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⊙, the inclination angle i, and the distance D): M=12.4−1.8+2.0M⊙, i=60∘±5∘, 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 1023 cm−2, which is consistent with the characteristics of “Compton-thick”.