NICER / ISS Science Nugget
for October 3, 2024

Aql X-1: Past and present accretion outbursts on display

A recent peer-reviewed paper - by K. G. Putha (Univ. of South Carolina) and collaborators in the UK journal Monthly Notices of the Royal Astronomical Society - describes outbursts of the accreting neutron-star binary Aquila X-1 observed by NICER in 2019 and 2020. Within a few weeks of its publication, Aql X-1 began its most recent outburst, which NICER is now tracking intensively. Accretion outbursts progress through states that are named according to whether low- or high-energy (soft or hard) X-ray emission predominates, and their relative strength is quantified through a "hardness ratio" (simply the ratio of X-ray counts detected in higher vs. lower energy bands). NICER observed the entire 2020 outburst, start to finish, but only the high and declining portions of the 2019 outburst. The evolution of Aql X-1's spectrum across states carries information about how the accretion disk fills in as matter is transferred from the companion, how close the edge of the disk comes to the neutron star, when the hot-electron "corona" becomes important, and many other physical clues to the accretion process. For example, the presence of characteristic line emission from fluorescing iron, near 6.4 keV photon energy, can be attributed to "reflection," the illumination and stimulation of atoms in the disk by photons energized within the corona. NICER data from the past outbursts shows that reflection is present during most of the outburst but not at the start or end, presumably because the accretion is far from the hot corona in the vicinity of the neutron star. Similarly, some observations show excess absorption of low-energy X-rays, suggesting that in some circumstances Aql X-1 ejects matter in a "wind" that partially obscures our view of the inner accretion flow.

The current outburst of Aql X-1 has garnered significant attention from observers across the spectrum, and NICER is coordinating its observations with other space-based telescopes as well as ground-based radio and optical facilities.