2010A&A...520A..21M

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. We aim at measuring the pulsar flux in the B band for which only an upper limit has been obtained with the HST. We used the deepest ground-based observations of the PSR B1055-52 field obtained with Very Large Telescope (VLT), available in the ESO archive. Due to the non-optimal image quality of the VLT data (∼1.2") we could not resolve the faint pulsar emission against the higher sky background induced by the presence of a bright star (Star A; m_V=14.6) ∼4.4" away from the pulsar position. We determined a 3σ upper limit of m_B∼25.3 on the pulsar brightness. This corresponds to an extinction-corrected flux ≲0.391µJy which is consistent with the HST spectrum and comparable with the HST flux upper limit at a similar wavelength. 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.