The type-I X-ray bursting low mass X-ray binary KS 1731−260 was recently detected for the first time in quiescence by Wijnands et al., following a τ outburst ≈13 yr outburst which ended in Feb 2001. We show that the emission area radius for a H atmosphere spectrum (possibly with a hard power-law component that dominates the emission above 3.5 keV) is consistent with that observed from other quiescent neutron star transients, R ∞ =23 +30 −15 (d/8 kpc) km, and examine possible IR counterparts for KS 1731−260. Unlike all other known transient neutron stars (NS), the duration of this recent (and the only observed) outburst is as long as the thermal diffusion time of the crust. The large amount of heat deposited by reactions in the crust will have heated the crust to temperatures much higher than the equilibrium core temperature. As a result, the thermal luminosity currently observed from the neutron star is dominated not by the core, but by the crust. This scenario implies that the mean outburst recurrence timescale found by Wijnands et al. (∼ 200 yr) is a lower limit. Moreover, because the thermal emission is dominated by the hot crust, the level and the time evolution of quiescent luminosity is determined mostly by the amount of heat deposited in the crust during the most recent outburst (for which reasonable constraints on the mass accretion rate exist), and is only weakly sensitive to the core temperature. Using estimates of the outburst mass accretion rate, our calculations of the quiescent flux immediately following the end of the outburst agree with the observed quiescent flux to within a factor of a few. In this paper, we present simulations of the evolution of the quiescent lightcurve for different scenarios of the crust microphysics, and demonstrate that monitoring observations (with currently flying instruments) spanning from 1-30 yr can measure the crust cooling timescale and the total amount of heat stored in the crust. These quantities have not been directly measured for any neutron star. This makes KS 1731−260 a unique laboratory for studying the thermal properties of the crust by monitoring the luminosity over the next few years to decades.

Observations of the Vela pulsar-wind nebula (PWN) with the Chandra X-Ray Observatory have revealed a complex, variable PWN structure, including inner and outer arcs, a jet in the direction of the pulsar's proper motion, and a counterjet in the opposite direction, embedded in diffuse nebular emission. The jet consists of a bright, 8 00 long inner jet, between the pulsar and the outer arc, and a dim, curved outer jet that extends up to $100 00 in approximately the same direction. From the analysis of 13 Chandra observations spread over %2.5 yr we found that this outer jet shows particularly strong variability, changing its shape and brightness. We observed bright blobs in the outer jet moving away from the pulsar with apparent speeds (0.3-0.6)c and fading on timescales of days to weeks. If the blobs are carried away by a flow along the jet, the observed variations suggest mildly relativistic flow velocities, about (0.3-0.7)c.

We obtained four Chandra/ACIS-S observations beginning 2 weeks after the end of the 2000 November outburst of the neutron star (NS) transient Aql X-1. Over the 5 month span in quiescence, the X-ray spectra are consistent with thermal emission from a NS with a pure hydrogen photosphere and R 1 ¼ 15:9 þ0:8 À2:9 (d=5 kpc) km at the optically implied X-ray column density. We also detect a hard power-law tail during two of the four observations. The intensity of Aql X-1 first decreased by 50% AE 4% over 3 months, then increased by 35% AE 5% in 1 month, and then remained constant (<6% change) over the last month. These variations in the first two observations cannot be explained by a change in either the power-law spectral component or the X-ray column density. Presuming a pure hydrogen atmosphere and that R 1 is not variable, the long-term changes can only be explained by variations in the NS effective temperature, from kT eff;1 ¼ 130 þ3 À5 eV, down to 113 þ3 À4 eV, and finally increasing to 118 þ9 À4 eV for the final two observations. During one of these observations, we observe two phenomena that were previously suggested as indicators of quiescent accretion onto the NS: short-timescale (<10 4 s) variability (at 32 þ8 À6 % rms) and a possible absorption feature near 0.5 keV. The possible absorption feature can potentially be explained as being due to a time-variable response in the ACIS detector. Even so, such a feature has not been detected previously from a NS and, if confirmed and identified, can be exploited for simultaneous measurements of the photospheric redshift and NS radius.

We report the results of the spectral analysis of two observations of the Vela pulsar with the Chandra X-Ray Observatory. The spectrum of the pulsar does not show statistically significant spectral lines in the observed 0.25-8.0 keV band. Similar to middle-aged pulsars with detected thermal emission, the spectrum consists of two distinct components. The softer component can be modeled as a magnetic hydrogen atmosphere spectrum-for the pulsar magnetic field G and neutron star mass and radius km, we

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.