A NICER View of a Highly-Absorbed Flare in GRS 1915+105

Abstract

After 26 years in outburst, the black hole X-ray binary GRS 1915+105 dimmed considerably in early 2018; its flux dropped sharply in mid-2019, and it has remained faint ever since. This faint period, the "obscured state," is punctuated by occasional X-ray flares, many of which have been observed by NICER as part of our regular monitoring program. Here we present detailed time-resolved spectroscopy of one bright flare, whose spectrum shows evidence of high column density partial covering absorption and extremely deep absorption lines (equivalent widths over 100 eV in some cases). We study the time-dependent ionization of the obscuring gas with XSTAR, ultimately attributing the absorption to a radially-stratified absorber of density 1e12-1e13 cm^-3 at a ~few x 1e11 cm from the black hole. We argue that a vertically-extended outer disk could explain this obscuration. We discuss several scenarios to explain the obscured state, including massive outflows, an increase in the mass accretion rate, and changes in the outer disk that herald the approach of quiescence, but none are entirely satisfactory. Alternative explanations, such as obscuration by the accretion stream impact point, may be testable with current or future data.

16 pages, 5 figures. Published in ApJ: https://iopscience.iop.org/article/10.3847/1538-4357/abb598

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• Testing the Kerr black hole hypothesis with the continuum-fitting and the iron line methods: the case of GRS 1915+105
• Modeling Magnetic Disk-Wind State Transitions in Black Hole X-ray Binaries
• Dips and eclipses in the X-ray binary Swift J1858.6-0814 observed with NICER
• ALMA/NICER observations of GRS 1915+105 indicate a return to a hard state
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• StrayCats: A catalog of NuSTAR Stray Light Observations
• Insight-HXMT observations of a possible fast transition from jet to wind dominated state during a huge flare of GRS~1915+105
• Time-resolved spectroscopy on the heartbeat state of GRS 1915+105 using AstroSat
• Mapping the X-ray variability of GRS1915+105 with machine learning
• Stratified wind from a super-Eddington X-ray binary is slower than expected
• The disk wind in GRS 1915+105 as seen by Insight-HXMT
• Exploring the broadband spectral and timing characteristics of GRS 1915+105 with AstroSat and NICER observations
• Time resolved spectroscopy of a GRS 1915+105 flare during its unusual low state using AstroSat
• Dimming GRS 1915+105 observed with NICER and Insight--HXMT
• Optical and X-ray Follow-Up to a Globular Cluster Ultraluminous X-ray Source in NGC 4472
• A large misalignment between continuous jet and discrete ejecta in microquasar GRS 1915+105 during its obscured phase
• Likely detection of magnetic field related LFQPO in the soft X-ray re-brightening of GRS~1915+105
• Spectral study of neutron star low mass X-ray binary source 1A 1744-361
• Exploring the accretion-ejection geometry of GRS 1915+105 in the obscured state with future X-ray spectro-polarimetry