Identification of the X-Ray Thermal Dominant State in an Ultraluminous X-Ray Source in M82

TL;DR

This paper reports the first identification of a thermal dominant state in a ULX, revealing similarities to Galactic black hole binaries and suggesting the presence of a nearly maximally spinning black hole of 200-800 solar masses in M82.

Contribution

It provides the first evidence of a thermal dominant state in a ULX, using simultaneous X-ray observations to analyze spectral and timing properties.

Findings

ULX in M82 exhibits a thermal dominant state during outbursts.

The spectrum can be modeled with a relativistic disk indicating a nearly maximally spinning black hole.

The black hole mass is estimated to be 200-800 solar masses.

Abstract

The thermal dominant state in black hole binaries (BHBs) is well understood but rarely seen in ultraluminous X-ray sources (ULXs). Using simultaneous observations of M82 with Chandra and XMM-Newton, we report the first likely identification of the thermal dominant state in a ULX based on the disappearance of X-ray oscillations, low timing noise, and a spectrum dominated by multicolor disk emission with luminosity varying to the 4th power of the disk temperature. This indicates that ULXs are similar to Galactic BHBs. The brightest X-ray spectrum can be fitted with a relativistic disk model with either a highly super-Eddington (L_disk/L_Edd = 160) non-rotating black hole or a close to Eddington (L_disk/L_Edd ~ 2) rapidly rotating black hole. The latter interpretation is preferred, due to the absence of such highly super-Eddington states in Galactic black holes and active galactic nuclei, and suggests that the ULX in M82 contains a black hole of 200-800 solar masses with nearly maximal spin. On long timescales, the source normally stays at a relatively low flux level with a regular 62-day orbital modulation and occasionally exhibits irregular flaring activity. The thermal dominant states are all found during outbursts.