Long-term investigation of γ Cas analogs

Context . The subcategory of γ Cas analogs gathers Be stars with bright and hard X-ray emission. Aims . This paper aims to enlarge the sample of γ Cas analogs studied on long timescales. Long-term variations are expected in such objects for two reasons. First, their Be disk builds and dissipates, leading to changes in optical emission lines, broad-band photometry and possibly even in the X-ray spectrum. Second, such stars are suspected long-period binaries hence velocity variations can be expected, helping to characterize the companions. Methods . Seven targets are analysed in this paper: five of them benefit from a spectroscopic monitoring in the visible (ESO, TIGRE, and amateur data) and three of them have been repeatedly observed at X-ray wavelengths (using XMM-Newton , Chandra , and Swift). Broad-band photometric data, sampling both long and short timescales, are also examined in support of the optical and X-ray spectroscopy. Results . We confirm the binary status of five targets (HD 44458, HD 110432, HD 119682, HD 161103, and HD 162718) and propose first orbital solutions for all of them (they remain preliminary for two cases). Their long periods (59–322 d) and small velocity amplitudes ( K ∼ 5 km s −1 ) imply low-mass (∼1 M ⊙ ) companions. With such properties, these systems appear similar to other Be binaries in general and γ Cas analogs in particular. They also agree with expectations from binary interaction models. In parallel, variations of the X-ray flux are detected in all three targets with a large dataset of X-ray observations. For NGC 6649 9 and HD 162718, these changes are modest (a factor of three) and uncorrelated to simultaneous optical broad-band photometry (which remains rather stable). In contrast, SS 397 varies by nearly one dex and the largest and best monitored X-ray changes correlate well with optical variations. At minimum flux, SS 397 keeps a hard X-ray spectrum despite a nearly normal L X /L BOL ratio, which has not been seen yet among γ Cas analogs. Finally, the photometric behaviours on short timescales of HD 161103, SS 397, and NGC 6649 9 appear linked to broad frequency groups, as typically found for Be stars (including γ Cas analogs). The frequency spectrum of HD 162718 displays a complex mix of (isolated) periodicities with the main one at 6.658 d −1 . This target is thus one of the rare γ Cas analogs to display a strong high-frequency signal typical of β Cep activity.