Discovery of an unidentified Fermi object as a black widow-like millisecond pulsar

TL;DR

This paper reports the discovery of a gamma-ray emitting millisecond pulsar that is radio-quiet, likely a black widow system with a heated companion, challenging previous assumptions about MSP radio visibility.

Contribution

It presents the first candidate of a radio-quiet gamma-ray millisecond pulsar with multi-wavelength observations, expanding the known diversity of MSP systems.

Findings

Discovered a gamma-ray MSP candidate with no radio detection.

Identified a 4.63-hour orbital period indicating a black widow-like system.

Estimated the MSP's spin period to be 3-5 ms based on gamma-ray luminosity.

Abstract

The Fermi Gamma-ray Space Telescope has revolutionized our knowledge of the gamma-ray pulsar population, leading to the discovery of almost 100 gamma-ray pulsars and dozens of gamma-ray millisecond pulsars (MSPs). Although the outer-gap model predicts different sites of emission for the radio and gamma-ray pulsars, until now all of the known gamma-ray MSPs have been visible in the radio. Here we report the discovery of a "radio-quiet" gamma-ray emitting MSP candidate by using Fermi, Chandra, Swift, and optical observations. The X-ray and gamma-ray properties of the source are consistent with known gamma-ray pulsars. We also found a 4.63-hr orbital period in optical and X-ray data. We suggest that the source is a black widow-like MSP with a ~0.1 solar-mass late-type companion star. Based on the profile of the optical and X-ray light-curves, the companion star is believed to be heated by the pulsar while the X-ray emissions originate from pulsar magnetosphere and/or from intra-binary shock. No radio detection of the source has been reported yet and although no gamma-ray/radio pulsation has been found, we estimated that the spin period of the MSP is ~3-5 ms based on the inferred gamma-ray luminosity.