The X-ray binary 4U 1608-52 contains a neutron star that is normally dormant, but awakens about every 2 years to feast on (i.e., accrete from) its companion star for a few weeks-months before it returns to quiescence again. In October 2014, 4U 1608-52 entered such an accretion outburst and we took the opportunity to observe the neutron star in action using NASA’s newly launched NuSTAR satellite.
NuSTAR was launched in 2012, and provides unprecedented sensitivity at X-ray energies of 3-79 keV. As such, it is an excellent tool to detect X-ray reflection off accretion disks in systems such as X-ray binaries and Active Galactic Nuclei (AGN). Such reflection manifests itself as a broad iron emission line around 6-7 keV, and a “Compton hump” near 20-40 keV. The ability of NuSTAR to detect both these features with excellent sensitivity allows to obtain detailed information about the accretion geometry, such as the inner radius of the accretion disk and the location of the illuminating X-ray source. Such studies are routinely performed for accreting supermassive black holes and stellar-mass black holes in X-ray binaries, but much less is known about the accretion geometry of neutron stars.
The NuSTAR observation of 4U 1608-52 revealed a reflection spectrum of unprecedented quality. This allowed us to match the observational data with detailed theoretical reflection models to infer information about the accretion geometry. Excitingly, we caught the neutron star accreting very slowly, at only about 1% of the Eddington limit. This is a regime that is difficult to capture, and therefore the accretion geometry is particularly uncertain. From our observation, we found that the accretion disk was extending very close to the neutron star, opposed to the expectation that the accretion disk is receding at slow accretion rates. Furthermore, our modeling showed that the X-ray source illuminating the accretion disk (often referred to as the “corona”) was located very close to the neutron star.
Paper link: ADS