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

Abstract

We report on a new XMM-Newton observation of NGC 247 from 2009 December. The galaxy contains a supersoft, ultraluminous X-ray source 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 10² s and longer with a ν^-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 root-mean-square 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 10⁹ 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.