XMM-Newton and NuSTAR Simultaneous X-Ray Observations of IGR J11215-5952

Abstract

We report the results of an XMM-Newton and NuSTAR coordinated observation of the Supergiant Fast X-ray Transient (SFXT) IGR J11215─5952, performed on 2016 February 14, during the expected peak of its brief outburst, which repeats every ∼165 days. Timing and spectral analysis were performed simultaneously in the energy band 0.4─78 keV. A spin period of 187.0 (±0.4) s was measured, consistent with previous observations performed in 2007. The X-ray intensity shows a large variability (more than one order of magnitude) on timescales longer than the spin period, with several luminous X-ray flares that repeat every 2─2.5 ks, some of which simultaneously observed by both satellites. The broadband (0.4─78 keV) time-averaged spectrum was well deconvolved with a double-component model (a blackbody plus a power law with a high energy cutoff) together with a weak iron line in emission at 6.4 keV (equivalent width, EW, of 40 ± 10 eV). Alternatively, a partial covering model also resulted in an adequate description of the data. The source time-averaged X-ray luminosity was 10<SUP>36</SUP> erg s<SUP>−1</SUP> (0.1─100 keV; assuming 7 kpc). We discuss the results of these observations in the framework of the different models proposed to explain SFXTs, supporting a quasi-spherical settling accretion regime, although alternative possibilities (e.g., centrifugal barrier) cannot be ruled out.