A thousand looks at X Persei: X-ray spectroscopy at high time resolution

Context. X Persei is a classical Be/X-ray binary composed of an O9.5III-B0V Be type star and a neutron star (NS). The NS exhibits coherent pulsations with a spin period of ∼837 s, orbiting the Be star with ∼250 d period. Its exceptionally hard X-ray emission extending beyond 100 keV makes X Persei especially notable. Due to the mild eccentricity of the orbit ∼0.11, the orbital separation varies between roughly 35 R★ (periastron) and 44 R★ (apastron). Aims. In this work, we analyzed five targeted observations obtained with the XMM-Newton and Chandra observatories taken over a ten-year time span, with the aim of investigating the structure and variability of the circumstellar disk surrounding the Be star. In particular, we focused on the presence of overdense areas known as clumps. Methods. We performed spectral and timing analyses, including the average and NS spin-resolved spectroscopy for three of the observations, producing individual spectra at intervals as short as 210 s that correspond to different epochs of the NS spin, resulting in approximately 1200 spectra. This detailed analysis is aimed at resolving emission line features otherwise diluted in averaged spectra and the evolution of continuum components along the NS spin. Results. The observed spectra were accurately modeled by a two-component continuum comprising a high-temperature blackbody and a power-law component. Phase-resolved spectroscopy revealed transient Fe Kα emission linked to clumps in the circumstellar disk during high density epochs, with an 8−9% prevalence. Dips in the X-ray light curve are tied to clump passages. Disk density modeling, based solely on X-ray data suggests a compact and dense disk, with a radial density exponent α of 2.4−3.3 and a high inner disk density of ρ0 ∼ (6 − 20)×10−10 g cm−3. This result is in agreement with previous studies carried out in the optical and infrared bands.