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)

Abstract:

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.