GRS 1915+105: A Changing-Look Microquasar?

ATel #12771; J. M. Miller, M. Balakrishnan, M. Reynolds (Univ. of Michigan), A. C. Fabian (Univ. of Cambridge), J. Kaastra (SRON), T. Kallman (NASA/GSFC)
on 17 May 2019; 07:40 UT
Credential Certification: Jon Miller (jonmm@umich.edu)

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We report on the most recent observation of GRS 1915+105 with the Neil Gehrels Swift Observatory, made through our high-cadence monitoring program. Observation 00034292136 was obtained on 2019 May 16 starting at 21:39:31 UT. The XRT "windowed timing" mode exposure was 0.86 ks in duration.

We examined the spectrum in the 1.3-8.5 keV band, after optimally binning the data (see Kaastra & Bleeker 2016). The spectrum is at least superficially similar to those obtained in many highly obscured Seyfert-2 AGN, and perhaps some Compton-thick AGN. Such spectra are characterized by a broad absorption trough between 2-5 keV, strong neutral reflection and/or scattering features in the Fe K band, and a very steep drop above the Fe K edge. A similar spectrum was previously observed in the stellar-mass black hole V4641 Sgr (Morningstar et al. 2014); in that case, the unusual obscuration could plausibly be attributed to inhomogeneities in the massive companion wind, but this may need to be reconsidered.

Using XSPEC, we fit the new XRT spectrum of GRS 1915+105 with a model given by:

tbabs*(zxipcf*cutoffpl + pexmon).

With continuum parameters linked between pexmon (Nandra et al. 2007) and the cut-off power-law, a good fit is found (chi^2/dof = 51.6/51). Crucially, the partial covering absorption is measured to have a column density of N_H = 1.8 (+1.6,-0.1) E+24 cm^-2. The covering fraction is measured to be 0.98 (+0.02,-0.03). The data weakly prefer an ionized absorber with a log ionization parameter of log xi = 2.8; the requirement of heavy obscuration is not changed if neutral obscuration is assumed.

With this model, the implied 1-10 keV flux is F = 1.4 E-10 erg/cm2/s. When the line of sight obscuration and internal obscuration are removed, the implied flux is an order of magnitude higher: F = 2.3 E-9 erg/cm2/s (L_unobscured ~ 2 E+37 erg/s at a distance of 8.6 kpc; Reid et al. 2014). It is possible that the low flux state that is currently observed in GRS 1915+105 is at least partly the result of internal obscuration. This behavior may represent a rare - but not unique - source state in stellar-mass black holes, tied to changes in the accretion flow.

For a view of the spectrum, please see:

https://sites.lsa.umich.edu/jonmm/2019/05/17/the-black-hole-grs-1915105-as-a-changing-look-micro-quasar

References:
Kaastra, J., & Bleeker, J., 2016, A&A, 587, 151
Morningstar, W., et al., 2014, ApJ, 786, L20
Nandra, K., et al., 2007, MNRAS, 382, 194
Reid, M., et al., 2014, ApJ, 796, 2