Spectral analysis of LMC X–2 with XMM/Newton: unveiling the emission process in the extragalactic Z-source

Dipartimento di Scienze Fisiche ed Astronomiche (DSFA), Università degli studi di Palermo, via Archirafi 36, 90123 Palermo (PA), Italy e-mail: giuseppe.lavagetto@fisica.unipa.it

Received: 6 June 2007
Accepted: 19 October 2007

Abstract

Aims.We present the results of the analysis of an archival observation of LMC X-2 performed with XMM/Newton. The spectra taken by high-precision instruments have never been analyzed before.

Methods.We find an X-ray position for the source that is inconsistent with the one obtained by ROSAT, but in agreement with the Einstein position and that of the optical counterpart. The correlated spectral and timing behaviour of the source suggests that the source is probably in the normal branch of its X-ray color-color diagram. The spectrum of the source can be fitted with a blackbody with a temperature 1.5 keV plus a disc blackbody at 0.8 keV. Photoelectric absorption from neutral matter has an equivalent hydrogen column of 4 10²⁰ cm^-2. An emission line, which we identify as the O VIII Lyman-α line, is detected, while no feature due to iron is detected in the spectrum.

Results.We argue that the emission of this source can be straightforwardly interpreted as the sum of the emission from a boundary layer between the NS and the disc and a blackbody component coming from the disc itself. Other canonical models that are used to fit Z-sources do not give a satisfactory fit to the data. The detection of the O VIII emission line (and the lack of detection of lines in the iron region) can be due to the fact that the source lies in the Large Magellanic Cloud.