Glancing through the accretion column of EXO 2030+375

TL;DR

This study uses XMM-Newton and Suzaku observations to analyze the accretion geometry and pulse profile features of the neutron star in EXO 2030+375, revealing a narrow dip likely caused by self-obscuration of the accretion stream.

Contribution

First detection of a narrow dip in the pulse profile linked to accretion stream self-obscuration, with analysis across different luminosities and phases.

Findings

Narrow dip feature in pulse profile at 2×10^36 erg/s.

Wider dip feature observed at higher luminosity (~2×10^37 erg/s).

Self-obscuration of accretion stream explains the dip features.

Abstract

We took advantage of the large collecting area and good timing capabilities of the EPIC cameras on-board 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. We carried out a timing and spectral analysis of the XMM-Newton observation as 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 fully the source and 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. The XMM-Newton data caught the source at an X-ray luminosity of $2\times10^{36}$ erg s$^{-1}$ and revealed the presence of 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. From 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 ($\gtrsim$$2\times10^{37}$ erg s$^{-1}$).