Key results from an XMM‐Newton and Chandra study of a new sample of extreme ULXs from the 2XMM catalogue

We present the results from an X‐ray and optical study of a new sample of eight extreme luminosity ultraluminous X‐ray source (ULX) candidates, which were selected as the brightest ULXs (with LX > 5 × 1040 erg s−1) located within 100 Mpc identified in a cross‐correlation of the 2XMM‐DR1 and RC3 catalogues. These objects are so luminous that they are difficult to describe with current models of super‐Eddington accretion on to all but the most massive stellar remnants; hence they are amongst the most plausible candidates to host larger, intermediate‐mass black holes (IMBHs). Two objects are luminous enough in at least one observation to be classed as hyperluminous X‐ray source (HLX) candidates, including one persistent HLX in an S0 galaxy that (at 3 × 1041 erg s−1) is the second most luminous HLX yet detected. The remaining seven sources are located in spiral galaxies, and several appear to be closely associated with regions of star formation as is common for many less luminous ULXs. However, the X‐ray characteristics of these extreme ULXs appear to diverge from the less luminous objects. They are typically harder, possessing absorbed power‐law continuum spectra with Γ∼ 1.7, and are potentially more variable on short time‐scales, with data consistent with ∼10–20 per cent rms variability on time‐scales of 0.2–2 ks (albeit at low to moderate significance in many data sets). These properties appear consistent with the sub‐Eddington hard state, which given the observed luminosities of these objects suggests the presence of IMBHs with masses in the range of . As such, this strengthens the case for these brightest ULXs as good candidates for the eventual conclusive detection of the highly elusive IMBHs in the present‐day Universe. However, we caution that a combination of the highest plausible super‐Eddington accretion rates and the largest permitted stellar black hole remnants cannot be ruled out without future, improved observations.

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“…1 1 This work builds and improves on the early results from this sample, published in Sutton, Roberts & Walton (2011). …”

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confidence: 54%

The most extreme ultraluminous X-ray sources: evidence for intermediate-mass black holes?

Sutton

Roberts

Walton

et al. 2012

Monthly Notices of the Royal Astronomical Society

141

191

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“…Because of the completeness of the current sample, we can extrapolate the results to somewhat larger volumes to predict properties of the expected population of ULXs more generally. Of particular interest are those objects that are missed in the current sample because they are too rare to appear within random small volumes, namely the very luminous ULXs of which a handful have been recently reported (e.g., Sutton et al 2011;Farrell et al 2009).…”

Section: Discussionmentioning

confidence: 99%

A Complete Sample of Ultraluminous X-Ray Source Host Galaxies

Swartz

Soria

Tennant

et al. 2011

ApJ

203

285

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One hundred seven ultraluminous X-ray (ULX) sources with 0.3−10.0 keV luminosities in excess of 10 39 ergs s −1 are identified in a complete sample of 127 nearby galaxies. The sample includes all galaxies within 14.5 Mpc above the completeness limits of both the Uppsala Galaxy Catalog and the Infrared Astronomical Satellite survey. The galaxy sample spans all Hubble types, a four decade range in mass, 7.5 < log(M/M ⊙ ) < 11.4, and in star-formation rate, 0.0002 < SFR(M ⊙ /yr) ≤ 3.6. ULXs are detected in this sample at rates of one per 3.2 × 10 10 M ⊙ , one per ∼0.5 M ⊙ yr −1 star-formation rate, and one per 57 Mpc 3 corresponding to a luminosity density of ∼2×10 37 erg s −1 Mpc −3 . At these rates we estimate as many as 19 additional ULXs remain undetected in fainter dwarf galaxies within the survey volume. An estimated 14 or 13%, of the 107 ULX candidates are expected to be background sources.The differential ULX luminosity function shows a power law slope α ∼ −1.2 to −2.0 with an exponential cutoff at ∼20×10 39 ergs s −1 with precise values depending on the model and on whether the ULX luminosities are estimated from their observed numbers of counts or, for a subset of candidates, from their spectral shapes. Extrapolating the observed luminosity function predicts at most one very luminous ULX, L X ∼ 10 41 ergs s −1 , within a distance as small as 100 Mpc. The luminosity distribution of ULXs within the local universe cannot account for the recent claims of luminosities in excess of 2×10 41 ergs s −1 requiring a new population class to explain these extreme objects. 5 Hence, the standard disk scaling relations between temperature, luminosity and black hole mass, well-tested for (sub-Eddington) Galactic black holes, would not be applicable to ULXs.

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“…This source was observed to reach a luminosity of L X = (1.53 ± 0.08) × 10 41 erg s −1 . These sources will be the subject of forthcoming work (Sutton, Roberts & Walton 2011; Sutton et al, in preparation), which will include recently obtained follow‐up observations of NGC 470 HLX1.…”

Section: Catalogue Productionmentioning

confidence: 99%

2XMM ultraluminous X-ray source candidates in nearby galaxies

Walton

Roberts

Mateos

et al. 2011

Monthly Notices of the Royal Astronomical Society

164

228

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Ultraluminous X‐ray sources (ULXs) are some of the most enigmatic X‐ray bright sources known to date. It is generally accepted that they cannot host black holes as large as those associated with active galaxies, but they appear to be significantly more luminous than their better understood Galactic X‐ray binary (XRB) cousins, while displaying an intriguing combination of differences and similarities with them. Through studying large, representative samples of these sources we may hope to enhance our understanding of them. To this end, we derive a large catalogue of 650 X‐ray detections of 470 ULX candidates, located in 238 nearby galaxies, by cross‐correlating the 2XMM Serendipitous Survey with the Third Reference Catalogue of Bright Galaxies. The presented dedicated catalogue offers a significant improvement over those previously published in terms of both the number and the contribution of background contaminants, e.g. distant quasars, which we estimate to be at most 24 per cent, but more likely ∼17 per cent. To undertake population studies, we define a ‘complete’ sub‐sample of sources compiled from observations of galaxies with sensitivity limits below 1039 erg s−1. The luminosity function of this sample is consistent with a simple power law of form N(>LX) ∝ L−0.96 ± 0.11X. Although we do not find any statistical requirement for a cut‐off luminosity of Lc∼ 1040 erg s−1, as has been reported previously, we are not able to rule out its presence. Also, we find that the number of ULXs per unit galaxy mass, Su, decreases with increasing galaxy mass for ULXs associated with spiral galaxies, and is well modelled with a power law of form Su ∝ M−0.64 ± 0.07. This is in broad agreement with previous results, and is likely to be a consequence of the decrease in specific star formation and increase in metallicity with increasing spiral galaxy mass. Su is consistent with being constant with galaxy mass for sources associated with elliptical galaxies, implying this older ULX population traces stellar mass rather than star formation.

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Key results from an XMM‐Newton and Chandra study of a new sample of extreme ULXs from the 2XMM catalogue

Cited by 7 publications

References 36 publications

The most extreme ultraluminous X-ray sources: evidence for intermediate-mass black holes?

The most extreme ultraluminous X-ray sources: evidence for intermediate-mass black holes?

A Complete Sample of Ultraluminous X-Ray Source Host Galaxies

2XMM ultraluminous X-ray source candidates in nearby galaxies

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