PSR J0537-6910: Exponential recoveries detected for 12 glitches

Pulsar glitches are unresolved increments of the rotation rate that sometimes trigger an enhancement of the spin-down rate. On occasions, the augmented spin-down decays gradually in an exponential manner, particularly after the largest glitch events. The young pulsar PSR J0537-6910 exhibits the highest known glitching rate, with 60 events detected in nearly 18 years of monitoring. Despite most PSR J0537-6910 glitches being large, only one exponential recovery has been reported, following its first discovered glitch. This is puzzling, as pulsars of similar characteristics typically present significant exponential recoveries. We aim to determine whether this reflects an intrinsic difference in PSR J0537-6910 or a detectability issue, for example due to its high glitch frequency. The full dataset, including recent NICER observations, was systematically searched for exponential relaxations. Each glitch was tested for evidence of a recovery over a broad range of trial timescales. Promising candidates were investigated further by comparing recovery models with and without an exponential term using Bayesian evidence. We discovered six new glitches, bringing the total to 66. Our criteria strongly indicates the presence of 11 previously undetected exponential recoveries. We presente updated glitch and timing solutions. Exponential recoveries are detected only for the largest glitches, though not all of them. The inferred timescales range from 4 to 37 d, with the decaying frequency increment generally below $1%$ of the total. We find that $\ddotν$ can remain stable across several glitches, with persistent changes associated with only some events. In particular, it tends to be lowest after glitches with exponential recoveries, yielding inter-glitch braking indices between 6 and 9. Following glitches without recoveries, $\ddotν$ is higher, leading to braking indices between 10 and 35.

💡 Research Summary

This paper presents a comprehensive re‑analysis of the glitch activity of the young, fast‑spinning X‑ray pulsar PSR J0537‑6910, focusing on the detection of post‑glitch exponential recoveries. PSR J0537‑6910 is notable for its extremely high spin‑down power (Ė ≈ 4.9 × 10³⁸ erg s⁻¹), a rotation frequency of about 62 Hz, and an unprecedented glitch rate of roughly three glitches per year. Over nearly 18 years of monitoring with RXTE and NICER, 60 glitches had previously been reported, most of them large (Δν > 10 µHz). However, only the first glitch was known to exhibit a clear exponential relaxation, leading to the impression that this pulsar rarely shows such recoveries.

The authors combined all available timing data, re‑processed the NICER observations, and identified six additional glitches, bringing the total to 66. For each glitch they fitted a pre‑glitch spin‑down model (including ν, ˙ν, and where possible ¨ν) and then searched the post‑glitch interval for evidence of an exponential term. The search involved scanning a wide range of trial relaxation timescales τ_d from one day up to four times the length of the post‑glitch data set, fitting a full glitch model with a fixed τ_d at each trial, and recording the reduced χ². A minimum in χ² as a function of τ_d indicated a candidate timescale. For those candidates, two nested models were compared using Bayesian evidence: Model 1 (no exponential) and Model 2 (including a single exponential component). The logarithmic Bayes factor ln B₂₁ was used to assess the strength of evidence, with ln B₂₁ > 5 taken as strong support for the exponential model.

Applying this systematic methodology, the authors found robust evidence for exponential recoveries in 12 glitches. All of these recoveries are associated with the largest events (Δν > 20 µHz). The inferred relaxation timescales span 4–37 days, and the decaying frequency increment Δν_d is typically ≤ 1 % of the total frequency step Δν, corresponding to recovery fractions Q ≈ 0.001–0.01. Not every large glitch shows a recovery; several large events lack any detectable exponential component, indicating that the presence of a recovery is not solely determined by glitch size.

A key result concerns the second derivative of the spin frequency (¨ν). The authors show that ¨ν tends to be lower after glitches that exhibit an exponential recovery, leading to inter‑glitch braking indices n ≈ 6–9. In contrast, after glitches without a recovery, ¨ν is higher, yielding n ≈ 10–35. This dichotomy suggests that the transient increase in the spin‑down torque associated with a glitch can either be partially relaxed (when a superfluid component recouples exponentially) or remain largely permanent (when no exponential term is present). The findings therefore provide new constraints on superfluid vortex dynamics, pinning strengths, and the coupling between the crust and interior superfluid in neutron stars.

The paper’s methodological contribution is significant: by employing a broad τ_d search combined with Bayesian model selection, the authors overcome the detection bias introduced by the high glitch frequency and irregular observation cadence that previously masked subtle recoveries. Their approach can be applied to other frequently glitching pulsars, potentially revising the statistics of exponential recoveries across the pulsar population.

In summary, PSR J0537‑6910 does exhibit exponential post‑glitch relaxations, but only in a subset of its largest glitches. The presence or absence of a recovery correlates with distinct long‑term spin‑down behavior, offering fresh insight into the internal physics of neutron stars and the mechanisms governing angular momentum exchange between superfluid components and the solid crust.