An ultraluminous supersoft source with a 4 hour modulation in NGC 4631

Abstract

Context. Supersoft X-ray sources (SSSs) are characterised by very low temperatures (< 100 eV). Classical SSSs have bolometric luminosities in the range of 10^36-10^38 erg/s and are modelled with steady nuclear burning of hydrogen on the surfaces of white dwarfs. However, several SSSs have been discovered with much higher luminosities. Their nature is still unclear. Aims. We report the discovery of a 4h modulation for an ultraluminous SSS in the nearby edge-on spiral galaxy NGC 4631, observed with XMM-Newton in 2002 June. Temporal and spectral analysis of the source is performed. Methods. We use a Lomb-Scargle periodogram analysis for the period search and evaluate the confidence level using Monte-Carlo simulations. We measure the source temperature, flux and luminosity through spectral fitting. Results. A modulation of 4.2+-0.4 h (3 sigma error) was found for the SSS with a confidence level >99%. Besides dips observed in the light curve, the flux decreased by a factor of 3 within ~10h. The spectrum can be described with an absorbed blackbody model with kT~67eV. The absorbed luminosity in the 0.2-2 kev energy band was 2.7x10^38 erg/sec while the bolometric luminosity was a hundred time higher (3.2x10^40 erg/s), making the source one of the most luminous of its class, assuming the best fit model is correct. Conclusions. This source is another very luminous SSS for which the standard white dwarf interpretation cannot be applied, unless a strong beaming factor is considered. A stellar-mass black hole accreting at a super Eddington rate is a more likely interpretation, where the excess of accreted matter is ejected through a strong optically-thick outflow. The 4 h modulation could either be an eclipse from the companion star or the consequence of a warped accretion disk.

4 pages, 4 figures, accepted as a Letter in Astronomy & Astrophysics

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Cited by in corpus (14)

• Ultraluminous X-ray Sources in the Chandra and XMM-Newton Era
• Optically thick outflows in ultraluminous supersoft sources
• Nature of the soft ULX in NGC 247: super-Eddington outflow and transition between the supersoft and soft ultraluminous regimes
• Revisiting the ultraluminous supersoft source in M101: an optically thick outflow model
• Black Holes in Short Period X-ray Binaries and the Transition to Radiatively Inefficient Accretion
• Exploring the Dust Content of Galactic Winds with Herschel. I. NGC 4631
• Different types of ultraluminous X-ray sources in NGC 4631
• XMM-Newton Observations of NGC 247: X-ray Population and a Supersoft Ultraluminous X-ray Source
• Hard and soft spectral states of ULXs
• The Spectral and Temporal Properties of Transient Sources in Early-Type Galaxies
• Chandra and HST Observations of the Supersoft ULX in NGC 247: Candidate for Standard Disk Emission
• Monitoring the Very-Long-Term Variability of X-ray Sources in the Giant Elliptical Galaxy M87
• Identification of three new ultraluminous X-ray sources in NGC 4631 and NGC 1097: Evidence for stellar-mass black holes
• New outburst from the luminous supersoft source SSS1 in NGC 300 with periodic modulation