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)
