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 (circle dot) and exhibits eclipsing of the pulsar signal. Though the distance to PSR J1853-0842A predicted from its dispersion measure (155.125 +/- 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) x 10(-21) s s(-1). The latter cannot be explained without the acceleration of the globular cluster (GC) and decisively supports the association between PSR J1853-0842A and NGC 6712. Considering the maximum GC acceleration, the Galactic acceleration, and the Shklovskii effect, we place an upper limit on the intrinsic spin-down rate to be 1.11 x 10(-20) s s(-1). From an analysis of the eclipsing observations, we estimate the electron density of the eclipse region to be about 1.88 x 10(6) cm(-3). We also place an upper limit of the accretion rate from the companion at about 3.05 x 10(-13) M (circle dot) yr(-1), which is comparable with some other BWs.

MILLISECOND PULSAR; GLOBULAR-CLUSTERS; DYNAMICAL FORMATION; PARAMETERS; MASS; VARIABILITY; HALO

Shanghai Astronomical Observatory, Chinese Academy of Sciences