NICER / ISS Science Nugget
for November 9, 2023

A squeezed disk

Accretion, the process of mass transfer onto a dense object -- a neutron star or black hole -- still presents a number of open questions, not least of which is the influence of the accreting object itself. Lacking a hard surface, black holes presumably allow matter to flow in an organized fashion (a disk of hot gas) right down to the vicinity of the event horizon (also known as the "gravitational radius," Rg), where gravity is so strong that nothing can escape. Neutron stars, on the other hand, have surfaces at radii larger than their corresponding event-horizon size, and they also typically have strong magnetic fields that can either channel or, in some cases, repel the flow of matter. Comparing the accretion processes in black hole vs. neutron stars binaries via their X-ray emissions, especially for a variety of companion-star types and orbit sizes, is valuable in assessing the relative importance of these differences.

So-called ultracompact binaries are a rare class that promise especially useful insights. NICER and NASA's NuSTAR telescope, which is sensitive to higher-energy X-rays, simultaneously observed the neutron-star binary system 4U 0614+091 (50-minute orbital period) four times from October 2021 to January 2022. Results from this General Observer investigation were recently described in a peer-reviewed paper by D. Moutard (Wayne State Univ.) and collaborators, published in The Astrophysical Journal. The team analyzed the datasets, which overlap in X-ray energy, using a technique known as "reflection spectroscopy", which employs a set of models to account for all known sources of X-ray emission from the tight binary. The accretion disk is necessarily small in such systems where the two stars are so close together, and the disk has its own warm glow in X-rays. Most importantly, much-higher-energy X-rays from a poorly-defined "corona" region are thought to illuminate the gas flowing in the disk, and are reprocessed there -- we observe both the direct coronal emission and the "reflected" emission from the disk. Because the disk material is moving rapidly and is in a deep gravitational well, much of the reflected emission is red-shifted and broadened in energy; the details of these modifications allow our models to recover information about the disk, most notably the radius of its inner edge.

In the 4U 0614 study, the investigators found that in three of the four observations the disk's inner edge was at essentially its innermost stable point, at 6 gravitational radii, but that in the remaining observation the disk was "truncated" -- i.e., its inner edge was further out -- at 11.5 Rg. This truncation occurred during the observation in which the overall brightness of 4U 0614 was at its lowest, while the coronal component by itself was at its brightest. An estimate of the neutron star's magnetic field strength suggests that it was not responsible for repelling the flow in the disk, but instead that some other mechanism, probably related to the input of matter into the disk from the companion star, was responsible for the pulling back of the disk. Such behavior, disk truncation during low-flux states, is commonly seen in black-hole binaries, suggesting a commonality in the accretion process irrespective of the nature of the accreting star.