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 normal-looking 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 and YMW16 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, $a_{l,\textrm{max}}/c \approx$ 2.3$\times10^{-18}$ s$^{-1}$, experienced by the pulsar and constraints on the magnetic field to be $<\sim$3$\times$10$^{8}$ G. This confirms the pulsar to be within NGC 6316 despite the lower-than-expected dispersion measure. 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.

Figures (3)

• Figure 1: The prepfold output of the first detection of PSR J1716$-$2808A from a GBT S$-$band observation. The residual curvature in the time vs pulse phase greyscale on the left indicates uncorrected orbital motion despite fitting for acceleration and jerk (i.e. $\dot p$ and $\Ddot{p}$).
• Figure 2: Maximum line of sight cluster acceleration profiles calculated using Eq. \ref{['phin3.5']} and Eq. \ref{['freire']} as a function of the total angular offset from the center of NGC 6316. The dashed vertical line denotes the core radius. The upper limit for the cluster acceleration from PSR J1716$-$2808A, assuming zero intrinsic spin-down, is shown via the inverted triangle and calculated via Eq. \ref{['ulim']}. Given the estimated upper limit of PSR J1716$-$2808A, we cannot constrain properties of the cluster without either detecting more MSPs or by measuring the orbital period derivative of PSR J1716$-$2808A through continued timing.
• Figure 3: Estimated scattering profiles for the flux calibrated summed L$-$band subbands, uncalibrated S$-$band subbands and C$-$band observation. The normalized subbands are plotted in black and the estimated scattering profile is plotted in red with: $\tau_{\textrm{1GHz}} \sim 1.4 \pm$ 0.1 ms.