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

Jon M. Miller, Liyi Gu, John Raymond, Laura Brenneman, Elena Gallo, Poshak Gandhi, Timothy Kallman, Shogo Kobayashi, Junjie Mao, Megumi Shidatsu, Yoshihiro Ueda, Xin Xiang, Abderahmen Zoghbi

First listed 2025-10-29 | Last updated 2025-11-19

Abstract

GRS 1915$+$105 was the stellar-mass black hole that best reproduced key phenomena that are also observed in Type-1 active galactic nuclei. In recent years, however, it has evolved to resemble a Type-2 or Compton-thick AGN. Herein, we report on the first XRISM observation of GRS 1915$+$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 $r \simeq 10^{6}~GM/c^{2}$. The Fe XXV He-$α$ and Fe XXVI Ly-$α$ lines have a broad base that may indicate some emission from $r \sim 3\times 10^{3}~GM/c^{2}$. 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, and possible explanations of some potentially novel spectral features.

Short digest

First XRISM/Resolve spectroscopy of GRS 1915+105 catches the source in a micro–Type 2/CTAGN-like state, where a sub‑Eddington engine is veiled by warm, Compton‑thick gas that turns the 1.6–11.6 keV band into a forest of narrow H‑/He‑like lines with clear RRCs. Modeling requires three photoionized emission zones; bulk redshifts and modest line broadening indicate an origin in the outer disk atmosphere or a slow wind at r ≃ 10^6 GM/c^2, with a broad base under Fe XXV–XXVI hinting at emission from r ~ 3×10^3 GM/c^2. A weak 6.4 keV Fe Kα plus strong RRCs point to photoionized reprocessing, the heavy warm cover acting as a coronagraph that reveals the line-emitting gas. The authors favor obscuration by an irradiated, warped, precessing outer disk as the driver of the current state.

Key figures to inspect

• Figure 1: Use the full-band Resolve spectrum to see how narrow H-/He-like lines dominate over the continuum and to spot RRCs; note the weak, narrow 6.4 keV Fe Kα that flags some cold gas at intermediate radii.
• Figure 2: In the 2 keV slices, compare line centroids to the marked lab energies to gauge the systematic redshift of the bulk emitter and to trace RRC features across the band.
• Figure 3: Inspect the Fe-peak complex (Cr–Ni) where lines tower 8–10× above the local continuum; check the Fe RRCs near 8.8 and 9.3 keV that verify photoionization.
• Figure 4: Examine the decomposition of Fe XXV He-α and Fe XXVI Ly-α into three pion components to see why multiple photoionized zones are required, and to assess the redshift and the broad base hinting at r ~ 3×10^3 GM/c^2 emission.