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ABSTRACT Pulse profile modelling is a relativistic ray-tracing technique that can be used to infer masses, radii, and geometric parameters of neutron stars. In a previous study, we looked at the performance of this technique when applied to thermonuclear burst oscillations from accreting neutron stars. That study showed that ignoring the variability associated with burst oscillation sources resulted in significant biases in the inferred mass and radius, particularly for the high count rates that are nominally required to obtain meaningful constraints. In this follow-on study, we show that the bias can be mitigated by slicing the bursts into shorter segments where variability can be neglected, and jointly fitting the segments. Using this approach, the systematic uncertainties on the mass and radius are brought within the range of the statistical uncertainty. With about 106 source counts, this yields uncertainties of approximately 10 per cent for both the mass and radius. However, this modelling strategy requires substantial computational resources. We also confirm that the posterior distributions of the mass and radius obtained from multiple bursts of the same source can be merged to produce outcomes comparable to that of a single burst with an equivalent total number of counts. 

Abstract We present the results of a deep study of the neutron star (NS) population in the globular cluster M28 (NGC 6626), using the full 330 ks 2002–2015 ACIS data set from the Chandra X-ray Observatory and coordinated radio observations taken with the Green Bank Telescope (GBT) in 2015. We investigate the X-ray luminosity ( L X ), spectrum, and orbital modulation of the seven known compact binary millisecond pulsars in the cluster. We report two simultaneous detections of the redback PSR J1824−2452I (M28I) and its X-ray counterpart at L X = [8.3 ± 0.9] × 10 31 erg s −1 . We discover a double-peaked X-ray orbital flux modulation in M28I during its pulsar state, centered around pulsar inferior conjunction. We analyze the spectrum of the quiescent NS low-mass X-ray binary to constrain its mass and radius. Using both hydrogen and helium NS atmosphere models, we find an NS radius of R = 9.2–11.5 km and R = 13.0–17.5 km, respectively, for an NS mass of 1.4 M ⊙ (68% confidence ranges). We also search for long-term variability in the 46 brightest X-ray sources and report the discovery of six new variable low-luminosity X-ray sources in M28. 

ABSTRACT We study the effects of the time-variable properties of thermonuclear X-ray bursts on modelling their millisecond-period burst oscillations. We apply the pulse profile modelling technique that is being used in the analysis of rotation-powered millisecond pulsars by the Neutron Star Interior Composition Explorer to infer masses, radii, and geometric parameters of neutron stars. By simulating and analysing a large set of models, we show that overlooking burst time-scale variability in temperatures and sizes of the hot emitting regions can result in substantial bias in the inferred mass and radius. To adequately infer neutron star properties, it is essential to develop a model for the time-variable properties or invest a substantial amount of computational time in segmenting the data into non-varying pieces. We discuss prospects for constraints from proposed future X-ray telescopes. 

Abstract We have used X-ray data from the Neutron Star Interior Composition Explorer (NICER) to search for long-timescale temporal correlations (“red noise”) in the pulse times of arrival (TOAs) from the millisecond pulsars PSR J1824−2452A and PSR B1937+21. These data more closely track intrinsic noise because X-rays are unaffected by the radio-frequency-dependent propagation effects of the interstellar medium. Our search yields strong evidence (natural log Bayes factor of 9.634 ± 0.016) for red noise in PSR J1824−2452A, but the search is inconclusive for PSR B1937+21. In the interest of future X-ray missions, we devise and implement a method to simulate longer and higher-precision X-ray data sets to determine the timing baseline necessary to detect red noise. We find that the red noise in PSR B1937+21 can be reliably detected in a 5 yr mission with a TOA error of 2μs and an observing cadence of 20 observations per month compared to the 5μs TOA error and 11 observations per month that NICER currently achieves in PSR B1937+21. We investigate detecting red noise in PSR B1937+21 with other combinations of observing cadences and TOA errors. We also find that time-correlated red noise commensurate with an injected stochastic gravitational-wave background having an amplitude ofA_GWB= 2 × 10⁻¹⁵and spectral index of timing residuals ofγ_GWB= 13/3 can be detected in a pulsar with similar TOA precision to PSR B1937+21. This is with no additional red noise in a 10 yr mission that observes the pulsar 15 times per month and has an average TOA error of 1μs.