Pulsars are the dense, rapidly spinning remains of stars much more massive than the Sun. To really get a pulsar revolving quickly, it needs a companion star: matter stripped from the partner falls onto the pulsar, speeding it up until it can rotate hundreds of times every second. Astronomers discovered these millisecond pulsars by their radio emissions, but many of them are also very strong gamma ray sources.

Astronomers have now used the Fermi Gamma-Ray Space Telescope to identify a "black widow" pulsar that's stripping mass off a close companion star while simultaneously evaporating it by emitting intense radiation. It's having these dramatic effects because the pulsar and its companion orbit each other so closely that they complete an orbit once every 93 minutes, making this the tightest black widow binary yet discovered.

Many radio telescopes are well suited to hunting for pulsars, since they can scan large parts of the sky and pick out the periodic flashes of light these bodies emit. Gamma ray observations are harder for a number of reasons, not least of which is that Earth's atmosphere is (thankfully) opaque to gamma radiation (which means we need space-based telescopes). The only existing gamma ray instrument with the sensitivity to spot millisecond pulsars is the Large Area Telescope (LAT) on the orbiting Fermi observatory.

To find pulsars in the maps provided by LAT, Holger Pletsch and colleagues sifted through nearly four years of data using a "blind search," sorting through all the data without targeting a specific source. That method would allow them to identify sources that might be faint or otherwise difficult to spot, but it was also computationally intensive. They began by taking 12-day chunks of time from four years of LAT data, looking for periodic fluctuations.

The hunt was complicated by the nature of millisecond pulsars themselves: they fluctuate on a very short time scale, and the specific physical parameters—rate of pulsation, amount of change in that rate, and the properties of the binary system—cannot be known in advance. In other words, it's not as easy as looking for a flashing light on a dark night: it's more like scanning grainy film footage one frame at a time, looking for the same flashing light.

Despite these difficulties, the researchers found a clear candidate, now known as PSR J1311-3430. They measured the pulsar's rotational period to be about 2.5 milliseconds, meaning it spins 400 times every second. In fact, the pulsar corresponded to a bright gamma ray source (2FGL J1311-3430 for you gamma ray enthusiasts) seen in the Fermi all-sky map. Since this object (along with many others) had no corresponding radio emission, it was considered an unidentified source.

Using the Fermi data and optical observations, the astronomers determined the companion star was extremely close to the pulsar, with the binary completing their mutual orbit in 93 minutes. Such a tight orbit means that not only is the pulsar accreting mass from its companion to increase its spin rate, but the intense radiation from the pulsar is blasting away the outer layers of the star. The pulsar heated up its companion so much that the low-mass star has inflated to a large size. This type of system is known as a black widow, because the pulsar is devouring its mate.

The particulars of the system also explained why there was no identifiable radio source at the same location. The wild winds of material stripped from the companion star would tend to scatter and obscure radio emission, so even if the pulsar was emitting radio pulses (a very likely scenario), they wouldn't be coherent as seen from Earth.

All of this leads to hope that other bright, unidentified gamma ray sources might also be millisecond pulsars. While the hunting process is intensive and time-consuming, the researchers expressed optimism, not least since many unknown objects are known and can provide immediate targets rather than using blind searches. Furthermore, such binary systems provide tests of the most extreme physics, including gravitational radiation and the properties of the pulsars themselves.

Science, 2012. DOI: 10.1126/science.1229054 (About DOIs).