a large catalogue of ultraluminous x-ray source candidates in nearby galaxies

A Large Catalogue of Ultraluminous X-ray Source

Candidates in Nearby Galaxies

Madrid: 2010

DOM WALTON

IoA, Cambridge, UK

In collaboration withJeanette Gladstone, Tim Roberts

and Andy Fabian

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

WHY ARE ULXs INTERESTING?

Madrid: 2010

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

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.)

BASIC FILTERING

Dom Walton Madrid: 2010 NGC 5194 & 5195

REMOVING LLAGN


LX > 1042 ergs s-1

1039 < LX < 1042 ergs s-1

THE CATALOGUE


Limitations

Incompleteness and source confusion

• 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

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 %

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)



Basic 2XMM Energy Bands:

Hardness Ratios:

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

Band 3: 1.0 – 2.0 keVBand 4: 2.0 – 4.5 keVBand 5: 4.5 – 12.0 keV

HR3=C3

C4

HR4=C4

C5

GALAXY DISTRIBUTIONS


DATA QUALITY DISTRIBUTIONS




Curvature Parameter:

totN

nNnNC

33

C (x 10-2)<NH> (cm-2) 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



• 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

DISC REFLECTION


Caballero-Garcia & Fabian, 2010

OPTICALLY THICK CORONAE


Gladstone, Roberts & Done, 2009

(Done & Kubota, 2006)

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

Dom Walton

Top: XMM, Bottom: SDSS

EPIC-pnEPIC-mos (combined)

Madrid: 2010

NGC 4517 ULX1 SPECTRUM


- Powerlaw- Disc Reflection

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 R

G, super solar iron abundance, steep ionising

continuum

- Comptonisation: low coronal electron temperature, high coronal optical depth

MODEL COMPARISON


SPOT THE SIMILARITY


Dom Walton

SPOT THE SIMILARITY

Madrid: 2010

ARABIAN CAMEL


ARABIAN CAMEL

Hump


REFLECTION SPECTRUM


REFLECTION SPECTRUM

Hump


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

a large catalogue of ultraluminous x-ray source candidates in nearby galaxies

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