Interpreting the large amplitude X-ray variation of GRS 1915+105 and IGR J17091-3624 as modulations of an accretion disc

Abstract

Using the flux resolved spectroscopy for the first time, we analyse the RXTE/PCA data of the black hole X-ray binaries GRS 1915+105 and IGR J17091-3624, when both sources show large amplitude, quasi-regular oscillations in 2.0-60.0 keV X-ray light curves. For different observations, we extract spectra during the peak (spectrally soft) and dip (spectrally hard) intervals of the oscillation, and find that their spectra are phenomenologically complex, requiring at least two distinct spectral components. Besides a thermal Comptonization component, we find that the disc emission is better modelled by an index-free multicolour disc blackbody component (p-free disc model) rather than that from a standard accretion disc. While the peak and dip spectra are complex, remarkably, their difference spectra constructed by treating dip spectra as the background spectra of the peak spectra, can be modelled as a single p-free disc component. Moreover, the variability at different time-scales and energy bands of the peak flux level is always greater than or equal to the variability of the dip flux level, which strengthens the possibility that the peak flux level may be due to an independent spectral component added to the dip one. We also verify it using joint spectral analysis of peak and dip spectra with a variable emission component. Our result substantiates the standard hypothesis that the oscillations are due to the limit cycle behaviour of an unstable radiation pressure dominated inner disc. However, in this interpretation, the flux variation of the unstable disc can be several order of magnitudes as expected from some theoretical simulations and need not be fine tuned to match the factor ~10 variation seen between the peak and dip levels.

12 pages, 4 figures, 5 tables, accepted for publication in the MNRAS

References in corpus (8)

• X-ray Properties of Black-Hole Binaries
• The faint "heartbeats" of IGR J17091-3624: an exceptional black-hole candidate
• Two Years of INTEGRAL monitoring of GRS 1915+105 Part 2: X-Ray Spectro-Temporal Analysis
• Orbital Period and Outburst Luminosity of Transient Low Mass X-ray Binaries
• Radiation Pressure and Mass Ejection in Rho-like States of GRS 1915+105
• Studies of Thermally Unstable Accretion Disks around Black Holes with Adaptive Pseudo-Spectral Domain Decomposition Method I. Limit-Cycle Behavior in the Case of Moderate Viscosity
• Stabilization of radiation pressure dominated accretion disks by viscous fluctuations
• Evidence of two unique variability classes from IGR J17091-3624

Cited by in corpus (6)

• AstroSat/LAXPC reveals the high energy variability of GRS 1915+105 in the chi class
• IC 3599 did it again: A second outburst of the X-ray transient Seyfert 1.9 Galaxy
• Chaotic and stochastic processes in the accretion flows of the black hole X-ray binaries revealed by recurrence analysis
• On the energy dependence of the QPO phenomenon in the black hole system MAXI J1535-571
• Detection of distinct power spectra in soft and hard X-ray bands in the hard state of GRS 1915+105
• Swift/XRT, Chandra and XMM-Newton observations of IGR J17091-3624 as it returns into quiescence