X-ray binaries are most easily studied when they are devouring a lot of gas and therefore produce bright radiation at X-ray, UV, optical, infrared and radio wavelengths. During such outburst episodes, the gas that is being stripped off from the companion forms a disk that swirls around the black hole or neutron star. We think that this accretion disk extends very close to the cannibal star, maybe even touching it.
During quiescent episodes, X-ray binaries are consuming much less gas and are therefore orders of magnitude dimmer at all wavelengths. Current theories of accretion prescribe that during quiescence, the accretion disk cannot extend close to the black hole or the neutron star and must lie (tens of) thousands of kilometers away. In between the edge of this disk and the compact star, the gas flow might be very hot and vertically extended. However, because this gas is very tenuous and not producing strong radiation, it is very hard to test this idea with observations.
Accretion theory thus predicts that as an X-ray binary starts to fade from outburst to quiescence, the geometry of the accretion flow is strongly changing. However, it is highly challenging to measure this because 1) it’s not predictable exactly when X-ray binaries transition to quiescence, 2) once they do, the transition happens very rapidly, typically on a timescale of 1-2 weeks, and is therefore easy to miss, and 3) as X-ray binaries decay into quiescence they become increasingly dim and therefore long/sensitive observations are needed in order to study the properties of the accretion stream. Ready to take up this challenge, we recently designed an ambitious observing campaign aimed to reveal the changing accretion flow in an X-ray binary called 4U 1608-52.
One powerful way to measure the location of the inner edge of an accretion disk is X-ray reflection. This produces prominent emission features at certain X-ray energies that can be studied with sensitive X-ray telescope. NuSTAR is one of the telescopes that is optimally suited to study X-ray reflection. In a previous study, we observed the X-ray binary 4U 1608-52 during one of its accretion outbursts. Our NuSTAR observation revealed a beautiful X-ray reflection spectrum that allowed us to determine that the accretion disk was extending very close to the neutron star. Taking advantage of the fact that 1) this X-ray binary goes into outburst once every few years (opposed to some sources for which we have observed only 1 outburst over 5 decades), and 2) it produces a very strong reflection spectrum, we designed an observing campaign to capture the source again with NuSTAR, but then at a factor ~10 lower luminosity. The main aim was to use the reflection spectrum to determine if with this change in brightness, the accretion geometry changes a lot.
When our target 4U 1608-52 was seen to enter a new accretion outburst in 2018, we closely monitored how its brightness evolved by looking at the data obtained with the Japanese MAXI X-ray telescope that is installed on the International Space Station. Once MAXI showed that the X-ray brightness of 4U 1608-52 was decreasing, we performed observations with the much more sensitive Swift and NICER telescopes. This allowed us to continue watching our target once it became too faint to be detected with MAXI. We analyzed each new Swift observation immediately after it was performed and tried to predict how the brightness of our target would decay onward. With a few days lead time, we then triggered our NuSTAR observation, hoping that it would observe our target at exactly the right time.
After an intense 2 weeks of watching 4U 1608-52 closely every day, we succeeded to have NuSTAR point to our target at exactly the right time. Interestingly, our new NuSTAR observation showed that with a factor ~10 change in brightness, the reflection spectrum completely disappeared. We think that this is because the accretion flow is changing from a (flat) disk into a hot (spherical) structure. This constitutes one of the very few observations that supports standard accretion models, so we are very excited about these results.
van den Eijnden, Degenaar, Ludlam, Parikh, Miller, Wijnands, Gendreau, Arzoumanian, Chakrabarty, Bult, submitted to MNRAS: A strongly changing accretion morphology during the outburst decay of the neutron star X-ray binary 4U 1608-52