NICER / ISS Science Nugget
for March 4, 2021
NICER probes unusual accretion conditions
X-ray binaries evolve from pairs of stars, wherein one member of the pair is especially massive and leaves behind a neutron star or black hole. Gas flowing onto these compact objects forms a geometrically thin accretion disk if the transfer rate is high, or results in a thicker flow if the transfer rate is low. In both circumstances, the orbiting gas provides an opportunity to study spacetime and matter in extremes that can test fundamental theories. X-ray observations of these special binaries can lead to constraints on the fundamental properties of neutron stars and black holes - their size, mass, spin, etc. - and an understanding of how gas behaves when subjected to strong gravitational and magnetic fields.
MAXI J1848-015 is an X-ray binary system discovered by the JAXA ISS payload MAXI in December 2020; it is located within the GLIMPSE-C01 stellar cluster, about 3.3 kpc (11,000 light-years) from Earth. It is currently unknown whether MAXI J1848 harbors a neutron star or black hole. Observations with NICER, in response to a Target of Opportunity request in late February, have found that the source is accreting at less than 0.03% of its maximum rate (its "Eddington limit"). In this circumstance, theory predicts that a thin accretion disk may be impossible close to the compact object. Disks sometimes expel gas in a wind; if a disk is absent, such outflows should not be seen. Yet, spectroscopy with NICER reveals a complex set of emission lines from partially ionized iron atoms, as reported by Jon Miller (U. of Michigan) et al. in Astronomer's Telegram #14429. They point to the presence of a thin accretion disk and an associated wind. Discoveries like this may force revisions to key theories, or the development of entirely new theories.
Figure: NICER spectrum from the recently discovered X-ray binary MAXI J1848-015, in the region of emission from the K-shell of iron atoms. The top panel shows the measured spectrum; the red trace represents a continuum (i.e., non-line) spectral model that is subtracted to form the spectrum shown in the lower panel, highlighting the narrow emission lines between 6.5 and 7.5 keV in photon energy.