NICER / ISS Science Nugget
for December 19, 2024

Coordination with NuSTAR enabling NICER black-hole studies

The black-hole X-ray binary (BHXRB) MAXI J1803-298 was first discovered in a months-long outburst in May 2021. It exhibited many of the hallmarks of BHXRB outbursts - e.g., evolution through the hard, intermediate, and soft accretion states, referring to the dominance of high-energy (hard) and low-energy (soft) X-ray photons in the emitted spectrum - as well as a few less common features, such as high-speed "wind" outflows of material from the disk of accreting matter around the black hole. A peer-reviewed paper recently published by O. Adegoke (Caltech) and collaborators in The Astrophysical Journal describes these phenomena using data acquired nearly simultaneously with NICER and NASA's NuSTAR telescope (which is sensitive to higher-energy X-rays) at multiple points during the outburst.

Adegoke et al. report the detection, during the initial hard accretion state, of short-lived dips in the X-ray brightness of MAXI J1803 recurring at intervals of approximately 7 hours. Based on analysis of the X-ray spectrum measured during these dips, and by drawing similarities to dipping behavior seen in other X-ray binary systems, the authors conclude that the 7-hour timescale represents the orbital period of the binary and that the dips are likely due to absorption of X-rays - originating in the hot accretion disk close to the black hole - by material in the vicinity of the companion star, likely the result of intense irradiation that expands and liberates some of the material in the star's outer layers. The team notes that this absorption is distinct from that seen (and independently reported in another paper) in later accretion stages that is evidence for high-speed, blue-shifted winds from the outer part of the accretion disk.

Another aspect of the Adegoke spectral analysis probes the origin of X-ray "reflection" from the disk: in addition to the disk's own thermal glow, some atoms within the hot flow are often found to fluoresce at X-ray energies characteristic of iron and other elements. What stimulates this fluorescence can vary from system to system, depending on local conditions such as the black hole's spin (and many other factors), and may include emission from a high-temperature, magnetized plasma called the "corona" or, to some extent, even outgoing X-rays from the disk itself that are bent back by the black hole's strong gravity and return to stimulate other parts of the disk. Adegoke et al. find the reflection spectrum of MAXI J1803 indicates that approximately 21% of the reflection-stimulating radiation is due to just such "self-irradiation" from gravitational light-bending.