Possible optical detection of a fast, nearby radio pulsar PSR B1133+16

Aims.We performed deep optical observations of the field of an old, fast-moving radio pulsar PSR B1133+16 in an attempt to detect its optical counterpart and a bow shock nebula. Methods.The observations were carried out using the direct imaging mode of FORS1 at the ESO VLT/UT1 telescope in the B, R, and Hα bands. We also used archival images of the same field obtained with the VLT in the B band and with the Chandra/ACIS in X-rays. Results. In the B band we detected a faint (B = 28$.\!\!^{\rm m}$1 ± 0$.\!\!^{\rm m}$3) source that may be the optical counterpart of PSR B1133+16, as it is positionally consistent with the radio pulsar and with the X-ray counterpart candidate published earlier. Its upper limit in the R band implies a color index $B-R$ $\la$ 0$.\!\!^{\rm m}$5, which is compatible with the index values for most pulsars identified in the optical range. The derived optical luminosity and its ratio to the X-ray luminosity of the candidate are consistent with expected values derived from a sample of pulsars detected in both spectral domains. No Balmer bow shock was detected, implying a low density of ambient matter around the pulsar. However, in the X-ray and Hα images we found the signature of a trail extending ~$ 4^{\prime\prime}{-}5^{\prime\prime}$ behind the pulsar and coinciding with the direction of its proper motion. If confirmed by deeper studies, this is the first time such a trail has been seen in the optical and X-ray wavelengths. Conclusions.Further observations at later epochs are necessary to confirm the identification of the pulsar by the candidate's proper motion measurements