Probing heartbeat oscillations from the black hole X-ray binary GRS 1915+105 using spectral-timing analysis

Abstract

GRS 1915+105 is a black hole X-ray binary whose $\rho$-class ("heartbeat") oscillations ($\sim$50--100 s) are attributed to radiation-pressure instabilities in the inner accretion disk at near-Eddington luminosities. We present a phase-resolved spectral and timing analysis of 24 Swift XRT observations (1--10 keV) and broadband AstroSat SXT+LAXPC data (0.8--30 keV), dividing each cycle into five phases. The narrow-band XRT fits show an apparent anti-correlation between the inner disk temperature ($T_{\rm in} \sim 1.7$--$1.5$ keV) and apparent radius ($R_{\rm in} \sim 22$--$38$ km) across the cycle. The broadband AstroSat fits, however, are statistically consistent with a constant disk temperature: a joint fit with $T_{\rm in}$ tied across all five phases gives $T_{\rm in} = 1.275 \pm 0.020$ keV ($\chi^2_\nu = 1.003$; $\Delta\chi^2 = +4.3$ for 4 added constraints), whereas tying the disk normalization as well is rejected ($\Delta\chi^2 = +175.9$), leaving a $\sim$20% variation in apparent $R_{\rm in}$ ($18.1 \pm 0.7$ to $21.9 \pm 0.7$ km). The coronal electron temperature rises from $\sim$6 to $\sim$14.5 keV approaching the burst, with the photon index tracking it. We attribute the larger XRT disk swings to its limited bandpass, where coronal Comptonization is unconstrained and the disk parameters absorb coronal variability; the dominant variability is therefore coronal, consistent with Vadawale et al. (2001), and the residual $R_{\rm in}$ change is plausibly a color-correction effect (Zoghbi et al. 2016). Hardness--intensity and color--color diagrams show clear spectral hysteresis. Our broadband coverage provides a phase-resolved test of disk constancy and favors coronal evolution as the driver of the spectral variability across the $\rho$ cycle.