http://arxiv.org/abs/1912.06833
The shape of the iron K${\alpha}$ fluorescence line in the spectrum of accreting black holes is observed to be distorted by relativistic effects, providing a diagnostic for accretion disc geometry. This can be converted into a black hole spin measurement by associating the disc inner edge with the innermost stable circular orbit (ISCO) of the black hole. However, the disc around accreting stellar mass black holes may be truncated outside of the ISCO for some spectral states. Here we analyse a Nuclear Spectroscopic Telescope ARray observation of GRS 1915+105 that exhibits an iron line so broad that previous modelling yielded a near-maximal spin measurement. However, the ionization state of the disc was assumed to be independent of radius, in line with nearly all previous studies in the literature. We show that accounting for the inner disc being more ionized than the outer disc due to irradiation allows for a truncated disc geometry. Although the value of the disc inner radius depends on the details of our physical assumptions, we conclude that the spin of GRS 1915+105 is not currently constrained by reflection spectroscopy. Our result suggests that existing spin measurements of active galactic nuclei may be biased.
S. Shreeram and A. Ingram
Tue, 17 Dec 19
7/89
Comments: Accepted for publication in MNRAS
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