Unconventional AGN in hard X‐ray surveys

Abstract. This paper presents the analysis of a statistically complete sample of 28 serendipitous X-ray sources selected in 82 pointed XMM-Newton fields down to a count-rate of 0.002 counts s −1 (4.5-7.5 keV energy band). This is the first sample selected in this energy range to have complete spectroscopic identifications and redshift determinations for all the objects. Apart from one Galactic source (an interacting binary), all the objects are AGNs. Their optical and X-ray properties (derived from the spectral analysis of the XMM-EPIC data) are compared together. The good correlation between the optical spectral type and the X-ray absorption properties supports the AGN unified model. Only one object that does not fit the relation between optical and X-ray absorption is found, namely a Seyfert 1.9 with no evidence of obscuration in the X-ray band (N H < 1.3 × 10 20 cm −2 ). In total, 7 sources out of 27 are heavily obscured in the X-ray (N H > 10 22 cm −2 ), corresponding to a surface density of 0.7 deg −2 at the flux limit the sample (4−7 × 10 −14 erg s −1 cm −2 in the 4.5-7.5 keV energy band). Among these obscured objects, two sources show a large (intrinsic) luminosity (L [2−10 keV] > 10 44 erg s −1 ) and are thus classified as type 2 QSO. Finally, we have compared the fraction of X-ray absorbed AGNs (26%) with that predicted by the current XRB synthesis models at the flux limit of the survey. We find that the models significantly (∼90% confidence level) over predict the fraction of absorbed AGNs thus confirming also in this hard energy band (4.5-7.5 keV) similar results recently obtained in the 2-10 keV band.

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“…12). This peculiar X-ray-to-optical flux ratio found for this type of sources has been already noted, for instance, by Comastri et al (2003).…”

Section: X-ray Bright Optically Normal Galaxies (Xbong)supporting

confidence: 75%

The XMM-NewtonHBS28 sample: Studying the obscuration in hard X-ray selected AGNs

Caccianiga

Severgnini

Braito

et al. 2004

A&A

109

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“…Many extreme X-ray to optical ratio sources could then be highly obscured quasars, i.e. type 2 QSO (Fiore et al 2003;Comastri et al 2003). The observed high values of the X/O are therefore at least qualitatively consistent with those expected by a population of high redshift, absorbed AGN with X-ray column densities in the range N H = 10 22 −10 24 cm −2 (see also Sect.…”

Section: X-ray To Optical/near-infrared Properties Of Agn Erossupporting

confidence: 75%

XMM-Newtonobservations of Extremely Red Objects and the link with luminous, X-ray obscured quasars

Brusa

Comastri

Daddi

et al. 2005

A&A

Self Cite

108

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Abstract. We present the results of a deep (about 80 ks) XMM-Newton survey of the largest sample of near-infrared selected Extremely Red Objects (R − K > 5) available to date to K S < ∼ 19.2. At the relatively bright X-ray fluxes (F 2−10 keV > ∼ 4 × 10 −15 erg cm −2 s −1 ) and near-infrared magnitude probed by the present observations, the fraction of AGN (i.e. X-ray detected) among the ERO population is small (∼3.5%); conversely, the fraction of EROs among hard X-ray selected sources is much higher (∼20%). The X-ray properties of EROs detected in our XMM-Newton observation indicate absorption in excess of 10 22 cm −2 in a large fraction of them. We have also considered additional samples of X-ray detected EROs available in the literature. X-ray spectral analysis of the highest S/N sources unambiguously indicates that large columns of cold gas (even >10 23 cm −2 ) are the rule rather than the exception. The X-ray, optical and near-infrared properties of those X-ray selected EROs with a spectroscopic or photometric redshift nicely match those expected for type 2 quasars, the high-luminosity, high-redshift obscured AGNs predicted in baseline XRB synthesis models. A close correlation is detected between X-and K-band fluxes. For the AGN EROs this is consistent, under reasonable assumptions, with the relation established locally between the host galaxies and their central black holes. This suggests that the majority of EROs are powered by massive black holes accreting, on average, at about 0.03−0.1 of the Eddington limit.

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“…Nevertheless, our galaxies span a range of X‐ray to optical flux ratios (see Fig. 2) similar to those probed by other XBONG samples compiled from XMM–Newton and Chandra surveys (Comastri, Brusa & Mignoli 2003), suggesting that they are representative of the XBONG population.…”

Section: The Samplesupporting

confidence: 74%

X-ray bright optically inactive galaxies inXMM-Newton/Sloan Digital Sky Survey fields: more diluted than absorbed?

Georgantopoulos

Georgakakis

2005

Monthly Notices of the Royal Astronomical Society

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We explore the properties of X-ray bright optically inactive galaxies (XBONGs) detected in the 0.5-8 keV spectral band in 20 public XMM-Newton fields overlapping with the Sloan Digital Sky Survey (SDSS). We constrain our sample to optically extended systems with log f X / f opt > −2 that have spectroscopic identifications available from the SDSS (r < 19.2 mag). The resulting sample contains 12 objects with L X (0.5-8 keV) = 5 × 10 41 -2 × 10 44 erg s −1 in the redshift range 0.06 < z < 0.45. The X-ray emission in four cases is extended, suggesting the presence of hot gas associated with a cluster or group of galaxies. The X-ray spectral fits show that two additional sources are best fit with a thermal component emission (kT ∼ 1 keV). Three sources are most likely associated with active galactic nuclei (AGNs): their X-ray spectrum is described by a steep photon index ∼ 1.9 typical of unobscured AGNs, while they are very luminous in X-rays [L X (0.5-8 keV) ≈ 10 43 -10 44 erg s −1 ]. Finally, three more sources could be associated with either normal galaxies or unobscured low-luminosity AGNs (L X < 10 42 erg s −1 ). We find no evidence for significant X-ray absorbing columns in any of our XBONGs. The above suggest that XBONGs, selected in the total 0.5-8 keV band, comprise a mixed bag of objects primarily including normal elliptical galaxies and type 1 AGNs whose optical nuclear spectrum is probably diluted by the strong stellar continuum. Nevertheless, as our sample is not statistically complete we cannot exclude the possibility that a fraction of optically fainter XBONGs may be associated with heavily obscured AGNs.

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Unconventional AGN in hard X‐ray surveys

Cited by 22 publications

References 16 publications

The XMM-NewtonHBS28 sample: Studying the obscuration in hard X-ray selected AGNs

The XMM-NewtonHBS28 sample: Studying the obscuration in hard X-ray selected AGNs

XMM-Newtonobservations of Extremely Red Objects and the link with luminous, X-ray obscured quasars

X-ray bright optically inactive galaxies inXMM-Newton/Sloan Digital Sky Survey fields: more diluted than absorbed?

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