Some brave neutron stars are consuming gas from a big companion star that is about ten times as massive as the tiny neutron star itself. In these so-called Be X-ray binaries, the neutron star is typically in a very wide, eccentric orbit and only able to swallow gas from its companion when it’s making its closest approach. Usually this results in modest accretion outbursts. Occasionally, however, very bright and powerful “giant outbursts” are observed during which the neutron star has a much bigger banquet. X-ray binaries that harbor massive companion stars are younger than those with small companion stars (millions of years compared to billions of years). Furthermore, neutron stars accompanied by a massive star are more strongly magnetized (by a factor of 1000 or so) and rotate much slower (seconds versus milliseconds) than when they are fed by a small companion.

When neutron stars feed off a small companion, their 1-km thick crust is heated and cools on a timescale of years after a meal. This heating and cooling sequence provides very valuable information about the tantalizing interior of neutron stars, but there are many unanswered questions about these processes. For instance, it is not yet established how the spin rate and magnetic field strength of a neutron star affects its temperature changes. Given their widely different properties compared to neutron stars with small companions, searching for heating and cooling in Be X-ray binaries can potentially shed new light on these open questions. Giant outbursts should then provide the best opportunity for this quest, as we might expect a neutron star to become more severely heated (and hence more clearly cooling) when more matter is consumed.

Using the X-ray telescope on board of NASA’s Swift satellite, we followed the behavior two Be X-ray binaries, V0332+53 and 4U 0115+63, after their giant accretion outbursts. Interestingly, we discovered that the X-ray brightness of these neutron stars was higher in the months after they finished their meals than it was before they started eating. This could indeed be a sign of a heated crust! However, neutron stars with massive companions are generally more messy eaters than those with small donors; the quality of our data did not allow us to exclude the possibility that their X-ray emission was elevated because these neutron stars were still scraping for food after finishing their main course. Such behavior is in itself relatively unexplored and interesting to study. Our pilot project therefore warrants follow-up investigation to explore either of these exciting possibilities.

Wijnands & Degenaar 2016, MNRAS Letters 463, L46: Meta-stable low-level accretion rate states or neutron star crust cooling in the Be/X-ray transients V0332+53 and 4U 0115+63

Paper link: ADS

Artist impression of a Be X-ray binary; a neutron star in a wide, eccentric orbit around a massive companion. Image credit: Walt Feimer, NASA/Goddard Space Flight Center.