X-ray flares from the ultra-luminous X-ray source in NGC 5408

¹ Mullard Space Science Laboratory, University College London, Holmbury St Mary, Surrey, RH5 6NT, UK e-mail: Roberto.Soria@mssl.ucl.ac.uk
² Observatoire Astronomique, UMR 7550 CNRS, 11 rue de l'Université, 67000 Strasbourg, France
³ Department of Physics and Astronomy, Leicester University, Leicester LE1 7RH, UK
⁴ School of Physics and Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK

Accepted: 22 April 2004

Abstract

We have studied an ultra-luminous X-ray source (ULX) in the dwarf galaxy NGC 5408 with a series of XMM-Newton observations, between 2001 July and 2003 January. We find that its X-ray spectrum is best fitted with a power law of photon index and a thermal component with blackbody temperature  keV. These spectral features, and the inferred luminosity  erg s^-1 in the  keV band, are typical of bright ULXs in nearby dwarf galaxies. The blackbody plus power-law model is a significantly better fit than either a simple power law or a broken power law (although the latter model is also acceptable at some epochs). Doppler-boosted emission from a relativistic jet is not required, although we cannot rule out this scenario. Our preliminary timing analysis shows flaring behaviour which we interpret as variability in the power-law component, on timescales of ~10² s. The hard component is suppressed during the dips, while the soft thermal component is consistent with being constant. The power density spectrum is flat at low frequencies, has a break at  mHz, and has a slope  at higher frequencies. A comparison with the power spectra of Cyg X-1 and of a sample of other BH candidates and AGN suggests a mass of ~. It is also possible that the BH is at the upper end of the stellar-mass class (), in a phase of moderately super-Eddington accretion. The formation of such a massive BH via normal stellar evolution may have been favoured by the very metal-poor environment of NGC 5408.