Tests of gravity theories with pulsar timing

Over the last few years, a set of new results from pulsar timing has introduced much tighter constraints on violations of the strong equivalence principle (SEP), either via a direct verification of the universality of free fall for a pulsar in a triple star system, or from tests of the nature of gravitational waves, in particular a search for dipolar gravitational wave emission in a variety of binary pulsars with different masses. No deviations from the SEP have been detected in our experiments. These results introduce some of the most stringent constraints on several classes of alternative theories of gravity and complement recent results from the ground-based gravitational wave detectors.Comment: 5 pages, 3 figures, contribution to the 2022 Gravitation session of the 56th Rencontres de Morion

An algorithm for determining the rotation count of pulsars

We present here a simple, systematic method for determining the correct global rotation count of a radio pulsar; an essential step for the derivation of an accurate phase-coherent ephemeris. We then build on this method by developing a new algorithm for determining the global rotational count for pulsars with sparse timing data sets. This makes it possible to obtain phase-coherent ephemerides for pulsars for which this has been impossible until now. As an example, we do this for PSR J0024-7205aa, an extremely faint MSP recently discovered in the globular cluster 47 Tucanae. This algorithm has the potential to significantly reduce the number of observations and the amount of telescope time needed to follow up on new pulsar discoveries.Comment: 13 pages in MNRAS emulation format, 7 figures. Accepted for publication in MNRA

A Massive Neutron Star in the Globular Cluster M5

We report the results of 19 years of Arecibo timing for two pulsars in the globular cluster NGC 5904 (M5), PSR B1516+02A (M5A) and PSR B1516+02B (M5B). This has resulted in the measurement of the proper motions of these pulsars and, by extension, that of the cluster itself. M5B is a 7.95-ms pulsar in a binary system with a > 0.13 solar mass companion and an orbital period of 6.86 days. In deep HST images, no optical counterpart is detected within ~2.5 sigma of the position of the pulsar, implying that the companion is either a white dwarf or a low-mass main-sequence star. The eccentricity of the orbit (e = 0.14) has allowed a measurement of the rate of advance of periastron: (0.0142 +/-0.0007) degrees per year. We argue that it is very likely that this periastron advance is due to the effects of general relativity, the total mass of the binary system then being 2.29 +/-0.17 solar masses. The small measured mass function implies, in a statistical sense, that a very large fraction of this total mass is contained in the pulsar: 2.08 +/- 0.19 solar masses (1 sigma); there is a 5% probability that the mass of this object is < 1.72 solar masses and a 0.77% probability that is is between 1.2 and 1.44 solar masses. Confirmation of the median mass for this neutron star would exclude most ``soft'' equations of state for dense neutron matter. Millisecond pulsars (MSPs) appear to have a much wider mass distribution than is found in double neutron star systems; about half of these objects are significantly more massive than 1.44 solar masses. A possible cause is the much longer episode of mass accretion necessary to recycle a MSP, which in some cases corresponds to a much larger mass transfer.Comment: 10 pages in ApJ emulate format, 2 tables, 6 figures. Added February 2008 data, slightly revised mass limits. Accepted for publication in Ap

The millisecond pulsar mass distribution: Evidence for bimodality and constraints on the maximum neutron star mass

The mass function of neutron stars (NSs) contains information about the late evolution of massive stars, the supernova explosion mechanism, and the equation-of-state of cold, nuclear matter beyond the nuclear saturation density. A number of recent NS mass measurements in binary millisecond pulsar (MSP) systems increase the fraction of massive NSs (with $M > 1.8$ M$_{\odot}$) to $\sim 20\%$ of the observed population. In light of these results, we employ a Bayesian framework to revisit the MSP mass distribution. We find that a single Gaussian model does not sufficiently describe the observed population. We test alternative empirical models and infer that the MSP mass distribution is strongly asymmetric. The diversity in spin and orbital properties of high-mass NSs suggests that this is most likely not a result of the recycling process, but rather reflects differences in the NS birth masses. The asymmetry is best accounted for by a bimodal distribution with a low mass component centred at $1.393_{-0.029}^{+0.031}$ M$_{\odot}$ and dispersed by $0.064_{-0.025}^{+0.064}$ M$_{\odot}$, and a high-mass component with a mean of $1.807_{-0.132}^{+0.081}$ and a dispersion of $0.177_{-0.072}^{+0.115}$ M$_{\odot}$. We also establish a lower limit of $M_{max} \ge 2.018$ M$_{\odot}$ at 98% C.L. for the maximum NS mass, from the absence of a high-mass truncation in the observed masses. Using our inferred model, we find that the measurement of 350 MSP masses, expected after the conclusion of pulsar surveys with the Square-Kilometre Array, can result in a precise localization of a maximum mass up to 2.15 M$_{\odot}$, with a 5% accuracy. Finally, we identify possible massive NSs within the known pulsar population and discuss birth masses of MSPs.Comment: submitted to ApJ; 21 pages in aastex6 two-column format, 12 figures, 5 tables. Comments are welcom

GMRT Discovery of A Millisecond Pulsar in a Very Eccentric Binary System

We report the discovery of the binary millisecond pulsar J0514-4002A, which is the first known pulsar in the globular cluster NGC 1851 and the first pulsar discovered using the Giant Metrewave Radio Telescope (GMRT). The pulsar has a rotational period of 4.99 ms, an orbital period of 18.8 days, and the most eccentric pulsar orbit yet measured (e = 0.89). The companion has a minimum mass of 0.9 M_sun and its nature is presently unclear. After accreting matter from a low-mass companion star which spun it up to a (few) millisecond spin period, the pulsar eventually exchanged the low-mass star for its more massive present companion. This is exactly the same process that could form a system containing a millisecond pulsar and a black hole; the discovery of NGC 1851A demonstrates that such systems might exist in the Universe, provided that stellar mass black holes exist in globular clusters.Comment: 12 pages (referee format), 3 figures, accepted for publication in Astrophysical Journal Letter