The miniJPAS survey: White dwarf science with 56 optical filters

Aims. We analyze the white dwarf population in miniJPAS, the first square degree observed with 56 medium-band, 145 angstrom in width optical filters by the Javalambre Physics of the accelerating Universe Astrophysical Survey (J-PAS), to provide a data-based forecast for the white dwarf science with low-resolution (R  similar to  50) photo-spectra. Methods. We define the sample of the bluest point-like sources in miniJPAS with r  <  21.5 mag, a point-like probability larger than 0.5, (u - r)< 0.80 mag, and (g - i)< 0.25 mag. This sample comprises 33 sources with spectroscopic information: 11 white dwarfs and 22 quasi-stellar objects (QSOs). We estimate the effective temperature (T-eff), the surface gravity, and the composition of the white dwarf population by a Bayesian fitting to the observed photo-spectra. Results. The miniJPAS data are sensitive to the Balmer series and the presence of polluting metals. Our results, combined with those from the Javalambre Photometric Local Universe Survey (J-PLUS) which has a lower spectral resolution but has already observed thousands of white dwarfs, suggest that J-PAS photometry would permit - down to r  similar to  21.5 mag and at least for sources with 7000  <  T-eff  <  22  000 K - both the classification of the observed white dwarfs into H-dominated and He-dominated with 99% confidence and the detection of calcium absorption for equivalent widths larger than 15 angstrom. The effective temperature is estimated with a 2% uncertainty, which is close to the 1% from spectroscopy. A precise estimation of the surface gravity depends on the available parallax information. In addition, the white dwarf population at T-eff  >  7000 K can be segregated from the bluest extragalactic QSOs, providing a clean sample based on optical photometry alone. Conclusions. The J-PAS low-resolution photo-spectra would produce precise effective temperatures and atmospheric compositions for white dwarfs, complementing the data from Gaia. J-PAS will also detect and characterize new white dwarfs beyond the Gaia magnitude limit, providing faint candidates for spectroscopic follow-up.