VLT observations of the middle-aged pulsar PSR B1055-52

Context. The 535 kyear old radio pulsar B1055–52 is the oldest of the “Three Musketeers”, the group of the middle-aged pulsars (the other two being PSR B0656+14 and Geminga) with bright soft X-ray and high-energy γ-ray emission. The identification of its optical counterpart has been only recently confirmed through observations performed with the Hubble Space Telescope (HST), which provided a first characterisation of the pulsar spectrum. Aims. We aim at measuring the pulsar flux in the B band for which only an upper limit has been obtained with the HST. Methods. We used the deepest ground-based observations of the PSR B1055-52 field obtained with Very Large Telescope (VLT), available in the ESO archive. Results. Due to the non-optimal image quality of the VLT data (~$1\farcs2$) we could not resolve the faint pulsar emission against the higher sky background induced by the presence of a bright star (Star A; mV = 14.6) ~$4\farcs4$ away from the pulsar position. We determined a 3 σ upper limit of mB ~ 25.3 on the pulsar brightness. This corresponds to an extinction-corrected flux $\la$0.391 μJy which is consistent with the HST spectrum and comparable with the HST flux upper limit at a similar wavelength. Conclusions. The presence of Star A makes it improbable to detect PSR B1055-52 in the optical bands through ground-based observations, unless performed under exceptional seeing conditions. The situation might be more promising through adaptive optics high-spatial resolution observations in the near-infrared, where the HST spectrum hints at a steep power-law which predicts an higher flux at longer wavelengths