We re-analyze the available X-ray spectral data of the type I bursting neutron star transients Aql X−1, Cen X−4, and 4U 1608−522 using realistic hydrogen atmosphere models. Previous spectral fits assumed a blackbody spectrum; because the free-free dominated photospheric opacity decreases with increasing frequency, blackbody spectral fits overestimate the effective temperature and underestimate, by as much as two orders of magnitude, the emitting area. Hydrogen atmosphere spectral models, when fit to the available observational data, imply systematically larger emission area radii, consistent with the canonical 10 km radius of a neutron star. This suggests that a substantial fraction of the quiescent luminosity is thermal emission from the surface of the neutron star. The magnitude of the equivalent hydrogen column density toward these systems, however, presents a considerable systematic uncertainty, which can only be eliminated by high signalto-noise X-ray spectral measurements (e.g., with AXAF or XMM ) which would permit simultaneous determination of the equivalent hydrogen column density, emission area, and thermal temperature.

The central pointlike X-ray source of the Cassiopeia A supernova remnant was discovered in the Chandra first light observation and found later in the archival ROSAT and Einstein images. The analysis of these data does not show statistically significant variability of the source. Because of the small number of photons detected, different spectral models can fit the observed spectrum. The power-law fit yields the photon index -4.1, g = 2.6 and luminosity L(0.1-5.0 keV 2-ergs s Ϫ1 for kpc. The power-law index is higher, and 34 ) = ( 60) # 10 d = 3.4 the luminosity lower, than those observed from very young pulsars. One can fit the spectrum equally well with a blackbody model with -8 MK, -0.5 km, and -ergs s Ϫ1 . The inferred radii 33 T = 6 R = 0.2 L = (1.4 1.9) # 10 bol are too small, and the temperatures too high, for the radiation to be interpreted as emitted from the whole surface of a uniformly heated neutron star. Fits with the neutron star atmosphere models increase the radius and reduce the temperature, but these parameters are still substantially different from those expected for a young neutron star. One cannot exclude, however, the possibility that the observed emission originates from hot spots on a cooler neutron star surface. An upper limit on the (gravitationally redshifted) surface temperature is -2.3 ϱ T ! 1.9 s

Abstract. ESO 4 m class telescope and VLT deep imaging of the isolated neutron star RX J0720.4-3125 reveals a proper motion of µ = 97 ± 12 mas/yr and a blue U − B color index. We show that a neutron star atmosphere model modified to account for a limited amount of hydrogen on the star's surface can well represent both the optical and X-ray data without invoking any additional thermal component. The large proper motion almost completely excludes the possibility that accretion from the interstellar medium is the powering mechanism of the X-ray emission. It also implies that the proposed spin down is entirely due to magnetic dipole losses. RX J0720.4-3125 is thus very likely a middle aged cooling neutron star. Its overall properties are quite similar to some of the long period radio pulsars recently discovered, giving further support to the idea that RX J0720.4-3125 may be a pulsar whose narrow radio beam does not cross the Earth.

Abstract. We present the Chandra LETGS X-ray spectrum of the nearby ( 60 pc) neutron star RXJ1856.5-3754. Detailed spectral analysis of the combined X-ray and optical data rules out the nonmagnetic neutron star atmosphere models with hydrogen, helium, iron and solar compositions. We also conclude that strongly magnetized atmosphere models are unable to represent the data. The data can be explained with a two-component blackbody model. The harder component with temperature of kT ∞ bb 63 eV and a radius R ∞ bb 2.2 km of the emitting region well fits the X-ray data and can be interpreted as radiation from a hot region on the star's surface.

Neutron stars and black holes often reside in binaries where the accretion rate onto the compact object varies by orders of magnitude. These "X-ray transients" are observed both in outburst (when the high accretion rate makes them X-ray bright) and quiescence (when the accretion rate is very low, or potentially zero). In a previous paper, we showed that the quiescent X-ray emission from three neutron star transients (Aql X-1, Cen X-4, and 4U 1608−522) were well represented by thermal emission from the neutron star's hydrogen atmosphere and that the emitting area was consistent with the whole surface. Previous black-body spectral fits (which are not accurate representations of the thermal spectrum) severely underestimated the true emitting area. In this paper, we fit hydrogen atmosphere models to the X-ray data for four neutron stars (the three from the previous paper, plus 4U 2129+47) and six black hole candidates (A0620−00, GS 2000+25, GS 1124−68, GS 2023+33, GRO J1655−40, and GRO J0422+32) with masses ∼ > 3 M ⊙ . While the neutron stars are similar in their intrinsic X-ray spectra (that is, similar effective temperatures and emission area radii ∼ 10 km), the spectra of two black hole candidates are significantly different, and the spectra of the remaining four are consistent with a very large parameter space that includes the neutron stars. The spectral differences between the neutron stars and black hole candidates favors the interpretation that the quiescent neutron star emission -2is predominantly thermal emission from the neutron star surface. Higher quality data from Chandra, XMM, and ASTRO-E will yield a much better contrast.There are many transients which do not have clear neutron star characteristics (such as type I X-ray bursts or coherent pulsations) and where the mass of the compact object is not constrained. In these cases, it is ambiguous as to whether the compact object is a neutron star or black hole. Our work suggests that an X-ray spectral comparison in quiescence provides an additional means for distinguishing between neutron stars and black holes. The faint X-ray sources in globular clusters -thought to be either cataclysmic variables or quiescent neutron stars -are a class of objects which can be investigated in this manner.