Fermi Gamma-ray Space Telescope

Searching a Thousand Radio Pulsars to Find Faint Gamma-ray Pulsations

Matthew Kerr
(D.A. Smith, P. Bruel, L. Guillemot, M. Keith, and S. Johnston, on behalf of the Fermi LAT Collaboration and Pulsar Timing Consortium)


In its 11th year after launch, the rate at which the Large Area Telescope (LAT) on the Fermi satellite discovers gamma-rays from pulsars remains constant, at about 24 per year. The more recent ones tend to be fainter and fainter in gamma-rays, partly because the LAT's all-sky survey accumulates deeper and deeper exposure, but also because of analysis improvements. In particular, a new, highly simplified way to "weight" gamma-ray photons works well even when a pulsar's signal-to-noise is too low to determine its position and spectrum reliably, or even to detect it as a phase-averaged point source. "Weighting" means, combining the photon's energy and direction with the LAT's energy-dependent PSF to find the probability that it came from a known pulsar position. The probability is used to weight the photon rotational phases folded into a histogram. We used over a thousand rotation ephemerides obtained mainly from the Parkes, Lovell, and Nançay radio telescopes to search for gamma-ray pulsations. We report 16 discoveries of gamma-ray pulsations: 4 are millisecond pulsars, the rest are unrecycled. One, PSR J2208+4056, has a spindown power of 8E32 erg/s, 3 times lower than the previously observed "deathline" value. The large number of ephemerides combined with the fast weighting method allows us to sample not just a larger space volume and higher-background regions, but also neutron star magnetosphere configurations that generate broad and/or faint beams, thus reducing the selection bias compared to previous searches.