XMM-Newton RGS spectrum of RX J0720.4-3125: An absorption feature at 0.57 keV

TL;DR

This paper reports the detection of a narrow 0.57 keV absorption feature in the X-ray spectrum of neutron star RX J0720.4-3125, likely caused by highly ionized oxygen in its surrounding medium, suggesting a dense circumstellar environment.

Contribution

First detection of an oxygen absorption line at 0.57 keV in the spectrum of RX J0720.4-3125, indicating a dense ambient medium near the neutron star.

Findings

Absorption feature at 0.57 keV with equivalent width 1.35 eV

Likely origin in highly ionized oxygen (OVII) in ambient medium

Optical flux excess may originate from a dense slab near the star

Abstract

We found a narrow absorption feature at 0.57keV in the co-added RGS spectrum of the isolated neutron star RX J0720.4-3125 with an equivalent width of 1.35+/-0.3eV and FWHM ~6.0eV. The feature was identified with an absorption line of highly ionized oxygen OVII, most probably originating in the ambient medium of RX J0720.4-3125. An extensive investigation with the photo-ionization code CLOUDY indicates the possibility that the optical flux excess observed in the spectrum of RX J0720.4-3125 at least partially originates in a relatively dense (e.g. nH~10^8 cm^-3) slab, located in the vicinity of the neutron star (e.g. ~10^10cm).