Thursday, August 12, 2010

Newly discovered pulsar

Today it was announced that Einstein@home has made it's first discovery (a public version of the paper can be found here) - unfortunately not a gravitational wave signal (as it was initial designed to solely search for), but a radio pulsar spinning at 41Hz with the snappy title of J2007+2722. This is still very exciting news as it's "...the first time an astronomical object has been discovered by this kind of distributed-computing project,". In brief pulsars are neutron stars, which are the ultra dense, rapidly rotating, remnants of stars several times more massive than our Sun leftover after they have ended their normal life via a supernova explosion.

The Einstein@home project was set up in 2005 as a distributed computing effort (like the more well known SETI@home, which has a screen saver that searches for extraterrestrial life in radio data) to make use of the public's spare compute cycles to search for gravitational waves from pulsars using data from the LIGO gravitational wave detectors. It's since become one of the largest distributed computing projects there is. The sensitivity of data from the LIGO detectors is currently such that the chances of Einstein@home finding a gravitational waves from an unknown pulsar are quite slim (although more sensitive data in the next few years will give far higher chances), so it was decided a couple of years ago to turn some of Einstein@home's computing power towards searching radio data for pulsars.

Surveys with large radio telescopes are the prime way of finding pulsars (although some can also be seen in other parts of the electromagnetic spectrum) - radio data from these surveys is searched to look for regularly spaced pulses, although these can be weak and the pulse time of arrivals at the telescope will be dispersed over different observation radio frequencies. The spacing of the pulses will also change due to the Doppler effect as the Earth revolves and orbits the Sun, or also if the pulsar is itself in orbit around another star in a binary system. For pulsars in extreme orbits, where the objects are very close together and circling each other with periods of minutes to hours (the current smallest binary orbital period for a pulsar is about 2 hours), the Doppler effect can be very large and cause the pulse spacing to change rapidly. Standard radio pulsar search techniques, which assume that the pulse spacing is only slowly varying, have a hard time time finding these objects. The Arecibo radio telescope has been conducting many surveys over the last few years, but there hasn't been the computing power to exhaustively search this data for these extreme binary pulsar systems. It is data from these surveys that has now been passed to Einstein@home, which is able to use it's large computing power to search for many different sizes of changing pulse spacing, included the rapid changes caused by the extreme systems.

Since starting searches for radio pulsars in Arecibo data with Einstein@home it has been able to find almost 120 pulsars that had previously been known about (although none are in extreme binary systems). However, this new announcement is for a pulsar that had not previously been known about - Einstein@home was the first search to find it! Using this initial discovery they were then able to get follow-up observations using the Green Bank radio telescope to confirm the pulsar signal and further study it. The pulsar itself isn't the most exciting object - it's not in a binary system, but it is reasonably rare as it's an isolated recycled pulsar. A recycled pulsar is one that has been "spun-up" from a slow spin-rate (probably about 1Hz, or one rotation per second) to a much faster rate by accreting material from a companion star (gravitationally pulling material from the other star onto itself). The only way for a pulsar to have a rotation rate as fast as this newly discovered pulsar is either for it to have been "recycled", or for it to still be spinning fast after it was born - this pulsar is slowing down it's rotation rate very slowly indicating it has a weak magnetic field, which generally is expected to not be the case for young, newborn, pulsar i.e. it must be an old, and therefore recycled, pulsar. However, as we saw above for a star to be recycled it must have had a binary companion, which from looking at the pulse spacings we know is not the case for this pulsar - so what's up? There are other known recycled pulsars that are isolated, i.e. not in a binary system, and it is thought that the system must have been disrupted due to the companion star going supernova and kicking it's own remnant out of the systm.

Anyway, this is the first discovery and it's a great boost to the Einstein@home project. Hopefully this will get more people to sign up. It should lead to more pulsar discoveries (maybe at the rate of a couple per year) and possibly some in extremely fast orbits. Ultimately we obviously hope that Einstein@home will also give us some gravitational wave discoveries.