474230main_Pulsar_Binary_Mod_1A few days ago (Feb. 5th) I wrote a post about a special source, named XSS J12270-4859, a cryptic name behind a binary system composed of a neutron star and a normal companion star slightly less massive than our Sun.

The special property of this system is that it belongs to a new family of binaries in which the neutron star is seen to switch between two different states. In the first state, the neutron star is a pulsar and the companion is destroyed by the deadly radiation emitted by the compact object. In the second state, the neutron star is stripping gas from its companion and an accretion disk is formed. The neutron star is called a radio millisecond pulsar in the former case, whereas it is a low mass X-ray binary in the latter (see also “The Mutant Star” for another system of the same kind).

In a recent paper, my team and I reported conclusive evidence that this binary had indeed changed state in December 2012. The most convincing explanation is that the source switched from a low-mass X-ray binary into a millisecond radio pulsar. However, despite our intense efforts to observe this source at radio wavelengths with the Parkes radio telescope, no millisecond pulsar was found. This did not particularly concern us for a number of reasons. For example, XSS J12270-4859 was expected to be a so-called “red-back” (from the name of a venomous spider) where pulsations are very often absorbed by a thick layer of gas surrounding the companion star. This material comes from the ablation process, which means that the pulsar is producing so much energy that it can make the companion star evaporate until it is completely destroyed (this is why the pulsar is called, by analogy, a red-back). The material lifted from the companion forms a cloud that absorbs radio waves and so the radio pulsations are often missed by our telescopes.

However, on February 15th, a group of researchers discovered very convincing pulsations from XSS J12270-4859 at a period of 1.69 ms!!! The most likely reason why they found the pulsations, whereas we did not, is that they used a different instrument (the Giant Meter Radio Telescope) which operated at a lower frequency than the Parkes telescopes. At those low frequencies, the signal was found to be stronger (as expected). This is a very exciting discovery because it confirms our expectations that indeed this source has turned on as a radio millisecond pulsar. We will now see whether more radio pulsations will be detected, as we hope. If they are, then we might be able to follow the evolution of this pulsar and understand more details of the puzzling properties of these deadly spiders in disguise.