Nature of the soft ULX in NGC 247: super-eddington outflow and transition between the supersoft and soft ultraluminous regimes

Journal Article · Thu Nov 10 04:00:00 EST 2016 · Astrophysical Journal

DOI:https://doi.org/10.3847/0004-637X/831/2/117· OSTI ID:22868505

Feng, Hua ^[1]; Tao, Lian ^[2]; Kaaret, Philip ^[3]; Grisé, Fabien ^[4]

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1. Department of Engineering Physics and Center for Astrophysics, Tsinghua University, Beijing 100084 (China)
2. Cahill Center for Astronomy and Astrophysics, California Institute of Technology, Pasadena, CA 91125 (United States)
3. Department of Physics and Astronomy, University of Iowa, Iowa City, IA 52242 (United States)
4. Observatoire astronomique de Strasbourg, Université Strasbourg, CNRS, UMR 7550, 11 rue de l’Université, F-67000 Strasbourg (France)

We report on XMM-Newton/Chandra/Swift/Hubble Space Telescope observations of the ultraluminous X-ray source (ULX) in NGC 247, which is found to make transitions between the supersoft ultraluminous (SSUL) regime with a spectrum dominated by a cool (∼0.1 keV) blackbody component and the soft ultraluminous (SUL) regime with comparable luminosities shared by the blackbody and power-law components. Multi-epoch observations revealed an anti-correlation between the blackbody radius and temperature, R{sub bb}∝T{sub bb}{sup −2.8±0.3}, ruling out a standard accretion disk as the origin of the soft X-ray emission. The soft X-ray emission is much more variable on both short and long timescales in the SSUL regime than in the SUL regime. We suggest that the SSUL regime may be an extension of the ultraluminous state toward the high accretion end, being an extreme case of the SUL regime, with the blackbody emission arising from the photosphere of thick outflows and the hard X-rays being emission leaked from the embedded accretion disk via the central low-density funnel or advected through the wind. However, the scenario that the supersoft ULXs are standard ULXs viewed nearly edge-on cannot be ruled out. Flux dips on a timescale of 200 s were observed. The dips cannot be explained by an increase of absorption, but could be due to the change of accretion rate or related to thermal fluctuations in the wind or disk. The optical emission of NGC 247 ULX exhibits a blackbody spectrum at a temperature of 19,000 K with a radius of 20 R{sub ⊙}, likely arising from an OB supergiant companion star.

• OSTI ID: 22868505
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 2 Vol. 831; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
ABSORPTION
ACCRETION DISKS
BLACK HOLES
COMPARATIVE EVALUATIONS
COSMIC X-RAY SOURCES
EMISSION
FLUCTUATIONS
GALAXIES
HARD X RADIATION
LUMINOSITY
PHOTOSPHERE
SOFT X RADIATION
SPACE
SPECTRA
STARS
STELLAR WINDS
TELESCOPES