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

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.