The Novel Obscured State of Stellar-mass Black Hole GRS 1915+105

TL;DR

This study reports the discovery of a new 'obscured state' in the stellar-mass black hole GRS 1915+105, characterized by increased internal obscuration and decreased accretion, based on a year-long Swift Observatory monitoring campaign.

Contribution

It introduces the first detailed observational evidence of an 'obscured state' in GRS 1915+105, highlighting a significant change in accretion behavior and internal obscuration mechanisms.

Findings

Gradual flux decrease by two orders of magnitude.

Dominant increase in internal obscuration with median N_H = 7×10^23 cm^-2.

Source appears to be Compton-thick in several observations.

Abstract

GRS 1915$+$105 is a stellar-mass black hole that is well known for exhibiting at least 12 distinct classes of X-ray variability and correlated multi-wavelength behavior. Despite such extraordinary variability, GRS 1915$+$105 remained one of the brightest sources in the X-ray sky. However, in early 2019, the source became much fainter, apparently entering a new accretion state. Here, we report the results of an extensive, year-long monitoring campaign of GRS 1915$+$105 with the Neil Gehrels Swift Observatory. During this interval, the flux of GRS 1915$+$105 gradually diminished; the observed count rate eventually dropped by two orders of magnitude. Simple but robust spectral fits to these monitoring observations show that this new state results from the combination of a dramatic and persistent increase in internal obscuration, and a reduced mass accretion rate. The internal obscuration is the dominant effect, with a median value of $N_{H} = 7\times 10^{23}~{\rm cm}^{-2}$. In a number of observations, the source appears to be Compton-thick. We suggest that this state should be identified as the "obscured state," and discuss the implications of this new (or rarely observed) accretion mode for black holes across the mass scale.