Chandra and HST Observations of the Supersoft ULX in NGC 247: Candidate for Standard Disk Emission

TL;DR

This study uses Chandra and HST observations to identify a likely accretion disk around a supersoft ULX in NGC 247, suggesting it may host an intermediate mass black hole in a stable thermal state.

Contribution

It provides multiwavelength evidence supporting a standard disk model for the ULX, indicating a potential intermediate mass black hole, which is a novel insight for supersoft ULXs.

Findings

The optical counterpart is point-like and uniquely identified.

The X-ray spectrum fits a standard thin disk model under Galactic extinction.

The ULX shows steady flux consistent with a thermal disk state.

Abstract

We report on multiwavelength observations of the supersoft ultraluminous X-ray source (ULX) in NGC 247 made with the Chandra X-ray Observatory and Hubble Space Telescope (HST). We aligned the X-ray and optical images using three objects present on both and identified a unique, point-like optical counterpart to the ULX. The X-ray to optical spectrum is well fitted with an irradiated disk model if the extinction measured for Cepheids in NGC 247 is used. Assuming only Galactic extinction, then the spectrum can be modeled as a standard thin accretion disk. Either result leads to the conclusion that a disk interpretation of the X-ray spectrum is valid, thus the source may be in the X-ray thermal state and contain an intermediate mass black hole of at least 600 solar masses. In contrast to other supersoft ULXs which are transient and exhibit a luminosity temperature relation inconsistent with a disk interpretation of the X-ray emission, the NGC 247 ULX has a relatively steady flux and all available X-ray data are consistent with emission from a disk in the thermal state.