Identification of the Optical Counterpart of Fermi Black Widow Millisecond Pulsar PSR J1544+4937

TL;DR

This paper reports the optical identification of the companion to the Fermi black widow millisecond pulsar PSR J1544+4937, revealing variable optical emission likely from a hot patch on the companion's surface, with implications for understanding the system.

Contribution

First optical identification of the companion to PSR J1544+4937, showing complex variability and challenging simple irradiation models, and providing initial distance constraints.

Findings

Detected a highly variable optical source at the pulsar's position.

Observed nearly achromatic light curves inconsistent with simple models.

Estimated the system's distance to be between 2 and 5 kpc.

Abstract

We report the optical identification of the companion to the {\it Fermi} black widow millisecond pulsar PSR J1544+4937. We find a highly variable source on Keck LRIS images at the nominal pulsar position, with 2 magnitude variations over orbital period in the B, g, R, and I bands. The nearly achromatic light curves are difficult to explain with a simply irradiated hemisphere model, and suggest that the optical emission is dominated by a nearly isothermal hot patch on the surface of the companion facing the pulsar. We roughly constrain the distance to PSR J1544+4937 to be between 2 and 5 kpc. A more reliable distance measurement is needed in order to constrain the composition of the companion.