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
Band 3: 1.0 – 2.0 keV
Band 4: 2.0 – 4.5 keV
Band 5: 4.5 – 12.0 keV
Basic 2XMM Energy Bands:
Hardness Ratios:
C3
HR3=
C4
C4
HR4=
C5
Adopting an absorbed powerlaw model, we use HR3 to estimate
C5 under 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
Dom Walton
Madrid: 2010
GALAXY DISTRIBUTIONS
Dom Walton
Madrid: 2010
DATA QUALITY DISTRIBUTIONS
Dom Walton
Madrid: 2010
HIGH ENERGY TURNOVER
Curvature Parameter:




N
n


3

N
n

3
C

N
tot
C (x 10-2)
<NH> (cm-2)
Dom Walton
1 x 1021
3 x 1021
Whole Catalogue
-17.4 ± 2.4
-8.3 ± 1.7
Spiral Galaxies
-21.4 ± 2.9
-10.1 ± 2.3
Elliptical Galaxies
-8.2 ± 2.5
-4.1 ± 1.7
High Quality
-33.5 ± 5.5
-14.3 ± 4.3
Medium Quality
-7.5 ± 2.2
-2.5 ± 1.3
Low Quality
-11.2 ± 2.8
8.1 ± 2.3
Madrid: 2010
HIGH ENERGY TURNOVER
Curvature Parameter:




N
n


3

N
n

3
C

N
tot
C (x 10-2)
<NH> (cm-2)
Dom Walton
1 x 1021
3 x 1021
Whole Catalogue
-17.4 ± 2.4
-8.3 ± 1.7
Spiral Galaxies
-21.4 ± 2.9
-10.1 ± 2.3
Elliptical Galaxies
-8.2 ± 2.5
-4.1 ± 1.7
High Quality
-33.5 ± 5.5
-14.3 ± 4.3
Medium Quality
-7.5 ± 2.2
-2.5 ± 1.3
Low Quality
-11.2 ± 2.8
8.1 ± 2.3
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:
- data from elliptical sources is poorer than spiral sources
- <NH> may be lower for elliptical galaxies
• We argue that high energy curvature is likely to be a
common intrinsic property of ULXs in all 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”
Top: XMM, Bottom: SDSS
• High quality EPIC spectrum
• Data above 2 keV favours spectral
curvature, like other high quality ULX
spectra
Dom Walton
EPIC-pn
EPIC-mos (combined)
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:
- Reflection:
χ2υ = 518/466 ~ 1.1
- Comptonisation:
χ2υ = 525/471 ~ 1.1
• Similar results are obtained to the application of
these models to other ULXs:
- 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