Serendipitous Discovery of Three Millisecond Pulsars with the GMRT in Fermi-directed Survey and Follow-up Radio Timing

Abstract

We report the discovery of three millisecond pulsars (MSPs): PSRs J1120−3618, J1646−2142, and J1828+0625 with the Giant Metrewave Radio Telescope (GMRT) at a frequency of 322 MHz using a 32 MHz observing bandwidth. These sources were discovered serendipitously while conducting the deep observations to search for millisecond radio pulsations in the directions of unidentified Fermi Large Area Telescope (LAT) γ-ray sources. We also present phase coherent timing models for these MSPs using ∼5 yr of observations with the GMRT. PSR J1120−3618 has a 5.5 ms spin period and is in a binary system with an orbital period of 5.6 days and minimum companion mass of 0.18 M⊙, PSR J1646−2142 is an isolated object with a spin period of 5.8 ms, and PSR J1828+0625 has a spin period of 3.6 ms and is in a binary system with an orbital period of 77.9 days and minimum companion mass of 0.27 M⊙. The two binaries have very low orbital eccentricities, in agreement with expectations for MSP-helium white dwarf systems. Using the GMRT 607 MHz receivers having a 32 MHz bandwidth, we have also detected PSR J1646−2142 and PSR J1828+0625, but not PSR J1120−3618. PSR J1646−2142 has a wide profile, with significant evolution between 322 and 607 MHz, whereas PSR J1120−3618 exhibits a single peaked profile at 322 MHz and PSR J1828+0625 exhibits a single peaked profile at both the observing frequencies. These MSPs do not have γ-ray counterparts, indicating that these are not associated with the target Fermi LAT pointing emphasizing the significance of deep blind searches for MSPs.