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A Large Catalogue of Ultraluminous X-ray Source Candidates in Nearby Galaxies. DOM WALTON IoA, Cambridge, UK In collaboration with Jeanette Gladstone, Tim Roberts and Andy Fabian. Madrid: 2010. OVERVIEW. General overview of the catalogue production
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A Large Catalogue of Ultraluminous X-ray Source Candidates in Nearby Galaxies DOM WALTON IoA, Cambridge, UK In collaboration with Jeanette Gladstone, Tim Roberts and Andy Fabian Madrid: 2010
OVERVIEW • General overview of the catalogue production • Present some analysis of the derived source population • Focus on and model the spectrum of a specific souce included in the catalogue • Demonstrate how it may be possible to distinguish between ULX spectral models in the future Dom Walton Madrid: 2010
ULX - DEFINITION • Two main criteria define a ULX: • Point source with LX > 1039 ergs s-1 • Extra-nuclear location in their host galaxies Dom Walton Madrid: 2010
WHY ARE ULXs INTERESTING? The extreme ULX luminosities appear to exceed the Eddington limit for a standard 10 solar mass black hole. This may be achieved by: - Intermediate mass black holes - Super-eddington emission - Anisotropic emission Dom Walton Madrid: 2010
CATALOGUE PRODUCTION • Began by cross correlating the 2XMM X-ray source and RC3 galaxy catalogues • Only kept sources within the elliptical D25 isophotes of the RC3 galaxies • Removed sources with LX < 1039 ergs s-1 (although sources with 1 agreement were retained) • Also removed sources flagged as extended • Removed known contaminants (AGN, stars, etc.) Dom Walton Madrid: 2010
BASIC FILTERING NGC 5194 & 5195 Dom Walton Madrid: 2010
REMOVING LLAGN LX > 1042 ergs s-1 1039 < LX < 1042 ergs s-1 Dom Walton Madrid: 2010
THE CATALOGUE • The final catalogue contains 655 detections of 475 discrete point sources • Most of these sources are located in spiral galaxies, despite the elliptical galaxies in 2XMM observations covering a much larger sky area • We find 5 new sources with LX > 1041 ergs s-1 Limitations Incompleteness and source confusion Dom Walton Madrid: 2010
ESTIMATING CONTAMINATION • The majority of the cosmic X-ray background can be resolved into point sources • Moretti et al. (2003) studied the number of background sources resolved with observation sensitivity (per deg2) • Using sensitivity maps of the 2XMM observations (provided by Univ. of Leicester), we can estimate fractional contamination contained in the catalogue Contamination ~ 18 % Dom Walton Madrid: 2010
HIGH ENERGY TURNOVER • The highest quality ULX spectra have been shown to display a smooth spectral turnover at high energies (Stobbart et al. 2006; Gladstone et al. 2009) • This is often seen around ~6 keV • Such curvature is not seen in the spectra of the standard accretion states of X-ray binaries • This turnover was suggested to define an 'ultraluminous' accretion state (Roberts 2007; Gladstone et al. 2009) Dom Walton Madrid: 2010
HIGH ENERGY TURNOVER Basic 2XMM Energy Bands: Hardness Ratios: Adopting an absorbed powerlaw model, we use HR3 to estimate C5under the assumption of no turnover, and compare this with the observed value of C5. This calculation is performed for <NH> = 1 and 3 x 1021 cm-2 Band 3: 1.0 – 2.0 keV Band 4: 2.0 – 4.5 keV Band 5: 4.5 – 12.0 keV Dom Walton Madrid: 2010
GALAXY DISTRIBUTIONS Dom Walton Madrid: 2010
DATA QUALITY DISTRIBUTIONS Dom Walton Madrid: 2010
HIGH ENERGY TURNOVER Curvature Parameter: Dom Walton Madrid: 2010
HIGH ENERGY TURNOVER Curvature Parameter: Dom Walton Madrid: 2010
HIGH ENERGY TURNOVER • The fraction of sources with observable curvature increases with data quality • At first it seems the turnover is more prominent in sources found in spiral galaxies. However, it is important to note: • We argue that high energy curvature is likely to be a common intrinsic property of ULXs in all galaxies -data from elliptical sources is poorer than spiral sources - <NH> may be lower for elliptical galaxies Dom Walton Madrid: 2010
DISC REFLECTION Caballero-Garcia & Fabian, 2010 Dom Walton Madrid: 2010
OPTICALLY THICK CORONAE (Done & Kubota, 2006) Gladstone, Roberts & Done, 2009 Dom Walton Madrid: 2010
NGC 4517 ULX1 • LX ~ 1040 ergs s-1 (assuming association with NGC 4517) • Offset from the nucleus by 43” • High quality EPIC spectrum • Data above 2 keV favours spectral curvature, like other high quality ULX spectra Top: XMM, Bottom: SDSS EPIC-pn EPIC-mos (combined) Dom Walton Madrid: 2010
NGC 4517 ULX1 SPECTRUM - Powerlaw - Disc Reflection Dom Walton Madrid: 2010
NGC 4517 ULX1 - MODELS • The data are represented equally well by both models: • Similar results are obtained to the application of these models to other ULXs: - Reflection: - Comptonisation: χ2υ= 518/466 ~ 1.1 χ2υ= 525/471 ~ 1.1 - Reflection: most of the emission located within a few RG, super solar iron abundance, steep ionising continuum - Comptonisation: low coronal electron temperature, high coronal optical depth Dom Walton Madrid: 2010
MODEL COMPARISON Dom Walton Madrid: 2010
MODEL COMPARISON Dom Walton Madrid: 2010
SPOT THE SIMILARITY Dom Walton Madrid: 2010
SPOT THE SIMILARITY Dom Walton Madrid: 2010
ARABIAN CAMEL Dom Walton Madrid: 2010
ARABIAN CAMEL Hump Dom Walton Madrid: 2010
REFLECTION SPECTRUM Dom Walton Madrid: 2010
REFLECTION SPECTRUM Hump Dom Walton Madrid: 2010
SUMMARY • By cross-correlating the 2XMM and RC3 catalogues, we have compiled a catalogue of 655 detections of 475 ULX candidates • With a simple hardness ratio analysis, we argue that spectral curvature above ~3 keV is a common feature in ULXs • Reliable data above 10 keV should be able to distinguish between the recent disc reflection and Comptonisation interpretations for this curvature Dom Walton Madrid: 2010
