Self-consistent Black Hole Accretion Spectral Models and the Forgotten Role of Coronal Comptonization of Reflection Emission

Steiner

et al. 2020

ApJ

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We present the analysis of several observations of the black hole binary GX 339-4 during its bright intermediate states from two different outbursts (2002 and 2004), as observed by RXTE/PCA. We perform a consistent study of its reflection spectrum by employing the relxill family of relativistic reflection models to probe the evolutionary properties of the accretion disk including the inner disk radius (R in ), ionization parameter (ξ), temperatures of the inner disk (T in ), corona (kT e ), and its optical depth (τ ). Our analysis indicates that the disk inner edge approaches the inner-most stable circular orbit (ISCO) during the early onset of bright hard state, and that the truncation radius of the disk remains low (< 9R g ) throughout the transition from hard to soft state. This suggests that the changes observed in the accretion disk properties during the state transition are driven by variation in accretion rate, and not necessarily due to changes in the inner disk's radius. We compare the aforementioned disk properties in two different outbursts with state transitions occurring at dissimilar luminosities, and find identical evolutionary trends in the disk properties, with differences only seen in corona's kT e and τ . We also perform an analysis by employing a self-consistent Comptonized accretion disk model accounting for the scatter of disk photons by the corona, and measure low inner disk truncation radius across the bright intermediate states, using the temperature dependent values of spectral hardening factor, thereby independently confirming our results from the reflection spectrum analysis.

Section: Introductionmentioning

confidence: 99%

Evolution of the Accretion Disk–Corona during the Bright Hard-to-soft State Transition: A Reflection Spectroscopic Study with GX 339–4

Sridhar

Steiner

et al. 2020

ApJ

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“…The model simplcut (Steiner et al 2017) is a variant of the model simpl (Steiner et al 2009), and functions as a coronal plasma, inverse-Compton (IC) scattering the disk photons in a convolution kernel. The simplcut model includes a coronal electron temperature (kT e ) and thus contains a high-energy cutoff in the power-law continuum.…”

Section: Modelingmentioning

confidence: 99%

Evidence for Returning Disk Radiation in the Black Hole X-Ray Binary XTE J1550–564

Connors

Dauser

et al. 2020

ApJ

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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 (RXTE), 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 (ISCO) throughout the bright-hard-to-soft states (10s 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.

“…It has four physical parameters: the scattered fraction f sc , the spectral index Γ, the electron temperature kT e , and the reflection fraction R f . We follow the procedures in Steiner et al (2017), but we do not implement any linking between the diskbb parameters in the hard and soft states. In XSPEC notation, the model we adopt is constant * Tbabs * [simplcut * (diskbb+relxillCp) +xillverCp] (2015-M2).…”

Section: Model 2: Taking the Comptonization Of Reflection Into Accountmentioning

confidence: 99%

The Evolution of GX 339-4 in the Low-hard State as Seen by NuSTAR and Swift

Wang

Steiner

et al. 2018

ApJ

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We analyze 11 Nuclear Spectroscopic Telescope Array and Swift observations of the black hole X-ray binary GX339-4in the hard state, 6 of which were taken during the end of the 2015 outburst and 5 during a failed outburst in 2013. These observations cover luminosities from 0.5% to 5% of the Eddington luminosity. Implementing the most recent version of the reflection model relxillCp, we perform simultaneous spectral fits on both data sets to track the evolution of the properties in the accretion disk, including the inner edge radius, the ionization, and the temperature of the thermal emission. We also constrain the photon index and electron temperature of the primary source (the "corona"). We observe a maximum truncation radius of 37R g in the preferred fit for the 2013 data set, and a marginal correlation between the level of truncation and luminosity. We also explore a self-consistent model under the framework of coronal Comptonization, and find consistent results regarding the disk truncation in the 2015 data, providing a more physical preferred fit for the 2013 observations.