V. Suleimanov

A NEUTRON STAR STIFF EQUATION OF STATE DERIVED FROM COOLING PHASES OF THE X-RAY BURSTER 4U 1724−307

Thermal emission during X-ray bursts is a powerful tool for determining neutron star (NS) masses and radii if the Eddington flux and the apparent radius in the cooling tail can be measured accurately and distances to the sources are known. We propose here an improved method of determining the basic stellar parameters using the data from the cooling phase of photospheric radius expansion (PRE) bursts covering a large range of luminosities. Because at that phase the blackbody apparent radius depends only on the spectral hardening factor (color correction), we suggest fitting the theoretical dependences of the color correction versus flux in Eddington units to the observed variations of the inverse square root of the apparent blackbody radius with the flux. For that we use a large set of atmosphere models for burst luminosities varying by three orders of magnitude and for various chemical compositions and surface gravities. We show that spectral variations observed during a long PRE burst from 4U 1724-307 are entirely consistent with the theoretical expectations for the passively cooling NS atmospheres. Our method allows us to more reliably determine both the Eddington flux (which is found to be smaller than the touchdown flux by 15%) and the ratio of the stellar apparent radius to the distance. We then find a lower limit on the NS radius of 14 km for masses below 2.3 M ☉, independently of the chemical composition. These results suggest that the matter inside NSs is characterized by a stiff equation of state. We also find evidence in favor of hydrogen-rich accreting matter and obtain an upper limit to the distance of 7 kpc. We finally show that the apparent blackbody emitting area in the cooling tails of the short bursts from 4U 1724-307 is two times smaller than that for the long burst and their evolution does not follow the theory. This makes their usage for determining the NS parameters questionable and casts serious doubt on the results of previous works that used similar bursts from other sources for analysis.

X-ray bursting neutron star atmosphere models: spectra and color corrections

Aims. X-ray bursting neutron stars in low mass X-ray binaries cons titute an appropriate source class to constrain masses and r adii of neutron stars, but a su fficiently extended set of corresponding model atmospheres is necessary for these investigations. Methods. We computed such a set of model atmospheres and emergent spec tra in a plane-parallel, hydrostatic, and LTE approximation with Compton scattering taken into account. Results. The models were calculated for six di fferent chemical compositions: pure hydrogen and pure helium atmospheres, and atmospheres with solar mix of hydrogen and helium, and various heavy element abundances Z = 1, 0.3, 0.1, and 0.01Z⊙. For each chemical composition the models are computed for three valu es of surface gravity, log=14.0, 14.3, and 14.6, and for 20 values of the luminosity in units of the Eddington luminosity, L/LEdd, in the range 0.001–0.98. The emergent spectra of all models ar redshifted and fitted by a diluted blackbody in the RXTE/PCA 3–20 keV energy band, and corresponding values of the col or rrection (hardness factors) fc are presented. Conclusions. Theoretical dependences fc–L/LEdd can fitted to the observed dependence K−1/4–F of the blackbody normalization K on flux during cooling stages of X-ray bursts to determine the Eddington flux and the ratio of the apparent neutron star radi us to the source distance. If the distance is known, these parameters can be transformed to the constraints on neutron star mass an d radius. The theoretical atmosphere spectra can also be used for direct c omparison with the observed X-ray burst spectra.Aims. X-ray bursting neutron stars in low-mass X-ray binaries constitute an appropriate source class for constraining the masses and radii of neutron stars, but a sufficiently extended set of corresponding model atmospheres is necessary for these investigations. Methods. We computed such a set of model atmospheres and emergent spectra in a plane-parallel, hydrostatic, and LTE approximation with Compton scattering taken into account. Results. The models were calculated for six different chemical compositions: pure hydrogen, pure helium, and a solar mix of hydrogen and helium with various heavy element abundances Z = 1, 0.3, 0.1, and 0.01 Z� . For each chemical composition the models are computed for three values of surface gravity, log g =14.0, 14.3, and 14.6, and for 20 values of the luminosity in units of the Eddington luminosity, L/LEdd, in the range 0.001–0.98. The emergent spectra of all models are redshifted and fitted by a diluted blackbody in the RXTE/PCA 3–20 keV energy band, and corresponding values of the color correction (hardness factors) fc are presented. Conclusions. Theoretical dependences fc–L/LEdd can be fitted to the observed dependence K −1/4 –F of the blackbody normalization K on flux during cooling stages of X-ray bursts to determine the Eddington flux and the ratio of the apparent neutron star radius to the source distance. If the distance is known, these parameters can be transformed to the constraints on neutron star mass and radius. Theoretical atmosphere spectra can also be used for direct comparison with the observed X-ray burst spectra.

CHANDRA DISCOVERY OF LUMINOUS SUPERSOFT X-RAY SOURCES IN M81

A Chandra ACIS-S imaging observation of the nearby galaxy M81 (NGC 3031) reveals nine luminous soft X-ray sources. The local environments, X-ray spectral properties, and X-ray light curves of the sources are presented and discussed in the context of prevailing physical models for supersoft sources. It is shown that the sample falls within expectations based on population synthesis models taken from the literature, although the high observed luminosities (Lobs � 2 � 10 36 –3 � 10 38 ergs s � 1 in the 0.2–2.0 keV band) and equivalent blackbody temperatures (Teff � 40 80 eV) place the brightest detected M81 objects at the high-luminosity end of the class of supersoft sources defined by previous ROSAT and Einstein studies of nearby galaxies. This is interpreted as a natural consequence of the higher sensitivity of Chandra to hotter and more luminous systems. Most of the sources can be explained as canonical supersoft sources: accreting white dwarfs powered by steady surface nuclear burning with X-ray spectra well fitted by hot white dwarf local thermodynamic equilibrium atmosphere models. An exceptionally bright source is scrutinized in greater detail since its estimated bolometric luminosity, Lbol � 1:5 � 10 39 ergs s � 1 , greatly exceeds theoretical estimates for supersoft sources. This source may be beyond the stability limit and undergoing a phase of mass outflow under extreme conditions. Alternatively, a model in which the observed X-ray spectrum arises from an accretion disk around a black hole of mass � 1200=ðcos iÞ 1=2 M� (viewed at an inclination angle i) cannot be excluded. Subject headings: binaries: symbiotic — stars: atmospheres — stars: evolution — white dwarfs — X-rays: stars

On the maximum accretion luminosity of magnetized neutron stars: connecting X-ray pulsars and ultraluminous X-ray sources

We study properties of luminous X-ray pulsars using a simplified model of the accretion column. The maximal possible luminosity is calculated as a function of the neutron star (NS) magnetic field and spin period. It is shown that the luminosity can reach values of the order of

Spectra of the spreading layers on the neutron star surface and constraints on the neutron star equation of state

10^{40}\,{\rm erg/s}

The effect of accretion on the measurement of neutron star mass and radius in the low-mass X-ray binary 4U 1608−52

for the magnetar-like magnetic field (

A reflection model for the cyclotron lines in the spectra of X-ray pulsars

B\gtrsim 10^{14}\,{\rm G}

The neutron star in HESS J1731−347: Central compact objects as laboratories to study the equation of state of superdense matter

) and long spin periods (

New constraints on the cooling of the Central Compact Object in Cas A

P\gtrsim 1.5\,{\rm s}

Witnessing the magnetospheric boundary at work in Vela X−1

). The relative narrowness of an area of feasible NS parameters which are able to provide higher luminosities leads to the conclusion that