Revisiting the ultraluminous supersoft source in M101: an optically thick outflow model

TL;DR

This paper investigates the nature of the ultraluminous supersoft source in M101, proposing an optically thick outflow model that explains its spectral and variability properties, and clarifies its relation to other ultraluminous X-ray sources.

Contribution

It introduces a new outflow model for ULSs, demonstrating how super-critical accretion and thick outflows produce observed spectral features and variability.

Findings

Spectral and timing properties inconsistent with standard disk emission.

Thermal emitter radius varies from ~10,000 km to ~100,000 km.

Spectral residuals suggest clumpy, multi-temperature outflow.

Abstract

The M101 galaxy contains the best-known example of an ultraluminous supersoft source (ULS), dominated by a thermal component at kT ~ 0.1 keV. The origin of the thermal component and the relation between ULSs and standard (broad-band spectrum) ultraluminous X-ray sources (ULXs) are still controversial. We re-examined the X-ray spectral and timing properties of the M101 ULS using archival Chandra and XMM-Newton observations. We show that the X-ray time-variability and spectral properties are inconsistent with standard disk emission. The characteristic radius R_{bb} of the thermal emitter varies from epoch to epoch between ~10,000 km and ~100,000 km; the colour temperature kT_{bb} varies between ~50 eV and ~140 eV; and the two quantities scale approximately as R_{bb} ~ T_{bb}^{-2}. In addition to the smooth continuum, we also find (at some epochs) spectral residuals well fitted with thermal plasma models and absorption edges: we interpret this as evidence that we are looking at a clumpy, multi-temperature outflow. We suggest that at sufficiently high accretion rates and inclination angles, the super-critical, radiatively driven outflow becomes effectively optically thick and completely thermalizes the harder X-ray photons from the inner part of the inflow, removing the hard spectral tail. We develop a simple, spherically symmetric outflow model and show that it is consistent with the observed temperatures, radii and luminosities. A larger, cooler photosphere shifts the emission peak into the far-UV and makes the source dimmer in X-rays but possibly ultraluminous in the UV. We compare our results and interpretation with those of Liu et al. (2013).