Measuring the Spin of GRS 1915+105 with Relativistic Disk Reflection

TL;DR

This study uses Suzaku X-ray observations and relativistic disk reflection models to measure the spin of the black hole in GRS 1915+105, finding it to be high and significantly different from zero.

Contribution

It provides new spin measurements of GRS 1915+105 using broadband X-ray spectra and relativistic reflection modeling, refining previous estimates.

Findings

Black hole spin is constrained to a/M ≈ 0.56 from soft X-ray data.

Full broadband spectrum suggests a higher spin, a/M ≈ 0.98.

Zero spin is excluded at 2-4 sigma confidence levels.

Abstract

GRS 1915+105 harbors one of the most massive known stellar black holes in the Galaxy. In May 2007, we observed GRS 1915+105 for 117 ksec in the low/hard state using Suzaku. We collected and analyzed the data with the HXD/PIN and XIS cameras spanning the energy range from 2.3-55 keV. Fits to the spectra with simple models reveal strong disk reflection through an Fe K emission line and a Compton back-scattering hump. We report constraints on the spin parameter of the black hole in GRS 1915+105 using relativistic disk reflection models. The model for the soft X-ray spectrum (i.e. < 10 keV) suggests a/M = 0.56(2) and excludes zero spin at the 4 sigma level of confidence. The model for the full broadband spectrum suggests that the spin may be higher, a/M = 0.98(1) (1 sigma confidence), and again excludes zero spin at the 2 sigma level of confidence. We discuss these results in the context of other spin constraints and inner disk studies in GRS 1915+105.