arXiv subjects

W. W. Zhang

Publications and source records attributed to W. W. Zhang.

At least 37 records · Page 2Linked to original sources

arXiv2016

NuSTAR and Swift observations of the very high state in GX 339-4: Weighing the black hole with X-rays

We present results from spectral fitting of the very high state of GX~339-4 with NuSTAR and Swift. We use relativistic reflection modelling to measure the spin of the black hole and inclination of the inner disk, and find a spin of $a=0.95^{+0.02}_{-0.08}$ and inclination of $30$°$\pm1$ (statistical errors). These values agree well with previous results from reflection modelling. With the exceptional sensitivity of NuSTAR at the high-energy side of the disk spectrum, we are able to constrain multiple physical parameters simultaneously using continuum fitting. By using the constraints from reflection as input for the continuum fitting method, we invert the conventional fitting procedure to estimate the mass and distance of GX~339-4 using just the X-ray spectrum, finding a mass of $9.0^{+1.6}_{-1.2}M_\odot$ and distance of $8.4\pm0.9$ kpc (statistical errors).

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arXiv2016

The first X-ray imaging spectroscopy of quiescent solar active regions with NuSTAR

We present the first observations of quiescent active regions (ARs) using NuSTAR, a focusing hard X-ray telescope capable of studying faint solar emission from high temperature and non-thermal sources. We analyze the first directly imaged and spectrally resolved X-rays above 2~keV from non-flaring ARs, observed near the west limb on 2014 November 1. The NuSTAR X-ray images match bright features seen in extreme ultraviolet and soft X-rays. The NuSTAR imaging spectroscopy is consistent with isothermal emission of temperatures $3.1-4.4$~MK and emission measures $1-8\times 10^{46}$~cm$^{-3}$. We do not observe emission above 5~MK but our short effective exposure times restrict the spectral dynamic range. With few counts above 6~keV, we can place constraints on the presence of an additional hotter component between 5 and 12~MK of $\sim 10^{46}$cm$^{-3}$ and $\sim 10^{43}$ cm$^{-3}$, respectively, at least an order of magnitude stricter than previous limits. With longer duration observations and a weakening solar cycle (resulting in an increased livetime), future NuSTAR observations will have sensitivity to a wider range of temperatures as well as possible non-thermal emission.

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arXiv2016

NuSTAR, Swift, and GROND observations of the flaring MeV blazar: PMN J0641$-$0320

MeV blazars are a sub--population of the blazar family, exhibiting larger--than--average jet powers, accretion luminosities and black hole masses. Because of their extremely hard X--ray continua, these objects are best studied in the X-ray domain. Here, we report on the discovery by the $Fermi$ Large Area Telescope and subsequent follow-up observations with $NuSTAR$, $Swift$ and GROND of a new member of the MeV blazar family: PMN J0641$-$0320. Our optical spectroscopy provides confirmation that this is a flat--spectrum radio quasar located at a redshift of $z=1.196$. Its very hard $NuSTAR$ spectrum (power--law photon index of $\sim$1 up to $\sim$80 keV) indicates that the emission is produced via inverse Compton scattering off photons coming from outside the jet.The overall spectral energy distribution of PMN J0641$-$0320 is typical of powerful blazars and by reproducing it with a simple one-zone leptonic emission model we find the emission region to be located either inside the broad line region or within the dusty torus.

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arXiv2016

NuSTAR observations of water megamaser AGN

Aims. Study the connection between the masing disk and obscuring torus in Seyfert 2 galaxies. Methods. We present a uniform X-ray spectral analysis of the high energy properties of 14 nearby megamaser Active Galactic Nuclei observed by NuSTAR. We use a simple analytical model to localize the maser disk and understand its connection with the torus by combining NuSTAR spectral parameters with available physical quantities from VLBI mapping. Results. Most of the sources analyzed are heavily obscured, showing a column density in excess of $\sim 10^{23}$ cm$^{-2}$. In particular, $79\%$ are Compton-thick ($N_{\rm H} > 1.5 \times 10^{24}$ cm$^{-2}$). Using column densities measured by NuSTAR, with the assumption that the torus is the extension of the maser disk, and further assuming a reasonable density profile, the torus dimensions can be predicted. They are found to be consistent with mid-IR interferometry parsec-scale observations of Circinus and NGC 1068. In this picture, the maser disk is intimately connected to the inner part of the torus. It is probably made of a large number of molecular clouds connecting the torus and the outer part of the accretion disk, giving rise to a thin disk rotating in most cases in Keplerian or sub-Keplerian motion. This toy model explains the established close connection between water megamaser emission and nuclear obscuration as a geometric effect.

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arXiv2016

IC 751: a new changing-look AGN discovered by NuSTAR

We present the results of five NuSTAR observations of the type 2 active galactic nucleus (AGN) in IC 751, three of which were performed simultaneously with XMM-Newton or Swift/XRT. We find that the nuclear X-ray source underwent a clear transition from a Compton-thick ($N_{\rm\,H}\simeq 2\times 10^{24}\rm\,cm^{-2}$) to a Compton-thin ($N_{\rm\,H}\simeq 4\times 10^{23}\rm\,cm^{-2}$) state on timescales of $\lesssim 3$ months, which makes IC 751 the first changing-look AGN discovered by NuSTAR. Changes of the line-of-sight column density at a $\sim2σ$ level are also found on a time-scale of $\sim 48$ hours ($ΔN_{\rm\,H}\sim 10^{23}\rm\,cm^{-2}$). From the lack of spectral variability on timescales of $\sim 100$ ks we infer that the varying absorber is located beyond the emission-weighted average radius of the broad-line region, and could therefore be related either to the external part of the broad-line region or a clumpy molecular torus. By adopting a physical torus X-ray spectral model, we are able to disentangle the column density of the non-varying absorber ($N_{\rm\,H}\sim 3.8\times 10^{23}\rm\,cm^{-2}$) from that of the varying clouds [$N_{\rm\,H}\sim(1-150)\times10^{22}\rm\,cm^{-2}$], and to constrain that of the material responsible for the reprocessed X-ray radiation ($N_{\rm\,H} \sim 6 \times 10^{24}\rm\,cm^{-2}$). We find evidence of significant intrinsic X-ray variability, with the flux varying by a factor of five on timescales of a few months in the 2-10 and 10-50 keV band.

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arXiv2015

Multiwavelength Study of Quiescent States of Mrk 421 with Unprecedented Hard X-Ray Coverage Provided by NuSTAR in 2013

We present coordinated multiwavelength observations of the bright, nearby BL Lac object Mrk 421 taken in 2013 January-March, involving GASP-WEBT, Swift, NuSTAR, Fermi-LAT, MAGIC, VERITAS, and other collaborations and instruments, providing data from radio to very-high-energy (VHE) gamma-ray bands. NuSTAR yielded previously unattainable sensitivity in the 3-79 keV range, revealing that the spectrum softens when the source is dimmer until the X-ray spectral shape saturates into a steep power law with a photon index of approximately 3, with no evidence for an exponential cutoff or additional hard components up to about 80 keV. For the first time, we observed both the synchrotron and the inverse-Compton peaks of the spectral energy distribution (SED) simultaneously shifted to frequencies below the typical quiescent state by an order of magnitude. The fractional variability as a function of photon energy shows a double-bump structure which relates to the two bumps of the broadband SED. In each bump, the variability increases with energy which, in the framework of the synchrotron self-Compton model, implies that the electrons with higher energies are more variable. The measured multi-band variability, the significant X-ray-to-VHE correlation down to some of the lowest fluxes ever observed in both bands, the lack of correlation between optical/UV and X-ray flux, the low degree of polarization and its significant (random) variations, the short estimated electron cooling time, and the significantly longer variability timescale observed in the NuSTAR light curves point toward in-situ electron acceleration, and suggest that there are multiple compact regions contributing to the broadband emission of Mrk 421 during low-activity states.

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arXiv2015

The NuSTAR view of reflection and absorption in NGC 7582

NGC 7582 is a well-studied X-ray bright Seyfert 2 with moderately heavy ($N_{\text{H}}\sim10^{23}-10^{24}$~cm$^{-2}$), highly variable absorption and strong reflection spectral features. The spectral shape changed around the year 2000, dropping in observed flux and becoming much more highly absorbed. Two scenarios have been put forth to explain this spectral change: 1) the central X-ray source partially ``shut off'' around this time, decreasing in intrinsic luminosity, with a delayed decrease in reflection features due to the light-crossing time of the Compton-thick material or 2) the source became more heavily obscured, with only a portion of the power law continuum leaking through. NuSTAR observed NGC~7582 twice in 2012, two weeks apart, in order to quantify the reflection using high-quality data above 10 keV. We find that the most plausible scenario is that NGC 7582 has recently become more heavily absorbed by a patchy torus with a covering fraction of $\sim\,80-90\%$ and an equatorial column density of $\sim 3 \times10^{24}$ cm$^{-2}$. We find the need for an additional highly variable full-covering absorber with $N_{\text{H}}= 4-6 \times10^{23}$ cm$^{-2}$ in the line of sight, possibly associated with a hidden broad line region.

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arXiv2015

A Soft X-Ray Spectral Episode for the Clocked Burster, GS 1826-24 as Measured by Swift and NuSTAR

We report on NuSTAR and Swift observations of a soft state of the neutron star low-mass X-ray binary GS 1826-24, commonly known as the "clocked" burster. The transition to the soft state was recorded in 2014 June through an increase of the 2-20 keV source intensity measured by MAXI, simultaneous with a decrease of the 15-50 keV intensity measured by Swift/BAT. The episode lasted approximately two months, after which the source returned to its usual hard state. We analyze the broad-band spectrum measured by Swift/XRT and NuSTAR, and estimate the accretion rate during the soft episode to be about 13% of Eddington, within the range of previous observations. However, the best fit spectral model, adopting the double Comptonization used previously, exhibits significantly softer components. We detect seven type-I X-ray bursts, all significantly weaker (and with shorter rise and decay times) than observed previously. The burst profiles and recurrence times vary significantly, ruling out the regular bursts that are typical for this source. One burst exhibited photospheric radius expansion, and we estimate the source distance at about (5.7 / xi_b^1/2) kpc, where xi_b parameterizes the possible anisotropy of the burst emission. Interpreting the soft state as a transition from an optically thin inner flow to an optically thick flow passing through a boundary layer, as is commonly observed in similar systems, is contradicted by the lower optical depth measured for the double Comptonization model we find for this soft state. The effect of a change in disk geometry on the burst behavior remains unclear.

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arXiv2015

Dark matter line emission constraints from NuSTAR observations of the Bullet Cluster

Line emission from dark matter is well motivated for some candidates e.g. sterile neutrinos. We present the first search for dark matter line emission in the 3-80keV range in a pointed observation of the Bullet Cluster with NuSTAR. We do not detect any significant line emission and instead we derive upper limits (95% CL) on the flux, and interpret these constraints in the context of sterile neutrinos and more generic dark matter candidates. NuSTAR does not have the sensitivity to constrain the recently claimed line detection at 3.5keV, but improves on the constraints for energies of 10-25keV.

astro-ph.CO↗

arXiv2015

NuSTAR Reveals Extreme Absorption in z < 0.5 Type 2 Quasars

The intrinsic column density (NH) distribution of quasars is poorly known. At the high obscuration end of the quasar population and for redshifts z<1, the X-ray spectra can only be reliably characterized using broad-band measurements which extend to energies above 10 keV. Using the hard X-ray observatory NuSTAR, along with archival Chandra and XMM-Newton data, we study the broad-band X-ray spectra of nine optically selected (from the SDSS), candidate Compton-thick (NH > 1.5e24 cm^-2) type 2 quasars (CTQSO2s); five new NuSTAR observations are reported herein, and four have been previously published. The candidate CTQSO2s lie at z<0.5, have observed [OIII] luminosities in the range 8.4 < log (L_[OIII]/L_solar) < 9.6, and show evidence for extreme, Compton-thick absorption when indirect absorption diagnostics are considered. Amongst the nine candidate CTQSO2s, five are detected by NuSTAR in the high energy (8-24 keV) band: two are weakly detected at the ~ 3 sigma confidence level and three are strongly detected with sufficient counts for spectral modeling (>~ 90 net source counts at 8-24 keV). For these NuSTAR-detected sources direct (i.e., X-ray spectral) constraints on the intrinsic AGN properties are feasible, and we measure column densities ~2.5-1600 times higher and intrinsic (unabsorbed) X-ray luminosities ~10-70 times higher than pre-NuSTAR constraints from Chandra and XMM-Newton. Assuming the NuSTAR-detected type 2 quasars are representative of other Compton-thick candidates, we make a correction to the NH distribution for optically selected type 2 quasars as measured by Chandra and XMM-Newton for 39 objects. With this approach, we predict a Compton-thick fraction of f_CT = 36^{+14}_{-12} %, although higher fractions (up to 76%) are possible if indirect absorption diagnostics are assumed to be reliable.

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arXiv2015

The complex accretion geometry of GX 339-4 as seen by NuSTAR and Swift

We present spectral analysis of five NuSTAR and Swift observations of GX 339-4 taken during a failed outburst in summer 2013. These observations cover Eddington luminosity fractions in the range ~0.9-6%. Throughout this outburst, GX 339-4 stayed in the hard state, and all five observations show similar X-ray spectra with a hard power-law with a photon index near 1.6 and significant contribution from reflection. Using simple reflection models we find unrealistically high iron abundances. Allowing for different photon indices for the continuum incident on the reflector relative to the underlying observed continuum results in a statistically better fit and reduced iron abundances. With a photon index around 1.3, the input power-law on the reflector is significantly harder than that which is directly observed. We study the influence of different emissivity profiles and geometries and consistently find an improvement when using separate photon indices. The inferred inner accretion disk radius is strongly model dependent, but we do not find evidence for a truncation radius larger than 100 r_g in any model. The data do not allow independent spin constraints but the results are consistent with the literature (i.e., a>0). Our best-fit models indicate an inclination angle in the range 40-60 degrees, consistent with limits on the orbital inclination but higher than reported in the literature using standard reflection models. The iron line around 6.4 keV is clearly broadened, and we detect a superimposed narrow core as well. This core originates from a fluorescence region outside the influence of the strong gravity of the black hole and we discuss possible geometries.

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arXiv2015

NuSTAR and Suzaku observations of the hard state in Cygnus X-1: locating the inner accretion disk

We present simultaneous Nuclear Spectroscopic Telescope Array (NuSTAR ) and Suzaku observations of the X-ray binary Cygnus X-1 in the hard state. This is the first time this state has been observed in Cyg X-1 with NuSTAR, which enables us to study the reflection and broad-band spectra in unprecedented detail. We confirm that the iron line cannot be fit with a combination of narrow lines and absorption features, and instead requires a relativistically blurred profile in combination with a narrow line and absorption from the companion wind. We use the reflection models of Garcia et al. (2014) to simultaneously measure the black hole spin, disk inner radius, and coronal height in a self-consistent manner. Detailed fits to the iron line profile indicate a high level of relativistic blurring, indicative of reflection from the inner accretion disk. We find a high spin, a small inner disk radius, and a low source height, and rule out truncation to greater than three gravitational radii at the 3σ confidence level. In addition, we find that the line profile has not changed greatly in the switch from soft to hard states, and that the differences are consistent with changes in the underlying reflection spectrum rather than the relativistic blurring. We find that the blurring parameters are consistent when fitting either just the iron line or the entire broad-band spectrum, which is well modelled with a Comptonized continuum plus reflection model.

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arXiv2015

NuSTAR and multifrequency study of the two high-redshift blazars S5 0836+710 and PKS 2149-306

The most powerful blazars are the flat spectrum radio quasars whose emission is dominated by a Compton component peaking between a few hundred keV and a few hundred MeV. We selected two bright blazars, PKS 2149-306 at redshift z=2.345 and S5 0836+710 at z=2.172, in order to observe them in the hard X-ray band with the NuSTAR satellite. In this band the Compton component is rapidly rising almost up to the peak of the emission. Simultaneous soft-X-rays and UV-optical observations were performed with the Swift satellite, while near-infrared (NIR) data were obtained with the REM telescope. To study their variability, we repeated these observations for both sources on a timescale of a few months. While no fast variability was detected during a single observation, both sources were found to be variable in the X-ray band, up to 50%, between the two observations, with larger variability at higher energies. No variability was detected in the optical/NIR band. These data together with Fermi-LAT, WISE and other literature data are then used to study the overall spectral energy distributions (SEDs) of these blazars. Although the jet non-thermal emission dominates the SED, it leaves the UV band unhidden, allowing us to detect the thermal emission of the disc and to estimate the mass of the black hole. The non-thermal emission is well reproduced by a one-zone leptonic model. The non-thermal radiative processes are synchrotron, self-Compton and external Compton using seed photons from both the broad-line region (BLR) and the torus. We find that our data are better reproduced if we assume that the location of the dissipation region of the jet, R_diss, is in-between the torus, (at R_torus), and the BLR (R_torus>R_diss>R_BLR). The observed variability is explained by changing a minimum number of model parameters by a very small amount.

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arXiv2015

The 0.3-30 keV Spectra of Powerful Starburst Galaxies: NuSTAR and Chandra Observations of NGC 3256 and NGC 3310

We present nearly simultaneous Chandra and NuSTAR observations of two actively star-forming galaxies within 50 Mpc: NGC 3256 and NGC 3310. Both galaxies are detected by both Chandra and NuSTAR, which together provide the first-ever spectra of these two galaxies spanning 0.3-30 keV. The X-ray emission from both galaxies is spatially resolved by Chandra; we find that hot gas dominates the E < 1-3 keV emission while ultraluminous X-ray sources (ULXs) dominate at E > 1-3 keV. The NuSTAR galaxy-wide spectra of both galaxies follow steep power-law distributions with Gamma ~ 2.6 at E > 5-7 keV, similar to the spectra of bright individual ULXs and other galaxies that have been studied by NuSTAR. We find that both NGC 3256 and NGC 3310 have X-ray detected sources coincident with nuclear regions; however, the steep NuSTAR spectra of both galaxies restricts these sources to be either low luminosity AGN or non-AGN in nature (e.g., ULXs or crowded X-ray sources that reach L2-10 keV ~ 10^40 erg/s cannot be ruled out). Combining our constraints on the 0.3-30 keV spectra of NGC 3256 and NGC 3310 with equivalent measurements for nearby star-forming galaxies M83 and NGC 253, we analyze the SFR-normalized spectra of these starburst galaxies. The spectra of all four galaxies show sharply declining power-law slopes above 3-6 keV due to ULX populations. Our observations therefore constrain the average spectra of luminous accreting binaries (i.e., ULXs). This result is similar to the super-Eddington accreting ULXs that have been studied individually in a targeted NuSTAR ULX program. We also find that NGC 3310 exhibits a factor of ~3-10 elevation of X-ray emission over the other star-forming galaxies. We argue that the excess is most likely explained by the relatively low metallicity of the young stellar population in NGC 3310.

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arXiv2015

Rapid Variability of Blazar 3C 279 during Flaring States in 2013-2014 with Joint Fermi-LAT, NuSTAR, Swift, and Ground-Based Multi-wavelength Observations

We report the results of a multi-band observing campaign on the famous blazar 3C 279 conducted during a phase of increased activity from 2013 December to 2014 April, including first observations of it with NuSTAR. The $γ$-ray emission of the source measured by Fermi-LAT showed multiple distinct flares reaching the highest flux level measured in this object since the beginning of the Fermi mission, with $F(E > 100\,{\rm MeV})$ of $10^{-5}$ photons cm$^{-2}$ s$^{-1}$, and with a flux doubling time scale as short as 2 hours. The $γ$-ray spectrum during one of the flares was very hard, with an index of $Γ_γ= 1.7 \pm 0.1$, which is rarely seen in flat spectrum radio quasars. The lack of concurrent optical variability implies a very high Compton dominance parameter $L_γ/L_{\rm syn} > 300$. Two 1-day NuSTAR observations with accompanying Swift pointings were separated by 2 weeks, probing different levels of source activity. While the 0.5$-$70 keV X-ray spectrum obtained during the first pointing, and fitted jointly with Swift-XRT is well-described by a simple power law, the second joint observation showed an unusual spectral structure: the spectrum softens by $ΔΓ_{\rm X} \simeq 0.4$ at $\sim$4 keV. Modeling the broad-band SED during this flare with the standard synchrotron plus inverse Compton model requires: (1) the location of the $γ$-ray emitting region is comparable with the broad line region radius, (2) a very hard electron energy distribution index $p \simeq 1$, (3) total jet power significantly exceeding the accretion disk luminosity $L_{\rm j}/L_{\rm d} \gtrsim 10$, and (4) extremely low jet magnetization with $L_{\rm B}/L_{\rm j} \lesssim 10^{-4}$. We also find that single-zone models that match the observed $γ$-ray and optical spectra cannot satisfactorily explain the production of X-ray emission.

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arXiv2015

The Multi-Layer Variable Absorbers in NGC 1365 Revealed by XMM-Newton and NuSTAR

Between July 2012 and February 2013, NuSTAR and XMM-Newton performed four long-look joint obser- vations of the type 1.8 Seyfert, NGC 1365. We have analyzed the variable absorption seen in these observations in order to characterize the geometry of the absorbing material. Two of the observations caught NGC 1365 in an unusually low absorption state, revealing complexity in the multi-layer absorber which had previously been hidden. We find the need for three distinct zones of neutral absorption in addition to the two zones of ionized absorption and the Compton-thick torus previously seen in this source. The most prominent absorber is likely associated with broad line region clouds with column densities of around $\sim\,$10$^{23}$ cm$^{-2}$ and a highly clumpy nature as evidenced by an occultation event in February 2013. We also find evidence of a patchy absorber with a variable column around $\sim\,10^{22}$ cm$^{-2}$ and a line of sight covering fraction of 0.3$-$0.9 which responds directly to the intrinsic source flux, possibly due to a wind geometry. A full-covering, constant absorber with a low column density of $\sim\,1 \times$ 10$^{22}$ cm$^{-2}$ is also present, though the location of this low density haze is unknown.

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arXiv2015

Determining the covering factor of Compton-thick active galactic nuclei with NuSTAR

The covering factor of Compton-thick obscuring material associated with the torus in active galactic nuclei (AGN) is at present best understood through the fraction of sources exhibiting Compton-thick absorption along the line of sight ($N_{H}>1.5\times10^{24}$ cm$^{-2}$) in the X-ray band, which reveals the average covering factor. Determining this Compton-thick fraction is difficult however, due to the extreme obscuration. With its spectral coverage at hard X-rays ($>$10 keV), NuSTAR is sensitive to the AGN covering factor since Compton scattering of X-rays off optically thick material dominates at these energies. We present a spectral analysis of 10 AGN observed with NuSTAR where the obscuring medium is optically thick to Compton scattering, so called Compton-thick (CT) AGN. We use the torus models of Brightman & Nandra which predict the X-ray spectrum from reprocessing in a torus and include the torus opening angle as a free parameter and aim to determine the covering factor of the Compton-thick gas in these sources individually. Across the sample we find mild to heavy Compton-thick columns, with $N_{H}$ measured from $10^{24}-10^{26}$ cm$^{-2}$, and a wide range of covering factors, where individual measurements range from 0.2-0.9. We find that the covering factor, $f_{c}$, is a strongly decreasing function of the intrinsic 2-10 keV luminosity, $L_{X}$, where $f_{c}=(-0.41\pm0.13)$log$_{10}$($L_{X}$/erg s$^{-1}$)$+18.31\pm5.33$, across more than two orders of magnitude in $L_{X}$ (10$^{41.5}-10^{44}$ erg s$^{-1}$). The covering factors measured here agree well with the obscured fraction as a function of $L_{X}$ as determined by studies of local AGN with $L_{X}>10^{42.5}$ erg s$^{-1}$.

astro-ph.HE↗

arXiv2015

Simultaneous NuSTAR/Chandra observations of the Bursting Pulsar GRO J1744-28 during its third reactivation

We report on a 10 ks simultaneous Chandra/HETG-NuSTAR observation of the Bursting Pulsar, GRO J1744-28, during its third detected outburst since discovery and after nearly 18 years of quiescence. The source is detected up to 60 keV with an Eddington persistent flux level. Seven bursts, followed by dips, are seen with Chandra, three of which are also detected with NuSTAR. Timing analysis reveals a slight increase in the persistent emission pulsed fraction with energy (from 10% to 15%) up to 10 keV, above which it remains constant. The 0.5-70 keV spectra of the persistent and dip emission are the same within errors, and well described by a blackbody (BB), a power-law with an exponential rolloff, a 10 keV feature, and a 6.7 keV emission feature, all modified by neutral absorption. Assuming that the BB emission originates in an accretion disc, we estimate its inner (magnetospheric) radius to be about 4x10^7 cm, which translates to a surface dipole field B~9x10^10 G. The Chandra/HETG spectrum resolves the 6.7 keV feature into (quasi-)neutral and highly ionized Fe XXV and Fe XXVI emission lines. XSTAR modeling shows these lines to also emanate from a truncated accretion disk. The burst spectra, with a peak flux more than an order of magnitude higher than Eddington, are well fit with a power-law with an exponential rolloff and a 10~keV feature, with similar fit values compared to the persistent and dip spectra. The burst spectra lack a thermal component and any Fe features. Anisotropic (beamed) burst emission would explain both the lack of the BB and any Fe components.

astro-ph.HE↗