Glancing through the accretion column of EXO 2030+375

Context. The current generation of X-ray instruments progressively reveals more and more details about the complex magnetic field topology and the geometry of the accretion flows in highly magnetized accretion-powered pulsars.
Aims: We took advantage of the large collecting area and good timing capabilities of the EPIC cameras onboard XMM-Newton to investigate the accretion geometry onto the magnetized neutron star hosted in the high-mass X-ray binary EXO 2030+375 during the rise of a source type I outburst in 2014.
Methods: We carried out a timing and spectral analysis of the XMM-Newton observation as a function of the neutron star spin phase. We used a phenomenological spectral continuum model comprising the required fluorescence emission lines. Two neutral absorption components are present: one covering the source fully, one only partially. The same analysis was also carried out on two Suzaku observations of the source performed during outbursts in 2007 and 2012, to search for possible spectral variations at different luminosities.
Results: The XMM-Newton data caught the source at an X-ray luminosity of 2 × 10³⁶ erg s^-1 and revealed a narrow dip-like feature in its pulse profile that was never reported before. The width of this feature corresponds to about one hundredth of the neutron star spin period. Based on the results of the phase-resolved spectral analysis we suggest that this feature can be ascribed to the self-obscuration of the accretion stream passing in front of the observer line of sight. We inferred from the Suzaku observation carried out in 2007 that the self-obscuration of the accretion stream might produce a significantly wider feature in the neutron star pulsed profile at higher luminosities (≳2 × 10³⁷ erg s^-1).
Conclusions: This discovery allowed us to derive additional constraints on the physical properties of the accretion flow in this object at relatively small distances from the neutron star surface. The narrow dip-like feature in the pulse profile is so far unique among all known high mass X-ray binaries hosting strongly magnetized neutron stars.