Pulsar searches of Fermi -LAT gamma-ray sources with the MWA

Abstract

Searches of unassociated gamma-ray sources in the Fermi -LAT catalogues have led to the discoveries of around a fifth of all known millisecond pulsars (MSPs). These searches have almost exclusively been performed at radio frequencies above 300MHz, where dispersion and scattering in the interstellar medium are less significant. We report on a shallow survey for pulsars targeting 308 unassociated Fermi -LAT sources in archival MurchisonWidefield Array (MWA) observations from the Southern-sky MWA Rapid Two-metre (SMART) pulsar survey at 154MHz. This is the largest radio survey of unassociated Fermi -LAT sources to date, and only the second to be conducted below 300MHz after a survey with the Low Frequency Array (LOFAR) that discovered three MSPs. Each source was observed for 20 min by digitally beamforming the MWA tile voltages. Searches were then performed using a new pipeline that implements a semi-coherent dispersion removal scheme for MWA data, enabling greater sensitivities to MSPs than is possible with fully-incoherent dispersion removal (e.g. 2–3 times better sensitivity for dispersion measures between 20–40 cm ⁻³ pc). The pipeline was tested by blindly detecting five known MSPs, four of which are in short-orbit binaries. No new pulsars were identified in the survey, which we attribute to insufficient sensitivity. We estimate flux density limits of approximately 30–220 mJy at 154MHz (or 0.7–5.2 mJy at 1.4GHz) for a spin period of 2 ms and a duty cycle of 28%, with a dependence on the sky temperature and the offset from the phase-centre of the primary beam. We discuss how the improved instantaneous sensitivity from the Phase III upgrade of the MWA will increase the number of detectable gamma-ray pulsars by ∼ 30% for the same integration time. Additionally, the real-time beamformer (under development) will enable longer observations with sensitivities that are more competitive with previous surveys of Fermi -LAT sources. The semi-coherent search pipeline we have developed will also be useful for searches of supernova remnants, globular clusters, and pulsar candidates identified in imaging surveys, all of which will help to inform the significance of future surveys with SKA-Low.