Insight-HXMT observations of a possible fast transition from jet to wind dominated state during a huge flare of GRS~1915+105

TL;DR

This study analyzes a giant flare from GRS 1915+105 observed by Insight-HXMT, revealing a rapid transition from jet to wind dominance possibly driven by accretion disk evolution and magnetic fields.

Contribution

It provides the first detailed spectral analysis of a giant flare during a low-luminosity state, indicating a fast jet-to-wind transition in GRS 1915+105.

Findings

Flare spectrum dominated by a power-law component.

Detection of a hard tail in bright epoch and absorption line at 6.78 keV in faint epoch.

Inner disk radius appears larger during faint epoch.

Abstract

We present the analysis of the brightest flare that was recorded in the \emph{Insight}-HMXT data set, in a broad energy range (2$-$200 keV) from the microquasar GRS~1915+105 during an unusual low-luminosity state. This flare was detected by \emph{Insight}-HXMT among a series of flares during 2 June 2019 UTC 16:37:06 to 20:11:36, with a 2-200 keV luminosity of 3.4$-$7.27$\times10^{38}$ erg s$^{-1}$. Basing on the broad-band spectral analysis, we find that the flare spectrum shows different behaviors during bright and faint epochs. The spectrum of the flare can be fitted with a model dominated by a power-law component. Additional components show up in the bright epoch with a hard tail and in the faint epoch with an absorption line $\sim$ 6.78 keV. The reflection component of the latter is consistent with an inner disk radius $\sim$ 5 times larger than that of the former. These results on the giant flare during the "unusual" low-luminosity state of GRS~1915+105 may suggest that the source experiences a possible fast transition from a jet-dominated state to a wind-dominated state. We speculate that the evolving accretion disk and the large-scale magnetic field may play important roles in this peculiar huge flare.