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 Gamma ~ 2.6--2.9 and a thermal component with blackbody temperature kT_bb ~ 0.12--0.14 keV. These spectral features, and the inferred luminosity ~ 10^{40} erg/s 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 ~ 10^2 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 f_b ~ 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 ~ 10^2 M_sun. It is also possible that the BH is at the upper end of the stellar-mass class (M ~ 50 M_sun), 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.

Submitted to A&A (2nd version, after taking into account the referee's comments). 9 pages, size 400 KB

References in corpus (5)

• The formation of massive black holes through collision runaway in dense young star clusters
• Displacement of X-Ray Sources from Star Clusters in Starburst Galaxies
• Probable intermediate mass black holes in NGC4559: XMM-Newton spectral and timing constraints
• On the origin of the various types of radio emission in GRS 1915+105
• A transition to a low/soft state in the ultra-luminous compact X-ray source Holmberg II X-1

Cited by in corpus (37)

• Ultraluminous X-Ray Sources
• X-ray source populations in galaxies
• Ultraluminous X-ray Sources in the Chandra and XMM-Newton Era
• Observational evidence for intermediate-mass black holes
• The ultraluminous X-ray sources NGC 1313 X-1 and X-2: a broadband study with NuSTAR and XMM-Newton
• X-ray observations of ultraluminous X-ray sources
• Low metallicity natal environments and black hole masses in Ultraluminous X-ray Sources
• The hard X-ray spectral evolution in X-ray binaries and its application to constrain the black hole mass of ultraluminous X-ray sources
• Quasi-Periodic Variability in NGC 5408 X-1
• Black Hole Mass of the Ultraluminous X-ray source M82 X-1
• A Systematic Study of Variability in a Sample of Ultraluminous X-ray Sources
• XMM-Newton Observations of Ultraluminous X-Ray Sources in Nearby Galaxies
• Evidence for an Intermediate Mass Black Hole in NGC 5408 X-1
• Optical emission of the ultraluminous X-ray source NGC 5408 X-1: donor star or irradiated accretion disk?
• XMM-Newton EPIC observations of the ultraluminous X-ray source NGC 5204 X-1
• XMM-Newton campaign on the ultraluminous X-ray source NGC 247 ULX-1: outflows
• A deep XMM-Newton observation of the ultraluminous X-ray source HoII X-1 : the case against a 1000 solar mass black hole
• Irradiation models for ULXs and fits to optical data
• A ULX microquasar in NGC 5408?
• The Lense-Thirring timing-accretion plane for ULXs
• A long-term X-ray monitoring of the ultraluminous X-ray source NGC 5408 X-1 with Swift reveals the presence of dips but no orbital period
• Chandra monitoring observations of the ultraluminous X-ray source NGC 5204 X-1
• Ultra-luminous X-ray Sources in nearby galaxies from ROSAT HRI observations II. statistical properties
• Milliarcsec-scale radio emission of ultraluminous X-ray sources: steady jet emission from an intermediate-mass black hole?
• A kinematic study of the neutral and ionised gas in the irregular dwarf galaxies IC 4662 and NGC 5408
• The ultraluminous X-ray source population of NGC 4485/4490
• A Two Hour Quasi-Period in an Ultra-luminous X-Ray source in NGC628
• Evidence for Quasi-periodic X-Ray Dips from an Ultraluminous X-Ray Source: Implications for the Binary Motion
• On the artificial nature of aperiodic variability in XMM-Newton observations of M31 X-ray sources and the ultraluminous X-ray source NGC 4559 ULX-7
• The rare X-ray flaring activity of the Ultraluminous X-ray source NGC 4559 X7
• A new pulsating neutron star in the Ultraluminous X-ray source NGC 4559 X7?
• Multiwavelength study of the bright X-ray source population in the interacting galaxies NGC5774/NGC5775
• Measuring the black hole mass in Ultraluminous X-ray Sources with the X-ray Scaling Method
• Revealing the nature of the ULX and X-ray population of the spiral galaxy NGC 4088
• Confirming the 115.5-day periodicity in the X-ray light curve of ULX NGC 5408 X-1
• Detection of X-ray elongated emission from a ultraluminous X-ray source in the interacting pair of galaxies NGC 5953/5954
• XMM-Newton multi-year campaign on NGC 55 ULX-1: Resolving the wind and its variability with RGS