Neutron stars are the densest directly observable stellar objects in our universe and constitute ideal astrophysical laboratories to study matter under extreme physical conditions: immense gravitational fields, ultra-strong magnetic fields, vigorous radiation fields, and supra-nuclear densities. Many observable properties of neutron stars are set by the structure and composition of their crust. A promising way to investigate the properties of these outer stellar layers is to study neutron stars in low-mass X-ray binaries.

In these interacting binary star systems, the neutron star pulls off and accretes matter from a companion star that has a mass comparable to (or lower than) that of our Sun. Many of such X-ray binaries are transient and exhibit outbursts of accretion that last only a few weeks. These are interleaved by years-long episodes of quiescence during which little or no matter is being accreted onto the neutron star. During these quiescent episodes the thermal heat radiation from the glowing neutron star surface becomes visible. This effectively serves as a thermometer of the neutron star and provides a powerful probe of its interior properties.

Sensitive X-ray instruments aboard the Swift, Chandra and XMM-Newton satellites have revealed that outbursts of accretion can severely affect a neutron star’s temperature. Using sophisticated and tailored observing strategies, it has been shown that it causes the crust of a neutron star to be heated to millions of degrees Kelvin. This heat is produced in a cascade of nuclear reactions, including fusion of atomic nuclei (due to the high matter density), and other chemical processes. Once neutron stars stop swallowing matter, the crustal layers slowly cool until they return to their pre-outburst temperature after several years. Both the heating and the cooling encode unique information about the structure and composition of the neutron star’s crust.

With the aim to study the cooling-down of the neutron star in an X-ray binary called XTE J1709-267, we targeted this object in 2013 September shortly after it exhibited an accretion outburst, using the Swift and XMM-Newton satellites. We were expecting to see a gradual fading over the course of several years. Much to our surprise, however, we found that the thermal radiation from the neutron star was rapidly fading during our 8-hour long XMM-Newton observation. An intriguing explanation for this is that we were witnessing fast cooling of the very outer layers of the neutron star in real-time.

If this interpretation is correct, the time-scale of the observed decay places new, strong constraints on the amount of heat that was generated inside the neutron star, and at which depth. When taken at face value, the findings indicate that one particular process that causes atomic nuclei to separate out in different layers, is important in the heat generation. This has important implications for our understanding of the crust structure of accreting neutron stars. A plausible alternative explanation is that the rapid fading was caused by a rapid reduction of the matter supply onto neutron star, hence that our observations tracked the cessation of the accretion flow marking the end of the outburst.

Degenaar, Miller, Wijnands 2013, ApJ Letters 767, L31: A Direct Measurement of the Heat Release in the Outer Crust of the Transiently Accreting Neutron Star XTE J1709-267

Paper link: ADS

An artist impression of an X-ray binary.
Credit: Stuart Littlefair.