The case of PSR J1911-5958A in the outskirts of NGC 6752: signature of a black hole binary in the cluster core?

We have investigated different scenarios for the origin of the binary millisecond pulsar PSR J1911-5958A in NGC 6752, the most distant pulsar discovered from the core of a globular cluster to date. The hypothesis that it results from a truly primordial binary born in the halo calls for accretion-induced collapse and negligible recoil speed at the moment of neutron star formation. Scattering or exchange interactions off cluster stars are not consistent with both the observed orbital period and its offset position. We show that a binary system of two black holes with (unequal) masses in the range of 3-100 solar masses can live in NGC 6752 until present time and can have propelled PSR J1911-5958A into an eccentric peripheral orbit during the last ~1 Gyr.Comment: Accepted by ApJ Letter. 5 pages, 1 figure, 1 tabl

Formation of millisecond pulsars. I. Evolution of low-mass X-ray binaries with P > 2 days

We have performed detailed numerical calculations of the non-conservative evolution of close binary systems with low-mass (1.0-2.0 M_sun) donor stars and a 1.3 M_sun accreting neutron star. Rather than using analytical expressions for simple polytropes, we calculated the thermal response of the donor star to mass loss, in order to determine the stability and follow the evolution of the mass transfer. Tidal spin-orbit interactions and Reimers wind mass-loss were also taken into account. We have re-calculated the correlation between orbital period and white dwarf mass in wide binary radio pulsar systems. Furthermore, we find an anti-correlation between orbital period and neutron star mass under the assumption of the "isotropic re-emission" model and compare this result with observations. We conclude that the accretion efficiency of neutron stars is rather low and that they eject a substantial fraction of the transferred material even when accreting at a sub-Eddington level. The mass-transfer rate is a strongly increasing function of initial orbital period and donor star mass. For relatively close systems with light donors (P < 10 days and M_2 < 1.3 M_sun) the mass-transfer rate is sub-Eddington, whereas it can be highly super-Eddington by a factor of 10^4 for wide systems with relatively heavy donor stars (1.6 - 2.0 M_sun) as a result of their deep convective envelopes. We briefly discuss the evolution of X-ray binaries with donor stars in excess of 2 M_sun. Based on our calculations we present evidence that PSR J1603-7202 evolved through a phase with unstable mass transfer from a relatively heavy donor star and therefore is likely to host a CO white dwarf companion.Comment: Accepted for publication in A&A. 18 pages, 6 figures, 2 table

New Direct Observational Evidence for Kicks in SNe

We present an updated list of direct strong evidence in favour of kicks being imparted to newborn neutron stars. In particular we discuss the new cases of evidence resulting from recent observations of the X-ray binary Circinus X-1 and the newly discovered binary radio pulsar PSR J1141-6545. We conclude that the assumption that neutron stars receive a kick velocity at their formation is unavoidable (van den Heuvel & van Paradijs 1997).Comment: 2 pages, to appear in the proceedings of the IAU Colloq. 177 "Pulsar Astronomy - 2000 and beyond

Comment on "A non-interacting low-mass black hole -- giant star binary system"

Thompson et al. (Reports, 1 November 2019, p. 637, Science) interpreted the unseen companion of the red giant star 2MASS J05215658+4359220 as most likely a black hole. We argue that if the red giant is about one solar mass, its companion can be a close binary consisting of two main-sequence stars. This would explain why no X-ray emission is detected from the system.Comment: 3 pages, Author version of Technical Comment published in Science on 8 May, 202

Birth Kick Distributions and the Spin-Kick Correlation of Young Pulsars

Evidence from pulsar wind nebula symmetry axes and radio polarization observations suggests that pulsar motions correlate with the spin directions. We assemble this evidence for young isolated pulsars and show how it can be used to quantitatively constrain birth kick scenarios. We illustrate by computing several plausible, but idealized, models where the momentum thrust is proportional to the neutrino cooling luminosity of the proto-neutron star. Our kick simulations include the effects of pulsar acceleration and spin-up and our maximum likelihood comparison with the data constrains the model parameters. The fit to the pulsar spin and velocity measurements suggests that: i) the anisotropic momentum required amounts to ~10% of the neutrino flux, ii) while a pre-kick spin of the star is required, the preferred magnitude is small 10-20rad/s, so that for the best-fit models iii) the bulk of the spin is kick-induced with $\bar \Omega$ ~120rad/s and iv) the models suggest that the anisotropy emerges on a timescale $\tau$ ~1-3s.Comment: 37 pages, 13 figures, ApJ accepte

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

Limits on the Mass, Velocity and Orbit of PSR J1933−-6211

We present a high-precision timing analysis of PSR J1933$-$6211, a millisecond pulsar (MSP) with a 3.5-ms spin period and a white dwarf (WD) companion, using data from the Parkes radio telescope. Since we have accurately measured the polarization properties of this pulsar we have applied the matrix template matching approach in which the times of arrival are measured using full polarimetric information. We achieved a weighted root-mean-square timing residuals (rms) of the timing residuals of 1.23 $\rm \mu s$, 15.5$\%$ improvement compared to the total intensity timing analysis. After studying the scintillation properties of this pulsar we put constraints on the inclination angle of the system. Based on these measurements and on $\chi^2$ mapping we put a 2-$\sigma$ upper limit on the companion mass (0.44 M$_\odot$). Since this mass limit cannot reveal the nature of the companion we further investigate the possibility of the companion to be a He WD. Applying the orbital period-mass relation for such WDs, we conclude that the mass of a He WD companion would be about 0.26$\pm$0.01 M$_\odot$ which, combined with the measured mass function and orbital inclination limits, would lead to a light pulsar mass $\leqslant$ 1.0 M$_\odot$. This result seems unlikely based on current neutron star formation models and we therefore conclude that PSR J1933$-$6211 most likely has a CO WD companion, which allows for a solution with a more massive pulsar