My colleagues and I have recently studied a new peculiar binary system (XSS J12270-4859) formed by a neutron star and a low mass star that can be found in the constellation Centaurus, in the southern hemisphere. Our recent work, lead by Cees Bassa from ASTRON, can be found online at this link.
XSS J12270-4859 has been discovered in 2004 and classified as a low mass X-ray binary in 2009, i.e., a binary in which the companion star is lighter than our Sun and that is losing its outer layers of gas because of the deadly gravitational attraction of a neutron star. This system would be a quite standard X-ray binary like many others we know in our galaxy, but it shows two features that make this system a quite peculiar one.
The first is that XSS J12270-4859 emits very energetic gamma-ray radiation. This fact is quite extraordinary among low mass X-ray binaries as most of the photons emitted by the neutron star are powered by a phenomenon known as accretion: the gas stripped off the outer layers of the companion falls onto the neutron star and in the process it becomes tremendously hot and radiates X-ray photons. Despite the immense temperatures reached by the gas, the gamma ray photons are too energetic to be produced in the accretion process and usually require the existence of shocked plasma and highly relativistic streams of particles. Indeed there is only one other peculiar low mass X-ray binary known to emit such high energy radiation and the origin of gamma rays is unknown there too.
I have already discussed about such other peculiar low mass X-ray binary in this blog (“Missing Link Pulsar”): this is a system that transitions back and forth between a low mass X-ray binary and a radio millisecond pulsar state (see also “The Mutant Star” for another similar beast). When it is a radio millisecond pulsar, the system is classified as a “red-back”, which is a venomous deadly Australian spider known for its…well…red-back. Such scary name indicates that the pulsar is ferociously destroying its companion star by irradiating it with high energy particles and driving gas off its outer layers. We have now discovered that XSS J12270-4859 has also this other peculiarity: it behaves in a very similar way as the missing link pulsar, i.e., the system switches between a low mass X-ray binary phase and a very dim state where no accretion takes place (and the pulsar turns presumably on at radio wavelengths). However, despite a deep observational campaign that we have done with the Parkes radio telescope, the radio pulsations have not been detected as of today and so the rotational rate of the neutron star remains unknown. So it seems that XSS J12270-4859 might well be a “red-back” pulsar in disguise…
An interesting fact is that the transition has been caught in December 2012 thanks to the help of an amateur astronomer (Berto Monard, from the Center for Backyard Astronomy) who has observed and recorded the optical counterpart of XSS J12270-4859 for several years and was able to catch the exact moment of the transition. We then started an intense observational campaign to understand what had happened to XSS J12270-4859 and we observed the system with the X-Ray Telescope and the Ultra-Violet and Optical Telescope (XRT and UVOT) aboard the Swift satellite, the Optical Monitor aboard the XMM-Newton space telescope and the New Technology Telescope (NTT) which is a 3.5 meter optical telescope part of the La Silla observatory operated by ESO in Chile . With these observations (plus some archival data) we were able to track the behavior of this binary in the past two years and we indeed saw that the X-rays coming from accretion abruptly decreased at the same time as the optical transition observed by Berto Monard (December 2012). The optical NTT data also showed that after December 2012 there is no signature of an accretion disc in the optical spectrum. Thanks to the optical observations collected by Swift an XMM-Newton we were also able to improve a previous determination of the binary orbital period and infer a very precise orbit for the companion around the neutron star (one revolution every 6.913 hours).
In the past few weeks we have also observed the source in X-rays with the XMM-Newton and Chandra X-ray Observatory. A detailed analysis and new exciting findings will be posted in the very next future, so stay tuned !