2012MNRAS.423.2486M

Photometric and spectroscopic results are presented for the Be star X Per/HD 24534 from near-infrared (near-IR) monitoring in 2010–2011. The star is one of a sample of selected Be/X-ray binaries being monitored by us in the near-IR to study correlations between their X-ray and near-IR behaviour. Comparison of the star's present near-IR magnitudes with earlier records shows the star to be currently in a prominently bright state with mean J, H, K magnitudes of 5.49, 5.33 and 5.06, respectively. The JHK spectra are dominated by emission lines of He I and Paschen and Brackett lines of H I. Lines of O I 1.1287 and 1.3165 µm are also present and their relative strength indicates, since O I 1.1287 is the stronger among the two lines, that Lyman β fluorescence plays an important role in their excitation. Recombination analysis of the H I lines is done, which shows that the Paschen and Brackett line strengths deviate considerably from case B predictions. These deviations are attributed to the lines being optically thick, and this supposition is verified by calculating the line centre optical depths predicted by recombination theory. Similar calculations indicate that the Pfund and Humphrey series lines should also be expected to be optically thick, which is found to be consistent with observations reported in other studies. The spectral energy distribution of the star is constructed and shown to have an IR excess. Based on the magnitude of the IR excess, which is modelled using a free–free contribution from the disc, the electron density in the disc is estimated and shown to be within the range of values expected in Be star discs.