arXiv subjects

A. Akylas

Publications and source records attributed to A. Akylas.

49 records · Page 3Linked to original sources

arXiv2006

Chandra and Spitzer observations of CDFS X-ray obscured QSOs

We present Chandra and Spitzer data of the 186, extragalactic, hard 2-10 keV X-ray selected sources, which lie in the central part of the Chandra Deep Field South (CDFS). For the vast majority of sources (99.5%) there is a spectroscopic or photometric redshift available. We classify 17 sources as X-ray obscured QSOs, according to strictly X-ray criteria, i.e. defined as having large hydrogen column densities (N_H>10^22 cgs) and luminosities (Lx>10^44 cgs). The surface density of X-ray obscured QSOs is ~210 sq. deg. We find 18 candidate Compton thick N_H>10^24 cgs sources, of which three have QSO luminosities (L_x>10^44 cgs). The X-ray obscured QSO comprise a mixed bag of objects, covering the redshift range z=1.3-4.3. Eight of these show narrow line optical spectra, two show no obscuration in their optical spectra presenting Broad Lines, while for the other seven there is only a photometric redshift available. About half of the X-ray obscured QSOs show high X-ray to optical flux ratios, X/O>1, and red colours, I-3.6>4. Combination of the X-ray with the mid-IR 8 or 24 micron flux can be used as an additional diagnostic to sift out the heavily obscured AGN. All X-ray selected QSOs present red mid-IR colours and can be easily separated among mid-IR sources, demonstrating that mid-IR selection provides a powerful tool for the detection of obscured QSOs.

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arXiv2006

XMM and Chandra measurements of the AGN intrinsic absorption: dependence on luminosity and redshift

We combine bright XMM data with the Chandra Deep Field South observations in order to explore the behavior of the intrinsic AGN absorption, as a function of redshift and luminosity.Our sample consists of 359 sources selected in the hard 2-8 keV band, spanning the flux range 6\times10^{-16}-$3\times10^{-13} erg s^-1 cm^-2 with a high rate of spectroscopic or photometric redshift completeness (100 and 85 per cent respectively for the Chandra and XMM data. We derive the column density values using X-ray spectral fits. We find that the fraction of obscured AGN falls with increasing luminosity in agreement with previous findings. The fraction of obscured AGN shows an apparent increase at high redshifts (z>2). Simulations show that this effect can be most probably attributed to the fact that at high redshifts the column densities are overestimated.

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arXiv2006

X-ray number counts of normal galaxies

We use the number counts of X-ray selected normal galaxies to explore their evolution by combining the most recent wide-angle shallow and pencil-beam deep samples available. The differential X-ray number counts, dN/dS, for early and late-type normal galaxies are constructed separately and then compared with the predictions of the local X-ray luminosity function under different evolution scenarios. The dN/dS of early type galaxies is consistent with no evolution out to z~0.5. For late-type galaxies our analysis suggests that it is the sources with X-ray--to--optical flux ratio logfx/fopt>-2 that are evolving the fastest. Including these systems in the late-type galaxy sample yields evolution of the form ~(1+z)^{2.7} out to z~0.4. On the contrary late-type sources with logfx/fopt<-2 are consistent with no evolution. This suggests that the logfx/fopt>-2 population comprises the most powerful and fast evolving starbursts at moderate and high-z. We argue that although residual low-luminosity AGN contamination may bias our results toward stronger evolution, this is unlikely to modify our main conclusions.

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arXiv2005

Red AGN in XMM-Newton/SDSS fields

In this paper we combine archival and proprietary XMM-Newton observations (about 5deg^2) that overlap with the Sloan Digital Sky Survey to explore the nature of the moderate-z X-ray population. We focus on X-ray sources with optically red colours (g-r>0.4), which we argue are important for understanding the origin of the X-ray background. Firstly, these systems constitute a significant fraction, about 2/3, of the z<1 X-ray population to the limit f(2-8keV)~2e-14cgs. Secondly, their luminosity function under evolution of the form ~(1+z)^3 suggests that they could be responsible for about 17 per cent of the diffuse X-ray background to z=1. Thirdly, their stacked X-ray spectrum in the range 1-8keV is consistent with a power-law distribution with Gamma~1.4 (without fitting intrinsic absorption), i.e. similar to the diffuse X-ray background. We find that the optically red X-ray population comprises a mixed bag of objects, both obscured (N_H>1e22 cm^{-2}) and unobscured (N_H<1e22 cm^{-2}), with a wide range of X-ray luminosities up L_X~1e44cgs. We argue that dilution of the AGN light by the host galaxy may play a role in shaping the continuum optical emission of this population. Finally, we explore a possible association of these sources and the moderate-z red (J-Ks>2mag) AGNs identified in the Two Micron All Sky Survey (2MASS). The median N_H of the red X-ray sources studied here is ~1e21cm^{-2}, lower than that found for the 2MASS AGNs, suggesting different populations.

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arXiv2005

Fourier Resolved Spectroscopy of the XMM-Newton Observations of MCG -6-30-15

We study the Frequency Resolved Spectra of the Seyfert galaxy MCG -6-30-15 obtained during two recent XMM-Newton observations. Splitting the Fourier spectra in soft (<2 keV) and hard (>2 keV) bands, we find that the soft band has a variability amplitude larger than the hard one on time scales longer than 10 ksec, while the opposite is true on time scales shorter than 3 ksec. Both the soft and hard band spectra are well fitted by power laws of different indices. The spectra of the hard band become clearly softer as the Fourier Frequency decreases from 7x10^{-4} Hz to 10^{-5} Hz, while the spectral slope of the soft band power law component is independent of the Fourier frequency. The well known broad Fe Ka feature is absent at all frequency bins; this result implies that this feature is not variable on time scales shorter than ~10^5 sec, in agreement with recent line variability studies. Strong spectral features are also present in the soft X-ray band (at E~0.7), clearly discernible in all Fourier Frequency bins. This fact is consistent with the assumption that they are due to absorption by intervening matter within the source.

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arXiv2004

The XMM-Newton/2dF survey VII. Is there any X-ray absorption in optically selected QSOs?

We explore the X-ray properties of optically selected QSOs spectroscopically identified in the course of the 2dF QSO survey (2QZ). Our main goal is to expand to higher redshifts previous findings suggesting the presence of a fraction of X-ray obscured sources among the low redshift optically selected broad line AGN population. The X-ray data are from the wide field (~2.5 sq. deg), shallow $[f(0.5 - 8 keV) ~10^-14 egs s^-1 cm^-2 XMM-Newton/2dF survey. A total of 96 2QZ QSOs overlap with the area covered by our X-ray survey. 66 of them have X-ray counterparts while 30 remain undetected in our X-ray survey. The 66 X-ray detected QSOs have a mean photon index of ~2 suggesting little or no X-ray obscuration for most of these sources. Individual X-ray spectral fittings reveal only 1 source (intrinsic Lx(0.5-8 keV) ~ 10^44 erg s^-1 at z=0.82) that is likely to be obscured (NH~10^23 cm^-2) at the 90% confidence level. Additionally, there are 9 2QZ sources that show evidence for moderate absorption (mean observed NH of ~10^21 cm^-2). For the 30 QSOs that remain undetected in our X-ray survey we use stacking analysis to estimate a mean hardness ratio of -0.59 +/- 0.11 suggesting that the bulk of this population has NH consistent with the Galactic value. However, we cannot exclude the possibility that some of these sources have enhanced photoelectric absorption that is not revealed in the mean stacked spectrum. We estimate a lower limit to the fraction of optically selected QSO with X-ray absorption of about 10% (10 out of 96).

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arXiv2004

The XMM-Newton/2dF survey IV. The X-ray spectral properties of the hard sources

We present an analysis of the X-ray spectral properties of 61 hard X-ray selected (2-8 keV) sources from the bright ($f_{2-8 keV}>10^{-14}$ \funits) {\it XMM-Newton}/2dF survey. This comprises of 9 {\it XMM-Newton} pointings in the North Galactic Pole region ($\sim 1.6 \rm deg^2$) and overlaps with the SDSS, 2QZ and 2dFGRS surveys. Our sources contribute about 50 per cent of the 2-10 keV X-ray background down to the flux limit of $10^{-14}$ cgs. The hardness ratio distribution of the sample suggests a deficit of heavily absorbed sources. A spectral fit to the co-added total source spectrum yields a steep photon index, $Γ=1.83^{+0.04}_{-0.05}$. All but 8 sources have optical counterparts down to the SDSS photometric limit of $r\approx22.5$. Spectroscopic identifications exist for 34 sources. The vast majority are associated with Broad-Line (BL) AGN (24 sources) while only 7 present narrow or no emission lines. Five sources are probably associated with Galactic stars. Finally, for another 17 probable AGN we present photometric redshifts. The combined spectrum of the 24 spectroscopically identified BL AGN is steep ($Γ=2.02^{+0.04}_{-0.05}$), while that of the 7 AGN, which do not present broad lines is flatter with $Γ=1.64^{+0.11}_{-0.11}$. The spectrum of the 8 optically unidentified sources is flat with $Γ\approx 1.1$. Spectral fits to the individual BL AGN reveal large absorption (rest-frame column density $\rm>10^{22} cm^{-2}$) in only two cases. The individual spectra of the NL AGN present significant evidence for even a moderate absorption ($\rm 3\times 10^{21} cm^{-2}$) in only one case.

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arXiv2003

On the XMM-Newton spectra of soft X-ray selected QSOs

We study the XMM-Newton spectra of a sample of 32 soft X-ray selected QSOs. Our goal is to check, using the spectra of moderate redshift (z~1.5), faint (f (0.2-8 keV) > 5x10^-15 cgs) broad-line QSOs, previous claims for either significant intrinsic absorption or spectral hardening at high energies. We derive hardness ratios for all sources and furthermore we perform spectral fits for the 11 brighter sources. The majority of sources have steep spectra (photon index >1.9). We find a few QSOs with large amounts of intrinsic absorption, as high as N_H~10^23 cm-2. We find no strong evidence for spectral hardening above 2 keV. The coadded QSO spectrum is well described by a single power-law with photon index of ~1.9, demonstrating that, on average, any effects of absorption are not important. This suggests that the discrepancy between the X-ray background and the (soft X-ray selected) QSO spectrum holds well at the faint fluxes probed here.

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arXiv2003

XMM-Newton observations of an absorbed z=0.67 QSO: no dusty torus?

We present XMM-Newton observations of AXJ0341.4-4453, a mildly reddened A_V<7 QSO at a redshift of z=0.672. The XMM-Newton spectrum shows a large obscuring column N_H~10^23 cm^{-2} corresponding to A_V~70, in agreement with previous results based on the lower limit of the ASCA hardness ratio. The X-ray spectrum is represented by a 'scattering' model with Gamma~2.0 with the scattered power-law normalization being a few per cent of the hard component. No FeK line is detected with a 90 per cent upper limit on its equivalent width of ~360eV. The large discrepancy between the column density observed in X-rays and that inferred from the Balmer decrement can be explained by dust sublimation near the nucleus. Then, the X-ray and the optical obscuration come from two different regions: the X-ray close to the accretion disk while the optical at much larger >0.25pc scales.

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arXiv2002

BeppoSAX observations of LINER-2 galaxies

We present BeppoSAX observations of 6 ``type-2'' LINER and ``transition'' galaxies (NGC3379, NGC3627, NGC4125, NGC4374, NGC5195 and NGC5879)from the Ho et al. (1997) spectroscopic sample of nearby galaxies. All objects are detected in the 2-10 keV band, having luminosities in the range L(2-10 keV) ~ 1x10^{39}- 1x10^{40} erg s-1. The PDS upper limits above 10 keV place constraints on the presence of a heavily obscured AGN in the case of NGC3379 and NGC4125. No significant variability is detected in any of the objects. The spectra are described in most cases by a simple power-law model with a spectral slope of 1.7-2.5 while there is evidence neither for a significant absorption above the Galactic nor for an FeK emission line. Therefore, based on the spectral properties alone, it is difficult to differentiate between a low-luminosity AGN or a star-forming galaxy scenario. However, imaging observations of NGC3627 and NGC5195 with Chandra ACIS-S reveal very weak nuclear sources while most of the X-ray flux originates either in off-nuclear point sources or in diffuse emission. The above clearly argue in favour of a star-forming origin for the bulk of the X-ray emission, at least in the above two sources.

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arXiv2001

Monitoring RXTE Observations of Markarian 348: the origin of the column density variations

We analyze 37 RXTE observations of the type 2 Seyfert galaxy Mrk348 obtained during a period of 14 months. We confirm the spectral variability previous reported by Smith et al., in the sense that thecolumn density decreases by a factor of ~3 as the count rate increases. Column density variations could possibly originate either due to the random drift of clouds within the absorption screen, or due to photoionization processes. Our modeling of the observed variations implies that the first scenario is more likely. These clouds should lie in a distance of >2 light years from the source, having a diameter of a few light days and a density of >10^7 cm^(-3), hence probably residing outside the Broad Line Region.

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arXiv2001

BeppoSAX Observations of the Seyfert-2 Galaxies NGC 7172 and ESO 103-G35

We investigate the X-ray spectra of the type-2 Seyfert galaxies NGC 7172 and ESO 103-G35, using BeppoSAX observations, separated by approximately one year. We find that the X-ray spectra of both NGC 7172 and ESO 103-G35 can be well fitted using a power-law model with an Fe K emission line at 6.4 keV. We did not find any statistically significant evidence for the existence of a reflection component in the X-ray spectra of these two galaxies. The continuum flux has decreased by a factor of approximately two during this period, in both objects. However, the spectral index as well as the absorption column have remained constant. We find weak evidence for the decrease of the normalization of the Fe K emission line in a similar manner to the continuum in NGC7172. We also report evidence for a broad Fe K confirming previous observations. In contrast, in the case of ESO 103-G35 the line flux does not change while its width remains unresolved.

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arXiv2000

The angular correlation function of the ROSAT All Sky Survey Bright Source Catalogue

We have derived the angular correlation function of a sample of 2096 sources detected in the ROSAT All Sky Survey Bright Source Catalogue, in order to investigate the clustering properties of AGN in the local Universe. Our sample is constructed by rejecting all known stars, as well as extended X-ray sources. Areas with |b|<30 deg. and declination <-30 deg. are also rejected due to the high or uncertain neutral hydrogen absorption. Cross-correlation of our sample with the Hamburg/RASS optical identification catalogue, suggests that the vast majority of our sources are indeed AGN. A 4.1 sigma correlation signal between 0 and 8 degrees was detected with w(theta<8 deg.)=0.025 +- 0.006. Assuming the usual power-law form of the 2-point correlation function we find an angular correlationlength of 0.062 degrees. Deprojection on 3 dimensions, using the Limber's equation, yields a spatial correlation length of 6.0+- 1.6 h^-1 Mpc. This is consistent with the AGN clustering results derived at higher redshifts in optical surveys and suggests a comoving model for the clustering evolution.

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