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

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.

9 pages, 5 figures, accepted for publication in ApJ

References in corpus (13)

• Ubiquitous equatorial accretion disc winds in black hole soft states
• The ACS Nearby Galaxy Survey Treasury IX. Constraining asymptotic giant branch evolution with old metal-poor galaxies
• Ultraluminous X-ray Sources in the Chandra and XMM-Newton Era
• Reprocessing of X-rays in the outer accretion disc of the black hole binary XTE J1817--330
• Optical emission of the ultraluminous X-ray source NGC 5408 X-1: donor star or irradiated accretion disk?
• Identification of the X-Ray Thermal Dominant State in an Ultraluminous X-Ray Source in M82
• Optical Properties of the Ultraluminous X-ray Source Holmberg IX X-1 and its Stellar Environment
• The Araucaria Project. The Distance to the Sculptor Galaxy NGC 247 from Near-Infrared Photometry of Cepheid Variables
• Optical counterpart of HLX-1 during the 2010 outburst
• The Disk and Extraplanar Environment of NGC 247
• The Nature of the UV/optical Emission of the Ultraluminous X-Ray Source in Holmberg II
• An ultraluminous supersoft source with a 4 hour modulation in NGC 4631
• No periodicity revealed for an "eclipsing" ultraluminous supersoft X-ray source in M81

Cited by in corpus (14)

• Optically thick outflows in ultraluminous supersoft sources
• ULX spectra revisited: Accreting, highly magnetized neutron stars as the engines of ultraluminous X-ray sources
• Nature of the soft ULX in NGC 247: super-Eddington outflow and transition between the supersoft and soft ultraluminous regimes
• SDSS1133: An Unusually Persistent Transient in a Nearby Dwarf Galaxy
• Revisiting the ultraluminous supersoft source in M101: an optically thick outflow model
• The Local Environment of Ultraluminous X-ray Sources Viewed by XMM Newton's Optical Monitor
• The chameleon on the branches: spectral state transition and dips in NGC 247 ULX-1
• The young stellar population in NGC 247 Main properties and hierarchical clustering
• VLT/FORS2 observations of four high-luminosity ULX candidates
• The Void in the Sculptor Group Spiral Galaxy NGC 247
• Identification of a Helium Donor Star in NGC 247 ULX-1
• A Luminous X-ray Flare From The Nucleus of The Dormant Bulgeless Spiral Galaxy NGC 247
• PHL 6625: A Minor Merger-Associated QSO Behind NGC 247
• A Synthetic Population of Ultra-Luminous X-ray Sources: Optical-X-ray Correlation