XMM-Newton Observations of NGC 247: X-ray Population and a Supersoft Ultraluminous X-ray Source

TL;DR

This study presents XMM-Newton observations of NGC 247, identifying a supersoft ultraluminous X-ray source with properties suggesting an intermediate mass black hole, along with analysis of the galaxy's X-ray source population and absorption features.

Contribution

First detailed X-ray spectral and variability analysis of NGC 247's ULX, proposing the presence of an intermediate mass black hole and exploring alternative emission models.

Findings

The ULX spectrum is dominated by thermal emission at 0.1 keV.

Detected variability consistent with black hole binary states.

X-ray source luminosity function matches low mass X-ray binaries.

Abstract

We report on a new XMM-Newton observation of NGC 247 from December 2009. The galaxy contains a supersoft, ultraluminous X-ray source (ULX) whose spectrum consists of a thermal component with a temperature about 0.1 keV and a power-law tail with a photon index around 2.5. The thermal emission is absolutely the dominant component, contributing 96% of the total luminosity in the 0.3-10 keV band. Variability is detected at timescales of 10^2 s and longer with a \nu^-1 power spectrum. These properties are consistent with black hole binaries in the thermal state and suggest the presence of an intermediate mass black hole of at least 600 solar masses. However, the integrated rms power is much higher than typically found in the thermal state. An alternative explanation of the emission could be a photosphere with a radius about 10^9 cm. A possible absorption feature around 1 keV is detected, which may be due to absorption of highly ionized winds. X-ray sources within the disk of NGC 247 have a luminosity function consistent with that found in low mass X-ray binaries. We confirm previous results that X-rays from the quasar PHL 6625 may be absorbed by gas in NGC 247, mainly at energies below 0.3 keV.