Publication Search Results

ID: 159430
Type: article
Authors: Connors, Riley M. T.; García, Javier A.; Tomsick, John; Hare, Jeremy; Dauser, Thomas; Grinberg, Victoria; Steiner, James F.; Mastroserio, Guglielmo; Sridhar, Navin; Fabian, Andrew C.; Jiang, Jiachen; Parker, Michael L.; Harrison, Fiona; Kallman, Timothy R.
Abstract: We present the analysis of X-ray observations of the black hole binary 4U 1630-47 using relativistic reflection spectroscopy. We use archival data from the Rossi X-ray Timing Explorer, Neil Gehrels Swift Observatory, and Nuclear Spectroscopic Telescope Array observatories, taken during different outbursts of the source between 1998 and 2015. Our modeling includes two relatively new advances in modern reflection codes: high-density disks, and returning thermal disk radiation. Accretion disks around stellar-mass black holes are expected to have densities well above the standard value assumed in traditional reflection models (i.e., n_e ∼ 10¹⁵ cm^-3). New high-density reflection models have important implications in the determination of disk truncation (i.e., the disk inner radius). This is because one must retain self-consistency in the irradiating flux and corresponding disk ionization state, which is a function of disk density and system geometry. We find that the disk density is n_e ≥ 10²⁰ cm^-3 across all spectral states. This density, combined with our constraints on the ionization state of the material, implies an irradiating flux impinging on the disk that is consistent with the expected theoretical estimates. Returning thermal disk radiation-the fraction of disk photons that bend back to the disk producing additional reflection components-is expected predominantly in the soft state. We show that returning radiation models indeed provide a better fit to the soft-state data, reinforcing previous results that show that in the soft state, the irradiating continuum may be blackbody emission from the disk itself.

ID: 159426
Type: article
Authors: Wang, Jingyi; Mastroserio, Guglielmo; Kara, Erin; García, Javier A.; Ingram, Adam; Connors, Riley; van der Klis, Michiel; Dauser, Thomas; Steiner, James F.; Buisson, Douglas J. K.; Homan, Jeroen; Lucchini, Matteo; Fabian, Andrew C.; Bright, Joe; Fender, Rob; Cackett, Edward M.; Remillard, Ron A.
Abstract: We analyze five epochs of Neutron star Interior Composition Explorer (NICER) data of the black hole X-ray binary MAXI J1820+070 during the bright hard-to-soft state transition in its 2018 outburst with both reflection spectroscopy and Fourier-resolved timing analysis. We confirm the previous discovery of reverberation lags in the hard state, and find that the frequency range where the (soft) reverberation lag dominates decreases with the reverberation lag amplitude increasing during the transition, suggesting an increasing X-ray emitting region, possibly due to an expanding corona. By jointly fitting the lag-energy spectra in a number of broad frequency ranges with the reverberation model reltrans, we find the increase in reverberation lag is best described by an increase in the X-ray coronal height. This result, along with the finding that the corona contracts in the hard state, suggests a close relationship between spatial extent of the X-ray corona and the radio jet. We find the corona expansion (as probed by reverberation) precedes a radio flare by ∼5 days, which may suggest that the hard-to-soft transition is marked by the corona expanding vertically and launching a jet knot that propagates along the jet stream at relativistic velocities.

ID: 159431
Type: article
Authors: Zhao, Xueshan; Gou, Lijun; Dong, Yanting; Zheng, Xueying; Steiner, James F.; Miller-Jones, James C. A.; Bahramian, Arash; Orosz, Jerome A.; Feng, Ye
Abstract: Cygnus X-1 is a well-studied persistent black hole X-ray binary. Recently, the three parameters needed to estimate the black hole spin of this system, namely the black hole mass M, the orbital inclination i, and the source distance D, have been updated. In this work we redetermine the spin parameter using the continuum-fitting technique for those updated parameter values. Based on the assumption that the spin axis of the black hole is aligned with the orbital plane, we fit the thermal disk component to a fully relativistic thin accretion disk model. The error in the spin estimate arising from the combined observational uncertainties is obtained via Monte Carlo simulations. We demonstrate that, without considering the counteracting torque effect, the new spin parameter is constrained to be a_* > 0.9985 (3σ), which confirms that the spin of the black hole in Cygnus X-1 is extreme.

ID: 157489
Type: article
Authors: Alabarta, K.; Altamirano, D.; Méndez, M.; Cúneo, V. A.; Zhang, L.; Remillard, R.; Castro, A.; Ludlam, R. M.; Steiner, James F.; Enoto, T.; Homan, J.; Arzoumanian, Z.; Bult, P.; Gendreau, K. C.; Markwardt, C.; Strohmayer, T. E.; Uttley, P.; Tombesi, F.; Buisson, D. J. K.
Abstract: We present a detailed X-ray spectral and variability study of the full 2018 outburst of MAXI J1727-203 using NICER observations. The outburst lasted approximately four months. Spectral modelling in the 0.3-10 keV band shows the presence of both a soft thermal and a hard Comptonised component. The analysis of these components shows that MAXI J1727-203 evolved through the soft, intermediate, and hard spectral states during the outburst. We find that the soft (disc) component was detected throughout almost the entire outburst, with temperatures ranging from ∼0.4 keV, at the moment of maximum luminosity, to ∼0.1 keV near the end of the outburst. The power spectrum in the hard and intermediate states shows broad-band noise up to 20 Hz, with no evidence of quasi-periodic oscillations. We also study the rms spectra of the broad-band noise at 0.3-10 keV of this source. We find that the fractional rms increases with energy in most of the outburst except during the hard state, where the fractional rms remains approximately constant with energy. We also find that, below 3 keV, the fractional rms follows the same trend generally observed at energies >3 keV, a behaviour known from previous studies of black holes and neutron stars. The spectral and timing evolution of MAXI J1727-203, as parametrised by the hardness-intensity, hardness-rms, and rms-intensity diagrams, suggest that the system hosts a black hole, although we could not rule out a neutron star.

ID: 157313
Type: article
Authors: Connors, Riley M. T.; García, Javier A.; Dauser, Thomas; Grinberg, Victoria; Steiner, James F.; Sridhar, Navin; Wilms, Jörn; Tomsick, John; Harrison, Fiona; Licklederer, Stefan
Abstract: We explore the accretion properties of the black hole X-ray binary XTE J1550-564 during its outbursts in 1998/99 and 2000. We model the disk, corona, and reflection components of X-ray spectra taken with the Rossi X-ray Timing Explorer, using the relxill suite of reflection models. The key result of our modeling is that the reflection spectrum in the very soft state is best explained by disk self-irradiation, I.e., photons from the inner disk are bent by the strong gravity of the black hole and reflected off the disk surface. This is the first known detection of thermal disk radiation reflecting off the inner disk. There is also an apparent absorption line at ∼6.9 keV, which may be evidence of an ionized disk wind. The coronal electron temperature (kT_e) is, as expected, lower in the brighter outburst of 1998/99, explained qualitatively by more efficient coronal cooling due to irradiating disk photons. The disk inner radius is consistent with being within a few times the innermost stable circular orbit throughout the bright-hard-to- soft states (10 s of r_g in gravitational units). The disk inclination is low during the hard state, disagreeing with the binary inclination value, and very close to 90° in the soft state, recovering to a lower value when adopting a blackbody spectrum as the irradiating continuum.

ID: 156292
Type: article
Authors: Dong, Yanting; García, Javier A.; Steiner, James F.; Gou, Lijun
Abstract: 4U 1543-47 is a low-mass X-ray binary that harbours a stellar-mass black hole located in our Milky Way galaxy. In this paper, we revisit seven data sets that were in the Steep Power Law state of the 2002 outburst. The spectra were observed by the Rossi X-ray Timing Explorer. We have carefully modelled the X-ray reflection spectra and made a joint-fit to these spectra with relxill for the reflected emission. We found a moderate black hole spin, which is 0.67_{-0.08}^{+0.15} at 90 per cent statistical confidence. Negative and low spins (<0.5) at more than 99 per cent statistical confidence are ruled out. In addition, our results indicate that the model requires a supersolar iron abundance: 5.05_{-0.26}^{+1.21}, and the inclination angle of the inner disc is 36.3_{-3.4}^{+5.3} deg. This inclination angle is appreciably larger than the binary orbital inclination angle (∼21 deg); this difference is possibly a systematic artefact of the artificially low density employed in the reflection model for this X-ray binary system.

ID: 156787
Type: article
Authors: Fabian, A. C.; Buisson, D. J.; Kosec, P.; Reynolds, C. S.; Wilkins, D. R.; Tomsick, J. A.; Walton, D. J.; Gandhi, P.; Altamirano, D.; Arzoumanian, Z.; Cackett, E. M.; Dyda, S.; Garcia, J. A.; Gendreau, K. C.; Grefenstette, B. W.; Homan, J.; Kara, E.; Ludlam, R. M.; Miller, J. M.; Steiner, James F.
Abstract: The Galactic black hole X-ray binary MAXI J1820+070 had a bright outburst in 2018 when it became the second brightest X-ray source in the sky. It was too bright for X-ray CCD instruments such as XMM-Newton and Chandra, but was well observed by photon-counting instruments such as Neutron star Inner Composition Explorer (NICER) and Nuclear Spectroscopic Telescope Array(NuSTAR). We report here on the discovery of an excess-emission component during the soft state. It is best modelled with a blackbody spectrum in addition to the regular disc emission, modelled as either diskbb or kerrbb. Its temperature varies from about 0.9 to 1.1 keV, which is about 30-80 per cent higher than the inner disc temperature of diskbb. Its flux varies between 4 and 12 per cent of the disc flux. Simulations of magnetized accretion discs have predicted the possibility of excess emission associated with a non-zero torque at the innermost stable circular orbit (ISCO) about the black hole, which, from other NuSTAR studies, lies at about 5 gravitational radii or about 60 km (for a black hole, mass is 8 M_{☉}). In this case, the emitting region at the ISCO has a width varying between 1.3 and 4.6 km and would encompass the start of the plunge region where matter begins to fall freely into the black hole.

ID: 157794
Type: article
Authors: Ricci, C.; Kara, E.; Loewenstein, M.; Trakhtenbrot, B.; Arcavi, I.; Remillard, R.; Fabian, A. C.; Gendreau, K. C.; Arzoumanian, Z.; Li, R.; Ho, L. C.; MacLeod, Chelsea L.; Cackett, E.; Altamirano, D.; Gandhi, P.; Kosec, P.; Pasham, D.; Steiner, James; Chan, C. -H
Abstract: We present the drastic transformation of the X-ray properties of the active galactic nucleus (AGN) 1ES 1927+654, following a changing-look event. After the optical/ultraviolet outburst the power-law component, produced in the X-ray corona, disappeared, and the spectrum of 1ES 1927+65 instead became dominated by a blackbody component (kT ∼ 80-120 eV). This implies that the X-ray corona, ubiquitously found in AGNs, was destroyed in the event. Our dense ∼450 days long X-ray monitoring shows that the source is extremely variable in the X-ray band. On long timescales the source varies up to ∼4 dex in ∼100 days, while on short timescales up to ∼2 dex in ∼8 hr. The luminosity of the source is found to first show a strong dip down to $\sim {10}^{40}\,\mathrm{erg}\,{{\rm{s}}}^{-1}$?--> , and then a constant increase in luminosity to levels exceeding the pre-outburst level ≳300 days after the optical event detection, rising up asymptotically to $\sim 2\times {10}^{44}\,\mathrm{erg}\,{{\rm{s}}}^{-1}$?--> . As the X-ray luminosity of the source increases, the X-ray corona is recreated, and a very steep power-law component (Γ ≃ 3) reappears, and dominates the emission for 0.3-2 keV luminosities $\gtrsim {10}^{43.7}\,\mathrm{erg}\,{{\rm{s}}}^{-1}$?--> , ∼300 days after the beginning of the event. We discuss possible origins of this event, and speculate that our observations could be explained by the interaction between the accretion flow and debris from a tidally disrupted star. Our results show that changing-look events can be associated with dramatic and rapid transformations of the innermost regions of accreting supermassive black holes.

ID: 157488
Type: article
Authors: Tripathi, Ashutosh; Zhou, Menglei; Abdikamalov, Askar B.; Ayzenberg, Dimitry; Bambi, Cosimo; Gou, Lijun; Grinberg, Victoria; Liu, Honghui; Steiner, James F.
Abstract: The iron-line and continuum-fitting methods are currently the two leading techniques for measuring black-hole spins with electromagnetic radiation. They can be naturally extended for probing the spacetime geometry around black holes and testing general relativity in the strong field regime. In the past couple of years, there has been significant work to use the iron-line method to test the nature of black holes. Here we use the continuum-fitting method and we show its capability of constraining the spacetime geometry around black holes by analyzing 17 Rossi X-ray Timing Explorer data of the X-ray binary LMC X-1.

ID: 156911
Type: article
Authors: Wang, Jingyi; Abdikamalov, Askar B.; Ayzenberg, Dimitry; Bambi, Cosimo; Dauser, Thomas; García, Javier A.; Nampalliwar, Sourabh; Steiner, James F.
Abstract: Signatures of X-ray reprocessing (reflection) out of an accretion disk are commonly observed in the high-energy spectrum of accreting black holes, and can be used to probe the strong gravity region around these objects. In this paper, we extend previous work in the literature and we employ a full emission model for relativistic reflection in non-Kerr spacetime to demonstrate an approach that tests the Kerr black hole hypothesis. We analyze a composite spectrum obtained with the Proportional Counter Array in the Rossi X-ray Timing Explorer (RXTE), of the stellar-mass black hole GX 339−4 in its brightest hard state. With a remarkable sensitivity of ~0.1% and 40 million counts in the 3─45 keV band to capture the faint features in the reflection spectrum, we demonstrate that it is possible with existing data and an adequate model to place constraints on the black hole spin a_* and the deformation parameter that quantifies the departure from the Kerr metric. Our measurement obtained with the best fit model, which should be regarded as principally a proof of concept, is a_*=0.92^+0.07_−0.12 and α₁₃=−0.76^+0.78_−0.60 with a 90% confidence level and is consistent with the hypothesis that the compact object in GX 339−4 is a Kerr black hole. We also discuss how the physical model choice and the emissivity profile adopted could make an impact on the constraints of α₁₃ and spin. To enable Kerr metric test using X-ray reflection spectroscopy, it is essential to improve our astrophysical understanding of accreting black holes, e.g., the natures of accretion flow and corona.

ID: 157490
Type: article
Authors: Wang, Jingyi; Kara, Erin; Steiner, James F.; García, Javier A.; Homan, Jeroen; Neilsen, Joseph; Marcel, Grégoire; Ludlam, Renee M.; Tombesi, Francesco; Cackett, Edward M.; Remillard, Ron A.
Abstract: We analyze seven Neutron Star Interior Composition Interior Explorer (NICER) and Nuclear Spectroscopic Telescope Array epochs of the black hole X-ray binary GX 339-4 in the hard state during its two most recent hard-only outbursts in 2017 and 2019. These observations cover the 1-100 keV unabsorbed luminosities between 0.3% and 2.1% of the Eddington limit. With NICER's negligible pileup, high count rate, and unprecedented time resolution, we perform a spectral-timing analysis and spectral modeling using relativistic and distant reflection models. Our spectral fitting shows that as the inner disk radius moves inward, the thermal disk emission increases in flux and temperature, the disk becomes more highly ionized, and the reflection fraction increases. This coincides with the inner disk increasing its radiative efficiency around ∼1% Eddington. We see a hint of the hysteresis effect at ∼0.3% of Eddington; the inner radius is significantly truncated during the rise (>49R_g), while only a mild truncation (∼5R_g) is found during the decay. At higher frequencies (2-7 Hz) in the highest-luminosity epoch, a soft lag is present whose energy dependence reveals a thermal reverberation lag with an amplitude similar to previous findings for this source. We also discuss the plausibility of the hysteresis effect and the debate of the disk truncation problem in the hard state.

ID: 158739
Type: article
Authors: Zhang, L.; Altamirano, D.; Cúneo, V. A.; Alabarta, K.; Enoto, T.; Homan, J.; Remillard, R. A.; Uttley, P.; Vincentelli, F. M.; Arzoumanian, Z.; Bult, P.; Gendreau, K. C.; Markwardt, C.; Sanna, A.; Strohmayer, T. E.; Steiner, James F.; Basak, A.; Neilsen, J.; Tombesi, F.
Abstract: We studied the outburst evolution and timing properties of the recently discovered X-ray transient MAXI J1348-630 as observed with NICER. We produced the fundamental diagrams commonly used to trace the spectral evolution, and power density spectra to study the fast X-ray variability. The main outburst evolution of MAXI J1348-630 is similar to that commonly observed in black hole transients. The source evolved from the hard state (HS), through hard- and soft-intermediate states, into the soft state in the outburst rise, and back to the HS in reverse during the outburst decay. At the end of the outburst, MAXI J1348-630 underwent two reflares with peak fluxes approximately one and two orders of magnitude fainter than the main outburst, respectively. During the reflares, the source remained in the HS only, without undergoing any state transitions, which is similar to the so-called 'failed outbursts'. Different types of quasi-periodic oscillations (QPOs) are observed at different phases of the outburst. Based on our spectral-timing results, we conclude that MAXI J1348-630 is a black hole candidate.

ID: 154537
Type: article
Authors: Bogdanov, Slavko; Guillot, Sebastien; Ray, Paul S.; Wolff, Michael T.; Chakrabarty, Deepto; Ho, Wynn C. G.; Kerr, Matthew; Lamb, Frederick K.; Lommen, Andrea; Ludlam, Renee M.; Milburn, Reilly; Montano, Sergio; Miller, M. Coleman; Bauböck, Michi; Özel, Feryal; Psaltis, Dimitrios; Remillard, Ronald A.; Riley, Thomas E.; Steiner, James F.; Strohmayer, Tod E.; Watts, Anna L.; Wood, Kent S.; Zeldes, Jesse; Enoto, Teruaki; Okajima, Takashi; Kellogg, James W.; Baker, Charles; Markwardt, Craig B.; Arzoumanian, Zaven; Gendreau, Keith C.
Abstract: We present the set of deep Neutron Star Interior Composition Explorer (NICER) X-ray timing observations of the nearby rotation-powered millisecond pulsars PSRs J0437−4715, J0030+0451, J1231−1411, and J2124−3358, selected as targets for constraining the mass─radius relation of neutron stars and the dense matter equation of state (EoS) via modeling of their pulsed thermal X-ray emission. We describe the instrument, observations, and data processing/reduction procedures, as well as the series of investigations conducted to ensure that the properties of the data sets are suitable for parameter estimation analyses to produce reliable constraints on the neutron star mass─radius relation and the dense matter EoS. We find that the long-term timing and flux behavior and the Fourier-domain properties of the event data do not exhibit any anomalies that could adversely affect the intended measurements. From phase-selected spectroscopy, we find that emission from the individual pulse peaks is well described by a single- temperature hydrogen atmosphere spectrum, with the exception of PSR J0437−4715, for which multiple temperatures are required.

ID: 154430
Type: article
Authors: Connors, Riley M. T.; García, Javier A.; Steiner, James F.; Grinberg, Victoria; Dauser, Thomas; Sridhar, Navin; Gatuzz, Efrain; Tomsick, John; Markoff, Sera B.; Harrison, Fiona
Abstract: The dynamical characteristics of XTE J1550-564, a black hole X-ray binary, are well established, and the broadband spectral evolution of the source has been well studied. Its orbital inclination is known to be high, at ̃75°, with the jet estimated to align well with the orbital axis. We explore simultaneous observations made with the Advanced Satellite for Cosmology and Astrophysics and Rossi X-ray Timing Explorer covering the 1-200 keV band during the early stages of the first outburst of XTE J1550-564 in its hard-intermediate state on 1998 September 23/24. We show that the most up-to-date reflection models applied to these data yield an inclination estimate much lower than that found in previous studies at ̃40°, grossly disagreeing with the dynamically estimated orbital inclination. We discuss the possible explanations for this disagreement and its implications for reflection models, including possible physical scenarios in which either the inner disk is misaligned with both the binary orbit and the outer jet or the inner accretion flow, corona, and/or jet have vertical structure that leads to lower inferred disk inclination through various physical means.

ID: 150438
Type: article
Authors: García, Javier A.; Kara, Erin; Walton, Dominic; Beuchert, Tobias; Dauser, Thomas; Gatuzz, Efrain; Baloković, Mislav; Steiner, James F.; Tombesi, Francesco; Connors, Riley M. T.; Kallman, Timothy R.; Harrison, Fiona A.; Fabian, Andrew; Wilms, Jörn; Stern, Daniel; Lanz, Lauranne; Ricci, Claudio; Ballantyne, David R.
Abstract: We present the analysis of the first Nuclear Spectroscopic Telescope Array observations (∼220 ks), simultaneous with the last Suzaku observations (∼50 ks), of the active galactic nucleus of the bright Seyfert 1 galaxy Mrk 509. The time-averaged spectrum in the 1–79 keV X-ray band is dominated by a power-law continuum (Γ ∼ 1.8–1.9), a strong soft excess around 1 keV, and signatures of X-ray reflection in the form of Fe K emission (∼6.4 keV), an Fe K absorption edge (∼7.1 keV), and a Compton hump due to electron scattering (∼20–30 keV). We show that these data can be described by two very different prescriptions for the soft excess: a warm (kT ∼ 0.5–1 keV) and optically thick (τ ∼ 10–20) Comptonizing corona or a relativistically blurred ionized reflection spectrum from the inner regions of the accretion disk. While these two scenarios cannot be distinguished based on their fit statistics, we argue that the parameters required by the warm corona model are physically incompatible with the conditions of standard coronae. Detailed photoionization calculations show that even in the most favorable conditions, the warm corona should produce strong absorption in the observed spectrum. On the other hand, while the relativistic reflection model provides a satisfactory description of the data, it also requires extreme parameters, such as maximum black hole spin, a very low and compact hot corona, and a very high density for the inner accretion disk. Deeper observations of this source are thus necessary to confirm the presence of relativistic reflection and further understand the nature of its soft excess.

ID: 145650
Type: article
Authors: Choudhury, Kishalay; García, Javier A.; Steiner, James F.; Bambi, Cosimo
Abstract: The reflection spectroscopic model RELXILL is commonly implemented in studying relativistic X-ray reflection from accretion disks around black holes. We present a systematic study of the model's capability to constrain the dimensionless spin and ionization parameters from ~6000 Nuclear Spectroscopic Telescope Array (NuSTAR) simulations of a bright X-ray source employing the lamp-post geometry. We employ high-count spectra to show the limitations in the model without being confused with limitations in signal-to-noise. We find that both parameters are well-recovered at 90% confidence with improving constraints at higher reflection fraction, high spin, and low source height. We test spectra across a broad range-first at 10⁶-10⁷ and then ~10⁵ total source counts across the effective 3-79 keV band of NuSTAR, and discover a strong dependence of the results on how fits are performed around the starting parameters, owing to the complexity of the model itself. A blind fit chosen over an approach that carries some estimates of the actual parameter values can lead to significantly worse recovery of model parameters. We further stress the importance to span the space of nonlinear-behaving parameters like {log} ? carefully and thoroughly for the model to avoid misleading results. In light of selecting fitting procedures, we recall the necessity to pay attention to the choice of data binning and fit statistics used to test the goodness of fit by demonstrating the effect on the photon index ?. We re-emphasize and implore the need to account for the detector resolution while binning X-ray data and using Poisson fit statistics instead while analyzing Poissonian data.

ID: 142796
Type: article
Authors: Steiner, James F.; García, Javier A.; Eikmann, Wiebke; McClintock, Jeffrey E.; Brenneman, Laura W.; Dauser, Thomas; Fabian, Andrew C.
Abstract: Continuum and reflection spectral models have each been widely employed in measuring the spins of accreting black holes. However, the two approaches have not been implemented together in a photon-conserving, self-consistent framework. We develop such a framework using the black hole X-ray binary GX 339-4 as a touchstone source, and we demonstrate three important ramifications. (1) Compton scattering of reflection emission in the corona is routinely ignored, but is an essential consideration given that reflection is linked to the regimes with strongest Comptonization. Properly accounting for this causes the inferred reflection fraction to increase substantially, especially for the hard state. Another important impact of the Comptonization of reflection emission by the corona is the downscattered tail. Downscattering has the potential to mimic the relativistically broadened red wing of the Fe line associated with a spinning black hole. (2) Recent evidence for a reflection component with a harder spectral index than the power-law continuum is naturally explained as Compton-scattered reflection emission. (3) Photon conservation provides an important constraint on the hard state's accretion rate. For bright hard states, we show that disk truncation to large scales R\gg {R}_{ISCO} is unlikely as this would require accretion rates far in excess of the observed \dot{M} of the brightest soft states. Our principal conclusion is that when modeling relativistically broadened reflection, spectral models should allow for coronal Compton scattering of the reflection features, and when possible, take advantage of the additional constraining power from linking to the thermal disk component.

ID: 139995
Type: article
Authors: Chen, Zihan; Gou, Lijun; McClintock, Jeffrey E.; Steiner, James F.; Wu, Jianfeng; Xu, Weiwei; Orosz, Jerome A.; Xiang, Yanmei
Abstract: The bright soft X-ray transient Nova Muscae 1991 was intensively observed during its entire eight-month outburst using the Large Area Counter on board the Ginga satellite. Recently, we obtained accurate estimates of the mass of the black hole primary, the orbital inclination angle of the system, and the distance. Using these crucial input data and Ginga X-ray spectra, we have measured the spin of the black hole using the continuum-fitting method. For four X-ray spectra of extraordinary quality we have determined the dimensionless spin parameter of the black hole to be {a}_* ={0.63}_-0.19^+0.16 (1s confidence level), a result that we confirm using 11 additional spectra of lower quality. Our spin estimate challenges two published results: it is somewhat higher than the value predicted by a proposed relationship between jet power and spin; and we find that the spin of the black hole is decidedly prograde, not retrograde as has been claimed.

ID: 141990
Type: article
Authors: García, Javier A.; Fabian, Andrew C.; Kallman, Timothy R.; Dauser, Thomas; Parker, Michael L.; McClintock, Jeffrey E.; Steiner, James F.; Wilms, Jörn
Abstract: Current models of the spectrum of X-rays reflected from accretion discs around black holes and other compact objects are commonly calculated assuming that the density of the disc atmosphere is constant within several Thomson depths from the irradiated surface. An important simplifying assumption of these models is that the ionization structure of the gas is completely specified by a single, fixed value of the ionization parameter ?, which is the ratio of the incident flux to the gas density. The density is typically fixed at n_e = 10¹⁵ cm^-3. Motivated by observations, we consider higher densities in the calculation of the reflected spectrum. We show by computing model spectra for n_e ? 10¹⁷ cm^-3 that high-density effects significantly modify reflection spectra. The main effect is to boost the thermal continuum at energies ? 2 keV. We discuss the implications of these results for interpreting observations of both active galactic nuclei and black hole binaries. We also discuss the limitations of our models imposed by the quality of the atomic data currently available.

ID: 139334
Type: article
Authors: García, Javier A.; Grinberg, Victoria; Steiner, James F.; McClintock, Jeffrey E.; Pottschmidt, Katja; Rothschild, Richard E.
Abstract: We have developed a correction tool to improve the quality of Rossi X-ray Timing Explorer (RXTE) High Energy X-ray Timing Experiment (HEXTE) spectra by employing the same method we used earlier to improve the quality of RXTE Proportional Counter Array (PCA) spectra. We fit all of the hundreds of HEXTE spectra of the Crab individually to a simple power-law model, some 37 million counts in total for Cluster A and 39 million counts for Cluster B, and we create for each cluster a combined spectrum of residuals. We find that the residual spectrum of Cluster A is free of instrumental artifacts while that of Cluster B contains significant features with amplitudes ˜1% the most prominent is in the energy range 30-50 keV, which coincides with the iodine K edge. Starting with the residual spectrum for Cluster B, via an iterative procedure we created the calibration tool hexBcorr for correcting any Cluster B spectrum of interest. We demonstrate the efficacy of the tool by applying it to Cluster B spectra of two bright black holes, which contain several million counts apiece. For these spectra, application of the tool significantly improves the goodness of fit, while affecting only slightly the broadband fit parameters. The tool may be important for the study of spectral features, such as cyclotron lines, a topic that is beyond the scope of this paper.