A black widow lurks 3000 light years away
Astronomers love analogies from the animal kingdom. For a fast rotating neutron star, which feeds on its life partner, they have coined the term “black widow”, although neutron stars are of course neither black nor widows. The star from which these pulsars (which otherwise would quickly come to rest on an astronomical scale) draw fresh energy for their rotation is still alive. Normally, such systems – about two dozen are known in the Milky Way alone – are identified by the X-rays and gamma rays that the pulsars emit like celestial lighthouses. However, not every pulsar also radiates in our direction. In fact, it’s much more likely that we’ll never see the high-energy radiation. And since neutron stars are only ten to twenty kilometers across, they cannot be observed with an optical teescope.
An international team of researchers has now come up with a clever idea for finding such black widows anyway: by looking at their victim. The day side of the companion star – the side that is constantly facing the pulsar – should, after all, be many times hotter than its night side, because this is where the pulsar’s high-energy radiation is constantly hitting it. “I figured instead of looking directly for the pulsar, we should look for the star it’s cooking,” explains physicist Kevin Burdge, who co-authored the team’s study. To test their theory, Burdge and his colleagues evaluated optical data taken by the Zwicky Transient Facility, an observatory in California that takes wide-angle images of the night sky. The team examined the brightness of stars to determine whether they changed dramatically by a factor of 10 or more over a period of an hour or less – signs that indicate the presence of a companion star closely orbiting a pulsar.
The team initially found a dozen known black widows using this method, which confirmed their accuracy. But then they discovered a star whose brightness changes by a factor of 13 every 62 minutes – likely making it the poor victim of a black widow. ZTF J1406+1222 is located 3000 light-years from Earth. But that’s not all. In reviewing decades-old measurements of the star from the Sloan Digital Sky Survey, the team found that the system is being tracked by another distant star. According to their calculations, this third star appears to orbit the inner binary star every 10,000 years, like an observer watching the black widow feast from a safe distance.
This is exciting because it takes a lot of imagination to explain the formation of such a system. The astronomical team suggests the following birth story: The triple system probably formed from a dense cluster of old stars, a globular cluster. This particular cluster may have drifted into the center of the Milky Way, where the gravity of the central black hole pulled the cluster apart, leaving the triple black widow intact. “It’s a complicated birth scenario,” Burdge says. “This system has probably been floating around in the Milky Way longer than the Sun.”