Insight-HXMT Observations of a Possible Fast Transition from the Jet- to Wind-dominated State during a Huge Flare of GRS 1915+105

We present the analysis of the brightest flare that was recorded in the 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 Insight-HXMT among a series of flares during 2019 June 2 UTC 16:37:06–20:11:36 with a 2–200 keV luminosity of 3.4–7.27 × 1038 erg s−1. Basing on the broadband 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 of ∼6.78 keV. The reflection component of the latter is consistent with an inner disk radius ∼five 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.