A. Danilenko, A. Kirichenko, R. E. Mennickent, G. Pavlov, Yu. Shibanov, S. Zharikov, D. Zyuzin
PSR J1357--6429 is a Vela-like radio pulsar that has been recently detected
in X-rays and gamma-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. A point-like source
has been detected in V, R, and I bands whose centre position is within the
1-sigma error circle of the X-ray position of the pulsar, and whose colours are
distinct from those of ordinary stars. We consider it as a candidate optical
counterpart of the pulsar. If it is indeed the counterpart, its 5-sigma 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
pulsar's X-ray tail, suggesting that this is a jet. The tentative optical
luminosity and efficiency of the pulsar are similar to those of the Vela
pulsar, which also supports the optical identification. However, the candidate
shows an unusually steep dereddened flux increase towards the infrared with a
spectral index of about 5, not typical for optical pulsars. It implies 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.
View original:
http://arxiv.org/abs/1202.0437
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