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

Probing heartbeat oscillations from the black hole X-ray binary GRS 1915+105 using spectral-timing analysis
Karan Akbari (St. Xavier’s College Mumbai), Chintan Patel (St. Xavier’s College Mumbai), Sayantan Bhattacharya (TIFR Mumbai), Sudip Bhattacharyya (TIFR Mumbai), Manojendu Choudhury (St. Xavier’s College Mumbai)
arXiv:2604.18601v1 Announce Type: new
Abstract: GRS 1915+105 is a black hole X-ray binary exhibiting quasi-periodic $rho$-class (“heartbeat”) oscillations with periods of $sim$50-100 s, thought to arise from radiation-pressure-driven instabilities in the inner accretion disk at near-Eddington luminosities. The coupled disk-corona response across this instability cycle has lacked simultaneous broadband phase-resolved observational constraints. We present phase-resolved spectral and timing analysis using 24 Swift XRT observations (2014-2016; 1-10 keV) and AstroSat SXT+LAXPC data (2017; 0.8-30 keV), dividing each cycle into five phases (three rise, two decay). We find a systematic anti-correlation between inner disk temperature ($T_{rm in}$) and apparent inner radius ($R_{rm in}$): $T_{rm in}$ decreases from $sim$1.7 to $sim$1.5 keV as $R_{rm in}$ increases from $sim$22 to $sim$38 km through Phases 1-3, before $R_{rm in}$ decreases to $sim$23 km at the burst peak (Phase 4) and $sim$18 km post-burst (Phase 5). The broadband fits reveal that the coronal electron temperature $kT_{rm e}$ rises from $sim$10.5 to $sim$14.5 keV through Phases 1-3 and drops to $sim$6 keV after the burst, while Hardness-Intensity and Color-Color Diagrams show clear spectral hysteresis, with Phase 3 appearing softest in XRT/SXT but hardest above $sim$10 keV in LAXPC. This evolution is consistent with radiation-pressure instability driving the cyclic $T_{rm in} – R_{rm in}$ variations, with coronal heating naturally explained by seed photon starvation via the Haardt-Maraschi mechanism as $R_{rm in}$ increases. Our 0.8-30 keV coverage provides the first phase-resolved characterization of both the thermal disk and Comptonized corona within a single $rho$ cycle, directly revealing the disk-corona coupling that drives the heartbeat oscillation and is inaccessible to narrow-band observations alone.arXiv:2604.18601v1 Announce Type: new
Abstract: GRS 1915+105 is a black hole X-ray binary exhibiting quasi-periodic $rho$-class (“heartbeat”) oscillations with periods of $sim$50-100 s, thought to arise from radiation-pressure-driven instabilities in the inner accretion disk at near-Eddington luminosities. The coupled disk-corona response across this instability cycle has lacked simultaneous broadband phase-resolved observational constraints. We present phase-resolved spectral and timing analysis using 24 Swift XRT observations (2014-2016; 1-10 keV) and AstroSat SXT+LAXPC data (2017; 0.8-30 keV), dividing each cycle into five phases (three rise, two decay). We find a systematic anti-correlation between inner disk temperature ($T_{rm in}$) and apparent inner radius ($R_{rm in}$): $T_{rm in}$ decreases from $sim$1.7 to $sim$1.5 keV as $R_{rm in}$ increases from $sim$22 to $sim$38 km through Phases 1-3, before $R_{rm in}$ decreases to $sim$23 km at the burst peak (Phase 4) and $sim$18 km post-burst (Phase 5). The broadband fits reveal that the coronal electron temperature $kT_{rm e}$ rises from $sim$10.5 to $sim$14.5 keV through Phases 1-3 and drops to $sim$6 keV after the burst, while Hardness-Intensity and Color-Color Diagrams show clear spectral hysteresis, with Phase 3 appearing softest in XRT/SXT but hardest above $sim$10 keV in LAXPC. This evolution is consistent with radiation-pressure instability driving the cyclic $T_{rm in} – R_{rm in}$ variations, with coronal heating naturally explained by seed photon starvation via the Haardt-Maraschi mechanism as $R_{rm in}$ increases. Our 0.8-30 keV coverage provides the first phase-resolved characterization of both the thermal disk and Comptonized corona within a single $rho$ cycle, directly revealing the disk-corona coupling that drives the heartbeat oscillation and is inaccessible to narrow-band observations alone.