We observed 1E 1207.4Ϫ5209, a neutron star in the center of the supernova remnant PKS 1209Ϫ51/52, with the ACIS detector aboard the Chandra X-Ray Observatory and detected two absorption features in the source spectrum. The features are centered near 0.7 and 1.4 keV; their equivalent widths are about 0.1 keV. We discuss various possible interpretations of the absorption features and exclude some of them. A likely interpretation is that the features are associated with atomic transitions of once-ionized helium in the neutron star atmosphere with a strong magnetic field. The first clear detection of absorption features in the spectrum of an isolated neutron star provides an opportunity to measure the mass-to-radius ratio and to constrain the equation of state of the superdense matter.

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 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 show that the soft X-ray spectra and light curves observed with the ROSAT and EU V E from the closest known millisecond pulsar J0437-4715 can be interpreted as thermal radiation from two hot polar caps whose emitting layers (atmospheres) are comprised of hydrogen. The simplest model yields a uniform temperature of (0.8 − 0.9) × 10 6 K within a cap radius of 0.7 − 0.9 km. The spectral fits indicate that the temperature may be nonuniformly distributed along the cap surface. The distribution can be approximated by a central core heated up to (1 − 2) × 10 6 K within a radius of 0.2 − 0.4 km, surrounded by a colder rim with temperatures (3 − 5)× 10 5 K extending out to 2−6 km. The polar cap interpretation implies low column densities, (1 − 3) × 10 19 cm −2 , and a high degree of ionization, > 20%, of the interstellar hydrogen towards the pulsar. The inferred bolometric luminosity of the polar caps, (1.0 − 1.6) × 10 30 erg s −1 , is in excellent agreement with the predictions of the slot-gap model of radio pulsars developed by Arons and his coworkers. Similar polar cap radiation should be emitted by other millisecond pulsars, although in some of them (e. g., PSR B1821-24) the soft X-ray flux is dominated by the nonthermal radiation from pulsar magnetospheres.

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