxies in the Context of Black Hole Demographics
The majority of nearby early-type galaxies contain detectable amounts ofemission-line gas at their centers. The nuclear gas kinematics form avaluable diagnostic of the central black hole (BH) mass. Here we analyzeand model Hubble Space Telescope STIS observations of a sample of 27galaxies; 16 Fanaroff-Riley Type I radio galaxies and 11 (more) normalearly-type galaxies. We focus here on what can be learned from thenuclear velocity dispersion (line width) of the gas as a complement tothe many studies dealing with gas rotation velocities. We find that thedispersion in a STIS aperture of ~0.1"-0.2" generally exceeds thelarge-scale stellar velocity dispersion of the galaxy. This isqualitatively consistent with the presence of central BHs but raises thequestions of whether the excess gas dispersion is of gravitational ornongravitational origin and whether the implied BH masses are consistentwith our current understanding of BH demography (as predicted by theM-σ relation between BH mass and stellar velocity dispersion). Toaddress this we construct purely gravitational axisymmetric dynamicalmodels for the gas, both thin-disk models and models with more generalaxis ratios and velocity anisotropies. For the normal galaxies thenuclear gas dispersions are adequately reproduced assuming disks aroundthe BHs with masses that follow the M-σ relation. In contrast, thegas dispersions observed for the radio galaxies generally exceed thosepredicted by any of the models. We attribute this to the presence ofnongravitational motions in the gas that are similar to or larger thanthe gravitational motions. The nongravitational motions are presumablydriven by the active galactic nucleus (AGN), but we do not find arelation between the radiative output of the AGN and thenongravitational dispersion. Given the uncertainties about the dynamicalstate of the gas, it is not possible to uniquely determine the BH massfor each galaxy from its nuclear gas dispersion. However, for the sampleas a whole the observed dispersions do not provide evidence forsignificant deviations from the M-σ relation.Based on observations with the NASA/ESA Hubble Space Telescope obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS5-26555.
| Low-Luminosity Active Galaxies and Their Central Black Holes
Central black hole masses for 117 spiral galaxies representingmorphological stages S0/a through Sc and taken from the largespectroscopic survey of Ho et al. are derived using Ks-banddata from the Two Micron All Sky Survey. Black hole masses are foundusing a calibrated black hole-Ks bulge luminosity relation,while bulge luminosities are measured by means of a two-dimensionalbulge-disk decomposition routine. The black hole masses are correlatedagainst a variety of parameters representing properties of the nucleusand host galaxy. Nuclear properties such as line width (FWHM [N II]), aswell as emission-line ratios (e.g., [O III]/Hβ, [O I]/Hα, [NII]/Hα, and [S II]/Hα), show a very high degree ofcorrelation with black hole mass. The excellent correlation with linewidth supports the view that the emission-line gas is in virialequilibrium with either the black hole or bulge potential. The very goodemission-line ratio correlations may indicate a change in ionizingcontinuum shape with black hole mass in the sense that more massiveblack holes generate harder spectra. Apart from theinclination-corrected rotational velocity, no excellent correlations arefound between black hole mass and host galaxy properties. Significantdifferences are found between the distributions of black hole masses inearly-, mid-, and late-type spiral galaxies (subsamples A, B, and C) inthe sense that early-type galaxies have preferentially larger centralblack holes, consistent with observations that Seyfert galaxies arefound preferentially in early-type systems. The line width distributionsshow a marked difference among subsamples A, B, and C in the sense thatearlier type galaxies have larger line widths. There are also cleardifferences in line ratios between subsamples A+B and C that likely arerelated to the level of ionization in the gas. Finally, aKs-band Simien & de Vaucouleurs diagram shows excellentagreement with the original B-band relation, although there is a largedispersion at a given morphological stage.
| XMM-Newton observations of a sample of γ-ray loud active galactic nuclei
Aims.To understand the nature of γ-ray loud active galactic nuclei(AGN) and the mechanisms for the generation of high-energyγ-rays. Methods: .We performed a homogeneous and systematicanalysis of simultaneous X-ray and optical/UV properties of a group of15 γ-ray loud AGN, using observations performed with XMM-Newton.The sample is composed of 13 blazars (6 BL Lac and 7 Flat-Spectrum RadioQuasar) and 2 radio galaxies that are associated with detections atenergies >100 MeV. The data for 7 of them are analyzed here for thefirst time, including the first X-ray observation of PKS 1406-706. Thespectral characteristics of the sources in the present sample werecompared with those in previous catalogs of blazars and other AGN, tosearch for difference or long term changes. Results: .All theselected sources appear to follow the classic "blazar sequence" and thespectral energy distributions (SED) built with the present X-ray andoptical/UV data and completed with historical data, confirm the findingsof previous studies on this type of source. Some sources displayinteresting features: four of them, namely AO 0235+164, PKS 1127-145, S50836+710 and PKS 1830-211 show the presence of an intervening absorptionsystem along the line of sight, but only the last is known to begravitationally lensed. AO 0235+164 was detected during an outburst andits SED shows a clear shift of the synchrotron peak. 3C 273 shows achange in state with respect to the previous BeppoSAX observations thatcan be interpreted as an increase of the Seyfert-like component and acorresponding decline of the jet emission. This is consistent with themonitoring at radio wavelengths performed during the same period. PKS1406-706 is detected with a flux higher than in the past, but with acorresponding low optical flux. Although it is classified as FSRQ, theSED can be modelled with a simple synchrotron self-Compton model.
| Supermassive Black Holes in Galactic Nuclei: Past, Present and Future Research
This review discusses the current status of supermassive black holeresearch, as seen from a purely observational standpoint. Since theearly ‘90s, rapid technological advances, most notably the launchof the Hubble Space Telescope, the commissioning of the VLBA andimprovements in near-infrared speckle imaging techniques, have not onlygiven us incontrovertible proof of the existence of supermassive blackholes, but have unveiled fundamental connections between the mass of thecentral singularity and the global properties of the host galaxy. It isthanks to these observations that we are now, for the first time, in aposition to understand the origin, evolution and cosmic relevance ofthese fascinating objects.
| A relativistic model of the radio jets in NGC 315
We apply our intrinsically symmetrical, decelerating relativistic jetmodel to deep Very Large Array imaging of the inner +/-70arcsec of thegiant low-luminosity radio galaxy NGC315. An optimized model accuratelyfits the data in both total intensity and linear polarization. We inferthat the velocity, emissivity and field structure in NGC315 are verysimilar to those of the other low-luminosity sources we have modelled,but that all of the physical scales are larger by a factor of about 5.We derive an inclination to the line of sight of 38°+/- 2° forthe jets. Where they first brighten, their on-axis velocity isβ=v/c~ 0.9. They decelerate to β~ 0.4 between 8 and 18kpc fromthe nucleus and the velocity thereafter remains constant. The speed atthe edge of the jet is ~0.6 of the on-axis value where it is bestconstrained, but the transverse velocity profile may deviatesystematically from the Gaussian form we assume. The proper emissivityprofile is split into three power-law regions separated by shortertransition zones. In the first of these, at ~3kpc (the flaring point)the jets expand rapidly at constant emissivity, leading to a largeincrease in the observed brightness on the approaching side. At ~10kpc,the emissivity drops abruptly by a factor of 2. Where the jets are wellresolved, their rest-frame emission is centre brightened. The magneticfield is modelled as random on small scales but anisotropic and we ruleout a globally ordered helical configuration. To a first approximation,the field evolves from a mixture of longitudinal and toroidal componentsto predominantly toroidal, but it also shows variations in structurealong and across the jets, with a significant radial component inplaces. Simple adiabatic models fail to fit the emissivity variations.
| Chandra and XMM-Newton observations of NGC 6251
We present new X-ray observations of the nucleus, jet and extendedemission of the nearby radio galaxy NGC 6251 using the Chandra/ACIS-Scamera, together with a reanalysis of archival Chandra/ACIS-I andXMM-Newton/EPIC data. We find that the nuclear X-ray spectrum is wellfitted with an absorbed power law, and that there is tentative, but nothighly significant, evidence for Fe Kα emission. We argue that theobserved nuclear X-ray emission is likely to originate in a relativisticjet, based on the double-peaked nature, and our synchrotron self-Comptonmodelling, of the radio to X-ray spectral energy distribution. However,we cannot rule out a contribution from an accretion flow. We resolveX-ray jet emission in three distinct regions, and argue in favour of asynchrotron origin for all three; inverse Compton emission models arepossible but require extreme parameters. We detect thermal emission onboth galaxy and group scales, and demonstrate that hot gas can confinethe jet, particularly if relativistic beaming is important. We showevidence that the radio lobe has evacuated a cavity in theX-ray-emitting gas, and suggest that the lobe is close to the plane ofthe sky, with the jet entering the lobe close to the surface nearest tothe observer.
| A high-energy view of radio-loud AGN
Seyfert galaxies and quasars were first discovered through optical andradio techniques, but in recent years high-energy emission, that canpenetrate central gas and dust, has become essentially the definingcharacteristic of an AGN. AGNs with extended radio jets are ofparticular interest, since the jets signal source orientation. However,the jets extend into the cores, where they are faster and more compact.Special-relativistic effects then cause jet brightness and variabilitytime-scales across the electromagnetic spectrum to be strong functionsof jet orientation. Jet X-ray emission is confused, to varying degrees,with that from the central engine, but can be measured, at least in astatistical sense, through considerations of the multiwaveband spectrumand the level of intrinsic absorption. The rich high-energy structuresfound in jets which are resolved with Chandra and HST inform ourinterpretation of the inner structures. In particular, it is found thatshocks are prevalent and don't necessarily disrupt jets, and thatone-zone models of emission near shocks are an over-simplification.
| Molecular Gas and Nuclear Activity in Radio Galaxies Detected by IRAS
This paper reports the latest results from a millimeter-wave (CO)spectroscopic survey of IRAS-detected radio galaxies withL1.4GHz~1023-1028 W Hz-1 inthe redshift range z~0.02-0.15. The IRAS flux-limited sample contains 33radio galaxies with different radio morphologies and a broad range ofinfrared luminosities (LIR=109-1012Lsolar), allowing for an investigation of (1) whether low-zradio-selected active galactic nuclei (AGNs) reside in moleculargas-rich host galaxies and (2) whether the CO properties are correlatedwith the properties of the host galaxy or the AGN. All of the radiogalaxies in Mazzarella et al. and Mirabel et al. have been reobserved.Three new CO detections have been made, raising the total number of COdetections to nine and setting the survey detection rate at ~25%. Manyof the CO lines have double-peaked profiles, and the CO line widths arebroad (average ΔvFWHM~500+/-130 km s-1),exceeding the average CO widths of both ultraluminous infrared galaxies(300+/-90 km s-1) and Palomar-Green QSOs (260+/-160 kms-1), and thus being indicative of massive host galaxies. TheCO luminosities translate into molecular gas masses of~(0.4-7)×109 Msolar, however, the 3 σCO upper limits for nondetections do not rule out a molecular gas massas high as that of the Milky Way (~3×109Msolar). Optical images of eight out of nine moleculargas-rich radio galaxies show evidence of close companions and/or tidalfeatures. Finally, there is no obvious correlation between radio powerand molecular gas mass. However, it is notable that only one F-R IIgalaxy out of 12 is detected in this CO survey; the remaining detectionsare of galaxies hosting F-R I and compact radio jets.
| Observational Evidence of Jet Precession in Galactic Nuclei Caused by Accretion Disks
We show that the observational data of extragalactic radio sources tendto support the theoretical relationship between the jet precessionperiod and the optical luminosity of the sources, as predicted by themodel in which an accretion disk causes the central black hole toprecess.
| Is the Core of M87 the Source of Its TeV Emission? Implications for Unified Schemes
M87 has recently been shown to be a TeV source that is likely to bevariable. Based on this and on contemporaneous optical and X-raymonitoring, we argue that the source of the TeV emission is the core ofM87 and not one of two jet knots (HST-1 and A), with an X-ray brightnesscomparable to that of the core. We model the TeV emission in the core asinverse Compton (IC) emission from the base of the jet. Homogeneousmodels fail to reproduce the spectral energy distribution (SED) and, inparticular, the TeV flux. They also fail to comply with the unifiedscheme of BL Lacertae (BL) objects and FR I radio galaxies. A jet thatdecelerates from a Lorentz factor Γ~20 down to Γ~5 over alength of ~0.1 pc reproduces the observed SED of the M87 core and, whenaligned with the line of sight, produces a SED similar to those of TeVBLs. The TeV flux in the decelerating jet model is successfullyreproduced as upstream Compton (UC) scattering, a recently identifiedemission mechanism, in which energetic electrons of the upstream fasterflow upscatter the low-energy photons produced in the slower downstreampart of the flow.
| Unveiling a Population of AGNs Not Detected in X-Rays
We define a sample of 27 radio-excess AGNs in the Chandra DeepField-North by selecting galaxies that do not obey the radio/infraredcorrelation for radio-quiet AGNs and star-forming galaxies.Approximately 60% of these radio-excess AGNs are undetected in X-rays inthe 2 Ms Chandra catalog, even at exposures of >=1 Ms; 25% lack even2 σ X-ray detections. The absorbing columns to the faint objectsdetected in X-rays are 1022cm-2
| Are Quasar Jets Dominated by Poynting Flux?
The formation of relativistic astrophysical jets is presumably mediatedby magnetic fields threading accretion disks and central, rapidlyrotating objects. As it is accelerated by magnetic stresses, the jet'skinetic energy flux grows at the expense of its Poynting flux. However,it is unclear how efficient the conversion from magnetic to kineticenergy is and whether there are any observational signatures of thisprocess. We address this issue in the context of jets in quasars. Usingdata from all spatial scales, we demonstrate that in these objects theconversion from Poynting flux-dominated to matter-dominated jets is verylikely to take place closer to the black hole than in the region wheremost of the Doppler-boosted radiation observed in blazars is produced.We briefly discuss the possibility that blazar activity could be inducedby global MHD instabilities, e.g., via the production of localizedvelocity gradients that lead to dissipative events such as shocks ormagnetic reconnection, in which acceleration of relativistic particlesand production of nonthermal flares is taking place.
| Evidence of an Untruncated Accretion Disk in the Broad-Line Radio Galaxy 4C 74.26
We present evidence of a broad ionized Fe Kα line in theXMM-Newton spectrum of the broad-line radio galaxy (BLRG) 4C 74.26. Thisis the first indication that the innermost regions of the accretion flowin BLRGs contain thin, radiatively efficient disks. Analysis of the 35ks XMM-Newton observation finds a broad line with an inner radius closeto the innermost stable circular orbit for a maximally spinning blackhole. The outer radius of the relativistic line is also found to bewithin 10 gravitational radii. The Fe Kα line profile gives aninclination angle of ~40°, consistent with the radio limit. Thereare two narrow components to the Fe Kα complex: one at 6.4 keVfrom neutral Fe and one at 6.2 keV. These may form the blue and redhorns of a diskline from farther out on the disk, but a longerobservation is required to confirm this hypothesis. We discuss theimplications of this observation for models of jet production andsuggest that BLRGs and radio-loud quasars will have larger-than-averageblack hole masses, thus resulting in thicker accretion flows close tothe black hole.
| X-Ray Emission Properties of Large-Scale Jets, Hot Spots, and Lobes in Active Galactic Nuclei
We examine a systematic comparison of jet knots, hot spots, and radiolobes recently observed with Chandra and ASCA. This report discusses theorigin of their X-ray emissions and investigates the dynamics of thejets. The data were compiled at well-sampled radio (5 GHz) and X-ray (1keV) frequencies for more than 40 radio galaxies. We examine threemodels for the X-ray production: synchrotron (SYN), synchrotronself-Compton (SSC), and external Compton (EC) on cosmic microwavebackground (CMB) photons. For the SYN sources-mostly jet knots in nearbylow-luminosity radio galaxies-X-ray photons are produced byultrarelativistic electrons with energies 10-100 TeV that must beaccelerated in situ. For the other objects, conservatively classified asSSC or EC sources, a simple formulation of calculating the ``expected''X-ray fluxes under an equipartition hypothesis is presented. We confirmthat the observed X-ray fluxes are close to the expected ones fornonrelativistic emitting plasma velocities in the case of radio lobesand the majority of hot spots, whereas a considerable fraction of jetknots are too bright in X-rays to be explained in this way. We examinetwo possibilities to account for the discrepancy in a framework of theinverse Compton model: (1) the magnetic field is much smaller than theequipartition value, and (2) the jets are highly relativistic onkiloparsec and megaparsec scales. We conclude that if the inverseCompton model is the case, the X-ray-bright jet knots are most likelyfar from the minimum-power condition. We also briefly discuss the otherpossibility, namely, that the observed X-ray emission from all the jetknots is synchrotron in origin.
| Origin of Radio Emission from Nearby Low-Luminosity Active Galactic Nuclei
We use the observational data in radio, optical, and X-ray wave bandsfor a sample of active galactic nuclei (AGNs) with measured black holemasses to explore the origin of radio emission from nearbylow-luminosity active galactic nuclei (LLAGNs). The maximal luminosityof an advection-dominated accretion flow (ADAF) can be calculated for agiven black hole mass, as there is a critical accretion rate above whichthe ADAF is no longer present. We find that the radio luminosities arehigher than the maximal luminosities expected from the ADAF model formost sources in this sample. This implies that the radio emission ispredominantly from the jets in these sources. The radio emission from asmall fraction of the sources (15/60; referred to as radio-weak sources)in this sample can be explained by the ADAF model. However, comparingthe observed multiband emission data with the spectra calculated for theADAF or adiabatic inflow-outflow solution (ADIOS) cases, we find thatneither ADAF nor ADIOS models can reproduce the observed multibandemission simultaneously, with reasonable magnetic field strengths, forthese radio-weak sources. A variety of other possibilities arediscussed, and we suggest that the radio emission is probably dominatedby jet emission even in these radio-weak LLAGNs.
| The X-Ray Afterglow of Dark GRB 970815: A Common Origin for Gamma-Ray Bursts and X-Ray Flashes?
GRB 970815 is a well-localized gamma-ray burst (GRB) detected by theAll-Sky Monitor (ASM) on the Rossi X-Ray Timing Explorer (RXTE) forwhich no afterglow was identified despite follow-up ASCA and ROSATpointings and optical imaging to limiting magnitude R>23. Although anX-ray source, AX/RX J1606.8+8130, was detected just outside the ASMerror box, it was never associated with the GRB because it was notclearly fading and because no optical afterglow was ever found. Werecently obtained an upper limit for this source with Chandra that is atleast a factor of 100 fainter than the ASCA detection. We also made deepoptical observations of the AX/RX J1606.8+8130 position, which is blankto limits V>25.2 and I>24.0. In view of these extreme limits, weconclude that AX/RX J1606.8+8130 is indeed the afterglow of GRB 970815,which corresponds to an optically ``dark'' GRB. AX/RX J1606.8+8130 cantherefore be ruled out as the counterpart of the persistent EGRET source3EG J1621+8203. The early light curves from BATSE and the RXTE ASM showspectral softening between multiple peaks of prompt emission. We proposethat GRB 970815 might be a case in which the properties of an X-rayflash and a ``normal'' GRB coincide in a single event.
| The HST view of the nuclear emission line region in low luminosity radio-galaxies
We study the properties of the emission line regions in two samples oflow luminosity radio-galaxies, while focusing on the Compact EmissionLine Region (CELR) revealed to be a characteristic feature of theseobjects by HST narrow-band imaging. We find a strong correlation betweenline and optical continuum nuclear emission, which suggests that theoptical cores (most likely of non-thermal origin) can be directlyassociated to the source of ionizing photons, i.e. that we are seeing ajet-ionized narrow line region. A photon budget argument indicates thatthe optical nuclear sources produce sufficient photon flux provided thatthe covering factor of the circum-nuclear gas is rather large, onaverage 0.3. Analysis of HST images and spectra suggests that the CELRmay take the form of a pc-scale, high filling factor structure, possiblyan optically thin torus. Estimates of the CELR mass lead to values assmall as 10{-}10^3 Mȯ, and photon counting sets a limitto the Broad Line Region mass of M_BLR < 10-2Mȯ. When considered together with the low accretion rateand the tenuous torus structure, a general paucity of gas in theinnermost regions of low luminosity radio-galaxies emerges as the maincharacterizing difference from more powerful Active Galactic Nuclei.
| Investigating the EGRET-radio galaxies link with INTEGRAL: The case of 3EG J1621+8203 and NGC 6251
The analysis of an INTEGRAL AO2 observation of the error contours of theEGRET source 3EG J1621+8203 is presented. The only source found insidethe error contours for energies between 20 and 30 keV at 5σdetection significance is the FR I radio galaxy NGC 6251. This supportsthe identification of NGC 6251 with 3EG J1621+8203. The observed flux ishigher and softer than observed in the past, but consistent with avariable blazar-like spectral energy distribution.Based on observations with INTEGRAL, an ESA mission 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.
| Structured jets in TeV BL Lac objects and radiogalaxies. Implications for the observed properties
TeV BL Lacertae objects require extreme relativistic bulk motions in thegamma-ray emission region, but at the VLBI scale their radio knotshardly move. The same sources show evidence, in radio, of a structuremade of a fast spine plus a slow layer. We propose that this structureexists even on the spatial scale of regions responsible for thegamma-ray emission. One component sees the (beamed) radiation producedby the other, and this enhances the inverse Compton emission of bothcomponents. In addition, this allows the magnetic field to be nearly inequipartition with the emitting particles. The inverse Compton emissionof the spine is anisotropic in its frame, possibly producing adeceleration of the spine by the Compton rocket effect. In thisscenario, the slow layer is also a relatively strong high-energyemitter, and thus radiogalaxies become potentially detectable by GLAST.
| Is 3C 111, an apparently normal radio galaxy, the counterpart of 3EG J0416+3650?
The Third EGRET Catalog (3EG) lists 66 high-confidence identificationsof sources with Active Galactic Nuclei (AGN). All are classified asbelonging to the blazar class, with the only exception of the nearbyradio galaxy Centaurus A. We report and strengthen the association ofanother radio galaxy, 3C 111, with the EGRET source 3EG J0416+3650. Atthe time of the compilation of the 3EG catalogue, 3C 111 has beenconsidered as a low-confidence counterpart of 3EG J0416+3650, beinglocated outside the 99% γ-ray probability contour. Since thisfirst suggestion, no other counterparts have been reported nor the EGRETerror box has been searched for likely candidates. 3C 111 has never beenconsiderated or cited in literature as a radiogalaxy counterpart of anEGRET source. We report a detailed multiwavelength study of the EGRETerror box as well as for the first time the overall spectral energydistribution of 3C 111, which appears to be intriguingly similar tothose of blazars, suggesting that the radiogalaxy 3C 111 is the likelycounterpart of 3EG J0416+3650.
| Radio observations of a few giant sources
We present multifrequency observations with the Giant Metrewave RadioTelescope (GMRT) and the Very Large Array (VLA) of a sample of 17largely giant radio sources (GRSs). These observations have eitherhelped clarify the radio structures or provided new information at adifferent frequency. The broad line radio galaxy, J0313+413, has anasymmetric, curved radio jet and a variable radio core, consistent witha moderate angle of inclination to the line of sight. We attempt toidentify steep spectrum radio cores (SSCs), which may be a sign ofrecurrent activity, and find four candidates. If confirmed, this wouldindicate a trend for SSCs to occur preferentially in GRSs. From thestructure and integrated spectra of the sources, we suggest that thelobes of emission in J0139+399 and J0200+408 may be owing to an earliercycle of nuclear activity. We find that inverse-Compton losses with thecosmic microwave background radiation dominate over synchrotronradiative losses in the lobes of all the sources, consistent withearlier studies. We also show that the prominence of the bridge emissiondecreases with increasing redshift, possibly owing to inverse-Comptonlosses. This could affect the appearance and identification of GRSs atlarge redshifts.
| Implications for unified schemes from the quasar fraction and emission-line luminosities in radio-selected samples
We use a principal components analysis of radio-selected (3CRR, 6CE and7CRS) active galactic nuclei (AGN) data sets to define two parametersrelated to low-frequency (151-MHz) radio luminosity L151 and[OIII] luminosity L[OIII]: a parameter α encoding theL151-L[OIII] correlation and a parameter βencoding scatter about this correlation. We describe methods forconstructing generalized luminosity functions (GLFs) based on α,β, redshift and schemes for unifying quasars and radio galaxies.These GLFs can be used to generate radio luminosity functions (RLFs)which improve on those of Willott et al. (2001a), mostly because theyincorporate scatter and are therefore much smoother.Luminosity-dependent unified schemes (e.g. a receding-torus scheme) havebeen invoked to explain the low quasar-to-radio galaxy fraction at lowα and the differences in emission-line luminosities of radioquasars and radio galaxies. With the constraints of the 3CRR, 6CE and7CRS data sets and radio source counts, our GLF approach was used todetermine whether a receding-torus-like scheme is required if there aretwo populations of radio sources: one at low α, consisting of`starved AGN' the other at high α, consisting of `Eddington-tunedAGN'. Because of the overlap between these two populations and theeffects of the β parameter, schemes with or without a recedingtorus can produce a low quasar fraction at low α and differencesin [OIII] luminosity between radio galaxies and quasars. The recedingtorus may be a physical process important in one or more populations ofradio sources, but this is not yet proved either by the quasar fractionor the emission-line properties of radio-selected samples.
| Construction of a Celestial Coordinate Reference Frame from VLBI Data
A large number (2 million) of VLBI observations have been reducedin order to refine the measured coordinates of the observed radiosources. The data reduction was carried out in the OCCAM package usingthe least squares colocation method. Corrections to the coordinates of642 objects were derived. The accuracy of the catalog is no worse than0.2 milliseconds of arc for stable sources.
| A Green Bank Telescope Search for Water Masers in Nearby Active Galactic Nuclei
Using the Green Bank Telescope, we have conducted a survey for 1.3 cmwater maser emission toward the nuclei of nearby active galaxies, themost sensitive large survey for H2O masers to date. Among 145galaxies observed, maser emission was newly detected in 11 sources andconfirmed in one other. Our survey targeted nearby (v<12,000 kms-1), mainly type 2 active galactic nuclei (AGNs) north ofδ=-20deg and includes a few additional sources as well.We find that more than one-third of Seyfert 2 galaxies have strong maseremission, although the detection rate declines beyond v~5000 kms-1 because of sensitivity limits. Two of the masersdiscovered during this survey are found in unexpected hosts: NGC 4151(Seyfert 1.5) and NGC 2782 (starburst). We discuss the possiblerelations between the large X-ray column to NGC 4151 and a possiblehidden AGN in NGC 2782 to the detected masers. Four of the masersdiscovered here, NGC 591, NGC 4388, NGC 5728, and NGC 6323, havehigh-velocity lines symmetrically spaced about the systemic velocity, alikely signature of molecular gas in a nuclear accretion disk. The masersource in NGC 6323, in particular, reveals the classic spectrum of a``disk maser'' represented by three distinct groups of Dopplercomponents. Future single-dish and VLBI observations of these fourgalaxies could provide a measurement of the distance to each galaxy andof the Hubble constant, independent of standard candle calibrations.
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