A magnetic model for low/hard state of black hole binaries

A magnetic model for the low/hard state(LHS) of two black hole X-ray binaries(BHXBs),H1743–322 and GX 339–4, is proposed based on transport of the magnetic field from a companion into an accretion disk around a black hole(BH). This model consists of a truncated thin disk with an inner advection-dominated accretion flow(ADAF). The spectral profiles of the sources are fitted in agreement with the data observed at four different dates corresponding to the rising phase of the LHS. In addition,the association of the LHS with a quasi-steady jet is modeled based on transport of magnetic field, where the Blandford-Znajek(BZ) and Blandford-Payne(BP) processes are invoked to drive the jets from BH and inner ADAF. It turns out that the steep radio/X-ray correlations observed in H1743–322 and GX 339–4 can be interpreted based on our model.     

A disk-corona model for the low/hard state of black hole X-ray binaries

A disk-corona model for fitting the low/hard（LH）state of the associated steady jet in black hole X-ray binaries（BHXBs）is proposed based on the large-scale magnetic field configuration that arises from the coexistence of the Blandford-Znajek（BZ）and Blandford-Payne（BP）processes,where the magnetic field configuration for the BP process is determined by the requirement of energy conversion from Poynting energy flux into kinetic energy flux in the jet.It is found that corona current is crucial to guarantee the consistency of the jet launching from the accretion disk.The relative importance of the BZ and BP processes in powering jets from black hole accretion disks is discussed,and the LH state of several BHXBs is fitted based on our model.In addition,we suggest that magnetic field configuration can be regarded as the second parameter for governing the state transition of BHXBs.     

High energy properties of the flat spectrum radio quasar 4C 50.11

We investigate theγ-ray and X-ray properties of the flat spectrum radio quasar(FSRQ)4C 50.11 at redshift z=1.517.The Fermi-LAT data indicate that this source was in an active state since July 2013.During this active period,the source’s emission appeared harder inγ-rays,with the flux having increased by more than a factor of three.We analyze two distinct flares seen in the active state and find that the variability is as short as several hours.The Swift-XRT data show that the source was variable at X-ray energies,but no evidence is found for flux or spectral changes related to theγ-ray activity.The broad-band X-ray spectrum obtained with Swift-XRT and Nu STAR is described well by a broken power law model,with an extremely flat spectrum(Γ_1~0.1)below the break energy,E_(break)~2.1 keV,and Γ_2~1.5 above the break energy.The spectral flattening below~3 keV is likely due to the low energy cut-off in the energy distribution of the photon-emitting electron population.We fit the broad-band spectral energy distribution of the source during both the active and quiescent states.The X-ray andγ-ray emission from the jet is mainly due to the inverse-Compton scattering process,with seed photons provided from the broad line region,and the jet is estimated to be larger than the accretion power if the jet is mainly composed of electron-proton pairs.     

Inference on disk-jet connection of MAXI J1836–194 from spectral analysis with the TCAF solution

Galactic transient black hole candidate(BHC) MAXI J1836–194 was discovered on 2011 Aug30, by MAXI/GSC and Swift/BAT. The source activity during this outburst continued for ~ 3 months before entering into the quiescent state. It again became active in March 2012 and continued for another~ 2 months. In this paper, 3-25 keV RXTE/PCA spectra from the 2011 outburst and 0.5-10.0 keV Swift/XRT data during its 2012 outburst are analyzed with the two-component advective flow(TCAF)model based fits files in XSPEC. We calculate the X-ray contributions coming from jets/outflow using a newly developed method based on the deviation of the TCAF model normalization. We also study the correlation between observed radio and estimated jet X-ray fluxes. The correlation indices(b) are found to be 1.79 and 0.61, when the 7.45 GHz Very Large Array(VLA) radio flux is correlated with the total X-ray and jet X-ray fluxes in 3-25 keV range respectively. It has been found that the jet contributes in X-rays up to a maximum of 86% during its 2011 outburst. This makes the BHC MAXI J1836–194 strongly jet dominated during the initial rising phase.     

The pc-scale radio structure of MIR-observed radio galaxies

We investigated the relationship between the accretion process and jet properties by utilizing very long baseline array(VLBA) and mid-infrared(MIR) data for a sample of 45 3 CRR radio galaxies selected with a flux density at 178 MHz 16.4 Jy, 5 GHz very large array(VLA) core flux density≥7 m Jy and MIR observations. The pc-scale radio structure at 5 GHz is presented by using our VLBA observations for 21 sources acquired in February, 2016, the analysis of archival data for 16 objects and directly obtaining measurements for eight radio galaxies available from literatures. The accretion mode is constrained from the Eddington ratio with a dividing value of 0.01, which is estimated from the MIR-based bolometric luminosity and the black hole masses. While most Fanaroff-Riley type II radio galaxies(FRIIs) have higher Eddington ratio than Fanaroff-Riley type I radio galaxies(FRIs), we found that there is indeed no single correspondence between the FR morphology and accretion mode with eight FRIIs at low accretion rate and two FRIs at high accretion rate. There is a significant correlation between the VLBA core luminosity at 5 GHz and the Eddington ratio. Various morphologies are identified in our sample, including core only, single-sided core-jet and two-sided core-jet structures. We found that the higher accretion rate may be more likely related with the core-jet structure, thus generating a more extended jet. These results imply that the higher accretion rates are likely able to produce more powerful jets. There is a strong correlation between the MIR luminosity at 15 μm and VLBA 5 GHz core luminosity, in favor of the tight relation between the accretion disk and jets. In our sample, the core brightness temperature ranges from 10~9 to 10~(13.38) K with a median value of 10~(11.09) K, indicating that systematically the beaming effect may not be significant. The exceptional cases, FRIs at high accretion rates and FRIIs at low accretion rates, are exclusively at the high and low ends, respectively, of the distribution of the flux ratio for VLBA core to 178 MHz flux density. It is not impossible that the locations of these sources are due to the recent shining or weakening of their central engines(i.e., both accretion and jet).     

The nature of the γ-ray flare associated with blazar 3C 454.3

We have studied the simultaneous spectral energy distributions(SEDs) of the 2009 December flare and those of the quiescent state of blazar 3C 454.3 by constructing a multi-component model. We find that all six SEDs can be explained by a one-zone leptonic model involving synchrotron self-Compton(SSC) plus external Compton emission from an accretion disk(ECD) and that from a broad-line region(ECC). X-ray emission is dominated by the SSC mechanism, and the γ-ray spectrum is well represented by a combination of ECD and ECC processes. Our results indicate that the energy density of the magnetic field and electrons decrease with distance from the central engine, and the Doppler factor increases with the blob moving outward in the development of the 2009 December flare. The increase in the observed flux density is possibly due to the increase in the Doppler factor of the blob. The relation between the Doppler factor δb and the distance from the central black hole suggests the magnetically driven jets span a sub-pc scale, and the relation between the magnetic field B′ and the dimension of the emission region R′b is in good agreement with what is required by conservation of magnetic flux. The weak "harder-when-brighter" behavior of the γ-ray spectrum could be a result of the increase in Doppler factor during the outward motion of the blob. The parameters during the quiescent state obviously deviate from those during the flare state. We propose that the flare was likely caused by the ejection of a new blob. The gamma-ray emissions in different states are associated with the evolution of the blob.     

An investigation of a magnetic cataclysmic variable with a period of 14.1 ks

Magnetic cataclysmic variables(CVs) contain a white dwarf(WD) with a magnetic field strong enough to control the accretion flow from a late type secondary. In this paper, we identify a magnetic CV(CXOGSG J215544.4+380116) from the Chandra archive data. The X-ray light curves show a significant period of 14.1 ks, and the X-ray spectra can be described by a multi-temperature hot thermal plasma, suggesting the source is a magnetic CV. The broad dip in the X-ray light curve is due to the eclipse of the primary magnetic pole, and the additional dip in the bright phase of the soft and medium bands may be caused by the accretion stream crossing our line of sight to the primary pole.Follow-up optical spectra show features of an M2–M4 dwarf dominating the red band and a WD which is responsible for the weak upturn in the blue band. The mass(~ 0.4 M⊙) and radius(~0.4 R⊙) for the M dwarf are obtained using CV evolution models and empirical relations between the orbital period and the mass/radius. The estimated low X-ray luminosity and accretion rate may suggest the source is a low-accretion-rate polar. In addition, Very Large Array observations reveal a possible radio counterpart to the X-ray source, but with a low significance. Further radio observations with high quality are needed to confirm the radio counterpart and explore the properties of this binary system.     

Systematic analysis of low/hard state RXTE spectra of GX 339–4 to constrain the geometry of the system

One of the popular models for the low/hard state of black hole binaries is that the standard accretion disk is truncated and the hot inner region produces, via Comptonization, hard X-ray flux.This is supported by the value of the high energy photon index, which is often found to be small,～ 1.7( 2), implying that the hot medium is starved of seed photons. On the other hand, the suggestive presence of a broad relativistic Fe line during the hard state would suggest that the accretion disk is not truncated but extends all the way to the innermost stable circular orbit. In such a case, it is a puzzle why the hot medium would remain photon starved. The broad Fe line should be accompanied by a broad smeared reflection hump at ～ 30 ke V and it may be that this additional component makes the spectrum hard and the intrinsic photon index is larger, i.e. 2. This would mean that the medium is not photon deficient, reconciling the presence of a broad Fe line in the observed hard state. To test this hypothesis,we have analyzed the RXTE observations of GX 339–4 from the four outbursts during 2002–2011 and identify observations when the system was in the hard state and showed a broad Fe line. We have then attempted to fit these observations with models, which include smeared reflection, to understand whether the intrinsic photon index can indeed be large. We find that, while for some observations the inclusion of reflection does increase the photon index, there are hard state observations with a broad Fe line that have photon indices less than 2.     

Effect of non-stationary accretion on spectral state transitions:An example of a persistent neutron star LMXB 4U 1636–536

Observations of black hole and neutron star X-ray binaries show that the luminosity of the hard-to-soft state transition is usually higher than that of the soft-to-hard state transition,indicating additional parameters other than mass accretion rate are required to interpret spectral state transitions.It has been found in some individual black hole or neutron star soft X-ray transients that the luminosity corresponding to the hard-to-soft state transition is positively correlated with the peak luminosity of the following soft state. In this work,we report the discovery of the same correlation in the single persistent neutron star low mass X-ray binary(LMXB) 4 U 1636–536 based on data from the All Sky Monitor(ASM) on board RXTE,the Gas Slit Camera(GSC) on board MAXI and the Burst Alert Telescope(BAT) on board Swift. We also found such a positive correlation holds in this persistent neutron star LMXB in a luminosity range spanning about a factor of four. Our results indicate that non-stationary accretion also plays an important role in driving X-ray spectral state transitions in persistent accreting systems with small accretion flares,which is much less dramatic compared with the bright outbursts seen in many Galactic LMXB transients.     

Fermi/LAT observations of lobe-dominant radio galaxy 3C 207 and possible radiation region of γ-rays

3 C 207 is a lobe-dominant radio galaxy with a one sided jet and bright knots, spanning a kpcMpc scale, which have been resolved in the radio, optical and X-ray bands. This target was confirmed as a γ-ray emitter with Fermi/LAT, but it is uncertain whether the γ-ray emission region is the core or knots due to the low spatial resolution of Fermi/LAT. We present an analysis of its Fermi/LAT data acquired during the past 9 years. Different from the radio and optical emission from the core, it is found that theγ-ray emission is steady without detection of flux variation at over a 2σ confidence level. This likely implies that the γ-ray emission is from its knots. We collect the radio, optical and X-ray data of knot-A,the closest knot from the core at 1.4′′, and compile its spectral energy distribution(SED). Although the single-zone synchrotron+SSC+IC/CMB model that assumes knot-A is at rest can reproduce the SED in the radio-optical-X-ray band, the predicted γ-ray flux is lower than the LAT observations and the derived magnetic field strength deviates from the equipartition condition by 3 orders of magnitude. Assuming that knot-A is moving relativistically, its SED from radio to γ-ray bands would be represented well with the single-zone synchrotron+SSC+IC/CMB model under the equipartition condition. These results likely suggest that the γ-ray emission may be from knot-A via the IC/CMB process and the knot should have relativistical motion. The jet power derived from our model parameters is also roughly consistent with the kinetic power estimated with radio data.     

The composite X-ray spectra of radio-loud and radio-quiet SDSS quasars

We present a study of the X-ray emission for a sample of radio-detected quasars constructed from the cross-matches between SDSS,FIRST catalogs and XMM-Newton archives.A sample of radio-quiet SDSS quasars without FIRST radio detection is also assembled for comparison.We construct the optical and X-ray composite spectra normalized at rest frame 4215 A(or 2200 A)for both radio-loud quasars(RLQs)and radio-quiet quasars(RQQs)at z≤3.2,with matched X-ray completeness of 19%,redshift and optical luminosity.While the optical composite spectrum of RLQs is similar to that of RQQs,we find that RLQs have a higher X-ray composite spectrum than RQQs,consistent with previous studies in the literature.By dividing the radio-detected quasars into radio loudness bins,we find the X-ray composite spectra are generally higher with increasing radio loudness.Moreover,a significant correlation is found between the optical-to-X-ray spectral index and radio loudness,and there is a unified multi-correlation between the radio and X-ray luminosities and radio loudness in radio-detected quasars.These results could be possibly explained with the corona-jet model,in which the corona and jet are directly related.     

The Relation between the Inclinations of Broad Line Regions and the Accretion Disk

According to the standard model, an active galactic nucleus (AGN) consists of an inner accretion disk with a jet around a central massive black hole, and a number of outer broad line regions (BLRs) and narrow line regions (NLRs). The geometrical relationship between the BLRs and the accretion disk is not well understood. Assuming the motion of the BLRs is virialized and its configuration is disk-like, we derived its inclination to the line of sight for a sample of AGNs from their bulge stellar velocity dispersion, their size of the BLRs and their Hβ linewidth. Compared with the inclination of the accretion disk obtained from the X-ray Fe Kα emission lines, we found that there is no positive correlation between the two. Our results showed that BLRs are not coplanar with the accretion disk and that we should be cautious of using the BLRs inclination as the disk inclination. The non-coplanar geometry of the outer BLRs and the inner accretion disk provides clues to the origin of BLRs and the properties of the accretion disk. Our preferable interpretation is that BLRs arise out of the outer part of a warped accretion disk.     

Spectral lags of flaring events in LS I+61?303 from RXTE Observations

This work reports the first discovery of(negative) spectral lags in X-ray emission below 10 ke V from the gamma ray binary LS I+61?303 during large flaring episodes using Rossi X-ray Timing Explorer(RXTE) observations. It is found from the RXTE data that during the flares, low energy(3–5 ke V) variations lead the higher energy(8–10 ke V) variations by a few tens of seconds whereas no significant time lag is observed during the non-flaring states. The observed spectral lag features for flaring events suggest that inverse Compton scattering may be operating, at least in some part of the system. Another possibility is that the sites of particle acceleration may be different for flaring and non-flaring events such as in the microquasar model, in which the flaring radiation may come from hot spots sitting above the black hole while steady state emissions are due to the jets.     

X-ray Emission Lines in GRB Afterglows： Evidence for a Two-component Jet Model

X-ray emission lines have been observed in X-ray afterglows of several γ-ray bursts (GRBs). It is a major breakthrough for understanding the nature of the progenitors. It has been proposed that the X-ray emission lines can be well explained by the Geometry-Dominated models, but in these models the illuminating angle is much larger than that of the collimated jet of the GRB. For GRB 011211, we have obtained an illuminating angle of about θ- 45°, while the angle of the GRB jet is only 3.6°. So we propose that the outflow of GRBs with emission lines should have two distinct components: a wide component that illuminates the reprocessing material and produces the emission lines and a narrow one that produces the GRB. Observations show the energy for producing the emission lines is higher than that of the GRB. In this case, when the wide component dominates the afterglows, a bump should appear in the GRB afterglow. For GRB 011211, the bump should occur within 0.05 days of the GRB, which is obviously too early for the observation to catch it. Alongside the X-ray emission lines there should also be a bright emission component between the UV and the soft X-rays. These features can be tested by the Swift satellite in the near future.     

Variation of the inner disk radius during the onset of the 2010 outburst of MAXI J1659–152

Low mass black hole binaries are generally transient sources and spend most of their time in the quiescent state. It is believed that the inner accretion flow in the quiescent state is in the form of advection dominated accretion flow and the cold outer accretion disk is truncated far away from the central black hole. During the onset of an outburst, the disk gradually extends towards the central black hole.However, the observational evidence for this general picture is indirect at best. Here we present the results of a study performed to understand the variation of the inner disk radius during the early phase of an outburst. We investigated the variation of the inner disk radius during the 2010 outburst of the black hole candidate MAXI J1659-152 using the method of simultaneous spectral fitting. We found that the inner edge of the disk is truncated at a large radius in the beginning of the outburst when the source was in the hard state. We found a systematic decrease in the inner disk radius as the outburst progressed. We also estimated an upper limit on the mass of the black hole to be 8.1 ± 2.9 M within the uncertainty of the distance and inclination angle.     

MHD simulations of inward shocks in Cassiopeia A

Cassiopeia A, the brightest radio supernova remnant(SNR) in the sky, has several unique characteristics in comparison to its peers. Besides its radio brightness and prominent soft-concave radio spectrum,its γ-ray spectrum appears to have a low-energy cutoff near 2 GeV, and it is the only SNR with prominent hard X-ray emission. While the unusual radio properties may be attributed to strong emission from reverse shocks, the hard X-ray emission has been associated with high-speed inward shocks induced by high density gases. Then, the low-energy γ-ray spectral cutoff could be attributed to slow penetration of lower energy particles accelerated near the inward shocks into high-density emission zone. In this paper,we carry out magneto-hydrodynamic(MHD) simulations of shocks in Cassiopeia A and demonstrate that its inward shock structure can indeed be reproduced via shock interactions with clumps of gases with a density of ~ 20 cm~(-3).     

Understanding the X-ray spectrum of anomalous X-ray pulsars and soft gamma-ray repeaters

Hard X-rays above 10 ke V are detected from several anomalous X-ray pulsars(AXPs)and soft gamma-ray repeaters(SGRs),and different models have been proposed to explain the physical origin within the frame of either a magnetar model or a fallback disk system.Using data from Suzaku and INTEGRAL,we study the soft and hard X-ray spectra of four AXPs/SGRs:1RXS J170849–400910,1E 1547.0–5408,SGR 1806–20 and SGR 0501+4516.It is found that the spectra could be well reproduced by the bulk-motion Comptonization(BMC)process as was first suggested by Tr¨umper et al.,showing that the accretion scenario could be compatible with Xray emission from AXPs/SGRs.Simulated results from the Hard X-ray Modulation Telescope using the BMC model show that the spectra would have discrepancies from the power-law,especially the cutoff at～200 ke V.Thus future observations will allow researchers to distinguish different models of the hard X-ray emission and will help us understand the nature of AXPs/SGRs.     

Diagnostics from three rising submillimeter bursts

In this paper we investigate three novel rising submillimeter(THz) bursts that occurred sequentially in Super Active Region NOAA 10486. The average rising rate of the flux density above 200 GHz is only 20 sfu GHz-1(corresponding to spectral index α of 1.6) for the THz spectral components of the2003 October 28 and November 4 bursts, but it attained values of 235 sfu GHz-1(α = 4.8) in the 2003 November 2 burst. The steeply rising THz spectrum can be produced by a population of highly relativistic electrons with a low-energy cutoff of 1 Me V, but it only requires a low-energy cutoff of 30 ke V for the two slowly rising THz bursts, via gyrosynchrotron(GS) radiation based on our numerical simulations of burst spectra in the magnetic dipole field case. The electron density variation is much larger in the THz source than in the microwave(MW) source. It is interesting that the THz source radius decreased by 20%–50%during the decay phase for the three events, but the MW source increased by 28% for the 2003 November2 event. In the paper we will present a formula that can be used to calculate the energy released by ultrarelativistic electrons, taking the relativistic correction into account for the first time. We find that the energy released by energetic electrons in the THz source exceeds that in the MW source due to the strong GS radiation loss in the THz range, although the modeled THz source area is 3–4 orders smaller than the modeled MW source one. The total energies released by energetic electrons via the GS radiation in radio sources are estimated, respectively, to be 5.2 × 1033, 3.9 × 1033 and 3.7 × 1032 erg for the October 28, November 2and 4 bursts, which are 131, 76 and 4 times as large as the thermal energies of 2.9 × 1031, 2.1 × 1031and5.2 × 1031 erg estimated from soft X-ray GOES observations.     

Separation of Different Contributions to the Total X-ray Luminosity in Gamma-ray Loud Blazars

The relativistic beaming model has been successfully used to explain many of the observational properties of active galactic nuclei. In this model the total emission is formed by two components, one beamed, one unbeamed. However, the exact contribution from each component in unresolved sources is still not clear. In the radio band, the core and extended emissions are clearly separated. We adopt the method proposed by Kembhavi to separate the two contributions in the X-ray emissions in a sample of 19 gamma-ray loud blazars. It is clearly shown that the beamed emission dominates the X-ray flux and the unbeamed X-ray emission is correlated with the extended radio emission of the considered objects. We also find that the ratio of the beamed to the unbeamed X-ray luminosity is correlated with the X-ray spectral index, an effect that should be a consequence of the underlying X-ray emission mechanism.     

Broad-band spectroscopy of the transient X-ray binary pulsar KS 1947+300 during 2013 giant outburst: Detection of pulsating soft X-ray excess component

We present the results obtained from detailed timing and spectral studies of the Be/X-ray binary pulsar KS 1947+300 during its 2013 giant outburst. We used data from Suzaku observations of the pulsar at two epochs, i.e. on 2013 October 22(close to the peak of the outburst) and 2013 November 22. Xray pulsations at ~18.81 s were clearly detected in the light curves obtained from both observations. Pulse periods estimated during the outburst showed that the pulsar was spinning up. The pulse profile was found to be single-peaked up to ~10 ke V beyond which a sharp peak followed by a dip-like feature appeared at hard X-rays. The dip-like feature has been observed up to ~70 keV. The 1–110 ke V broad-band spectroscopy of both observations revealed that the best-fit model was comprised of a partially absorbed Negative and Positive power law with EXponential cutoff(NPEX) continuum model along with a blackbody component for the soft X-ray excess and two Gaussian functions at 6.4 and 6.7 ke V for emission lines. Both the lines were identified as emission from neutral and He-like iron atoms. To fit the spectra, we included the previously reported cyclotron absorption line at 12.2 keV. From the spin-up rate, the magnetic field of the pulsar was estimated to be ~1.2×10~(12)G and found to be comparable to that obtained from the detection of the cyclotron absorption feature. Pulse-phase resolved spectroscopy revealed the pulsating nature of the soft X-ray excess component in phase with the continuum flux. This confirms that the accretion column and/or accretion stream are the most probable regions of the soft X-ray excess emission in KS1947+300.The presence of the pulsating soft X-ray excess in phase with continuum emission may be the possible reason for not observing the dip at soft X-rays.