Radio Timing and Optical Photometry of the Black Widow System PSR J1953+1846A in the Globular Cluster M71

TL;DR

This study combines radio timing and optical photometry to identify and analyze the optical companion of the black widow pulsar PSR J1953+1846A in globular cluster M71, revealing heating effects and possible intra-binary shock interactions.

Contribution

First optical identification of a black widow pulsar companion in a globular cluster using combined radio and optical data, confirming heating effects and intra-binary shock presence.

Findings

Optical companion is faint, variable, and located close to the pulsar position.

Optical light curve shows heating effects consistent with pulsar wind interaction.

Possible intra-binary shock indicated by X-ray and optical data comparison.

Abstract

We report on the determination of the astrometric, spin and orbital parameters for PSR J1953+1846A, a "black widow" binary millisecond pulsar in the globular cluster M71. By using the accurate position and orbital parameters obtained from radio timing, we identified the optical companion in ACS/Hubble Space Telescope images. It turns out to be a faint (m_F606W>=24, m_F814W>=23) and variable star located at only ~0.06" from the pulsar timing position. The light curve shows a maximum at the pulsar inferior conjunction and a minimum at the pulsar superior conjunction, thus confirming the association with the system. The shape of the optical modulation suggests that the companion star is heated, likely by the pulsar wind. The comparison with the X-ray light curve possibly suggests the presence of an intra-binary shock due to the interaction between the pulsar wind and the material released by the companion. This is the second identification (after COM-M5C) of an optical companion to a black widow pulsar in a globular cluster. Interestingly, the two companions show a similar light curve and share the same position in the color magnitude diagram.