PSR J1357−6429 is a Vela-like radio pulsar that has been recently detected in X-rays and γ-rays. It powers a compact tail-like X-ray pulsar wind nebula and X-ray-radio plerion associated with an extended TeV source HESS J1356−645. We have performed deep optical observations with the VLT to search for an optical counterpart of the pulsar and its nebula.

The observations were carried out using the direct imaging mode in the V, R, and I bands. We also analyzed archival X-ray data obtained with the Chandra and XMM-Newton. In all three optical bands we detect a point-like source with V=27.3±0.3, R=25.52±0.07, and I=24.13±05 whose center position is consistent at 1-σ significance with the X-ray position of the pulsar and colours are distinct from those of ordinary stars. We consider it as a candidate to the optical counterpart of the pulsar. If it is the true counterpart, its 5-σ offset from the radio pulsar position, measured about 9 yr earlier, implies the transverse velocity of the pulsar in the range of 1600—2000 km s^-1 at the distance of 2–2.5 kpc, making it the fastest moving pulsar known. The direction of the estimated proper motion coincides with the extension of the X-ray tail from the pulsar, suggesting that this is actually a jet.

The tentative optical luminosity and efficiency of the pulsar are similar to those of the Vela-pulsar, which also supports the true optical identification. However, the candidate shows an unusually steep dereddened flux increase towards the infrared with a spectral index α_ν ∼5, not typical for optical pulsars. It suggests a strong double-knee spectral break in the pulsar emission between the optical and X-rays. The reasons for the spectral steepness are unclear. It may be caused by a possible nebula knot projected to the jet and strongly overlapped with the pulsar, as is observed for the Crab, where the knot has the spectrum significantly steeper than that of the pulsar. We find no other signs of the pulsar nebula in the optical. Alternatively, the detected source can be a faint AGN, not seen yet in other spectral domains.

The position, and peculiar colours of the detected source suggest that it is a possible optical counterpart of the pulsar. Further high spatial resolution infrared observations can help to verify its real nature.

Page created on November 9, 2011.