Discovery and Timing of the First Millisecond Pulsar in NGC 6316

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.

💡 Research Summary

The globular cluster NGC 6316, located in the Galactic bulge at a distance of ~11.3 kpc, has been identified as a bright γ‑ray source by the Fermi Large Area Telescope. Gamma‑ray spectral analyses have long suggested that the cluster should host dozens to a hundred millisecond pulsars (MSPs), yet no radio MSPs had been detected prior to this work. Using the Green Bank Telescope (GBT) in both C‑band and S‑band, the authors performed deep searches with the PRESTO software suite, employing high‑acceleration (zmax = 600) Fourier‑domain searches to capture pulsars in tight binaries. A highly accelerated candidate emerged with a spin period of 2.45 ms and a dispersion measure (DM) of 172 pc cm⁻³, significantly lower than the predictions of the NE2001 (≈191 pc cm⁻³) and YMW16 (≈371 pc cm⁻³) electron density models.

A rapid follow‑up observation with the Parkes telescope’s ultra‑wideband receiver (UWL) confirmed the signal, allowing the team to construct a family of circular orbital solutions. By combining 135 times‑of‑arrival (TOA) measurements from both GBT (including two 5‑hour L‑band sessions) and Parkes over a 3.09‑year span, they derived a phase‑connected timing solution using the ELL1 binary model. The resulting parameters are: orbital period Pb = 0.424976623 d, projected semi‑major axis a₁ sin i = 0.339253 lt‑s, minimum companion mass ≳0.08 M⊙ (assuming an edge‑on orbit), and an eccentricity upper limit e < 7 × 10⁻⁵, indicating a nearly circular, compact binary. The pulsar’s sky position lies only ~4.5″ from the cluster centre, well within the core radius (~10″).

Spin parameters are f = 408.0317456967 Hz and (\dot f) = 1.396 × 10⁻¹⁵ Hz s⁻¹, corresponding to a period derivative (\dot P) = −8.37 × 10⁻²¹ s s⁻¹. The negative (\dot P) is interpreted as being dominated by the line‑of‑sight gravitational acceleration of the cluster rather than intrinsic spin‑down. By assuming zero intrinsic (\dot P) the authors place an upper limit on the line‑of‑sight cluster acceleration of (a_{l,\mathrm{max}}/c ≈ 2.3 × 10^{-18}) s⁻¹. Contributions from the Shklovskii effect, Galactic potential, and nearby massive objects are estimated to be orders of magnitude smaller, confirming that the cluster potential is the primary driver. Using the acceleration limit together with Phinney (1993) and King (1962) models of the cluster potential, they infer that the pulsar’s surface magnetic field must be ≤ 3 × 10⁸ G.

Flux density measurements at 1.5 GHz yield S ≈ 43 µJy, with no detectable linear polarization, precluding a rotation measure determination. Scattering analysis across L‑, S‑, and C‑band data suggests a scattering timescale of τ₁GHz ≈ 1.4 ms, following a ν⁻⁴ scaling. The authors note that higher‑sensitivity observations with the new GBT Ultra‑Wideband Receiver would improve scattering, spectral index, and polarization constraints.

A search for γ‑ray pulsations in the Fermi‑LAT data, using the radio ephemeris, did not reveal significant pulsations, likely due to limited photon statistics and background confusion in the Galactic bulge.

In summary, this work reports the first MSP in NGC 6316, PSR J1716‑2808A, and provides a robust timing solution that confirms its cluster membership despite a lower‑than‑expected DM. The negative period derivative and derived acceleration limit give insight into the pulsar’s location on the far side of the cluster. The study validates the expectation that NGC 6316 harbors a substantial MSP population and demonstrates that deeper, more sensitive radio searches—potentially uncovering dozens of additional pulsars—will enable precise measurements of the cluster’s mass distribution, ionized gas content, and dynamical state.