Discovery and Timing of the First Millisecond Pulsar in NGC 6316

Abstract

NGC 6316 is a poorly studied, distant, and massive globular cluster (GC) with prominent gamma-ray emission detected via the \textit{Fermi} Large Area Telescope (LAT). Based on gamma-ray spectral studies, NGC 6316 is expected to host tens of millisecond pulsars (MSPs). Using the Green Bank Telescope (GBT) and Murriyang, CSIRO's Parkes radio telescope (Parkes), we present the discovery and a 3.1 yr duration timing solution of the first millisecond pulsar found in the cluster. PSR J1716$-$2808A has a rotational period of 2.45\,ms and is in a binary with a $\sim$0.1 M${}_{\odot }$ companion with an orbital period of 0.42 d. This is a typical cluster MSP within a compact orbit with no evidence of eclipses. PSR J1716$-$2808A has a dispersion measure DM = 172.26 pc cm${}^{-3}$, which is lower than predicted NE2001, YMW16 and NE2025 electron density model values. The MSP is located within half a core radius from the cluster center and has a negative period derivative, implying that it is on the back side of the cluster and is being accelerated towards us. Given the negative period derivative, we report an upper limit on the maximum line-of-sight cluster acceleration, $a_{l,\text{GC}}/c\approx -2.3\times 10^{-18}$ s${}^{-1}$, experienced by the pulsar and constraints on the magnetic field to be $<\sim$3$\times$10${}^8$ G. The presence of external acceleration strongly supports the pulsar to be within NGC 6316. We can better constrain NGC 6316's properties through longer-term timing of PSR~J1716$-$2808A or by finding more pulsars within the cluster. Based on the gamma-ray pulsar estimates and a cluster distance of 11.3 kpc, deeper, more sensitive searches would find many additional pulsars.

Accepted to ApJ, 9 pages, 4 figures, 2 tables

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