An Eclipsing Black Widow Pulsar in NGC 6712

Abstract

We report the discovery of the first radio pulsar associated with NGC 6712, an eclipsing black widow (BW) pulsar, J1853$-$0842A, found by high-sensitivity searches using the Five-hundred-meter Aperture Spherical radio Telescope. This 2.15 ms pulsar is in a 3.56 hr compact circular orbit with a very low mass companion likely of mass 0.018 to 0.036 $M_{\odot }$ and exhibits eclipsing of the pulsar signal. Though the distance to PSR J1853$-$0842A predicted from its dispersion measure ($155.125\pm 0.004$ cm${}^{-3}$ pc) and Galactic free electron density models are about 30\% smaller than that of NGC 6712 obtained from interstellar reddening measurements, this is likely due to limited knowledge about the spiral arms and Scutum stellar cloud in this direction. Follow-up timing observations spanning 445 days allow us to localize the pulsar's position to be 0.14 core radii from the center of NGC 6712 and measure a negative spin-down rate for this pulsar of $-2.39(2)\times 10^{-21}\mathrm{s}{\mathrm{s}}^{-1}$. The latter cannot be explained without the acceleration of the GC and decisively supports the association between PSR J1853--0842A and NGC 6712. Considering the maximum GC acceleration, Galactic acceleration, and Shklovskii effect, we place an upper limit on the intrinsic spin-down rate to be $1.11\times 10^{-20}\mathrm{s}{\mathrm{s}}^{-1}$. From an analysis of the eclipsing observations, we estimate the electron density of the eclipse region to be about $1.88\times 10^6\mathrm{c}{\mathrm{m}}^{-3}$. We also place an upper limit of the accretion rate from the companion is about $3.05\times 10^{-13}{M}_{\odot }\mathrm{y}{\mathrm{r}}^{-1}$ which is comparable with some other BWs.

11 pages, 5 figures, 3 tables; Accepted by ApJ

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