Orbital and Spin Phase-Resolved Spectroscopy of the Intermediate Polar EX Hya Using XMM-Newton Data

TL;DR

This study presents the first orbital and spin phase-resolved X-ray spectra of the intermediate polar EX Hya, revealing spectral variations, stable emission components, and evidence of reflection from the accretion disc.

Contribution

It provides the first detailed orbital and spin phase-resolved spectral analysis of EX Hya using XMM-Newton data, highlighting spectral stability and phase-dependent features.

Findings

Spectral components at 0.6-0.8 keV and 1.3-1.7 keV are nearly constant over phases.

The VMCFLOW component varies with phase and epochs, indicating shock zone changes.

A 6.4 keV Fe line appears at orbital minima, suggesting disc reflection.

Abstract

We present for the first time orbital phase-resolved spectra of an intermediate polar (IP), EX Hya, together with the spin phase-resolved spectra during two different epochs using the X-ray Multi-Mirror Mission, European Photon Imaging Camera (pn instrument). We find that the source at the two epochs has the same X-ray luminosity of $\sim$ 6.5 $\times$ 10$^{31}$ erg s$^{-1}$. We detect spectral variations between the 2000 and 2003 observations of the source. We fitted the spectrum using a neutral hydrogen absorption model with or without covering fraction together with Gaussians for emission lines, two collisional equilibrium plasma emission models (MEKAL) and a cooling-flow plasma emission model (VMCFLOW). We find that two of the three emission components ($kT$=0.6-0.8 keV and $kT$=1.3-1.7 keV) fitted by the MEKAL models are almost constant over the spin and orbital phases and also over the two different epochs with the normalisation varying directly proportional to the flux when the data are folded according to the orbital and spin phase indicating that the slight variation may be due to occultation. The emission modeled by the VMCFLOW changes over the spin and orbital phases and the 2000 and 2003 observations reveal two different ranges of temperatures (3-33 and 8-61 keV respectively) that model the shock zone in the accretion column/s. The ratios of the spin maximum to minimum and the orbital maximum to minimum spectra along with the increase in the plasma temperatures indicate that the spectrum gets harder in the minimum phases of both orbital and spin periods. In the 2003 observation, a 6.4 keV fluorescent Fe emission line is present at the orbital minima in a range of phases from 0.9 to 1.3 and it is absent otherwise. This indicates that there is reflection from the disc most likely from a large bulge at the accretion impact zone.