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A strangely light neutron star within a supernova remnant

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To constrain the equation of state of cold dense matter, astrophysical measurements are essential. These are mostly based on observations of neutron stars in the X-ray band, and, more recently, also on gravitational wave observations. Of particular interest are observations of unusually heavy or light neutron stars which extend the range of central densities probed by observations and thus permit the testing of nuclear-physics predictions over a wider parameter space. Here we report on the analysis of such a star, a central compact object within the supernova remnant HESS J1731-347. We estimate the mass and radius of the neutron star to be M=0.77−0.17+0.20M⊙\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$M=0.7{7}_{-0.17}^{+0.20}\,{M}_{\odot }$$\end{document} and R=10.4−0.78+0.86\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$R=10.{4}_{-0.78}^{+0.86}$$\end{document} km, respectively, based on modelling of the X-ray spectrum and a robust distance estimate from Gaia observations. Our estimate implies that this object is either the lightest neutron star known, or a ‘strange star’ with a more exotic equation of state. Adopting a standard neutron star matter hypothesis allows the corresponding equations of state to be constrained.
Equation of state predictions and observational constraints as a function of the radius and mass of the compact star The gray dashed contours show mass and radius constraints for PSR J0740+6620 and PSR J0030+0451 reported in refs. 51,55 based on the NICER data, and the black dotted line denotes constrains for 4U 1702-429 in ref. ²⁵. The black solid and dashed lines show constrains for the CCO in HESS J1731-347 obtained in this work. Here, the thick solid line corresponds to the case when only parallax priors and X-ray data are considered, whereas the thick dashed lines correspond to the joint fit including all prior information as discussed in the text. All contours are plotted for 68.3% and 95.4% credible intervals (including only statistical uncertainties). The collection of thin black lines represents the family of chiral EOSs considered in refs. 23,24, with red lines on top indicating the EOSs that are allowed by the combined analysis of priors in refs. 51,55, constraints based on the analysis of X-ray bursts from 4U 1702-429 in ref. ²⁵, and this work (both at 90% confidence). Two representative strange quark-matter EOSs are also plotted for completeness (red dashed lines). The error bars around 1.4M⊙ show the expected radius of a ‘standard’ neutron star allowed by EOS constraints in ref. ⁵¹ (black) and in this work (beige, both at 90% confidence level). The horizontal dashed line shows a lower limit on the expected astrophysical neutron star mass from ref. ²⁹.
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Nature Astronomy | Volume 6 | December 2022 | 1444–1451 1444
nature astronomy
A strangely light neutron star within a
supernova remnant
Victor Doroshenko , Valery Suleimanov , Gerd Pühlhofer and
Andrea Santangelo
To constrain the equation of state of cold dense matter, astrophysical
measurements are essential. These are mostly based on observations of
neutron stars in the X-ray band, and, more recently, also on gravitational
wave observations. Of particular interest are observations of unusually
heavy or light neutron stars which extend the range of central densities
probed by observations and thus permit the testing of nuclear-physics
predictions over a wider parameter space. Here we report on the analysis of
such a star, a central compact object within the supernova remnant HESS
J1731-347. We estimate the mass and radius of the neutron star to be
󰁝0󰅂17 M
 km, respectively, based on modelling of
the X-ray spectrum and a robust distance estimate from Gaia observations.
Our estimate implies that this object is either the lightest neutron star
known, or a ‘strange star’ with a more exotic equation of state. Adopting a
standard neutron star matter hypothesis allows the corresponding
equations of state to be constrained.
Central compact objects (CCOs) are isolated, radio-quiet,
non-accreting, thermally emitting neutron stars found at the centres of
supernova remnants (SNRs)
. Their thermal X-ray emission is believed
to be associated with the cooling of young, weakly magnetized4,5 neu-
tron stars and comes from the atmosphere covering a large fraction of
the surface. Most CCOs exhibit no pulsations, which suggests a rather
uniform temperature distribution over the neutron star surface. This
implies that the uncertainties associated with the geometry of the
emission region, the details of the accretion physics and the radiative
transfer in strong magnetic fields, which are all typical of accreting
neutron stars, are irrelevant for CCOs. Such simplicity makes CCOs ideal
laboratories to investigate the equation of state (EOS) of dense matter6,7.
On the other hand, CCOs are only observed as faint X-ray sources
and often lack reliable distance and luminosity estimates, which trans-
lates to large uncertainties in the emission area and neutron star radius
. Furthermore, the lack of detected pulsations does not
necessarily imply that the entire surface emits uniformly, and could
be simply due to an unfavourable orientation of the observer’s line of
sight with respect to the spin and magnetic dipole axes of the neutron
. The composition of the atmosphere also strongly influences
the estimated neutron star parameters. In particular, atmospheres
composed of carbon, or even heavier elements, have been suggested
for a large fraction of CCOs1315.
We emphasize, however, that the caveats outlined above are now
largely sorted out. With regard to atmosphere composition, hydrogen
atmospheres yield unreasonably small neutron star radii9,10, whereas
emission from atmospheres with even heavier compositions strongly
deviates from the observed black-body-like spectra13. Instead of deduc-
ing unreasonably small neutron star radii, one could assume that the
emission originates from a fraction of the neutron star surface and
pulsations are not observed only because of an unfavourable viewing
orientation. This is, however, in conflict with the existing limits on the
amplitude of potential pulsations (~8% for the CCO in HESS J1731-347
) and less than ~12% for the CCO in Cas A
, and in the range
3–50% for other objects). Note that although probabilities quoted
above for individual objects, including the CCO in HESS J1731-347, are
not negligible, missing pulsations from all non-pulsating CCOs are
highly unlikely, with a chance probability estimated to be ≤10
Considering that there are currently no other ideas to explain
the observed thermal non-pulsed emission, uniformly emitting car-
bon atmospheres appear to be the only viable option to explain the
observed spectra of CCOs. The largest remaining source of uncertainty
Received: 31 March 2022
Accepted: 1 September 2022
Published online: 24 October 2022
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Institut für Astronomie und Astrophysik, Tübingen, Germany. e-mail:
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... From the astrophysical point of view, a tremendous progress was done in the last decade. As such, NS masses in the range 0.8 ≲ M/M ⊙ ≲ 2.1 (Antoniadis et al. 2013;Arzoumanian et al. 2018;Brodie & Haber 2023;Cromartie et al. 2020;Demorest et al. 2010;Doroshenko et al. 2022;Fonseca et al. 2021) have been measured with unprecedented precision. Measurements of massive NSs are informative of the NS EOS stiffness and of utmost relevance for the onset of non-nucleonic particle degrees of freedom (Oertel et al. 2017;Sedrakian et al. 2023). ...
... At the other extremity, NS masses lower than 1 M ⊙ challenge our understanding of NS formation in core-collapse supernovae (Suwa et al. 2018). This is, in particular, the case of the NS in the supernova remnant (SNR) HESS J1731-347, whose mass was recently estimated to be M = 0.77 +0.20 −0.17 M ⊙ (or M = 0.83 +0.17 −0.13 M ⊙ ) (Brodie & Haber 2023;Doroshenko et al. 2022). Moreover, its very low radius R = 10.4 +0.86 −0.78 km (or R = 11.25 +0.53 −0.37 km) (Brodie & Haber 2023;Doroshenko et al. 2022) triggers questions about the composition of low mass NS cores or the behavior of NS EOS at low densities. ...
... This is, in particular, the case of the NS in the supernova remnant (SNR) HESS J1731-347, whose mass was recently estimated to be M = 0.77 +0.20 −0.17 M ⊙ (or M = 0.83 +0.17 −0.13 M ⊙ ) (Brodie & Haber 2023;Doroshenko et al. 2022). Moreover, its very low radius R = 10.4 +0.86 −0.78 km (or R = 11.25 +0.53 −0.37 km) (Brodie & Haber 2023;Doroshenko et al. 2022) triggers questions about the composition of low mass NS cores or the behavior of NS EOS at low densities. In relation with the first aspect, the occurrence of quark matter (Di Clemente et al. 2022) and ∆-resonances ) has been speculated. ...
The equation of state (EOS) is a key ingredient for understanding the structure and composition of neutron stars (NSs). Observation of several pulsars with masses $\approx$ 2 Msun, inference of tidal deformabilities from gravitational waves signals in binary neutron stars mergers and joint mass and radius estimates of two millisecond pulsars contributed to better constraining the behavior of NS EOS beyond the nuclear saturation density. We aim to build families of EOSs subjected to various minimal sets of constraints and identify the role some of these constraints play. We also aim to establish correlations between properties of nuclear matter (NM) and properties of NSs. The non-relativistic mean field theory of NM and the standard Skyrme parametrization of the nucleonic effective interactions are used to generate, within a Bayesian framework, models of EOSs. The constraints we pose come from empirical parameters of NM, ab initio calculations of pure neutron matter (PNM) and the 2 Msun lower limit on the maximum NS mass. EOSs also have to be causal. A purely nucleonic composition is hypothesized. EOSs are generated and investigated for five sets of constraints. Marginalized posteriors of the effective interaction's parameters; empirical parameters of NM; selected global properties of NSs are plotted and analyzed. Correlations among parameters in the isoscalar and isovector channels as well as with NS properties are studied. EOSs of NM and NSs are very sensitive to the set of constraints, including whether correlations among the values that the energy per nucleon in PNM takes at different densities are accounted for. In each of the five sets that we have generated there is a significant number of models that comply at 50\% credible region with joint mass and radius constraints from PSR J0030+045 and J0740+6620, while a tension exists with similar data from the NS in HESS J1731--34.
... These masses were computed using the orbital parameters and photometric information. To identify systems with possible neutron star companions, when masses are available, we kept systems with M 1 < M 2 and M 2 1.1 M e (see Doroshenko et al. 2022;Chamel et al. 2013, for a discussion of the minimum (0.77 M e ) and maximum (2.5 M e ) neutron star masses, respectively). In the present analysis, we will, however, consider 1.3 M e as the typical neutron star mass. ...
... The first is Gaia DR3 1131316620813278464 (system #11), even though the companion mass seems a bit small (1.17 ± 0.05 M e ). Although neutron stars have been observed with masses as low as 0.77 M e (Doroshenko et al. 2022), a massive white dwarf is more likely given the initial mass function. The second is Gaia DR3 1114090365983444480 (system #18), which is very likely to host a neutron star since the inferred mass is 1.25 ± 0.12 M e . ...
Full-text available
Supernova explosions attributed to the unseen companion in several binary systems identified by the Third Gaia Data Release (Gaia DR3) may be responsible for a number of well-known and well-studied features in the radio sky, including the low-latitude intermediate-velocity (LLIV) arch and the north celestial pole (NCP) loop. Slices from the longitude–latitude–velocity data cube of the λ -21 cm Galactic neutral hydrogen HI4PI survey show multiple signatures of an expanding shell. The source of this expansion, which includes the LLIV arch on the approaching side, may be the neutron star candidate in the Gaia DR3 1093757200530267520 binary. If we make the simplifying assumptions that the expansion of the cavity is uniform and spherically symmetric, then the explosion took place about 700,000 yr ago. The momentum is in reasonable agreement with recent model estimates for a supernova this old. The H i on the receding side of this cavity is interacting with the gas approaching us on the near side of a second cavity. The NCP loop appears to be located at the intersection of these two expanding features. The neutron star candidate in the Gaia DR3 1144019690966028928 binary may be (in part) responsible for this cavity. Explosions from other candidates may account for the observed elongation along the line of sight of this second cavity. We can use the primary star in these binaries to anchor the distances to the LLIV arch and NCP loop, which are ∼167 and ∼220 pc, respectively.
... 5-6 were taken from Ref. [76]. We consider 2σ error bars for the available data, otherwise a factor of 0.5 and 2 in both the tem- Ref. [76] was substituted by the updated results HESS J1731-347 from Ref. [72]. ...
... Figs. 5-6 depict the recently updated measurement of the HESS J1731-347 star (object number 8) which is reported to be the lightest and smallest compact object ever detected [72]. Our analysis shows that the surface temperature of the HESS J1731-347 compact star can be described by the light-element envelope, which is in agreement with the results of Sagun et al. [82]. ...
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We study the effect of asymmetric fermionic dark matter (DM) on the thermal evolution of neutron stars (NSs). No interaction between DM and baryonic matter is assumed, except the gravitational one. Using the two-fluid formalism, we show that DM accumulated in the core of a star pulls inwards the outer baryonic layers of the star, increasing the baryonic density in the NS core. As a result, it significantly affects the star's thermal evolution by triggering an early onset of the direct Urca process and modifying the photon emission from the surface caused by the decrease of the radius. Thus, due to the gravitational pull of DM, the direct Urca process becomes kinematically allowed for stars with lower masses. Based on these results, we discuss the importance of NS observations at different distances from the Galactic center. Since the DM distribution peaks towards the Galactic center, NSs in this region are expected to contain higher DM fractions that could lead to a different cooling behavior.
... The Tolman-Oppenheimer-Volkoff (TOV) equation (Oppenheimer & Volkoff 1939) was solved using the chosen EoSs to calculate the mass ( NS ) and radius ( NS ) of a NS for a given range of central energy densities ( c ); NS and NS were calculated for 150 linearly spaced values between 1.4 × 10 14 and 2.5 × 10 15 g cm −3 . The c values were chosen so that the calculations for NS and NS covered their full range of possible values within current observational constraints (Nättilä et al. 2017;Abbott et al. 2018;Riley et al. 2019;Miller et al. 2021;Doroshenko et al. 2022). We note that the EoSs investigated are only marginally compatible (90% confidence interval) with Doroshenko et al. (2022); see Di Clemente et al. (2022a) for other possible astrophysical paths to form such a compact object that better satisfies these constraints. ...
... The c values were chosen so that the calculations for NS and NS covered their full range of possible values within current observational constraints (Nättilä et al. 2017;Abbott et al. 2018;Riley et al. 2019;Miller et al. 2021;Doroshenko et al. 2022). We note that the EoSs investigated are only marginally compatible (90% confidence interval) with Doroshenko et al. (2022); see Di Clemente et al. (2022a) for other possible astrophysical paths to form such a compact object that better satisfies these constraints. Calculating NS and NS across all parameter space allowed for accurate interpolation of the NS -NS relation of NSs, and thus compactness, NS = NS / NS , across all parameter space. ...
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The merging of a binary system involving two neutron stars (NSs), or a black hole (BH) and a NS, often results in the emission of an electromagnetic (EM) transient. One component of this EM transient is the epic explosion known as a kilonova (KN). The characteristics of the KN emission can be used to probe the equation of state (EoS) of NS matter responsible for its formation. We predict KN light curves from computationally simulated BH-NS mergers, by using the 3D radiative transfer code \texttt{POSSIS}. We investigate two EoSs spanning most of the allowed range of the mass-radius diagram. We also consider a soft EoS compatible with the observational data within the so-called 2-families scenario in which hadronic stars coexist with strange stars. Computed results show that the 2-families scenario, characterized by a soft EoS, should not produce a KN unless the mass of the binary components are small ($M_{\rm BH} \leq 6M_{\odot}$, $M_{\rm NS} \leq 1.4M_{\odot}$) and the BH is rapidly spinning ($\chi_{\rm BH} \geq 0.3$). In contrast, a strong KN signal potentially observable from future surveys (e.g. VRO/LSST) is produced in the 1-family scenario for a wider region of the parameter space, and even for non-rotating BHs ($\chi_{\rm BH} = 0$) when $M_{\rm BH} = 4M_{\odot}$ and $M_{\rm NS} = 1.2M_{\odot}$. We also provide a fit that allows for the calculation of the unbound mass from the observed KN magnitude, without running timely and costly radiative transfer simulations. Findings presented in this paper will be used to interpret light curves anticipated during the fourth observing run (O4), of the advanced LIGO, advanced Virgo and KAGRA interferometers and thus to constrain the EoS of NS matter.
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We investigate the thermodynamical properties of color-flavor locked (CFL) quark matter at zero temperature, finite temperature, and strong magnetic field by using quasiparticle model. We find that considering CFL quark phase can significantly change the equation of state (EOS) as well as the properties of quark matter in quark stars (QSs) at finite temperature or under magnetic field within quasiparticle model. In particular, our results have shown that we can provide the large QSs within CFL quark phase from quasiparticle model by satisfying both the upper limit of \(\Lambda _{1.4}<800\) for the low-spin priors of 1.4 solar mass pulsars from GW170817 and the new estimates of the mass-radius region from PSR J0740 + 6620, PSR J0030 + 0451, HESS J1731-347, and 4U 1702-429, which cannot be obtained by considering the QSs with SQM within quasiparticle model.
We apply quadratic f ( R ) = R + ϵR ² field equations, where ϵ has a dimension [L ² ], to static spherical stellar model. We assume the interior configuration is determined by Krori-Barua ansatz and additionally the fluid is anisotropic. Using the astrophysical measurements of the pulsar PSR J0740+6620 as inferred by NICER and XMM observations, we determine ϵ ≈ ± 3 km ² . We show that the model can provide a stable configuration of the pulsar PSR J0740+6620 in both geometrical and physical sectors. We show that the Krori-Barua ansatz within f ( R ) quadratic gravity provides semi-analytical relations between radial, p r , and tangential, p t , pressures and density ρ which can be expressed as p r ≈ v r ² ( ρ - ρ 1 ) and p r ≈ v t ² ( ρ - ρ 2 ), where v r ( v t ) is the sound speed in radial (tangential) direction, ρ 1 = ρ s (surface density) and ρ 2 are completely determined in terms of the model parameters. These relations are in agreement with the best-fit equations of state as obtained in the present study. We further put the upper limit on the compactness, C = 2 GMR s ⁻¹ c ⁻² , which satisfies the f ( R ) modified Buchdahl limit. Remarkably, the quadratic f ( R ) gravity with negative ϵ naturally restricts the maximum compactness to values lower than Buchdahl limit, unlike the GR or f ( R ) gravity with positive ϵ where the compactness can arbitrarily approach the black hole limit C → 1. The model predicts a core density a few times the saturation nuclear density ρ nuc = 2.7 × 10 ¹⁴ g/cm ³ , and a surface density ρ s > ρ nuc . We provide the mass-radius diagram corresponding to the obtained boundary density which has been shown to be in agreement with other observations.
In this study, we investigate the impact of weakly interacting massive particles (WIMPs) dark matter (DM) on C–Λ universal relations, GW170817 posterior, and radial oscillations of neutron stars (NSs) by considering the interactions of uniformly trapped neutralinos as a DM candidate with the hadronic matter through the exchange of the Higgs boson within the framework of the Next-to-Minimal Supersymmetric Standard Model (NMSSM). The hadronic equation of state (EOS) is modelled using the relativistic mean-field (RMF) formalism with IOPB-I, G3, and quark–meson coupling (QMC)-RMF series parameter sets. The presence of DM softens the EOS at both the background and the perturbation levels that implies a small shift to the left in the posterior accompanied by a much larger jump in the left of the mass–radius curves with increasing DM mass. It is observed that EOSs with DM also satisfy the C–Λ universality relations among themselves but get slightly shifted to the right in comparison to that without considering DM. Additionally, we find that the inclusion of DM allows the mass–radius (M–R) curves to remain consistent with observational constraints for HESS J1731−347, indicating the possibility of classifying it as a dark matter-admixed neutron star (DMANS). Moreover, we explore the impact of DM on the radial oscillations of pulsating stars and investigate the stability of NSs. The results demonstrate a positive correlation between the mass of DM and the frequencies of radial oscillation modes.
In this work, we investigate the tidal deformability of a neutron star admixed with dark matter, modeled as a massive, self-interacting, complex scalar field. We derive the equations to compute the tidal deformability of the full Einstein-Hilbert-Klein-Gordon system self-consistently, and we probe the influence of the scalar field mass and self-interaction strength on the total mass and tidal properties of the combined system. We find that dark matter corelike configurations lead to more compact objects with smaller tidal deformability, and dark matter cloudlike configurations lead to larger tidal deformability. Electromagnetic observations of certain cloudlike configurations would appear to violate the Buchdahl limit. The self-interaction strength is found to have a significant effect on both mass and tidal deformability. We discuss observational constraints and the connection to anomalous detections. We also investigate how this model compares to those with an effective bosonic equation of state and find the interaction strength where they converge sufficiently.
The central compact object within HESS J1731-347 possesses unique mass and radius properties that renders it a compelling candidate for a self-bound star. In this research, we examine the capability of quark stars composed of color superconducting quark matter to explain the latter object by using its marginalized posterior distribution and imposing it as a constraint on the relevant parameter space. Namely, we investigate quark matter for Nf=2,3 in the color superconducting phase, incorporating perturbative QCD corrections, and we derive their properties accordingly. The utilized thermodynamic potential of this work possesses an MIT bag model formalism with the parameters being established as flavor-independent. In this instance, we conclude the favor of the 3-flavor over the 2-flavor color superconducting quark matter and we isolate our interest on the former, without neglecting the possible favor of the latter in a different framework. The parameter space is further confined due to the additional requirement for a high maximum mass (MTOV≥2.6M⊙), accounting for GW190814’s secondary companion. Our study places significant emphasis on the speed of sound and the trace anomaly which was proposed as a measure of conformality [Y. Fujimoto et al., Phys. Rev. Lett. 129, 252702 (2022).]. We conclude that it is possible for color-flavor locked quark stars to reach high masses without violating the conformal bound or the ⟨Θ⟩μB≥0, provided that the quartic coefficient α4 does not exceed an upper limit which depends on the established MTOV. For MTOV=2.6M⊙, we find that the limit reads α4≤0.594. Lastly, a further study takes place on the agreement of color-flavor locked quark stars with additional astrophysical objects including the GW170817 and GW190425 events, followed by a relevant discussion.
Numerical simulation of strange quark stars (QSs) is challenging due to the strong density discontinuity at the stellar surface. In this paper, we report successful simulations of rapidly rotating QSs and study their oscillation modes in full general relativity. Building on top of the numerical relativity code einstein toolkit, we implement a positivity-preserving Riemann solver and a dustlike atmosphere to handle the density discontinuity at the surface. The robustness of our numerical method is demonstrated by performing stable evolutions of rotating QSs close to the Keplerian limit and extracting their oscillation modes. We focus on the quadrupolar l=|m|=2 f-mode and study whether they can still satisfy the universal relations recently proposed for rotating neutron stars (NSs). We find that two of the three proposed relations can still be satisfied by rotating QSs. For the remaining broken relation, we propose a new relation to unify the NS and QS data by invoking the dimensionless spin parameter j. The onsets of secular instabilities for rotating QSs are also studied by analyzing the f-mode frequencies. Same as the result found previously for NSs, we find that QSs become unstable to the Chandrasekhar-Friedman-Schutz instability when the angular velocity of the star Ω≈3.4σ0 for sequences of constant central energy density, where σ0 is the mode frequency of the corresponding nonrotating configurations. For the viscosity-driven instability, we find that QSs become unstable when j≈0.881 for both sequences of constant central energy density and constant baryon mass. Such a high value of j cannot be achieved by realistic uniformly rotating NSs before reaching the Keplerian limit. The critical value for the ratio between the rotational kinetic energy and gravitational potential energy of rotating QSs for the onset of the instability, when considering sequences of constant baryon mass, is found to agree with an approximate value obtained for homogeneous incompressible bodies in general relativity to within 4%.
Full-text available
We report a precision measurement of the parity-violating asymmetry A_{PV} in the elastic scattering of longitudinally polarized electrons from ^{208}Pb. We measure A_{PV}=550±16(stat)±8(syst) parts per billion, leading to an extraction of the neutral weak form factor F_{W}(Q^{2}=0.00616 GeV^{2})=0.368±0.013. Combined with our previous measurement, the extracted neutron skin thickness is R_{n}-R_{p}=0.283±0.071 fm. The result also yields the first significant direct measurement of the interior weak density of ^{208}Pb: ρ_{W}^{0}=-0.0796±0.0036(exp)±0.0013(theo) fm^{-3} leading to the interior baryon density ρ_{b}^{0}=0.1480±0.0036(exp)±0.0013(theo) fm^{-3}. The measurement accurately constrains the density dependence of the symmetry energy of nuclear matter near saturation density, with implications for the size and composition of neutron stars.
The supernova remnant (SNR) HESS J1731−347 is one of the few objects exhibiting emission up to the TeV energy band and it stands as a prime target for the study of cosmic ray acceleration in SNRs. It also hosts a central compact object (CCO), which is of interest in the context of the ultra-dense matter equation of state in neutron stars. For both types of studies, however, the parameters of the respective models depend crucially on the assumed distance to the object and are affected to a certain extent by the assumed interstellar medium (ISM) properties around the SNR. Here, we report on the first quantitative analysis of the properties of the compact X-ray dust scattering halo that is assumed to be present around the CCO based on Chandra observations of the source. Our findings unambiguously confirm the presence of a compact halo around the CCO, and we show that the observed halo properties are consistent with expectations from independent measurements of the dust distribution along the line of sight and the distance to the source. Although we were not able to significantly improve those constraints, our results are important for future studies of the CCO itself. Indeed, the halo contribution is expected to affect the X-ray spectrum and the derived parameters of the neutron star when observed with moderate angular resolutions. Our results, which offer a quantitative characterization of the halo properties, will be useful in accounting for this effect.
Most young neutron stars belonging to the class of Central Compact Objects (CCOs) in supernova remnants do not have known periodicities. We investigated seven such CCOs to understand the common reasons for the absence of detected pulsations. Making use of XMM-Newton, Chandra, and NICER observations, we perform a systematic timing and spectral analysis to derive updated sensitivity limits for both periodic signals and multi-temperature spectral components that could be associated with radiation from hotspots on the neutron star surface. Based on these limits, we then investigated for each target the allowed viewing geometry that could explain the lack of pulsations. We find that it is unlikely (< 10 ⁻⁶ ) to attribute that we do not see pulsations to an unfavorable viewing geometry for five considered sources. Alternatively, the carbon atmosphere model, which assumes homogeneous temperature distribution on the surface, describes the spectra equally well and provides a reasonable interpretation for the absence of detected periodicities within current limits. The unusual properties of CCOs with respect to other young neutron stars could suggest a different evolutionary path, as that proposed for sources experiencing episodes of significant fallback accretion after the supernova event.
In the past few years, new observations of neutron stars (NSs) and NS mergers have provided a wealth of data that allow one to constrain the equation of state (EOS) of nuclear matter at densities above nuclear saturation density. However, most observations were based on NSs with masses of about 1.4 M ⊙ , probing densities up to ∼three to four times the nuclear saturation density. Even higher densities are probed inside massive NSs such as PSR J0740+6620. Very recently, new radio observations provided an update to the mass estimate for PSR J0740+6620, and X-ray observations by the NICER and XMM telescopes constrained its radius. Based on these new measurements, we revisit our previous nuclear physics multimessenger astrophysics constraints and derive updated constraints on the EOS describing the NS interior. By combining astrophysical observations of two radio pulsars, two NICER measurements, the two gravitational-wave detections GW170817 and GW190425, detailed modeling of the kilonova AT 2017gfo, and the gamma-ray burst GRB 170817A, we are able to estimate the radius of a typical 1.4 M ⊙ NS to be 11.94 − 0.87 + 0.76 km at 90% confidence. Our analysis allows us to revisit the upper bound on the maximum mass of NSs and disfavors the presence of a strong first-order phase transition from nuclear matter to exotic forms of matter, such as quark matter, inside NSs.
PSR J0740+6620 has a gravitational mass of 2.08 ± 0.07 M o˙, which is the highest reliably determined mass of any neutron star. As a result, a measurement of its radius will provide unique insight into the properties of neutron star core matter at high densities. Here we report a radius measurement based on fits of rotating hot spot patterns to Neutron Star Interior Composition Explorer (NICER) and X-ray Multi-Mirror (XMM-Newton) X-ray observations. We find that the equatorial circumferential radius of PSR J0740+6620 is 13.7-1.5+2.6 km (68%). We apply our measurement, combined with the previous NICER mass and radius measurement of PSR J0030+0451, the masses of two other ∼2 M o˙ pulsars, and the tidal deformability constraints from two gravitational wave events, to three different frameworks for equation-of-state modeling, and find consistent results at ∼1.5-5 times nuclear saturation density. For a given framework, when all measurements are included, the radius of a 1.4 M o˙ neutron star is known to ±4% (68% credibility) and the radius of a 2.08 M o˙ neutron star is known to ±5%. The full radius range that spans the ±1σ credible intervals of all the radius estimates in the three frameworks is 12.45 ± 0.65 km for a 1.4 M o˙ neutron star and 12.35 ± 0.75 km for a 2.08 M o˙ neutron star. © 2021. The American Astronomical Society. All rights reserved.
We present analysis of multiple Chandra and XMM-Newton spectra, separated by 9–19 years, of four of the youngest central compact objects (CCOs) with ages <2500yr: CXOU J232327.9+584842 (Cassiopeia A), CXOU J160103.1−513353 (G330.2+1.0), 1WGA J1713.4−3949 (G347.3−0.5), and XMMU J172054.5−372652 (G350.1−0.3). By fitting these spectra with thermal models, we attempt to constrain each CCO’s long-term cooling rate, composition, and magnetic field. For the CCO in Cassiopeia A, 14 measurements over 19 years indicate a decreasing temperature at a ten-year rate of 2.2 ± 0.2 or 2.8 ± 0.3 percent (1σ error) for a constant or changing X-ray absorption, respectively. We obtain cooling rate upper limits of 17 percent for CXOU J160103.1−513353 and 6 percent for XMMU J172054.5−372652. For the oldest CCO, 1WGA J1713.4−3949, its temperature seems to have increased by 4 ± 2 percent over a ten year period. Assuming each CCO’s preferred distance and an emission area that is a large fraction of the total stellar surface, a non-magnetic carbon atmosphere spectrum is a good fit to spectra of all four CCOs. If distances are larger and emission areas are somewhat smaller, then equally good spectral fits are obtained using a hydrogen atmosphere with B ≤ 7 × 1010G or B ≥ 1012G for CXOU J160103.1−513353 and B ≤ 1010G or B ≥ 1012G for XMMU J172054.5−372652 and non-magnetic hydrogen atmosphere for 1WGA J1713.4−3949. In a unified picture of CCO evolution, our results suggest most CCOs, and hence a sizable fraction of young neutron stars, have a surface magnetic field that is low early in their life but builds up over several thousand years.
The second data release of Gaia revealed a parallax zero-point offset of −0.029 mas based on quasars. The value depended on the position on the sky, and also likely on magnitude and colour. The offset and its dependence on other parameters inhibited improvement in the local distance scale using for example the Cepheid and RR Lyrae period–luminosity relations. Analysis of the recent Gaia Early Data Release 3 (EDR3) reveals a mean parallax zero-point offset of −0.021 mas based on quasars. The Gaia team addresses the parallax zero-point offset in detail and proposes a recipe to correct for it based on ecliptic latitude, G -band magnitude, and colour information. This paper presents a completely independent investigation into this issue focusing on the spatial dependence of the correction based on quasars and the magnitude dependence based on wide binaries. The spatial and magnitude corrections are connected to each other in the overlap region in the range 17 < G < 19. The spatial correction is presented at several spatial resolutions based on the HEALPix formalism. The colour dependence of the parallax offset is unclear and in any case secondary to the spatial and magnitude dependence. The spatial and magnitude corrections are applied to two samples of brighter sources, namely a sample of approximately 100 stars with independent trigonometric parallax measurements from Hubble Space Telescope data, and a sample of 75 classical cepheids using photometric parallaxes. The mean offset between the observed GEDR3 parallax and the independent trigonometric parallax (excluding outliers) is about −39 μas, and after applying the correction it is consistent with being zero. For the classical cepheid sample the analysis presented here suggests that the photometric parallaxes may be underestimated by about 5%.
We present a detailed spectroscopic and timing analysis of X-ray observations of the bright pulsar PSR B0656+14. The observations were obtained simultaneously with eROSITA and XMM-Newton during the calibration and performance verification phase of the Spektrum-Roentgen-Gamma mission (SRG). The analysis of the 100 ks deep observation of eROSITA is supported by archival observations of the source, including XMM-Newton, NuSTAR , and NICER. Using XMM-Newton and NICER, we first established an X-ray ephemeris for the time interval 2015 to 2020, which connects all X-ray observations in this period without cycle count alias and phase shifts. The mean eROSITA spectrum clearly reveals an absorption feature originating from the star at 570 eV with a Gaussian σ of about 70 eV that was tentatively identified in a previous long XMM-Newton observation. A second previously discussed absorption feature occurs at 260–265 eV and is described here as an absorption edge. It could be of atmospheric or of instrumental origin. These absorption features are superposed on various emission components that are phenomenologically described here as the sum of hot (120 eV) and cold (65 eV) blackbody components, both of photospheric origin, and a power law with photon index Γ = 2 from the magnetosphere. We created energy-dependent light curves and phase-resolved spectra with a high signal-to-noise ratio. The phase-resolved spectroscopy reveals that the Gaussian absorption line at 570 eV is clearly present throughout ~60% of the spin cycle, but it is otherwise undetected. Likewise, its parameters were found to be dependent on phase. The visibility of the line strength coincides in phase with the maximum flux of the hot blackbody. If the line originates from the stellar surface, it nevertheless likely originates from a different location than the hot polar cap. We also present three families of model atmospheres: a magnetized atmosphere, a condensed surface, and a mixed model. They were applied to the mean observed spectrum, whose continuum fit the observed data well. The atmosphere model, however, predicts distances that are too short. For the mixed model, the Gaussian absorption may be interpreted as proton cyclotron absorption in a field as high as 10 ¹⁴ G, which is significantly higher than the field derived from the moderate observed spin-down.
Gaia Early Data Release 3 (EDR3) provides trigonometric parallaxes for 1.5 billion stars, with reduced systematics compared to Gaia Data Release 2 and reported precisions better by up to a factor of 2. New to EDR3 is a tentative model for correcting the parallaxes of magnitude-, position-, and color-dependent systematics for five- and six-parameter astrometric solutions, Z 5 and Z 6. Using a sample of over 2000 first-ascent red giant branch stars with asteroseismic parallaxes, I perform an independent check of the Z 5 model in a Gaia magnitude range of 9 ≲ G ≲ 13 and color range of 1.4 μm-1 ≲ ν eff ≲ 1.5 μm-1. This analysis therefore bridges the Gaia team's consistency check of Z 5 for G > 13 and indications from independent analysis using Cepheids of a ≈15 μas overcorrection for G < 11. I find overcorrection sets in at G ≲ 10.8, such that Z 5-corrected EDR3 parallaxes are larger than asteroseismic parallaxes by 15 3 μas. For G ⪆ 10.8, EDR3 and asteroseismic parallaxes in the Kepler field agree up to a constant consistent with expected spatial variations in EDR3 parallaxes after a linear, color-dependent adjustment. I also infer an average underestimation of EDR3 parallax uncertainties in the sample of 22% 6%, consistent with the Gaia team's estimates at similar magnitudes and independent analysis using wide binaries. Finally, I extend the Gaia team's parallax spatial covariance model to brighter magnitudes (G < 13) and smaller scales (down to ≈0. 1), where systematic EDR3 parallax uncertainties are at least ≈3-4 μas. © 2021. The American Astronomical Society. All rights reserved.