X-ray flares from the ultra-luminous X-ray source in NGC 5408
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 Γ ≈ 2.6-2.9 and a thermal component with blackbody temperature kTbb ≈ 0.12-0.14 keV. These spectral features, and the inferred luminosity ≈ 1040 erg s-1 in the 0.3-12 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 νb ≈ 2.5 mHz, and has a slope ≈ -1 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 ∼102 M⊙. It is also possible that the BH is at the upper end of the stellar-mass class (M ∼ 50 M⊙), 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.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- September 2004
- DOI:
- 10.1051/0004-6361:20041145
- arXiv:
- arXiv:astro-ph/0404493
- Bibcode:
- 2004A&A...423..955S
- Keywords:
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- black hole physics;
- galaxies: individual: NGC 5408;
- X-rays: galaxies;
- X-ray: stars;
- accretion;
- accretion disks;
- Astrophysics
- E-Print:
- Submitted to A&