The energy distribution and the recent light history of X Persei

Recent UBVRI and near-infrared photometry of X Per, the optical counterpart of 3U 0352 + 30, is used to derive the physical parameters of the Be star and of the circumstellar envelope. A stellar radius of 4.9 R_⊙ and a luminosity of 2.1×10⁴L_⊙, 1 mag fainter than for a main sequence B0 star, are derived for the primary. The colour excess is $$E_{B-V}\leqq 0.46$$. During the stage of higher luminosity (1972–73) the energy distribution is characterized by excess of infrared radiation most likely produced by emission from a hot, probably disk-shaped, circumstellar plasma with an equatorial extension of 2.0 R_∗ and a mean electron density, of $$n_\text e = 1.5\times10^{12}\text {cm}^{-3}$$. The infrared excess is absent during the stage of lower luminosity (1974–75). The intrinsic energy distribution of the Be star is also anomalous with a variable Balmer excess. We suggest that the long-term photometric history of X Per is the result of a size variation of the atmospheric envelope of the Be star which probably affects both the X-ray emission and the infrared excess. Wind accretion on to a nearby compact companion (neutron star) might be responsible for the X-ray emission. A mass loss of about $$10^{-9}M_\odot\enspace\text{yr}^{-1}$$ is thus derived for the Be star.