Over last week, NuSTAR observed the stellar-mass black hole GRS 1915+105, which resides in the Milky Way Galaxy. A fraction of massive black holes in active galaxies are largely hidden from view by intervening gas. In most cases, this is likely due to obscuration by a distant reservoir of cold gas and dust; however, a fraction of cases may be hidden by gas that is much closer to the black hole. In surveys, such black holes can only be directly detected via hard X-ray emission above 15 keV and indirectly detected using neutral iron emission lines at an energy of around 6.5 keV. GRS 1915+105 is a stellar-mass black hole in the Milky Way that was originally famous for being bright, but is now highly obscured, like some massive black holes in the centers of distant galaxies. NuSTAR accepted a Director's Discretionary Time request to observe the stellar-mass black hole GRS 1915+105 simultaneously with JAXA/NASA/ESA’s XRISM mission. The pairing of the two missions delivers exactly the hard X-ray sensitivity and sharp line response needed to study how and why black holes become obscured. Starting in January 2026, scientists will be able to submit joint observing proposals with XRISM observing time made available in the NuSTAR General Observer (GO) program, and a reciprocal agreement for NuSTAR time available in XRISM GO Cycle 3. In the future, surveys of obscured black holes with NuSTAR and XRISM will advance our understanding of black hole fueling, and how much accretion power is hidden from view in the local Universe.

Author: Jon Miller (Professor of Astronomy, University of Michigan), Daniel Stern (NuSTAR Deputy PI, Caltech)