X-ray Energy Spectra Study and Black Hole Spin Estimate of GRS 1915+105

Abstract

GRS 1915+105 is a galactic low-mass X-ray binary. Its energy spectra and black hole spin have been extensively studied. Since 2018 June, it has declined into a low-flux X-ray level, occasionally interrupted by multi-wavelength flares. Using the data of Insight-HXMT (Insight-Hard X-ray Modulation Telescope) satellite from August 30 to Oct 13, 2020, the energy spectra characteristics of GRS 1915+105 is investigated. The results show that the energy spectra can be well fitted with a Comptonized multi-temperature blackbody model. The evolution of the hardness-intensity diagram during the outburst remains in the soft state. Adopting new dynamical parameters of GRS 1915+105 (including the black hole mass $M$, the Sun mass $M_{\odot }$, the inclination angle $i$, and the distance $D$): $M=12.4_{-1.8}^{+2.0}$ $M_{\odot }$, $i={60}^{\circ }\pm 5^{\circ }$, $D=8.6_{-1.6}^{+2.0}$ kpc, the group obtains a lower limit of the black hole spin, $a_{*}>0.9990$, confirming that GRS 1915+105 is an extremely-spinning black hole. Considering the effects of a local absorber, the group adds the absorption component “tbpcf” into the model. Its equivalent hydrogen column density reaches ${10}^{23}$ cm${}^{-2}$, which is consistent with the characteristics of “Compton-thick”.