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

TL;DR

This study analyzes high-quality X-ray data from M82 X-1, revealing a weakly broadened iron line and insights into the accretion disc structure, suggesting possible disc truncation near the black hole.

Contribution

First detailed spectral analysis disentangling host galaxy emission from the ULX, detecting a weakly broadened iron line and constraining the disc truncation radius.

Findings

Detection of a weakly broadened Fe K-alpha line with 30-80 eV EW.

Disc truncation radius estimated at 6-20 gravitational radii.

Host galaxy emission modeled with multi-phase hot plasma.

Abstract

In this paper we present the best quality XMM-Newton and Suzaku data from M82 X-1 so far. We analyze the spectra of this remarkable Ultra-Luminous X-ray 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 multi-phase 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 K-alpha 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 indicates 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.