Orbital phase-resolved spectroscopy of 4U 1538−52 with MAXI
1 Dept. of Physics, Systems Engineering and Sign Theory, University of Alicante, 03080 Alicante, Spain
2 University Institute of Physics Applied to Sciences and Technologies, University of Alicante, 03080 Alicante, Spain
3 MAXI team, Institute of Physical and Chemical Research (RIKEN), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
4 ISS Science Project Office, Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA), 2-1-1 Sengen, Tsukuba, Ibaraki 305-8505, Japan
5 School of Physics, Faculty of Science, Monash University, Clayton, 3800 Victoria, Australia
Received: 12 November 2014
Accepted: 10 July 2015
Context. 4U 1538−52, an absorbed high mass X-ray binary with an orbital period of ~3.73 days, shows moderate orbital intensity modulations with a low level of counts during the eclipse. Several models have been proposed to explain the accretion at different orbital phases by a spherically symmetric stellar wind from the companion.
Aims. The aim of this work is to study both the light curve and orbital phase spectroscopy of this source in the long term. In particular, we study the folded light curve and the changes in the spectral parameters with orbital phase to analyse the stellar wind of QV Nor, the mass donor of this binary system.
Methods. We used all the observations made from the Gas Slit Camera on board MAXI of 4U 1538−52 covering many orbits continuously. We obtained the good interval times for all orbital phase ranges, which were the input for extracting our data. We estimated the orbital period of the system and then folded the light curves, and we fitted the X-ray spectra with the same model for every orbital phase spectrum. We also extracted the averaged spectrum of all the MAXI data available.
Results. The MAXI spectra in the 2–20 keV energy range were fitted with an absorbed Comptonisation of cool photons on hot electrons. We found a strong orbital dependence of the absorption column density but neither the fluorescence iron emission line nor low energy excess were needed to fit the MAXI spectra. The variation in the spectral parameters over the binary orbit were used to examine the mode of accretion onto the neutron star in 4U 1538−52. We deduce a best value of Ṁ/v∞ = 0.65 × 10-9M⊙ yr-1/ (km s-1) for QV Nor.
Key words: X-rays: binaries / pulsars: individual: 4U 1538 / 52 / stars: mass-loss
© ESO, 2015