Abstract
Abstract 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 ≳2 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.
Highlights
The Fermi Large Area Telescope (LAT) source 4FGL J1653.6−0158 is a bright gamma-ray source, and the brightest remaining unassociated source (Saz Parkinson et al 2016). It was first seen by the Energetic Gamma Ray Experiment Telescope (EGRET, Hartman et al 1999), and was listed in the LAT Bright Gamma-ray source list (Abdo et al 2009) more than a decade ago
DISCOVERY OF PSR J1653−0158 THROUGH GAMMA RAYS. These moments determine the ultimate sensitivity to a particular pulse profile and pulsed fraction, as given in Eq (11) in Nieder et al (2020). We extended this dataset to February 23, 2020 (MJD 58,902), using the latest P8R3_SOURCE_V2 instrument response functions (IRFs) (Bruel et al 2018), a larger maximum zenith angle of 105◦, and using the Fermi-LAT Fourth Source Catalog as the RoI model for the photon probability weight computations
The searched ranges are guided by the known millisecond pulsar (MSP) population in the Australia Telescope National Facility (ATNF) Pulsar Catalogue3 (Manchester et al 2005)
Summary
The Fermi Large Area Telescope (LAT) source 4FGL J1653.6−0158 is a bright gamma-ray source, and the brightest remaining unassociated source (Saz Parkinson et al 2016). To identify the neutron star in 4FGL J1653.6−0158, we carried out a binary-pulsar search of the gamma rays, using the powerful GPU-accelerated distributed volunteer computing system Einstein@Home. Such searches are very computationally demanding, and would take decades to centuries on a single computer while still taking weeks or months on Einstein@Home. The companion’s pulsar-facing side is heated by the pulsar wind, leading to a periodically varying optical light curve This permits the orbital period Porb and other orbital parameters to be tightly constrained (for a feasible search the uncertainty ∆Porb needs to be less than a few milliseconds).
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