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Deep optical observations of the gamma-ray pulsar J0357+3205

A middle-aged radio-quiet pulsar J0357+3205 was discovered in gamma-rays with $Fermi$ and later in X-rays with $Chandra$ and $XMM$-$Newton$ observatories. It produces an unusual thermally-emitting pulsar wind nebula observed in X-rays. Deep optical observations were obtained to search for the pulsar optical counterpart and its nebula using the Gran Telescopio Canarias (GTC). The direct imaging mode in the Sloan $g'$ band was used. Archival X-ray data were reanalysed and compared with the optical data. No pulsar optical counterpart was detected down to $g'\geq~28_{\cdotp}^{\text{m}}1$. No pulsar nebula was either identified in the optical. We confirm early results that the X-ray spectrum of the pulsar consists of a nonthermal power-law component of the pulsar magnetospheric origin dominating at high energies and a soft thermal component from the neutron star surface. Using magnetised partially ionised hydrogen atmosphere models in X-ray spectral fits we found that the thermal component can come from entire surface of the cooling neutron star with a temperature of 36$^{+9}_{-6}$ eV, making it one of the coldest among cooling neutron stars known. The surface temperature agrees with the standard neutron star cooling scenario. The optical upper limit does not put any additional constraints on the thermal component, however it implies a strong spectral break for the nonthermal component between the optical and X-rays as is observed in other middle-aged pulsars. The thermal emission from the entire surface of the neutron star likely dominates over the nonthermal emission in the UV range. Observations of the PSR J0357+3205 in this range are promising to put more stringent constraints on its thermal properties.