In 1967, Jocelyn Bell, then a graduate student in astronomy at Cambridge University, noticed a strange signal, a series of sharp pulses that came every 1.3 seconds, in the data of her radio telescope that seemed too fast to be coming from anything like a star. Bell and her advisor Anthony Hewish initially thought they might have detected a signal from an extraterrestrial civilization that they named LGM-1, for “Little Green Men.” (It was later renamed.) It turned out not to be aliens, but rather the discovery of the first pulsar.
‘Spider’ Pulsar System
Now, more than 50 years later, an international research team searching for so-called ‘Spider’ pulsar systems – rapidly spinning neutron stars whose high-energy outflows are destroying their binary companion star – is at the core of a gamma ray pulsar known as PSR J2039-5617. The researchers utilized the enormous computing power of the citizen science project Einstein@Home to track down the neutron star’s faint gamma-ray pulsations in data from NASA’s Fermi Space Telescope. Their results show that the pulsar is in orbit with a stellar companion about a sixth of the mass of our Sun. The pulsar is slowly but surely evaporating its companion star. The team also found that the companion’s orbit varies slightly and unpredictably over time.
The discovery, published in the Monthly Notices of the Royal Astronomical Society, details how researchers found a neutron star rotating 377 times a second in an exotic binary system, requiring 10 years of precise data. The pulsars have been given arachnid names of ‘Black widows’ or ‘Redbacks’, after species of spider where the females kill the unfortunate smaller males after mating.
Existence Suspected for Years
“It had been suspected for years that there is a pulsar, a rapidly rotating neutron star, at the heart of the source we now know as PSR J2039-5617,” says Lars Nieder, a PhD student at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute; AEI) in Hannover. “But it was only possible to lift the veil and discover the gamma-ray pulsations with the computing power donated by tens of thousands of volunteers to Einstein@Home,” he adds. Their efforts identified a “black-widow” pulsar in data from the Large Area Telescope on board the Fermi satellite, resolving the nature of a mysterious source, first seen two decades ago by the EGRET satellite.
Two Main Processes at Work
“For J2039-5617, there are two main processes at work,” explains Dr. Colin Clark from Jodrell Bank Centre for Astrophysics, lead author of the study about the system’s variable brightness during an orbital period depending on which side of the neutron star’s companion is facing the Earth. “The pulsar heats up one side of the light-weight companion, which appears brighter and more bluish. Additionally, the companion is distorted by the pulsar’s gravitational pull causing the apparent size of the star to vary over the orbit. These observations allowed the team to get the most precise measurement possible of the binary star’s 5.5-hour orbital period, as well as other properties of the system.”
Einstein@Home –Saves 500 Years
With this information and the precise sky position from Release 2 of Gaia data, reports the University of Manchester, “the team used the aggregated computing power of the distributed volunteer computing project Einstein@Home for a new search of about 10 years of archival observations of NASA’s Fermi Gamma-ray Space Telescope. Improving on earlier methods they had developed for this purpose, they enlisted the help of tens of thousands of volunteers to search Fermi data for periodic pulsations in the gamma-ray photons registered by the Large Area Telescope onboard the space telescope. The volunteers donated idle compute cycles on their computers’ CPUs and GPUs to Einstein@Home.”
The search would have taken 500 years to complete on a single computer core. By using a part of the Einstein@Home resources it was done in 2 months.
The new knowledge of the frequency of the gamma-ray pulsations also allowed collaborators to detect radio pulsations in archival data from the Parkes radio telescope. Their results, also published in Monthly Notices of the Royal Astronomical Society, show that the pulsar’s radio emission is often eclipsed by material that has been blown off the companion star by its nearby Black Widow pulsar.
Image credit: Chandra X-Ray Observatory composite X-ray (red/white) and optical (green/blue) image reveals an elongated cloud, or cocoon, of high-energy particles flowing behind the rapidly rotating pulsar, B1957+20 (white point-like source). The pulsar, a.k.a. the “Black Widow” pulsar, is moving through the galaxy at a speed of almost a million kilometers per hour. A bow shock wave due to this motion is visible to optical telescopes, shown in this image as the greenish crescent shape. The pressure behind the bow shock creates a second shock wave that sweeps the cloud of high-energy particles back from the pulsar to form the cocoon.