Einstein@Home probes the Galactic center gamma-ray glow
The volunteer computing project searched for members of a proposed, hidden millisecond pulsar population near the center of the Milky Way.
Shortly after its launch, the Large Area Telescope (LAT) onboard NASA’s Fermi Gamma-ray Space Telescope detected an excess of gamma-ray emissions in the GeV energy range from the center of our galaxy. As of today, its origin remains unknown. One leading theory proposes that the emission is the combined gamma-ray output of a large population of millisecond pulsars near the Galactic Center. Now, an international team led by researchers at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute) in Hanover, Germany, has used their distributed volunteer computing project, Einstein@Home, to search for members of this proposed pulsar population. The team selected 55 unidentified pulsar-like sources from the Fermi-LAT 8-year source catalog. They identified four previously unknown gamma-ray pulsars among them, including the third millisecond pulsar ever discovered through its gamma-ray emission. The researchers used the full 16-year Fermi-LAT data set to characterize the pulsars. Follow-up searches in the radio band did not reveal any of the new pulsars. The team also searched for and set upper limits on the continuous gravitational-wave emission from the two fastest-rotating pulsars using public LIGO data. Although one of their discoveries is the pulsar with the smallest known angular separation from the Galactic Center of any gamma-ray pulsar, the team found that the four pulsars are most likely significantly closer to Earth than to the center of the Milky Way. Therefore, they are probably members of the standard Galactic disk population. This finding does not rule out the presence of a Galactic Center population. A new survey with a longer data set, a new source selection, and an improved data analysis may help uncover this population in the future.
Paper abstract
The Fermi Large Area Telescope (LAT) has revealed a mysterious extended excess of GeV gamma-ray emission around the Galactic Center, which can potentially be explained by unresolved emission from a population of pulsars, particularly millisecond pulsars (MSPs), in the Galactic bulge. We used the distributed volunteer computing system Einstein@Home to search the Fermi-LAT data for gamma-ray pulsations from sources in the inner Galaxy, to try to identify the brightest members of this putative population. We discovered four new pulsars, including one new MSP and one young pulsar whose angular separation to the Galactic Center of 0.93° is the smallest of any known gamma-ray pulsar. We demonstrate a phase-resolved difference imaging technique that allows the flux from this pulsar to be disentangled from the diffuse Galactic Center emission. No radio pulsations were detected from the four new pulsars in archival radio observations or during the MPIfR-MeerKAT Galactic Plane Survey. While the distances to these pulsars remain uncertain, we find that it is more likely that they are all foreground sources from the Galactic disk, rather than pulsars originating from the predicted bulge population. Nevertheless, our results are not incompatible with an MSP explanation for the GC excess, as only one or two members of this population would have been detectable in our searches.
