XRISM Spectroscopy of the Stellar-mass Black Hole GRS 1915+105

Abstract GRS 1915+105 was the stellar-mass black hole that best reproduced key phenomena that are also observed in type 1 active galactic nuclei (AGNs). In recent years, however, it has evolved to resemble a type 2 or Compton-thick AGN. Herein we report on the first XRISM observation of GRS 1915+105. The high-resolution Resolve calorimeter spectrum reveals that a sub-Eddington central engine is covered by a layer of warm, Compton-thick gas. With the obscuration acting as a coronagraph, numerous strong, narrow emission lines from He-like and H-like charge states of Si, S, Ar, Ca, Cr, Mn, Fe, and Ni dominate the spectrum. Radiative recombination continuum (RRC) features are also observed, signaling that much of the emitting gas is photoionized. The line spectrum can be fit by three photoionized emission zones, with broadening and bulk velocities suggestive of an origin in the outer disk atmosphere and/or a slow wind at r ≃ 10 6 GM / c 2 . The Fe XXV He α and Fe XXVI Ly α lines have a broad base that may indicate some emission from r ∼ 3 × 10 3 GM / c 2 . These results broadly support a picture wherein the current state in GRS 1915+105 is due to obscuration by the irradiated outer disk. This could arise through disk thickening if the Eddington fraction is higher than inferred, but it is more likely due to a warped, precessing disk that has brought the outer disk into the line of sight. We discuss the strengths and weaknesses of this interpretation and our modeling, as well as possible explanations of some potentially novel spectral features.