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Register a new userNew constraints on the nuclear equation of state from the thermal emission of neutron stars in quiescent low-mass X-ray binaries
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This paper presents a new analysis of the thermal emission from the neutron star surface to constrain the dense matter equation of state. It is based on the use of a Markov-Chain Monte Carlo algorithm combined with an empirical parametrization of the equation of state, as well as the consistent treatment of seven neutron star quiescent low-mass X-ray binaries in globular clusters with well-measured distances. Previous analyses have indicated that the thermal emission of these neutron stars tends to prefer low neutron star radii, questioning basic knowledge from nuclear physics. We show that it is possible to reconcile the thermal emission analyses with nuclear physics knowledge, with or without including a prior on the slope of the symmetry energy $L_{rm sym}$. We obtain radii of the order of about 12~km without worsening the fit statistic. With an empirical parametrization of the equation of state, we obtain the following values for the slope of the symmetry energy, its curvature $K_{rm sym}$, and the isoscalar skewness parameter $Q_{rm sat}$: $L_{rm sym}=37.2^{+9.2}_{-8.9}$ MeV, $K_{rm sym}=-85^{+82}_{-70}$ MeV, and $Q_{rm sat}=318^{+673}_{-366}$ MeV. For the first time, we measure the values of the empirical parameters $K_{rm sym}$ and $Q_{rm sat}$. These values are only weakly impacted by our assumptions, such as the distances or the number of free empirical parameters, provided they are taken within a reasonable range. We also study the weak sensitivity of our results to the set of sources analyzed, and we identify a group of sources that dominates the constraints. The resulting masses and radii obtained are also discussed in the context of the independent constraints from GW 170817 and its electromagnetic counterpart, AT 2017gfo.
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X-ray spectral analysis of quiescent low-mass X-ray binaries (LMXBs) has been one of the most common tools to measure the radius of neutron stars (NSs) for over a decade. So far, this method has been mainly applied to NSs in globular clusters, primarily because of their well-constrained distances. Here, we study Chandra data of seven transient LMXBs in the Galactic plane in quiescence to investigate the potential of constraining the radius (and mass) of the NSs inhabiting these systems. We find that only two of these objects had X-ray spectra of sufficient quality to obtain reasonable constraints on the radius, with the most stringent being an upper limit of $Rlesssim$14.5 km for EXO 0748-676 (for assumed ranges for mass and distance). Using these seven sources, we also investigate systematic biases on the mass/radius determination; for Aql X-1 we find that omitting a power-law spectral component when it does not seem to be required by the data, results in peculiar trends in the obtained radius with changing mass and distance. For EXO 0748-676 we find that a slight variation in the lower limit of the energy range chosen for the fit leads to systematically different masses and radii. Finally, we simulated Athena spectra and found that some of the biases can be lifted when higher quality spectra are available and that, in general, the search for constraints on the equation of state of ultra-dense matter via NS radius and mass measurements may receive a considerable boost in the future.
This paper presents the measurement of the neutron star (NS) radius using the thermal spectra from quiescent low-mass X-ray binaries (qLMXBs) inside globular clusters (GCs). Recent observations of NSs have presented evidence that cold ultra dense matter -- present in the core of NSs -- is best described by normal matter equations of state (EoSs). Such EoSs predict that the radii of NSs, Rns, are quasi-constant (within measurement errors, of ~10%) for astrophysically relevant masses (Mns > 0.5 Msun). The present work adopts this theoretical prediction as an assumption, and uses it to constrain a single Rns value from five qLMXB targets with available high signal-to-noise X-ray spectroscopic data. Employing a Markov-Chain Monte-Carlo approach, we produce the marginalized posterior distribution for Rns, constrained to be the same value for all five NSs in the sample. An effort was made to include all quantifiable sources of uncertainty into the uncertainty of the quoted radius measurement. These include the uncertainties in the distances to the GCs, the uncertainties due to the Galactic absorption in the direction of the GCs, and the possibility of a hard power-law spectral component for count excesses at high photon energy, which are observed in some qLMXBs in the Galactic plane. Using conservative assumptions,we found that the radius, common to the five qLMXBs and constant for a wide range of masses, lies in the low range of possible NS radii, Rns=9.1(+1.3)(-1.5) km (90%-confidence). Such a value is consistent with low-res equations of state. We compare this result with previous radius measurements of NSs from various analyses of different types of systems. In addition, we compare the spectral analyses of individual qLMXBs to previous works.
Neutrons stars are unique laboratories to discriminate between the various proposed equations of state of matter at and above nuclear density. One sub-class of neutron stars - those inside quiescent low-mass X-ray binaries (qLMXBs) - produce a thermal surface emission from which the neutron star radius (R_NS) can be measured, using the widely accepted observational scenario for qLMXBs, assuming unmagnetized H atmospheres. In a combined spectral analysis, this work first reproduces a previously published measurement of the rns, assumed to be the same for all neutron stars, using a slightly expanded data set. The radius measured is R_NS = 9.4 +/-1.2 km. On the basis of spectral analysis alone, this measured value is not affected by imposing an assumption of causality in the core. However, the assumptions underlying this R_NS measurement would be falsified by the observation of any neutron star with a mass >2.6 Msun, since radii <11 km would be rejected if causality is assumed, which would exclude most of the R_NS parameter space obtained in this analysis. Finally, this work directly tests a selection of dense matter equations of states: WFF1, AP4, MPA1, PAL1, MS0, and thr
We report the discovery of excess 4.5 and 8 micron emission from three quiescent black hole low-mass X-ray binaries, A 0620-00, GS 2023+338, and XTE J1118+480. The mid-infrared emission from GS 2023+338 probably originates in the accretion disk. However, the excess emission from A 0620-00 and XTE J1118+480 is brighter and peaks at longer wavelengths, and so probably originates from circumbinary dust that is heated by the light of the secondary star. We find that the inner edge of the dust distribution lies near 1.7 times the binary separation, which is the minimum radius at which a circumbinary disk would be stable against tidal disruption. The excess infrared emission is not detected at 24 micron, which implies that the dust does not extend beyond about 3 times the binary separation. The total masses of circumbinary material are between 10^22 and 10^24 g. The material could be the remains of fall-back disks produced in supernovae, or material from the companions injected into circumbinary orbits during mass transfer.
The first detection of gravitational waves from a neutron star-neutron star merger, GW170817, has opened up a new avenue for constraining the ultradense-matter equation of state (EOS). The deviation of the observed waveform from a point-particle waveform is a sensitive probe of the EOS controlling the merging neutron stars structure. In this topical review, I discuss the various constraints that have been made on the EOS in the year following the discovery of GW170817. In particular, I review the surprising relationship that has emerged between the effective tidal deformability of the binary system and the neutron star radius. I also report new results that make use of this relationship, finding that the radius inferred from GW170817 lies between 9.8 and 13.2 km at 90% confidence, with distinct likelihood peaks at 10.8 and 12.3 km. I compare these radii, as well as those inferred in the literature, to X-ray measurements of the neutron star radius. I also summarize the various maximum mass constraints, which point towards a maximum mass < 2.3 M_sun, depending on the fate of the remnant, and which can be used to additionally constrain the high-density EOS. I review the constraints on the EOS that have been performed directly, through Bayesian inference schemes. Finally, I comment on the importance of disentangling thermal effects in future EOS constraints from neutron star mergers.
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