2FGL J1311.7-3429 Joins the Black Widow Club

TL;DR

This paper identifies a likely black widow millisecond pulsar with a 1.56-hour orbit, exhibiting optical modulation, flares, and potential gamma-ray pulsations, suggesting it may be the shortest orbital period known for such objects.

Contribution

The discovery of a candidate black widow pulsar with an extremely short orbital period and detailed multi-wavelength characterization is a novel addition to pulsar studies.

Findings

Optical modulation with a 1.56-hour period observed.

Optical flares superimposed on the modulation.

Potential gamma-ray pulsations indicating a millisecond pulsar.

Abstract

We have found an optical/X-ray counterpart candidate for the bright, but presently unidentified, Fermi source 2FGL J1311.7-3429. This counterpart undergoes large amplitude quasi-sinusoidal optical modulation with a 1.56h (5626s) period. The modulated flux is blue at peak, with T_eff ~14,000K, and redder at minimum. Superimposed on this variation are dramatic optical flares. Archival X-ray data suggest modest binary modulation, but no eclipse. With the gamma-ray properties, this appears to be another black-widow-type millisecond pulsar. If confirmation pulses can be found in the GeV data, this binary will have the shortest orbital period of any known spin-powered pulsar. The flares may be magnetic events on the rapidly rotating companion or shocks in the companion-stripping wind. While this may be a radio-quiet millisecond pulsar, we show that such objects are a small subset of the gamma-ray pulsar population.