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

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. © 2022. The Author(s). Published by the American Astronomical Society

Discovery of a gamma-ray black widow pulsar by GPU-accelerated Einstein@Home

We report the discovery of 1.97 ms period gamma-ray pulsations from the 75 minute orbital-period binary pulsar now named PSR J1653−0158. The associated Fermi Large Area Telescope gamma-ray source 4FGL J1653.6−0158 has long been expected to harbor a binary millisecond pulsar. Despite the pulsar-like gamma-ray spectrum and candidate optical/X-ray associations—whose periodic brightness modulations suggested an orbit—no radio pulsations had been found in many searches. The pulsar was discovered by directly searching the gamma-ray data using the GPU-accelerated Einstein@Home distributed volunteer computing system. The multidimensional parameter space was bounded by positional and orbital constraints obtained from the optical counterpart. More sensitive analyses of archival and new radio data using knowledge of the pulsar timing solution yield very stringent upper limits on radio emission. Any radio emission is thus either exceptionally weak, or eclipsed for a large fraction of the time. The pulsar has one of the three lowest inferred surface magnetic-field strengths of any known pulsar with B surf ≈ 4 × 107 G. The resulting mass function, combined with models of the companion star's optical light curve and spectra, suggests a pulsar mass gsim2 M ⊙. The companion is lightweight with mass ~0.01 M ⊙, and the orbital period is the shortest known for any rotation-powered binary pulsar. This discovery demonstrates the Fermi Large Area Telescope's potential to discover extreme pulsars that would otherwise remain undetected

Mass estimates from optical modelling of the new TRAPUM redback PSR J1910−5320

Spider pulsars continue to provide promising candidates for neutron star mass measurements. Here we present the discovery of PSR J1910−5320, a new millisecond pulsar discovered in a MeerKAT observation of an unidentified Fermi-LAT gamma-ray source. This pulsar is coincident with a recently identified candidate redback binary, independently discovered through its periodic optical flux and radial velocity. New multicolour optical light curves obtained with ULTRACAM/New Technology Telescope in combination with MeerKAT timing and updated SOAR/Goodman spectroscopic radial velocity measurements allow a mass constraint for PSR J1910−5320. icarus optical light curve modelling, with streamlined radial velocity fitting, constrains the orbital inclination and companion velocity, unlocking the binary mass function given the precise radio ephemeris. Our modelling aims to unite the photometric and spectroscopic measurements available by fitting each simultaneously to the same underlying physical model, ensuring self-consistency. This targets centre-of-light radial velocity corrections necessitated by the irradiation endemic to spider systems. Depending on the gravity darkening prescription used, we find a moderate neutron star mass of either 1.6 ± 0.2 or 1.4 ± 0.2 M⊙. The companion mass of either 0.45 ± 0.04 or $0.43^{+0.04}_{-0.03}$M⊙ also further confirms PSR J1910−5320 as an irradiated redback spider pulsar