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 observed PSR J0437À4715 with the FUV-MAMA detector of the Space Telescope Imaging Spectrometer (STIS) to measure the pulsar's spectrum and pulsations. For the first time, UV emission from a millisecond pulsar has been detected. The measured flux, ð2:0 AE 0:2Þ Â 10 À15 ergs s À1 cm À2 in the 1150-1700 Å range, corresponds to the luminosity L FUV ¼ ð4:7 AE 0:5Þ Â 10 27 ergs s À1 , for the distance of 140 pc and negligible interstellar extinction. The shape of the observed spectrum suggests thermal emission from the neutron star surface with a surprisingly high temperature of about 1 Â 10 5 K, above the upper limit on the surface temperature of the younger ''ordinary'' pulsar J0108À1431. For the few-Gigayear-old J0437À4715, such a temperature requires a heating mechanism to operate. The spectrum of J0437À4715 shows marginal evidence of an emission line at 1372 Å , which might be a gravitationally redshifted Zeeman component of the hydrogen Ly line in a magnetic field of $7 Â 10 8 G. No pulsations are detected, with a 3 upper limit of 50% on the pulsed fraction.

The observations of the pulsar-wind nebula (PWN) around the Vela pulsar with the Advanced CCD Imaging Spectrometer aboard the Chandra X-ray Observatory, taken on 2000 April 30 and November 30, reveal its complex morphology reminiscent of that of the Crab PWN. Comparison of the two observations shows changes up to 30% in the surface brightness of the PWN features. Some of the PWN elements show appreciable shifts, up to a few arcseconds (∼ 10 16 cm), and/or spectral changes. To elucidate the nature of the observed variations, further monitoring of the Vela PWN is needed. Subject headings: supernova remnants: individual (Vela) -pulsars: individual (PSR B0833-45) -X-rays: individual (Vela pulsar-wind nebula)

Pulsar winds shocked in the ambient medium produce spectacular nebulae observable from the radio through gamma-rays. The shape and the spectrum of a pulsar wind nebula (PWN) depend on the angular distribution, magnetization and energy spectrum of the wind streaming from the pulsar magnetosphere, as well as on the pulsar velocity and the properties of the ambient medium. The advent of Chandra, with its unprecedented angular resolution and high sensitivity, has allowed us not only to detect many new PWNe, but also study their spatial and spectral structure and dynamics, which has significantly advanced our understanding of these objects. Here we overview recent observational results on PWNe, with emphasis on Chandra observations.Comment: 15 pages, 5 tables, 10 figures. To appear in the proceedings of "40 Years of Pulsars: Millisecond Pulsars, Magnetars, and More", August 12-17, 2007, McGill University, Montreal, Canada. Corrected typos in Table

We present extensive spectral and photometric observations of the recycled pulsar/white-dwarf binary containing PSR J0437−4715, which we analyzed together with archival X-ray and gammaray data, to obtain the complete mid-infrared to gamma-ray spectrum. We first fit each part of the spectrum separately, and then the whole multi-wavelength spectrum. We find that the opticalinfrared part of the spectrum is well fit by a cool white dwarf atmosphere model with pure hydrogen composition. The model atmosphere (T eff = 3950 ± 150 K, log g = 6.98 ± 0.15, R WD = (1.9 ± 0.2) × 10 9 cm) fits our spectral data remarkably well for the known mass and distance (M = 0.25 ± 0.02M ⊙ , d = 156.3 ± 1.3 pc), yielding the white dwarf age (τ WD = 6.0 ± 0.5 Gyr). In the UV, we find a spectral shape consistent with thermal emission from the bulk of the neutron star surface, with surface temperature between 1.25 × 10 5 and 3.5 × 10 5 K. The temprature of the thermal spectrum suggests that some heating mechanism operates throughout the life of the neutron star. The temperature distribution on the neutron star surface is non-uniform. In the X-rays, we confirm the presence of a high-energy tail which is consistent with a continuation of the cut-off power-law component (Γ = 1.56 ± 0.01, E cut = 1.1 ± 0.2 GeV) that is seen in gamma-rays and perhaps even extends to the near-UV. Subject headings: pulsars: individual (PSR J0437−4715); white dwarfs 7