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Context: Recently, Doroshenko and collaborators reported a very low-mass compact star, a Central Compact Object named XMMU J173203.3-344518 inside the supernova remnant HESS J1731-347. Its tiny mass is at odds with all calculations of minimum masses of neutron stars generated by iron cores, therefore (and even if not compellingly) it has been suggested to be a {\it strange star}. Besides the mass, a radius and surface temperature have been extracted from data, and the whole body of information should ultimately reveal if this object is truly consistent with an exotic composition. Aims: To understand the status of the compact object XMMU J173203.3-344518 in HESS J1731-347 within the existing models of strange stars, including its prompt formation. Methods: The information obtained on the mass, radius and surface temperature are compared to theoretical calculations performed within usual theoretical models using General Relativity as the assumed theory of gravitation and a handful of cooling scenarios. A qualitative discussion showing the consistency of the strange-matter driven supernova scenario with a low-mass compact star is provided. Results: We found that the object HESS J1731-347 fits within the same quark star models recently employed to explain the masses and radii of the NICER objects PSR J040+6620 and PSR J0030+0451, in which both quantities were simultaneously determined. It is also remarkable to find that a simple cooling scenario devised $30\text{ yr}$ ago with superconducting quarks provides an overall good explanation of the surface temperature. Conclusions: We conclude that XMMU J173203.3-344518 in the remnant HESS J1731-347 fits into a ``strange star'' scenario that is also consistent with heavier compact stars, which can also belong to the same class and constitute an homogeneous type of self-bound objects produced in Nature., Comment: Accepted by A&A Letters, in the press

The study of compact object populations has come a long way since the determination of the mass of the Hulse-Taylor pulsar, and we now count on more than 150 known Galactic neutron stars and black hole masses, as well as another 180 objects from binary mergers detected from gravitational-waves by the Ligo-Virgo-KAGRA Collaboration. With a growing understanding of the variety of systems that host these objects, their formation, evolution and frequency, we are now in a position to evaluate the statistical nature of these populations, their properties, parameter correlations and long-standing problems, such as the maximum mass of neutron stars and the black hole lower mass gap, to a reasonable level of statistical significance. Here, we give an overview of the evolution and current state of the field and point to some of its standing issues. We focus on Galactic black holes, and offer an updated catalog of 35 black hole masses and orbital parameters, as well as a standardized procedure for dealing with uncertainties., Comment: 39 pages, 12 figures, 3 tables. Published in the Galaxies Special Issue "Challenges of This Century in High-Density Compact Objects, High-Energy Astrophysics, and Multi-Messenger Observations. Quo Vadis?"

Binary population synthesis provides a direct way of studying the effects of different choices of binary evolution models and initial parameter distributions on present-day binary compact merger populations, which can then be compared to empirical properties such as observed merger rates. Samples of zero-age main sequence binaries to be evolved by such codes are typically generated from an universal IMF and simple, uniform, distributions for orbital period $P$, mass ratio $q$ and eccentricity $e$. More recently, however, mounting observational evidence has suggested the non-universality of the IMF and the existence of correlations between binary parameters. In this study, we implement a metallicity- and redshift-dependent IMF alongside correlated distributions for $P$, $q$ and $e$ in order to generate representative populations of binaries at varying redshifts, which are then evolved with the COMPAS code in order to study the variations in merger rates and overall population properties., Comment: 5 pages, 5 figures, 1 table. Proceedings contribution to IWARA2022, held in Antigua, Guatemala, in September 2022. To be published by Astron. Nach

The lack of objects between $2\,M_{\odot}$ and $5\,M_{\odot}$ in the joint mass distribution of compact objects has been termed "mass gap", and attributed mainly to the characteristics of the supernova mechanism precluding their birth. However, recent observations show that a number of candidates reported to lie inside the "gap" may fill it, suggesting instead a paucity that may be real or largely a result of small number statistics. We quantify in this work the individual candidates and evaluate the joint probability of a mass gap. Our results show that an absolute mass gap is not present, to a very high confidence level. It remains to be seen if a relative paucity of objects stands in the future, and how this population can be related to the formation processes, which may include neutron star mergers, collapse of a neutron star to a black hole and others., Comment: 20 pages, 7 figures, 5 tables. Accepted for publication in ApJ

We have analyzed in this work the updated sample of neutron star masses derived from the study of a variety of 96 binary systems containing at least one neutron star using Bayesian methods. After updating the multimodality of the distributions found in previous works, we determined the maximum mass implied by the sample using a robust truncation technique, with the result $m_{max} \sim 2.5-2.6 \, M_{\odot}$. We have checked that this mass is actually consistent by generating synthetic data and employing a Posterior Predictive Check. A comparison with seven published $m_{max}$ values inferred from the remnant of the NS-NS merger GW170817 was performed and the tension between the latter and the obtained $m_{max}$ value quantified. Finally, we performed a Local Outlier Factor test and verified that the result for $m_{max}$ encompasses the highest individual mass determinations with the possible exception of PSR J1748-2021B. The conclusion is that the whole distribution already points toward a high value of $m_{max}$, while several lower values derived from the NS-NS merger event are disfavored and incompatible with the higher binary system masses. A large $m_{max}$ naturally accommodates the lower mass component of the event GW190814 as a neutron star., Comment: 3 figures. Sumitted for publication

We revisit in this work a model for repeating Fast Radio Bursts based of the release of energy provoked by the magnetic field dynamics affecting a magnetar's crust. We address the basics of such a model by solving the propagation of the perturbation approximately, and quantify the energetics and the radiation by bunches of charges in the so-called {\it charge starved} region in the magnetosphere. The (almost) simultaneous emission of newly detected X-rays from SGR 1935+2154 is tentatively associated to a reconnection behind the propagation. The strength of $f$-mode gravitational radiation excited by the event is quantified, and more detailed studies of the non-linear (spiky) soliton solutions suggested., Comment: Submitted, discussion on GW enlarged, references added and physical pictured clarified

This research develops a well-established analytical solution of the Einstein-Maxwell field equations. We analyze the behavior of a spherically symmetric and static interior driven by a charged anisotropic matter distribution. The class I methodology is used to close the system of equations and a suitable relation between the anisotropy factor and the electric field is imposed. The inner geometry of this toy model is described using an ansatz for the radial metric potential corresponding to the well-known isotropic Buchdahl space-time. The main properties are explored in order to determine if the obtained model is appropriate to represent a real compact body such as neutron or quark star. {We have fixed the mass and radii using the data of the compact objects} SMC X-1 and LMC X-4. It was found that the electric field and electric charge have magnitudes of the order of $\sim 10^{21}\ [V/cm]$ and $\sim 10^{20}\ [C]$, respectively. The magnitude of the electric field and electric charge depends on the dimensionless parameter $\chi$. To observe these effects on the total mass, mass-radius ratio and surface gravitational red-shift, we computed numerical data for different values of $\chi$.

We suggest in this note that "spider" systems are the naturally expected progenitors of the highest neutron star masses, and possibly low-mass black holes, based on their long-term evolutionary features and actual mass measurements., Comment: To appear in Science China: Physics, Mechanics & Astronomy, News & Views section

The search for the true ground state of the dense matter remains open since Bodmer, Terazawa and other raised the possibility of stable quarks, boosted by Witten's $strange$ $matter$ hypothesis in 1984. Within this proposal, the strange matter is assumed to be composed of $strange$ quarks in addition to the usual $up$s and $down$s, having an energy per baryon lower than the strangeless counterpart, and even lower than that of nuclear matter. In this sense, neutron stars should actually be strange stars. Later work showed that a paired, symmetric in flavor, color-flavor locked (CFL) state would be preferred to the one without any pairing for a wide range of the parameters (gap $\Delta$, strange quark mass $m_s$, and bag constant B). We use an approximate, yet very accurate, CFL equation of state (EoS) that generalizes the MIT bag model to obtain two families of exact solutions for the static Einstein field equations constructing families anisotropic compact relativistic objects. In this fashion, we provide exact useful solutions directly connected with microphysics., Comment: Submitted to IJMPD on January 2020, 13 pages, 6 figures, anisotropy discussion enlarged and clarified