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|Formation of Magnetically Supported Disks during Hard-to-Soft Transitions in Black Hole Accretion Flows|
We carried out three-dimensional global resistive magnetohydrodynamicsimulations of the cooling instability in optically thin hot black holeaccretion flows by assuming bremsstrahlung cooling. General relativisticeffects are simulated using the pseudo-Newtonian potential. The coolinginstability grows when the density of the accretion disk becomessufficiently large. We found that as the instability grows the accretionflow changes from an optically thin, hot, gas pressure-supported state(low/hard state) to a cooler, magnetically supported, quasi-steadystate. During this transition, the magnetic pressure exceeds the gaspressure because the disk shrinks in the vertical direction while almostconserving the toroidal magnetic flux. Since further verticalcontraction of the disk is suppressed by magnetic pressure, the cooldisk stays in an optically thin, spectrally hard state. The magneticallysupported disk exists for a time scale much longer than the thermal timescale, and comparable to the accretion time scale. We examined thestability of the magnetically supported disk analytically, assuming thatthe toroidal magnetic flux is conserved, and found it to be thermallyand secularly stable. Our findings may explain why black hole candidatesstay in luminous, hard state even when their luminosity exceeds thethreshold for the onset of the cooling instability.
|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.
|Jet-dominated advective systems: radio and X-ray luminosity dependence on the accretion rate|
We present a novel method to measure the accretion rate of radioemitting X-ray binaries (XRBs) and active galactic nuclei (AGN)independently of the X-ray luminosity. The radio emission of the jet isused as a tracer for the accretion rate and is normalized using sourcesof known accretion rates: island state neutron stars (NSs) andefficiently radiating black holes (BHs) close to a state transition. Weshow that the radio power in BHs and NSs is comparable for a given massaccretion rate and verify empirically the assumed analytic scaling ofthe radio luminosity with accretion rate . As our accretion measure isindependent of the X-ray luminosities, we can search for radiativelyinefficient accretion in BHs by comparing the X-ray luminosities withthe accretion rate in XRBs and AGN. While the X-ray luminosity ofefficiently radiating objects scales linearly with accretion rate, thescaling of hard state BHs is quadratical, in agreement with theoreticalmodels. We show that the turnover from the inefficient quadratic scalingto the linear scaling has to occur at accretion rates of 1-10 per centEddington both in XRBs and AGN. The comparison of both accretion statessupports the idea that in a BH in the hard state some accretion power isadvected into the BH while the jet power exceeds the X-ray luminosity:these are therefore jet-dominated advective systems.
|A transient relativistic radio jet from Cygnus X-1|
We report the first observation of a transient relativistic jet from thecanonical black hole candidate, Cygnus X-1, obtained with theMulti-Element Radio-Linked Interferometer Network (MERLIN). The jet wasobserved in only one of six epochs of MERLIN imaging of the sourceduring a phase of repeated X-ray spectral transitions in 2004 Jan-Feb,and this epoch corresponded to the softest 1.5-12keV X-ray spectrum.With only a single epoch revealing the jet, we cannot formally constrainits velocity. Nevertheless, several lines of reasoning suggest that thejet was probably launched 0.5-4.0d before this brightening,corresponding to projected velocities of 0.2c <~ vapp<~ 1.6c, and an intrinsic velocity of >~0.3c. We also report theoccurrence of a major radio flare from Cyg X-1, reaching a flux densityof ~120mJy at 15GHz, and yet not associated with any resolvable radioemission, despite a concerted effort with MERLIN. We discuss theresolved jet in terms of the recently proposed `unified model' for thedisc-jet coupling in black hole X-ray binaries, and tentatively identifythe `jet line' for Cyg X-1. The source is consistent with the model inthe sense that a steady jet appears to persist initially when the X-rayspectrum starts softening, and that once the spectral softening iscomplete the core radio emission is suppressed and transientejecta/shock observed. However, there are some anomalies, and Cyg X-1clearly does not behave like a normal black hole transient inprogressing to the canonical soft/thermal state once the ejection eventhas happened.
|An ultraluminous X-ray microquasar in NGC5408?|
We studied the radio source associated with the ultraluminous X-raysource in NGC5408 (LX ~ 1040ergs-1).The radio spectrum is steep (index ~ -1), consistent with optically thinsynchrotron emission, not with flat-spectrum core emission. Its fluxdensity (~0.28 mJy at 4.8 GHz, at a distance of 4.8 Mpc) was the same inthe March 2000 and December 2004 observations, suggesting steadyemission rather than a transient outburst. However, it is orders ofmagnitude higher than expected from steady jets in stellar-massmicroquasar. Based on its radio flux and spectral index, we suggest thatthe radio source is either an unusually bright supernova remnant, or,more likely, a radio lobe powered by a jet from the black hole (BH).Moreover, there is speculative evidence that the source is marginallyresolved with a radius ~30 pc. A faint HII region of similar sizeappears to coincide with the radio and X-ray sources, but its ionizationmechanism remains unclear. Using a self-similar solution for theexpansion of a jet-powered electron-positron plasma bubble, in theminimum-energy approximation, we show that the observed flux and(speculative) size are consistent with an average jet power ~ 7 ×1038ergs-1 ~ 0.1LX ~0.1LEdd, an age ~105 yr, a current velocity ofexpansion ~80 km s-1. We briefly discuss the importance ofthis source as a key to understand the balance between luminosity andjet power in accreting BHs.
|Periodic long-term X-ray and radio variability of Cygnus X-1|
We present a comprehensive analysis of long-term periodic variability ofCyg X-1 using the method of multiharmonic analysis of variance appliedto available monitoring data since 1969, in X-rays from Vela 5B, Ariel5, Ginga, CGRO and RXTE satellites and in radio from the Ryle and GreenBank telescopes. We confirm a number of previously obtained results,and, for the first time, find an orbital modulation at 15GHz in the softstate and show the detailed non-sinusoidal shape of that modulation inthe hard state of both the 15-GHz emission and the X-rays from theRXTE/All-Sky Monitors (ASM). We find the CGRO/BATSE data are consistentwith the presence of a weak orbital modulation, in agreement with itstheoretical modelling as due to Compton scattering in the companionwind. We then confirm the presence of a ~150-d superorbital period inall of the data since ~1976, finding it in particular for the first timein the Ariel 5 data. Those data sets, covering >65 superorbitalcycles, show a remarkable constancy of both the period and the phase. Onthe other hand, we confirm the presence of a ~290-d periodicity in the1969-1979 Vela 5B data, indicating a switch from that period to itsfirst harmonic at some time <~1980. We find the superorbitalmodulation is compatible with accretion disc precession. Finally, wefind a significant modulation in the RXTE/ASM data at a period of 5.82d,which corresponds to the beat between the orbital and superorbitalmodulations provided the latter is prograde.
|XMM-Newton study of the complex and variable spectrum of NGC 4051|
We study the X-ray spectral variability of the narrow line Seyfert 1galaxy NGC 4051 as observed during two XMM-Newton observations. To gaininsight on the general behaviour, we first apply model-independenttechniques such as rms spectra and flux-flux plots. We then performtime-resolved spectral analysis by splitting the observations into 68spectra (2 ks each). The data show evidence for a neutral andconstant-reflection component and for constant emission fromphotoionized gas, which are included in all spectral models. The nuclearemission can be modelled both in terms of a `standard model' [pivotingpower-law plus a blackbody (BB) component for the soft excess] and of atwo-component one (power law plus ionized reflection from the accretiondisc). Both the models reproduce the source spectral variability andcannot be distinguished on a statistical ground. The distinction hasthus to be made on a physical basis. The standard model results indicatethat the soft excess does not follow the standard BB law (LBB~ T4), despite a variation in luminosity by about one orderof magnitude. The resulting temperature is consistent with beingconstant and has the same value as observed in the PG quasars. Moreover,although the spectral slope is correlated with flux, which is consistentwith spectral pivoting, the hardest photon indices are so flat (Γ~ 1.3-1.4) as to require rather unusual scenarios. Furthermore, the verylow flux states exhibit an inverted Γ-flux behaviour whichdisagrees with a simple pivoting interpretation. These problems can besolved in terms of the two-component model in which the soft excess isnot thermal, but due to the ionized reflection component. In thiscontext, the power law has a constant slope (about 2.2) and theslope-flux correlation is explained in terms of the relativecontribution of the power-law and reflection components which alsoexplains the shape of the flux-flux plot relationship. The variabilityof the reflection component from the inner disc closely follows thepredictions of the light-bending model, suggesting that most of theprimary nuclear emission is produced in the very innermost regions, onlya few gravitational radii (rg) from the central black hole.
|Opening angles, Lorentz factors and confinement of X-ray binary jets|
We present a collation of the available data on the opening angles ofjets in X-ray binaries, which in most cases are small (<~10°).Under the assumption of no confinement, we calculate the Lorentz factorsrequired to produce such small opening angles via the transverserelativistic Doppler effect. The derived Lorentz factors, which are inmost cases lower limits, are found to be large, with a mean >10,comparable to those estimated for active galactic nuclei (AGN) and muchhigher than the commonly assumed values for X-ray binaries of 2-5. Jetpower constraints do not, in most cases, rule out such high Lorentzfactors. The upper limits on the opening angles show no evidence forsmaller Lorentz factors in the steady jets of Cygnus X-1 andGRS1915+105. In those sources in which deceleration has been observed(notably XTEJ1550-564 and Cygnus X-3), some confinement of the jets mustbe occurring, and we briefly discuss possible confinement mechanisms. Itis however possible that all the jets could be confined, in which casethe requirement for high bulk Lorentz factors can be relaxed.
|INTEGRAL/RXTE high-energy observation of a state transition of GX 339-4|
On 2004 August 15, we observed a fast (shorter than 10 h) statetransition in the bright black hole transient GX 339-4 simultaneouslywith Rossi X-Ray Timing Explorer (RXTE) and INTEGRAL. This transitionwas evident both in timing and spectral properties. Combining the datafrom the Proportional Counter Array (PCA), the High-Energy X-ray TimingExperiment (HEXTE) and the Imager on Board the INTEGRAL Satellite(IBIS), we obtained good quality broad-band (3-200 keV) energy spectrabefore and after the transition. These spectra indicate that the hardcomponent steepened. Also, the high-energy cut-off that was present at~70 keV before the transition was not detected after the transition.This is the first time that an accurate determination of the broad-bandspectrum across such a transition has been measured on a shorttime-scale. It shows that, although some spectral parameters do notchange abruptly through the transition, the high-energy cut-offincreases/disappears rather fast. These results constitute a benchmarkon which to test theoretical models for the production of the hardcomponent in these systems.
|The flat synchrotron spectra of partially self-absorbed jets revisited|
Flat radio spectra with large brightness temperatures at the core ofactive galactic nuclei and X-ray binaries are usually interpreted as thepartially self-absorbed bases of jet flows emitting synchrotronradiation. Here we extend previous models of jets propagating at largeangles to our line of sight to self-consistently include the effects ofenergy losses of the relativistic electrons due to the synchrotronprocess itself and the adiabatic expansion of the jet flow. We also takeinto account energy gains through self-absorption. Two model classes arepresented. The ballistic jet flows, with the jet material travellingalong straight trajectories, and adiabatic jets. Despite the energylosses, both scenarios can result in flat emission spectra; however, theadiabatic jets require a specific geometry. No re-acceleration processalong the jet is needed for the electrons. We apply the models toobservational data of the X-ray binary Cygnus X-1. Both models can bemade consistent with the observations. The resulting ballistic jet isextremely narrow with a jet opening angle of only 5arcsec. Its energytransport rate is small compared to the time-averaged jet power andtherefore suggests the presence of non-radiating protons in the jetflow. The adiabatic jets require a strong departure from energyequipartition between the magnetic field and the relativistic electrons.These models also imply a jet power of two orders of magnitude higherthan the Eddington limiting luminosity of a 10-Msolar blackhole. The models put strong constraints on the physical conditions inthe jet flows on scales well below achievable resolution limits.
|Investigating a fluctuating-accretion model for the spectral-timing properties of accreting black hole systems|
The fluctuating-accretion model of Lyubarskii and its extension byKotov, Churazov & Gilfanov seek to explain the spectral-timingproperties of the X-ray variability of accreting black holes in terms ofinward-propagating mass accretion fluctuations produced at a broad rangeof radii. The fluctuations modulate the X-ray emitting region as theymove inwards and can produce temporal-frequency-dependent lags betweenenergy bands, and energy-dependent power spectral densities (PSDs) as aresult of the different emissivity profiles, which may be expected atdifferent X-ray energies. Here, we use a simple numerical implementationto investigate in detail the X-ray spectral-timing properties of themodel and their relation to several physically interesting parameters,namely the emissivity profile in different energy bands, the geometricalthickness and viscosity parameter of the accretion flow, the strength ofdamping on the fluctuations and the temporal coherence (measured by the`quality factor', Q) of the fluctuations introduced at each radius. Wefind that a geometrically thick flow with large viscosity parameter isfavoured, and we confirm that the predicted lags are quite robust tochanges in the emissivity profile and physical parameters of theaccretion flow, which may help to explain the similarity of the lagspectra in the low/hard and high/soft states of Cyg X-1. We alsodemonstrate the model regime where the light curves in different energybands are highly spectrally coherent. We compare model predictionsdirectly to X-ray data from the narrow line Seyfert 1 galaxy NGC 4051and the black hole X-ray binary (BHXRB) Cyg X-1 in its high/soft state,and we show that this general scheme can reproduce simultaneously thetime lags and energy-dependence of the PSD.
|A long-term modulation in XTEJ1716-389: an SS433-like system|
We report on the properties of a 99.3-d periodic modulation in the X-raytransient XTEJ1716-389. We associate this source with the transientKSJ1716-389, first detected by the Mir/Kvant module in 1994. Thespectral characteristics of XTEJ1716-389, a high intrinsic absorptioncolumn, strong emission features and a power-law spectrum, make it verysimilar to the class of highly absorbed X-ray binaries detected byINTEGRAL. We associate the 99.3-d periodic behaviour with thegeometrical obscuration that results from a precessing circumbinary discthat is moving in and out of the field of view, comparable to what hasbeen proposed for SS433. We therefore propose that XTEJ1716-389 is ahigh-mass X-ray binary with a supergiant companion that is similar notonly to SS433, but also to the new class of highly obscured X-raybinaries, suggesting that SS433 is a member of much wider populationthat is now being detected by INTEGRAL.
|Jets in neutron star X-ray binaries: a comparison with black holes|
We present a comprehensive study of the relation between radio and X-rayemission in neutron star (NS) X-ray binaries, use this to infer thegeneral properties of the disc-jet coupling in such systems and comparethe results quantitatively with those already established for black hole(BH) systems. There are clear qualitative similarities between the twoclasses of object: hard states below about 1 per cent of the Eddingtonluminosity produce steady jets, while transient jets are associated withoutbursting and variable sources at the highest luminosities. However,there are important quantitative differences: the NSs are less radioloud for a given X-ray luminosity (regardless of mass corrections) andthey do not appear to show the strong suppression of radio emission insteady soft states that we observe in BH systems. Furthermore, in thehard states, the correlation between radio and X-ray luminosities of theNS systems is steeper than the relation observed in BHs by about afactor of 2. This result strongly suggests that the X-ray emission inthe BH systems is radiatively inefficient, with an approximate relationof the form , consistent with both advection-dominated models and thejet-dominated scenario. In contrast, the jet power in both classes ofobject scales linearly with accretion rate. This constitutes some of thefirst observational evidence for the radiatively inefficient scaling ofX-ray luminosity with accretion rate in accreting BH systems. Moreover,based on simultaneous radio/X-ray observations of Z-type NSs (thebrightest of our Galaxy, always near or at the Eddington accretionrate), we draw a model that can describe the disc-jet coupling in suchsources, finding a possible association between a particular X-ray statetransition [horizontal branch to normal branch] and the emission oftransient jets.
|Bulk motion Comptonization in black hole accretion flows|
We study spectra generated by Comptonization of soft photons by coldelectrons radially free falling on to a black hole. We use a Monte Carlomethod involving a fully relativistic description of Comptonization inthe Kerr space-time. In agreement with previous studies, we find thatComptonization on the bulk motion of free fall gives rise to power-lawspectra with the photon index of Γ>~ 3. In contrast to someprevious studies, we find that these power-law spectra extend only toenergies <~100 keV rulesout bulk motion Comptonization as a main radiative process in softspectral states of black hole binaries. The normalization of the powerlaw (below the cut-off) with respect to the peak of the blackbodyemission of the surrounding disc is typically very low, except formodels with an overlap between the disc and the plasma, in which casethe spectra are very soft, Γ>~ 4.
|A deep XMM-Newton observation of the ultraluminous X-ray source Holmberg II X-1: the case against a 1000-Msolar black hole|
We present results from a 112-ks long look by XMM-Newton at theultraluminous X-ray source (ULX) Holmberg II X-1 (Ho II X-1), longthought to be the one of best candidates for the missing class ofintermediate-mass black holes (IMBHs). Our data comprises the firsthigh-quality XMM-Newton/RGS (reflection grating spectrometer) spectrumof an ULX, and an XMM-Newton/EPIC (European Photo Imaging Camera)spectrum with unprecedented signal-to-noise ratio. A detailed timinganalysis shows that any variability on time-scales of minutes to hoursis very weak (less than a few per cent fractional rms), though largeramplitude variations on much shorter time-scales could be hidden byphoton counting statistics. This result suggests that if Ho II X-1harbours an IMBH, then we are observing this source in a highly unusualand atypical state when compared with the known variability behaviour ofother accreting systems of large mass. Moreover, unlike galactic X-raybinaries, our spectral analysis indicates the possible presence of anoptically thick low-temperature corona. Taken together our timing andspectral analysis suggests that the compact companion is most likely ahigh-luminosity analogue of black hole binary systems similar to GRS1915+105, the galactic microquasar, harbouring a compact object of massno greater than 100 Msolar.
|Long-Term Variations of the Supergiant in the X-Ray Binary Cyg X-1|
The photometrical and spectral variations point to the supergiantparameters changes on the time scale of tens of years. Line profilenon-LTE simulations lead to the conclusion that the star radius hasgrown about 1-4 from 1997 to 2003-2004 and the temperature decreased by1300-2400 K
|INTEGRAL detection of a strong outburst in 4U 1954+31|
We report the dectection of a ~150 mCrab outburst in the transient highmass X-ray binary 4U 1954+31, observed in the Galactic Plane Survey(GPS) by INTEGRAL. The flux in the IBIS/ISGRI instrument reached 17.7counts/s (165 mCrab, 32-sigma detection) in the 20-40 keV band. Theobservations started at 00:14 UTC on Apr 15, 2006. The flux was already~15 counts/s (140 mcrab). It peaked 40 min later, and then fell to about10 counts/s (95 mcrab).
|Outburst of IGR J11215-5952 Observed with RXTE|
Sidoli et al. (2006, astro-ph/0203081) reported that the hard x-raytransient IGR J11215-5952 appeared to have a recurrence period of ~330days, based on three short outbursts observed by INTEGRAL, the mostrecent of which was actually the discovery observation (Lubinski et al., ATEL #469). This short outburst, and the presence in the INTEGRAL error circle of the blue supergiant HD 306414 (Negueruela et al.
|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.
|A Comprehensive Search for Gamma-Ray Lines in the First Year of Data from the INTEGRAL Spectrometer|
We have carried out an extensive search for gamma-ray lines in the firstyear of public data from the spectrometer (SPI) on the INTEGRAL mission.INTEGRAL has spent a large fraction of its observing time in theGalactic plane with particular concentration in the Galactic center (GC)region (~3 Ms in the first year). Hence the most sensitive searchregions are in the Galactic plane and center. The phase space of thesearch spans the energy range 20-8000 keV and line widths from 0 to 1000keV (FWHM). It includes both diffuse and pointlike emission. We havesearched for variable emission on timescales down to ~1000 s. Diffuseemission has been searched for on a range of different spatial scalesfrom ~20° (the approximate field of view of the spectrometer) up tothe entire Galactic plane. Our search procedures were verified by therecovery of the known gamma-ray lines at 511 and 1809 keV at theappropriate intensities and significances. We find no evidence for anypreviously unknown gamma-ray lines. The upper limits range from afew×10-5 to a few×10-2 cm-2s-1 depending on line width, energy, and exposure; regions ofstrong instrumental background lines were excluded from the search.Comparison is made between our results and various prior predictions ofastrophysical lines.
|Simultaneous Chandra and RXTE Spectroscopy of the Microquasar H1743-322: Clues to Disk Wind and Jet Formation from a Variable Ionized Outflow|
We observed the bright phase of the 2003 outburst of the Galactic blackhole candidate H1743-322 in X-rays simultaneously with Chandra and RXTEon four occasions. The Chandra HETGS spectra reveal narrow, variable(He-like) Fe XXV and (H-like) Fe XXVI resonance absorption lines. In thefirst observation, the Fe XXVI line has a FWHM of 1800+/-400 kms-1 and a blueshift of 700+/-200 km s-1,suggesting that the highly ionized medium is an outflow. Moreover, theFe XXV line is observed to vary significantly on a timescale of a fewhundred seconds in the first observation, which corresponds to theKeplerian orbital period at approximately 104rg(where rg=GM/c2). Our models for the absorptiongeometry suggest that a combination of changing ionizing flux andgeometric effects are required to account for the large changes in lineflux observed between observations and that the absorption likely occursat a radius between 102rg and104rg for a 10 Msolar black hole. Wesuggest that the absorption occurs in an inhomogeneous accretion diskwind. If the wind in H1743-322 has unity filling factor, the highestimplied mass outflow rate is 5% of the Eddington mass accretion rate.The observed wind may be a hotter, more ionized version of theSeyfert-like, outflowing warm absorber geometries recently found in theGalactic black holes GX 339-4 and XTE J1650-500. We discuss thesefindings in the context of ionized Fe absorption lines found in thespectra of other Galactic sources, and connections to warm absorbers,winds and jets in other accreting systems.
|Connection between the Accretion Disk and Superluminal Radio Jets and the Role of the Radio Plateau State in GRS 1915+105|
We investigate the association between the accretion disk during theradio plateau state and the following superluminal relativistic radiojets with peak intensity varying from 200 to 1000 mJy observed over aperiod of 5 yr and present evidence of a direct accretion disk-jetconnection in the microquasar GRS 1915+105. We have analyzed RXTEPCA/HEXTE X-ray data and have found that the accretion rate,m˙accr, as inferred from the X-ray flux, is very highduring the radio plateaux. We suggest that the accretion disk during theradio plateaux is always associated with radiation-driven wind, which ismanifested in the form of enhanced absorption column density for X-raysand depleted IR emission. We find that the wind density increases withthe accretion disk luminosity during the radio plateaux. The winddensity is similar to the density of the warm absorber proposed inextragalactic active galactic nuclei (AGNs) and quasars. We suggest asimple model for the origin of superluminal relativistic jets. Finally,we discuss the implications of this work for Galactic microquasars andextragalactic AGNs and quasars.
|Variability Study of Seyfert 2 Galaxies with XMM-Newton|
We present the results of timing analysis of XMM-Newton observations ofSeyfert 2 galaxies in order to search for differences in the meanproperties of Seyfert 1 galaxies and Seyfert 2 galaxies. We selected 13Seyfert 2 galaxies from the XMM-Newton archive that have hard X-raycomponents in their spectra and calculated the excess variance(σ2rms) in the 2-10 keV band. We found thatsix Seyfert 2 galaxies (3C 98, IRAS 05189-2524, MCG -5-23-16, NGC 6300,UGC 4203, and PKS 1814-637) have buried luminous nuclei and that thenuclei have timing properties similar to those of Seyfert 1 nuclei. Thisindicates that these galaxies are candidates for having buried Seyfert 1nuclei as expected by the unified Seyfert model. The first five galaxiesshow significant time variability. The amplitude of the time variabilityof IRAS 05189-2524 is similar to that of narrow-line Seyfert 1 galaxies.In contrast, the amplitude of variability of the seven other galaxies isquite small, much smaller than that of Seyfert 1 galaxies with similarX-ray luminosity. The lack of short time variability in these objects isexplained by the dominance of the reflection component in three galaxies(Mrk 3, Mrk 463, and NGC 7582), and by the presence of very massiveblack holes and an inferred low accretion rate in the other threegalaxies (NGC 1052, NGC 4507, and NGC 7172). For Mrk 348, thesignificant time variability that is expected based on the estimate ofthe central black hole mass was not detected.
|Evolution of Hard X-Ray Spectra along the Orbital Phase in Circinus X-1|
Using the data from the Proportional Counter Array and the High-EnergyX-Ray Timing Experiment on board the Rossi X-Ray Timing Explorersatellite, we investigate the evolution of the 3-200 keV spectrum of thepeculiar low-mass X-ray binary Circinus X-1 along its orbital phase. Apower-law hard component, dominating above ~30 keV as well ascontributing between 0.9% and 3.5% of the total flux, and the ironemission line are detected in the spectrum. It is found that the derivedphoton index (Γ) of the power-law hard component and thecharacteristics of the iron emission line are modulated by the orbitalphase: the hardest hard tail, the lowest line energy, and the largestline width of the iron emission line are detected at the periastron.However, the softest hard tail, the highest line energy, and the lowestline width of the iron emission line are detected after the apastron. Atthe periastron the source evolves along a complete track in thecolor-color diagram. These behaviors of orbital modulation arediscussed. A possible origin of the power-law hard component is alsodiscussed.
|Fourier-resolved Spectroscopy of 4U 1543-47 during the 2002 Outburst|
We have obtained Fourier-resolved spectra of the black hole binary 4U1543-47 in the canonical states (high/soft, very high, intermediate, andlow/hard) observed in this source during the decay of an outburst thattook place in 2002. Our objective is to investigate the variability ofthe spectral components generally used to describe the energy spectra ofblack hole systems, namely a disk component, a power-law componentattributed to Comptonization by a hot corona, and the contribution ofthe iron line due to reprocessing of the high-energy (E>~7 keV)radiation. We find that (1) the disk component is not variable ontimescales shorter than ~100 s, (2) the reprocessing emission asmanifest by the variability of the Fe Kα line responds to theprimary radiation variations down to timescales of ~70 ms in the highand very high states, but longer than 2 s in the low state, (3) thelow-frequency QPOs are associated with variations of the X-ray power-lawspectral component and not to the disk component, and (4) the spectracorresponding to the highest Fourier frequency are the hardest (show theflatter spectra) at a given spectral state. These results questionmodels that explain the observed power spectra as due to modulations ofthe accretion rate alone, as such models do not provide any apparentreason for a Fourier frequency dependence of the power-law spectralindices.
|Time-dependent Synchrotron and Compton Spectra from Jets of Microquasars|
Jet models for the high-energy emission of Galactic X-ray binary sourceshave regained significant interest with detailed spectral and timingstudies of the X-ray emission from microquasars, the recent detection bythe HESS collaboration of very high energy γ-rays from themicroquasar LS 5039, and the earlier suggestion of jet models forultraluminous X-ray sources observed in many nearby galaxies. Here westudy the synchrotron and Compton signatures of time-dependent electroninjection and acceleration, and adiabatic and radiative cooling in thejets of Galactic microquasars. Synchrotron, synchrotron self-Compton,and external Compton radiation processes with soft photons provided bythe companion star and the accretion disk are treated. An analyticalsolution is introduced to the electron kinetic equation for generalpower-law geometries of the jets for Compton scattering in the Thomsonregime. We pay particular attention to predictions concerning the rapidflux and spectral variability signatures expected in a variety ofscenarios, making specific predictions concerning possible spectralhysteresis, similar to what has been observed in several TeV blazars.Such predictions should be testable with dedicated monitoringobservations of Galactic microquasars and ultraluminous X-ray sourcesusing Chandra and/or XMM-Newton.
|Spitzer Reveals Infrared Optically Thin Synchrotron Emission from the Compact Jet of the Neutron Star X-Ray Binary 4U 0614+091|
Spitzer observations of the neutron star (ultracompact) X-ray binary(XRB) 4U 0614+091 with the Infrared Array Camera reveal emission ofnonthermal origin in the range 3.5-8 μm. The mid-infrared spectrum iswell fit by a power law with spectral index of α=-0.57+/-0.04(where the flux density is Fν~να).Given the ultracompact nature of the binary system, we exclude thepossibility that either the companion star or the accretion disk can bethe origin of the observed emission. These observations represent thefirst spectral evidence for a compact jet in a low-luminosity neutronstar XRB and furthermore of the presence, already observed in two blackhole (BH) XRBs, of a ``break'' in the synchrotron spectrum of suchcompact jets. We can derive a firm upper limit on the break frequency ofthe spectrum of νthin=3.7×1013 Hz, whichis lower than that observed in BH XRBs by at least a factor of 10.Assuming a high-energy cooling cutoff at ~1 keV, we estimate a total(integrated up to X-rays) jet power to X-ray bolometric luminosity ratioof ~5%, much lower than that inferred in BHs.
|The Nonlinear Behavior of the Black Hole System GRS 1915+105|
Using nonlinear time series analysis, along with surrogate dataanalysis, it is shown that the various types of long-term variabilityexhibited by the black hole system GRS 1915+105 can be explained interms of a deterministic nonlinear system with some inherent stochasticnoise. Evidence is provided for a nonlinear limit cycle origin of one ofthe low-frequency QPOs detected in the source, while some other types ofvariability could be due to an underlying low-dimensional chaoticsystem. These results imply that the partial differential equations thatgovern the magnetohydrodynamic flow of the inner accretion disk can beapproximated by a small number (~3-5) of nonlinear but ordinarydifferential equations. While this analysis does not reveal the exactnature of these approximate equations, they may be obtained in thefuture, after results of magnetohydrodynamic simulation of realisticaccretion disks become available.
|Comprehensive Analysis of RXTE Data from Cygnus X-1: Spectral Index-Quasi-Periodic Oscillation Frequency-Luminosity Correlations|
We present timing and spectral analysis of ~2.3 Ms of Rossi X-Ray TimingExplorer (RXTE) archival data from Cyg X-1. Using a genericComptonization model we reveal that the spectrum of Cyg X-1 consists ofthree components: a thermal seed photon spectrum, a Comptonized part ofthe seed photon spectrum, and the iron line. We find a strongcorrelation between the 0.1-20 Hz frequencies of quasi-periodicoscillations (QPOs) and the spectral index. The presence of two spectralphases (states) is clearly seen in the data when the spectral indicessaturate at low and high values of QPO frequencies. This saturationeffect was discovered earlier in a number of black hole candidate (BHC)sources, and now we strongly confirm this phenomenon in Cyg X-1. In thesoft state this index-QPO frequency correlation shows a saturation ofthe photon index Γ~2.1 at high values of the low frequencyνL. The saturation level of Γ~2.1 is the lowestvalue found yet in BHCs. The bolometric luminosity does not show clearcorrelation with the index. We also show that Fe Kα emission linestrength (equivalent width [EW]) correlates with the QPO frequency. TheEW increases from 200 eV in the low/hard state to 1.5 keV in thehigh/soft state. The observational correlations revealed compel us topropose a scenario for the spectral transition and iron line formationthat occur in BHC sources. We also present the spectral state (power-lawindex) evolution for 8 yr of Cyg X-1 observations by RXTE.
|Gamma Rays from Compton Scattering in the Jets of Microquasars: Application to LS 5039|
Recent High Energy Stereoscopic System (HESS) observations show thatmicroquasars in high-mass systems are sources of very high energyγ-rays. A leptonic jet model for microquasar γ-ray emissionis developed. Using the head-on approximation for the Compton crosssection and taking into account angular effects from the star's orbitalmotion, we derive expressions to calculate the spectrum of γ-rayswhen nonthermal jet electrons Compton scatter photons of the stellarradiation field. The spectrum of Compton scattered accretion diskradiation is also derived by approximating the accretion disk as a pointsource of radiation located behind the jet. Numerical results arecompared with simpler expressions obtained using δ-functionapproximations for the cross sections, from which beaming factors arederived. Calculations are presented for power-law distributions ofnonthermal electrons that are assumed to be isotropically distributed inthe comoving jet frame and applied to γ-ray observations of LS5039. We conclude that (1) the TeV emission measured with HESS cannotresult only from Compton scattered stellar radiation (CSSR), but couldbe synchrotron self-Compton (SSC) emission or a combination of CSSR andSSC; and (2) fitting both the HESS data and the EGRET data claimed to beassociated with LS 5039 requires a very improbable leptonic model with avery hard electron energy distribution. Because the γ-rays wouldbe variable in a leptonic jet model, the data sets are unlikely to berepresentative of a simultaneously measured γ-ray spectrum. Wetherefore attribute EGRET γ-rays primarily to CSSR emission andHESS γ-rays to SSC emission. Detection of periodic modulation ofthe TeV emission from LS 5039 would favor a leptonic SSC or cascadehadron origin of the emission in the inner jet, whereas stochasticvariability alone would support a more extended leptonic model. Thepuzzle of the EGRET γ-rays from LS 5039 will be quickly solvedwith GLAST.
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