Orbital phase-resolved spectroscopy of the intermediate polar FO Aqr using XMM-Newton Observatory data

TL;DR

This study analyzes XMM-Newton data of FO Aqr to understand how orbital motion affects X-ray spin modulation and absorption features, revealing ionized absorption from the accretion bulge and its impact on spectral variability.

Contribution

It provides the first orbital-phase resolved spectral analysis of FO Aqr, identifying ionized absorption components and their relation to the accretion structure.

Findings

Spin pulse amplitude varies with orbital phase, peaking at orbital maximum.

Absorption increases significantly at orbital minimum, indicating a bulge impact zone.

Ionized absorption from the accretion bulge is detected and characterized.

Abstract

We present the orbital-phase resolved analysis of an archival FO Aqr observation obtained using the X-ray Multi-Mirror Mission (XMM-Newton), European Photon Imaging Camera (pn instrument). We investigate the variation of the spin pulse amplitudes over the orbital period in order to account for the effects of orbital motion on spin modulation. The semi-amplitude variations are in phase with the orbital modulation, changing from (38.0 +/- 1.8)% at the orbital maximum to (13.3 +/- 3.7)% at the orbital minimum. The spectral parameters also show changes over the orbital period. One of the absorption components increase by a factor of 5 between the orbital minimum and maximum. We interpret that this absorption arises from the bulge where accretion stream from the secondary impacts the disk. The spectrum extracted from the orbital minima and maxima can be fitted with a warm absorber model yielding values N_H = 2.09 (+0.98 -1.09) \times 10^22 and 0.56 (+0.26 -0.15) \times 1022 cm^{-2} ; and log({\xi}) = 0.23 (+0.37 -0.26) and <0.30 erg cm s^{-1} respectively, indicating the existence of ionized absorption from the bulge at the impact zone which is spread out on the disk. The absorption due to accretion curtain and/or column which causes the spin modulation can be distinguished from the disk absorption via spectral modeling.