We review multiwavelength properties of pulsar wind nebulae created by supersonically moving pulsars and the effects of pulsar motion on the pulsar wind nebulae morphologies and the ambient medium. Supersonic pulsar wind nebulae are characterized by bow-shaped shocks around the pulsar and/or cometary tails filled with the shocked pulsar wind. In the past several years significant advances in supersonic pulsar wind nebula studies have been made in deep observations with the Chandra and XMM-Newton X-ray observatories and the Hubble Space Telescope. In particular, these observations have revealed very diverse supersonic pulsar wind nebula morphologies in the pulsar vicinity, different spectral behaviours of long pulsar tails, the presence of puzzling outflows misaligned with the pulsar velocity and far-UV bow shocks. Here we review the current observational status focusing on recent developments and their implications.
We observed a nearby millisecond pulsar J2124-3358 with the Hubble Space Telescope in broad far-UV (FUV) and optical filters. The pulsar is detected in both bands with fluxes F (1250-2000Å) = (2.5 ± 0.3) × 10 −16 erg s −1 cm −2 and F (3800 − 6000Å) = (6.4 ± 0.4) × 10 −17 erg s −1 cm −2 , which correspond to luminosities of ≈ 5.8 × 10 27 and 1.4 × 10 27 erg s −1 , for d = 410 pc and E(B − V ) = 0.03. The optical-FUV spectrum can be described by a power-law model, f ν ∝ ν α , with slope α = 0.18-0.48 for a conservative range of color excess, E(B − V ) = 0.01-0.08. Since a spectral flux rising with frequency is unusual for pulsar magnetospheric emission in this frequency range, it is possible that the spectrum is predominantly magnetospheric (power law with α < 0) in the optical while it is dominated by thermal emission from the neutron star surface in the FUV. For a neutron star radius of 12 km, the surface temperature would be between 0.5 × 10 5 and 2.1 × 10 5 K, for α ranging from −1 to 0, E(B − V ) = 0.01-0.08, and d = 340-500 pc. In addition to the pulsar, the FUV images reveal extended emission spatially coincident with the known Hα bow shock, making PSR J2124-3358 the second pulsar (after PSR J0437−4715) with a bow shock detected in FUV.
We report on a Hubble Space Telescope detection of the nearby, old pulsar B0950+08 (d 262 pc, spin-down age 17.5 Myr) in two far-ultraviolet (FUV) bands. We measured the mean flux densitiesf ν = 109 ± 6 nJy and 83 ± 14 nJy in the F125LP and F140LP filters (pivot wavelengths 1438 and 1528Å). Using the FUV data together with previously obtained optical-UV data, we conclude that the optical-FUV spectrum consists of two components -a nonthermal (presumably magnetospheric) power-law spectrum (f ν ∝ ν α ) with slope α ∼ −1.2 and a thermal spectrum emitted from the bulk of the neutron star surface with a temperature in the range of (1-3) × 10 5 K, depending on interstellar extinction and neutron star radius. These temperatures are much higher than predicted by neutron star cooling models for such an old pulsar, which means that some heating mechanisms operate in neutron stars. A plausible mechanism responsible for the high temperature of PSR B0950+08 is the interaction of vortex lines of the faster rotating neutron superfluid with the slower rotating normal matter in the inner neutron star crust (vortex creep heating).
We report non-detections of the ∼ 3 × 10 8 yr old, slow, isolated, rotation-powered pulsar PSR J2144-3933 in observations with the Hubble Space Telescope in one optical band (F475X) and two farultraviolet bands (F125LP and F140LP), yielding upper bounds F F475X < 22.7 nJy, F F125LP < 5.9 nJy, F F140LP < 19.5 nJy, at the pivot wavelengths 4940Å, 1438Å and 1528Å, respectively. Assuming a blackbody spectrum, we deduce a conservative upper bound on the surface (unredshifted) temperature of the pulsar of T < 42, 000 K. This makes PSR J2144-3933 the coldest known neutron star, allowing us to study thermal evolution models of old neutron stars. This temperature is consistent with models with either direct or modified Urca reactions including rotochemical heating, and, considering frictional heating from the motion of neutron vortex lines, it puts an upper bound on the excess angular momentum in the neutron superfluid, J < 10 44 erg s.
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