Weakly broadened iron line in the X-ray spectrum of the ultraluminous X-ray source M82 X-1

Abstract

We present the best quality XMM-Newton and Suzaku data from M82 X-1 so far. We analyse the spectra of this remarkable ultraluminous X-ray (ULX) source in a self-consistent manner. We have disentangled emission from the host galaxy, responsible for the soft X-ray emission (E≤ 2.5 keV), which is successfully described by a two-temperature thermal emission from a hot plasma in multiphase state, plus a narrow Fe line emission at 6.7 keV. This allowed us to properly study the intrinsic continuum emission from M82 X-1. The continuum of the Suzaku spectrum is curved, and the high-quality data of the Suzaku spectrum allowed us to significantly detect a weakly broadened Fe Kα emission line. The equivalent width is in the range 30-80 eV and it does not depend on the model applied for the continuum. Assuming that this line is coming from the ULX via disc fluorescence, the data indicate a disc truncation at a radius of 6-20 gravitational radii. This value is comparable to or even larger than the innermost stable circular orbit of a non-spinning (Schwarzschild) black hole. Future longer observations might test this scenario.