Observations of a radio-bright, X-ray obscured GRS 1915+105

TL;DR

This study reports on the recent radio and X-ray observations of GRS 1915+105 during its extended low-flux phase, revealing a mode change in radio flaring and evidence of heavy X-ray obscuration, suggesting a unique accretion state.

Contribution

It provides new insights into the low-luminosity and obscured accretion phases of GRS 1915+105, highlighting a transition to an unusual accretion state with heavy X-ray absorption.

Findings

Radio flaring increased by two orders of magnitude in 2019.

X-ray emission remained low, indicating heavy obscuration.

Radio-X-ray correlation suggests a super-Eddington accretion state.

Abstract

The Galactic black hole transient GRS1915+105 is famous for its markedly variable X-ray and radio behaviour, and for being the archetypal galactic source of relativistic jets. It entered an X-ray outburst in 1992 and has been active ever since. Since 2018 GRS1915+105 has declined into an extended low-flux X-ray plateau, occasionally interrupted by multi-wavelength flares. Here we report the radio and X-ray properties of GRS1915+105 collected in this new phase, and compare the recent data to historic observations. We find that while the X-ray emission remained unprecedentedly low for most of the time following the decline in 2018, the radio emission shows a clear mode change half way through the extended X-ray plateau in 2019 June: from low flux (~3mJy) and limited variability, to marked flaring with fluxes two orders of magnitude larger. GRS1915+105 appears to have entered a low-luminosity canonical hard state, and then transitioned to an unusual accretion phase, characterised by heavy X-ray absorption/obscuration. Hence, we argue that a local absorber hides from the observer the accretion processes feeding the variable jet responsible for the radio flaring. The radio-X-ray correlation suggests that the current low X-ray flux state may be a signature of a super-Eddington state akin to the X-ray binaries SS433 or V404 Cyg.