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

S E Motta,M Del Santo,A Segreto,I Heywood,D R A Williams,L Rhodes,P A Woudt,G Sivakoff,R Fender,D A Green,M Giustini,J J E Kajava

doi:10.1093/mnras/stab511

Abstract

ABSTRACT The Galactic black hole transient GRS 1915+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 GRS 1915+105 has declined into an extended low-flux X-ray plateau, occasionally interrupted by multiwavelength flares. Here, we report the radio and X-ray properties of GRS 1915+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 (∼3 mJy) and limited variability, to marked flaring with fluxes two orders of magnitude larger. GRS 1915+105 appears to have entered a low-luminosity canonical hard state, and then transitioned to an unusual accretion phase, characterized 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.

Highlights

In black hole (BH) X-ray binaries a stellar mass black hole accretes via an accretion disc formed by matter stripped from a low-mass companion star
Cold winds, which appear to co-exist with the radio jets, have been observed in the optical band during the hard state (Muñoz-Darias et al 2016), casting doubts on the idea according to which jets and winds are associated to different accretion states
Several characteristic X-ray variability patterns observed in the X-ray light curve of GRS 1915+105 (Belloni et al 2000) are believed to reflect transitions from and to three accretion states: two soft states (A and B), and a hard state, C, all slightly different from the canonical states seen in other BH binaries (Belloni & Motta 2016)

Summary

Introduction

In black hole (BH) X-ray binaries a stellar mass black hole accretes via an accretion disc formed by matter stripped from a low-mass companion star. BH X-ray binaries are typically transient systems, i.e. they alternate between long states of quiescence, characterised by a luminosity typically of the order of L ∼ 1034 erg s−1 (see Wijnands et al 2015), and relatively short outbursts, during which their luminosity can reach ∼ 1039 erg s−1 During outbursts these systems show clear repeating patterns of behaviour across various accretion states, each associated with mechanical feedback in the form of winds and relativistic jets (e.g., Fender et al 2009, Ponti et al 2012). State C shows high X-ray variability and no disc contribution to the X-ray spectrum, and is known to be associated with steady radio jets (Rushton et al 2010) Such jets appear as flat-top periods in the radio light curve, characterised by relatively high radio flux densities (∼ 100 mJy beam−1), an optically thick radio spectrum, and a flat low-flux X-ray light curve (Pooley & Fender 1997). Radio Plateaus are generally preceded and followed by flaring periods due to the launch of relativistic ejections (Rodríguez & Mirabel 1999), which have been repeatedly resolved as extended radio jets on a range of scales from ∼1 mas to hundreds of arcseconds (Dhawan et al 2000; Miller-Jones et al 2005; Rushton et al 2007; Fender et al 1999; Miller-Jones et al 2007)

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