Revisiting the ultraluminous supersoft source in M 101: An optically thick outflow model
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The M 101 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 are still controversial. We re-examined the X-ray spectral and timing properties of the M 101 ULS using archival Chandra and XMM–Newton observations. We show that the X-ray time-variability and spectral properties are inconsistent with standard-disc emission. The characteristic radius Rbb of the thermal emitter varies from epoch to epoch between ≈10 000 and ≈100 000 km; the colour temperature kTbb varies between ≈50 and ≈140 eV and the two quantities scale approximately as R bb ∝T −2 bb Rbb∝Tbb−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, multitemperature outflow. We suggest that at sufficiently high accretion rates and inclination angles, the supercritical, 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.
This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2016 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
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