Radiation Pressure and Mass Ejection in Rho-like States of GRS 1915+105

TL;DR

This paper proposes a unified model explaining the diverse X-ray lightcurves in GRS 1915+105's rho state through radiation pressure instabilities and disk ejections, supported by phase-resolved spectral analysis.

Contribution

It introduces a comprehensive scenario linking different heartbeat oscillations to a common physical mechanism involving radiation pressure and disk ejection events.

Findings

Different lightcurves can have similar spectral evolution.

Disk ejection is a universal feature of the rho state.

Mass accretion rate variations explain lightcurve differences.

Abstract

We present a unifying scenario to address the physical origin of the diversity of X-ray lightcurves within the rho variability class of the microquasar GRS 1915+105. This 'heartbeat' state is characterized by a bright flare that recurs every ~50-100 seconds, but the profile and duration of the flares varies significantly from observation to observation. Based on a comprehensive, phase-resolved study of heartbeats in the RXTE archive, we demonstrate that very different X-ray lightcurves do not require origins in different accretion processes. Indeed, our detailed comparison of the phase-resolved spectra of a double-peaked oscillation and a single-peaked oscillation shows that different cycles can have basically similar X-ray spectral evolution. We argue that all heartbeat oscillations can be understood as the result of a combination of a thermal-viscous radiation pressure instability, a local Eddington limit in the disk, and a sudden, radiation-pressure-driven evaporation or ejection event in the inner accretion disk. This ejection appears to be a universal, fundamental part of the rho state, and is largely responsible for a hard X-ray pulse seen in the lightcurve of all cycles. We suggest that the detailed shape of oscillations in the mass accretion rate through the disk is responsible for the phenomenological differences between different rho-type lightcurves, and we discuss how future time-dependent simulations of disk instabilities may provide new insights into the role of radiation pressure in the accretion flow.