We present the first astrophysical measurement of the pressure of cold matter above nuclear saturation density, based on recently determined masses and radii of three neutron stars. The pressure at higher densities are below the predictions of equations of state that account only for nucleonic degrees of freedom, and thus present a challenge to the microscopic theory of neutron star matter.PACS numbers: 97.60. Jd, 26.60.Kp, Neutron stars probe the dense QCD phase diagram at lower temperatures and higher baryon densities, in contrast to the higher temperature-lower density regime in the early universe and in ultrarelativistic heavy-ion collisions [1,2]. The baryon density, ρ, in stellar interiors may reach an order of magnitude beyond nuclear saturation density, ρ ns ≃ 2.7 × 10 14 g cm −3 ≃ 0.16 fm −3 , and cross into a regime where quark degrees of freedom are excited or matter undergoes a meson condensation phase transition. The pressure of matter at these high densities can, as we show here, be extracted from current neutron star mass-radius determinations [3], and crucially constrains calculations of high density neutron star matter.The equation of state (EoS) of supranuclear matter determines the dividing line between neutron stars and black holes, and directly impacts the mechanism as well as outcomes of supernova explosions and the numbers of neutron stars and black holes in the Galaxy [4,5]. In certain models, it affects the mechanism and duration of gamma-ray bursts [6]. Accurate evolutions of inspiraling neutron-star binaries and the collapse to black holes, needed to calculate gravitational wave signals, depend sensitively on the assumed EoS [7].Microscopic calculations of the EoS of neutron-star matter have been based on a variety of inputs. The approach most firmly founded on experiment in the region of ρ ns is to determine two-body potentials from nucleonnucleon scattering data below 350 MeV and properties of light nuclei, supplemented by a three-body potential [8,9]. Such calculations, accurate in the neighborhood of ρ ns , have fundamental limitations. Beyond a few times ρ ns the forces between particles can no longer be described via static few-body potentials; since the characteristic range of the nuclear forces is ∼ 1/2m π , where m π is the mass of the pion, the parameter measuring the relative importance of three and higher body forces is ∼ ρ/(2m π ) 3 ∼ 0.35ρ/ρ ns . Thus, at ρ ≫ ρ ns a well defined expansion in terms of two-, three-, or more, body forces no longer exists. EoS based on nucleons alone do not take into account the rich variety of hadronic degrees of freedom that enter with increasing density. In addition, pion condensates [8,10,11] or kaon condensates [12,13] can enter at higher densities. Field-theoretic models based on nucleons interacting via meson exchange include, e.g., Ref [14]; see Ref.[15] for a general summary of EoS. However, one cannot assume that matter at higher densities can even be described in terms of well-defined "asymptotic" laboratory particles. More realist...

We present a comprehensive study of spectroscopic radius measurements of twelve neutron stars obtained during thermonuclear bursts or in quiescence. We incorporate, for the first time, a large number of systematic uncertainties in the measurement of the apparent angular sizes, Eddington fluxes, and distances, in the composition of the interstellar medium, and in the flux calibration of X-ray detectors. We also take into account the results of recent theoretical calculations of rotational effects on neutron star radii, of atmospheric effects on surface spectra, and of relativistic corrections to the Eddington critical flux. We employ Bayesian statistical frameworks to obtain neutron star radii from the spectroscopic measurements as well as to infer the equation of state from the radius measurements. Combining these with the results of experiments in the vicinity of nuclear saturation density and the observations of M 2  neutron stars, we place strong and quantitative constraints on the properties of the equation of state between 2 8 -» times the nuclear saturation density. We find that around M M 1.5 , =  the preferred equation of state predicts radii between 10.1 and 11.1 km. When interpreting the pressure constraints in the context of high density equations of state based on interacting nucleons, our results suggest a relatively weak contribution of the three-body interaction potential.

Low mass X-ray binaries (LMXBs) that show thermonuclear bursts are ideal sources for constraining the equation of state of neutron star matter. The lack of independent distance measurements for most of these sources, however, prevents a systematic exploration of the masses and radii of the neutron stars, hence limiting the equation-of-state studies. We present here a measurement of the distance to the LMXB 4U 1608−52 that is based on the study of the interstellar extinction towards the source. We first model the individual absorption edges of the elements Ne and Mg in the high-resolution X-ray spectrum obtained with XMM-Newton. We then combine this information with a measurement of the run of reddening with distance using red clump stars and determine a minimum distance to the source of 3.9 kpc, with a most probable value of 5.8 kpc. Finally, we analyze time-resolved X-ray spectra of Type-I X-ray bursts observed from this source to measure the mass and the radius of the neutron star. We find a mass of M= 1.74 ± 0.14 M ⊙ and a radius of R= 9.3 ± 1.0 km, respectively. This mass and radius can be achieved by several multi-nucleon equations of state.Subject headings: stars: neutron -X-ray: individual (4U 1608−52)

A linear relation between the hydrogen column density (N H ) and optical extinction (A V ) in the Galaxy has long been observed. A number of studies found differing results in the slope of this relation. Here, we utilize the data on 22 supernova remnants that have been observed with the latest generation X-ray observatories and for which optical extinction and/or reddening measurements have been performed and find N H (cm −2 ) = (2.21 ± 0.09) × 10 21 A V (mag). We compare our result with the previous studies and assess any systematic uncertainties that may affect these results.

We present multiwavelength observations of the afterglow of GRB 130427A, the brightest (in total fluence) gamma-ray burst of the past 29 years. Optical spectroscopy from Gemini-North reveals the redshift of the GRB to be z = 0.340, indicating that its unprecedented brightness is primarily the result of its relatively close proximity to Earth; the intrinsic luminosities of both the GRB and its afterglow are not extreme in comparison to other bright GRBs. We present a large suite of multiwavelength observations spanning from 300 s to 130 d after the burst and demonstrate that the afterglow shows relatively simple, smooth evolution at all frequencies, with no significant latetime flaring or rebrightening activity. The entire dataset from 1 GHz to 10 GeV can be modeled as synchrotron emission from a combination of reverse and forward shocks in good agreement with the standard afterglow model, providing strong support to the applicability of the underlying theory and clarifying the nature of the GeV emission observed to last for minutes to hours following other very bright GRBs. A tenuous, wind-stratified circumburst density profile is required by the observations, suggesting a massive-star progenitor with a low mass-loss rate, perhaps due to low metallicity. GRBs similar in nature to GRB 130427A, inhabiting low-density media and exhibiting strong reverse shocks, are probably not uncommon but may have been difficult to recognize in the past owing to their relatively faint late-time radio emission; more such events should be found in abundance by the new generation of sensitive radio and millimeter instruments. 25 Here and elsewhere we assume a standard ΛCDM cosmological model with Ω Λ = 0.7, Ωm = 0.3, h = 0.7.