Stronger Reflection from Black Hole Accretion Disks in Soft X-ray States

Abstract

We analyze 15,000 spectra of 29 stellar-mass black hole candidates collected over the 16-year mission lifetime of RXTE using a simple phenomenological model. As these black holes vary widely in luminosity and progress through a sequence of spectral states, which we broadly refer to as hard and soft, we focus on two spectral components: The Compton power law and the reflection spectrum it generates by illuminating the accretion disk. Our proxy for the strength of reflection is the equivalent width of the Fe-K line as measured with respect to the power law. A key distinction of our work is that for all states we estimate the continuum under the line by excluding the thermal disk component and using only the component that is responsible for fluorescing the Fe-K line, namely the Compton power law. We find that reflection is several times more pronounced (~3) in soft compared to hard spectral states. This is most readily caused by the dilution of the Fe line amplitude from Compton scattering in the corona, which has a higher optical depth in hard states. Alternatively, this could be explained by a more compact corona in soft (compared to hard) states, which would result in a higher reflection fraction.

6 pages, 4 figures and 1 table. Accepted for publication in ApJL

References in corpus (9)

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• Average bolometric corrections and optical to X-ray flux measurements as a function of accretion rate for X-ray binaries
• Observing imprints of black hole event horizon on X-ray spectra
• X-ray spectral correlations in a sample of Low-mass black hole X-ray binaries in the hard state
• The spectral-timing analysis of Cygnus X-1 with Insight-HXMT
• The spin measurement of MAXI J1348-630 using the Insight-HXMT data
• Impact of reflection Comptonization on X-ray reflection spectroscopy: the case of EXO 1846-031
• Dimming GRS 1915+105 observed with NICER and Insight--HXMT
• Accretion Geometry of Black Hole X-ray Binaries: Insights from X-ray Observations
• Interaction between disk and extended corona in a general relativistic framework