Coronal properties of black hole X-ray binaries in the hard state as seen by NuSTAR and Swift

Abstract

In this work, we measure two important phenomenological parameters of corona (and hot accretion flow) in black hole X-ray binaries: the photon index $\Gamma$ and the electron temperature $\te$. Thanks to the capability of {\it NuSTAR} in hard X-rays, we measure these two parameters over six orders of magnitude in the 0.1-100 keV X-ray luminosity $\lx$, from $\sim 5\times10^{38} \ergs$ down to as low as $\sim 5\times10^{32} \ergs$. We confirm the existence of a "V"-shaped correlation between $\Gamma$ and $\lx$. Surprisingly, we observe a "$\Lambda$"-shaped correlation between $\te$ and $\lx$. The "cooler when brighter" branch in the high luminosity regime ($\lx \ga 3\times 10^{36} \ergs$) agrees with previous results and can be understood under the existing model of Compton scattering in the corona. On the other hand, the apparent "cooler when fainter" (positive $\te$-$\lx$ correlation) branch in the low-luminosity regime ($\lx \la 3\times 10^{36} \ergs$) is unexpected, thus it puts a new challenge to existing models of hot accretion flow/corona.

accepted for publication in ApJL

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• Investigating Changing-Look Active Galactic Nuclei with Long-term Optical and X-Ray Observations
• The hard to soft transition of GX 339-4 as seen by Insight-HXMT
• X-ray quasi-periodic oscillations in Lense-Thirring precession model -- II. variability of relativistic iron K$\alpha$ line
• Analysis of the reflection spectra of MAXI J1535-571 in the hard and intermediate states
• Disk-jet coupling changes as a possible indicator for outbursts from GX 339-4 remaining within the X-ray hard state
• X-ray spectral correlations in a sample of Low-mass black hole X-ray binaries in the hard state
• Broadband X-ray properties of black holes GRS 1758-258 and 1E 1740.7-2942: AstroSat and NuSTAR results
• Global Accretion Properties of Black Hole X-Ray Binaries: A Phenomenological Perspective
• Multi-wavelength properties of changing-state active galactic nuclei: I. the evolution of soft excess and X-ray continuum
• Insight-HXMT Detections of Hard X-ray Tails in Scorpius X-1
• Broadband X-ray Spectroscopy and Estimation of Spin of the Galactic Black Hole Candidate GRS 1758-258
• Accretion Disc-Jet Couplings in X-ray Binaries
• Hidden biases in flux-resolved X-ray spectroscopy
• Possible Coronal Geometry in the Hard and Soft State of Black Hole X-ray Binaries from MONK Simulations