Ultraluminous X‐Ray Sources in Nearby Galaxies from<i>ROSAT</i>HRI Observations. II. Statistical Properties

Liu, Jifeng; Bregman, Joel N.; Irwin, Jimmy A.

doi:10.1086/500787

Cited by 38 publications

(59 citation statements)

References 43 publications

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“…In fact, this contamination can be on average as high as 50% . Similar conclusions have been reached by Ptak & Colbert (2004) and Liu et al (2006). Analyzing the spatial distribution of ULX candidates, the contamination seems to be different according to the type of the putative host galaxy.…”

Section: Introductionsupporting

confidence: 80%

Eight more ultra luminous X-ray candidates unmasked

Gutiérrez

López-Corredoira

2007

A&A

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Aims. We aim to identify the nature of several objects previously catalogued as ultra-luminous X-ray (ULX) source candidates. Methods. Long-slit spectroscopy of the optical counterparts has been carried out. Results. The eight objects are at higher redshift than their putative parent galaxies. Conclusions. It is found that these eight previously catalogued ULX candidates, most of them projected in galactic halos are actually background unrelated objects. This result confirms the high degree of contamination present in current catalogues of ULXs.

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Section: Introductionsupporting

confidence: 80%

Eight more ultra luminous X-ray candidates unmasked

Gutiérrez

López-Corredoira

2007

A&A

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“…In addition, we note that LMXB ULXs seem to be preferentially associated with elliptical galaxies and their average luminosity is lower than that of ULXs in spiral galaxies (50). The fraction of ULXs associated with old stellar populations in spiral galaxies is rather low (51), implying that ULX LMXB systems possibly similar to NGC 5907 ULX might not be common.…”

Section: The Nature Of the Systemmentioning

confidence: 74%

An accreting pulsar with extreme properties drives an ultraluminous x-ray source in NGC 5907

Israel

Belfiore

Stella

et al. 2017

Science

420

386

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Ultraluminous x-ray sources (ULXs) in nearby galaxies shine brighter than any X-ray source in our Galaxy. ULXs are usually modeled as stellar-mass black holes (BHs) accreting at very high rates or intermediate-mass BHs. We present observations showing that NGC 5907 ULX is instead an x-ray accreting neutron star (NS) with a spin period evolving from 1.43 s in 2003 to 1.13 s in 2014. It has an isotropic peak luminosity of ∼1000 times the Eddington limit for a NS at 17.1 Mpc. Standard accretion models fail to explain its luminosity, even assuming beamed emission, but a strong multipolar magnetic field can describe its properties. These findings suggest that other extreme ULXs (x-ray luminosity ≥1041 erg s −1 ) might harbor NSs.Ultraluminous x-ray sources (ULXs) are observed in off-nucleus regions of nearby galaxiesand have x-ray luminosities in excess of a few 10 39 erg s −1 , which is the Eddington luminosity (L Edd ) for a black hole (BH) of 10 M (1). The L Edd sets an upper limit on the accretion luminosity (L acc ) of a compact object steadily accreting, since for L acc > L Edd accretion will be halted by radiation forces. For spherical accretion of fully ionized hydrogen, the limit can be written as, where σ T is the Thomson scattering cross section, m p is the proton mass, and M/M is the compact object mass in solar masses; for a 1.4 M neutron star (NS), the maximum accreting luminosity is ∼2×10 38 erg s −1 .The high luminosity of ULXs has thus been explained as accretion at or above the Eddington luminosity onto BHs of stellar origin (<80-100 M ), or onto intermediate-mass (10BHs (2, 3). However, if the emission of ULXs were beamed over a fraction b < 1 of the sky, their true luminosity, and thus also the compact object mass required not to exceed L Edd , would be reduced by the same factor. This possibility, together with the recent identification of two accreting NSs associated with the ∼10 40 erg s −1 M82 X-2 (4) and NGC 7793 P13 (5, 6) x-ray sources, have brought support to the view that most low-luminosity ULXs likely host a NS (7) 2 or a stellar-mass BH (8). For the most extreme ULXs with x-ray luminosity exceeding a few ×10 40 erg s −1 , BHs with masses in excess of 100 M are still commonly considered (9, 10).Despite several searches for coherent x-ray pulsations,no other ultraluminous x-ray source has been found to host a NS so far (11).Within the framework of "Exploring the X-ray Transient and variable Sky", EXTraS (12) Fig. 1 and Table 1). In all cases, a strong first period derivative term is present (see Table 1). The pulse shape is nearly sinusoidal, while the pulsed fraction (the semi-amplitude of the sinusoid divided by the average count rate)is energy dependent and increases from about 12% at low energies (<2.5 keV) to ∼20% in the hard band (>7 keV; Fig. 1).To derive constraints on the orbital period (P orb ), we applied a likelihood analysis to the two 2014 NuSTAR observations (see supplementary online text), which have the longest baseline. 3By assuming a circular orbit (as in the case of M...

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“…Ptak & Colbert (2004) calculated a similar contamination: around 44%. Liu et al (2006) calculated a contamination ratio of 30% within 2R 25 . Swartz et al (2004) conclude that ∼25% of the sources ULXs candidates from Chandra ACIS may be background objects including 14% of the ULX candidates in the sample of spiral galaxies and 44% of those in elliptical galaxies, but their numbers are not directly comparable with those of the C02 sample because in some cases the Chandra fields do not cover the circles within 2R 25 .…”

Section: Overall Contaminationmentioning

confidence: 99%

“…Pakull & Mirioni 2002;Gao et al 2003;Kaaret et al 2004;Zampieri et al 2004;Liu et al 2004;Smith et al 2005;Lehmann et al 2005), tentatively associated with intermediate-mass black holes. The ULX phenomenon is indeed closely connected to star formation activities, since ULXs preferentially occur in spiral arms of late-type or starbursts rather than in early-type galaxies (Kilgard et al 2002;Schwartz et al 2004;Liu et al 2006). Actually, 70% of starburst galaxies host at least one ULX.…”

Section: Introductionmentioning

confidence: 99%

Toward a clean sample of ultra-luminous X-ray sources

López-Corredoira

Gutiérrez

2006

A&A

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Context. Observational follow-up programmes for the characterization of ultra-luminous X-ray sources (ULXs) require the construction of clean samples of such sources in which the contamination by foreground/background sources is minimum. Aims. We calculate the degree of foreground/background contaminants among the ULX sample candidates in a published catalogue and compare these computations with available spectroscopic identifications. Methods. We use statistics based on known densities of X-ray sources and AGN/QSOs selected in the optical. The analysis is done individually for each parent galaxy. The existing identifications of the optical counterparts are compiled from the literature. Results. More than a half of the ULXs, within twice the distance of the major axis of the 25 mag/arcsec 2 isophote from RC3 nearby galaxies and with X-ray luminosities L X [2−10 keV] ≥ 10 39 erg/s, are expected to be high redshift background QSOs. A list of 25 objects (clean sample) confirmed to be real ULXs or to have a low probability of being contaminant foreground/background objects is provided.

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Ultraluminous X‐Ray Sources in Nearby Galaxies fromROSATHRI Observations. II. Statistical Properties

Cited by 38 publications

References 43 publications

Eight more ultra luminous X-ray candidates unmasked

Eight more ultra luminous X-ray candidates unmasked

An accreting pulsar with extreme properties drives an ultraluminous x-ray source in NGC 5907

Toward a clean sample of ultra-luminous X-ray sources

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