The HELLAS2XMM survey

Fiore, F.; Brusa, M.; Cocchia, F.; Baldi, A.; Carangelo, N.; Ciliegi, P.; Comastri, A.; Franca, F. La; Maiolino, R.; Matt, Giorgio; Molendi, S.; Mignoli, M.; Perola, G. C.; Severgnini, P.; Vignali, C.

doi:10.1051/0004-6361:20031031

Cited by 247 publications

(470 citation statements)

References 46 publications

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“…The region where most of unobscured Sy2 are found (i.e. X/O < −1) is usually populated by normal galaxies or low-luminosity AGN (Fiore et al 2003). This result confirms the decreasing contrast between the AGN light and the host starlight for the unobscured Sy2, suggesting the dilution hypothesis is possible if the BEL are weak.…”

Section: Dilution By the Host Galaxysupporting

confidence: 71%

A new sample of X-ray selected narrow emission-line galaxies

Pons

Watson

2016

A&A

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A sample of X-ray and optically selected narrow emission-line galaxies (769 sources) from the 3XMM catalogue cross-correlated with SDSS (DR9) catalogue has been studied. Narrow-emission line active galactic nuclei (AGN; type-2) have been selected on the basis of their emission line ratios and/or X-ray luminosity. We have looked for X-ray unobscured type-2 AGN. As X-ray spectra were not available for our whole sample, we have checked the reliability of using the X-ray hardness ratio (HR) as a probe of the level of obscuration and we found a very good agreement with full spectral fitting results, with only 2% of the sources with apparently unobscured HR turning out to have an obscured spectrum. Despite the fact that type-2 AGN are supposed to be absorbed based on the Unified Model, about 60% of them show no sign or very low level of X-ray obscuration. After subtraction of contaminants to the sample, that is Narrow-Line Seyfert 1 and Compton-thick AGN, the fraction of unobscured Sy2 drops to 47%. For these sources, we were able to rule out dust reddening and variability for most of them as an explanation of the absence of optical broad emission-lines. The main explanations remaining are the dilution of weak/very broad emission-lines by the host galaxy and the intrinsic absence of the broad-line region (BLR) due to low accretion rates (i.e. True Sy2). However, the number of True Sy2 strongly depends on the method used to verify the intrinsic lack of broad lines. Indeed using the optical continuum luminosity to predict the BLR properties gives a much larger fraction of True Sy2 (about 90% of the unobscured Sy2 sample) than the use of the X-ray 2 keV luminosity (about 20%). Nevertheless the number of AGN we securely detected as True Sy2 is at least three times larger that the previously confirmed number of True Sy2.

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Section: Dilution By the Host Galaxysupporting

confidence: 71%

A new sample of X-ray selected narrow emission-line galaxies

Pons

Watson

2016

A&A

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“…in the 2-10 keV energy range), even if exhibiting signatures of photoelectric absorption, is less attenuated (see e.g. Fiore et al 2003). Obscured QSOs are therefore expected to display large values of X-ray to optical flux ratio, defined here as:…”

Section: The Exo50 Samplementioning

confidence: 99%

“…For a fixed absorption or AGN/host ratio, the higher is the redshift, the higher is the f x / f o and the redder are the optical-to-infrared colours (cf. Fiore et al 2003). However from a deeper look to the data, this redshift effect alone is not able to explain the extreme properties of all the EXO50 objects; for example the source number 5 (namely 2XMMJ123204.9+215254 at z=0.76) can not be explained with the redshift effect but requires very high AGN/host ratios; a very high AGN/host galaxy ratio indeed may be required also for 2XMMJ100038.9+050955 and 2XMMJ135055.7+642857 (sources 2 and 6, respectively) if their redshifts turn out to be similar to that of 2XMMJ123204.9+215254.…”

Section: Are the Exo50 Type 2 Qso Objects Different From Standard X-rmentioning

confidence: 99%

“…Furthermore, since the dust extinction increases in the ultraviolet (UV) while the X-ray absorption goes in the opposite direction, i.e. strongly decreasing going towards the high energies, a redshift dependence is also expected (∼ (1 + z) 3.6 ; Fiore et al 2003). However, in spite of the outstanding progress, obtained by using medium and deep X-ray surveys (see e.g.…”

Section: Introductionmentioning

confidence: 99%

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Exploring the active galactic nuclei population with extreme X-ray-to-optical flux ratios (fx/fo > 50)

Ceca

Carrera

Caccianiga

et al. 2015

Monthly Notices of the Royal Astronomical Society

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The cosmic history of the growth of supermassive black holes in galactic centers parallels that of star-formation in the Universe. However, an important fraction of this growth occurs inconspicuously in obscured objects, where ultraviolet/optical/near-infrared emission is heavily obscured by dust. Since the X-ray flux is less attenuated, a high X-ray-to-optical flux ratio ( f x / f o ) is expected to be an efficient tool to find out these obscured accreting sources. We explore here via optical spectroscopy, X-ray spectroscopy and infrared photometry the most extreme cases of this population (those with f x / f o > 50, EXO50 sources hereafter), using a well defined sample of 7 X-ray sources extracted from the 2XMM catalogue. Five EXO50 sources (∼ 70 percent of the sample) in the bright flux regime explored by our survey ( f (2−10keV) 1.5 × 10 −13 erg cm −2 s −1 ) are associated with obscured AGN (N H > 10 22 cm −2 ), spanning a redshift range between 0.75 and 1 and characterised by 2-10 keV intrinsic luminosities in the QSO regime (e.g. well in excess to 10 44 erg s −1 ). We did not find compelling evidence of Compton Thick AGN. Overall the EXO50 Type 2 QSOs do not seem to be different from standard X-ray selected Type 2 QSOs in terms of nuclear absorption; a very high AGN/host galaxy ratio seems to play a major role in explaining their extreme properties. Interestingly 3 out of 5 EXO50 Type 2 QSO objects can be classified as Extreme Dust Obscured Galaxies (EDOGs, f 24 µm / f R 2000), suggesting that a very high AGN/host ratios (along with the large amount of dust absorption) could be the natural explanation also for a part of the EDOG population. The remaining 2 EXO50 sources are classified as BL Lac objects, having rather extreme properties, and which are good candidates for TeV emission.

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“…With the advent of the Chandra and XMM data, there have been a number of computations of the evolution of the HXLF with redshift (e.g., Cowie et al 2003;Steffen et al 2003;Ueda et al 2003;Hasinger 2003;Fiore et al 2003;Barger et al 2005), while Sazonov & Revnivtsev (2004) have used the RXTE data to compute the local 3-20 keV luminosity function. The latest HXLF determinations by Barger et al (2005) use only the spectroscopically identified X-ray sources with L 2−8 keV = 10 42 ergs s −1 in the CDF-N, CDF-S, and CLASXS fields.…”

Section: Evolution Of the Hard X-ray Luminosity Functionmentioning

confidence: 99%