arXiv subjects

G. C. Perola

Publications and source records attributed to G. C. Perola.

At least 19 recordsLinked to original sources

arXiv2009

The bolometric luminosity of type 2 AGN from extinction-corrected [OIII]: no evidence for Eddington-limited sources

There have been recent claims that a significant fraction of type 2 AGN accrete close or even above the Eddington limit. In type 2 AGN the bolometric luminosity (L_b) is generally inferred from the [OIII] emission line luminosity (L_OIII). The key issue, in order to estimate the bolometric luminosity in these AGN, is therefore to know the bolometric correction to be applied to L_OIII. A complication arises from the fact that the observed L_OIII is affected by extinction, likely due to dust within the narrow line region. The extinction-corrected [OIII] luminosity (L^c_OIII) is a better estimator of the nuclear luminosity than L_OIII. However, so far only the bolometric correction to be applied to the uncorrected L_OIII has been evaluated. This paper is devoted to estimate the bolometric correction C_OIII=L_b/L^c_OIII in order to derive the Eddington ratios for the type 2 AGN in a sample of SDSS objects. We have collected from the literature 61 sources with reliable estimate of both L^c_OIII and X-ray luminosities (L_X). To estimate C_OIII, we combined the observed correlation between L^c_OIII and L_X with the X-ray bolometric correction. We found, contrary to previous studies, a linear correlation between L^c_OIII and L_X. We estimated C_OIII using the luminosity-dependent X-ray bolometric correction of Marconi et al. (2004), and we found a mean value of C_OIII in the luminosity ranges log L_OIII=38-40, 40-42, and 42-44 of 87, 142 and 454 respectively. We used it to calculate the Eddington ratio distribution of type 2 SDSS AGN at 0.3<z<0.4 and we found that these sources are not accreting near their Eddington limit, contrary to previous claims.

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arXiv2008

X-ray absorption in Compton-thin AGN: the predictions of a model revisited

The evidence of a decrease with increasing luminosity of the fraction f_{abs} of absorbed and Compton-thin among X-ray (2-10 keV) selected AGN is observationally rather well supported, while that of an increase of f_{abs} with redshift is rather controversial. In Lamastra, Perola & Matt (2006) the gravitational effect of the SMBH on the molecular interstellar gas, in the central region of the host galaxy, was shown to predict an anti-correlation between f_{abs} and M_{BH}. The most recent findings on the distribution of the Eddington ratio λ=L_b/L_E as a function of M_{BH} and z are used to convert that relationship into one between f_{abs} and both bolometric (L_b) and X-ray (L_X) luminosities at various values of z. The findings on λ(M_{BH},z) are properly treated in order to ensure completeness in the prediction of f_{abs} above a certain luminosity, at values of z=0.1, 0.35, 0.7 and >1. To verify the consequence of these findings alone, we adopted in a first istance a distribution of gas surface density Σ, observed in a sample of local spiral galaxies, irrespective of the galaxy morphological type and z. Assuming in the λ(M_{BH},z) distribution the Eddington limit, λ=1, as a ``natural'' cut-off, the predictions are consistent with the existence of an anti-correlation between f_{abs} and L_X, but fail to reproduce an increase of f_{abs} with z. Because the early type galaxies on average are much poorer in molecular gas than late type ones, a quantitative agreement with the local value of f_{abs} requires the existence of a correlation between Σand the central activity. An increase of typical values of Σwith z, correlated with the activity, might explain an increase of f_{abs} with z. However, at the highest luminosities f_{abs} could hardly exceed about 0.3.

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arXiv2007

Testing the cosmic ray content in galaxy clusters

The effective content of cosmic rays (CR) in galaxy clusters remains elusive. This paper aims to estimate a maximum production of both secondary relativistic electrons, SRE, and gamma rays, GR, from the relativistic protons, RP, that have supposedly accumulated throughout the entire history of a cluster. The production rate is normalized by adopting a reference value of 0.3 for the ratio of RP to thermal pressure. The SRE content which obtains, when constrained to reproduce the observed radio brightness profile, yields univocally B(r), if the presence of primary RE were negligible. This procedure is applied to four radio halo clusters (Coma, A2163, A2255, A2319). In these objects, the central value B_0 required is consistent with typical, albeit rather uncertain, values derived from FR. On the other hand, B(r) typically increases beyond the thermal core, a hardly acceptable condition. This problem is alleviated by assuming a mix of SRE and of primary RE, with the latter becoming the dominant component beyond the thermal core. These results suggest that in clusters without a radio halo detected so far a diffuse radio emission should also be observable due to SRE alone, and therefore more centrally condensed. To encourage deeper radio observations of such clusters, some examples were selected that seem rather promising. Efforts in this direction, if accompanied by FR measurements, could provide highly significant constraints on the CR content in clusters, even before the GLAST mission will have accomplished the hard task of detecting the GR. A complementary result concerns the excess far UV in the Coma cluster, that some authors have attributed to IC emission from SRE. It is shown that this hypothesis can be excluded, because it requires a RP energy content in excess of the thermal one.

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arXiv2006

The HELLAS2XMM survey.VIII. Optical identifications of the extended sample

(Abridged) We present the results of the photometric and spectroscopic identification of 110 hard X-ray selected sources (6e-15 1000, while their Lx, L(R) and Nh are similar. This suggests that while the central engine of narrow-line AGNs and XBONGs looks similar, the narrow-line region in XBONGs could be strongly inhibited or obscured.

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arXiv2006

The broad band spectrum and variability of NGC 4151 observed by BeppoSAX

We present an analysis of all SAX observations of NGC 4151. This source was observed 5 times from 1996 to 2001 with durations ranging from a day to four days. The intrinsic continuum, is absorbed at low energies by a complex system: a cold patchy absorber plus a warm uniform screen photoionized by the central continuum. We find that this dual absorber is the main driver of the observed variability, up to a factor of eight, at 3 keV. In particular the covering fraction of the cold absorber changes on time scales of the order of a day, supporting its association with the Broad Line Region. The column density of the warm gas varies on a longer time scale (months to year). Some of the small amplitude spectral variability above 10 keV can be explained with an intrinsic variation (ΔΓ~0.2).The flux below 1 keV remains constant confirming an extended origin. Its spectrum is reproduced by a combination of a thermal component and a power law with the same slope as the intrinsic continuum but with an intensity a few per cent. A Compton reflection component is significantly detected in 1996 (averaged value of Ω/2π~0.4), with intensity decreasing on time scale of year, and it desappears in 2000 and 2001. The long time scale of variations argues for an association with an optically thick torus at a distance of few light years. An iron line was detected in all spectra. Its energy is consistent with fluorescence by cold iron. We find that the line is variable. Its behaviour is reproduced by a variable component proportional to the level of the reflection flux plus a constant component. The flux of the latter is consistent with the extended line emission observed by Chandra. We conclude that the first component is likely arising from the torus and the second is produced in the extended Narrow Line Region.

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arXiv2006

Opening a New Window to Fundamental Physics and Astrophysics: X-ray Polarimetry

An extensive theoretical literature predicts that X-ray Polarimetry can directly determine relevant physical and geometrical parameters of astrophysical sources, and discriminate between models further than allowed by spectral and timing data only. X-ray Polarimetry can also provide tests of Fundamental Physics. A high sensitivity polarimeter in the focal plane of a New Generation X-ray telescope could open this new window in the High Energy Sky.

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arXiv2005

The HELLAS2XMM survey. IX. Spectroscopic identification of super-EROs hosting AGNs

We present VLT near-IR spectroscopic observations of three X-ray sources characterized by extremely high X-ray-to-optical ratios (X/O>40), extremely red colors (6.3<R-K<7.4, i.e. EROs) and bright infrared magnitudes (17.6<K<18.3). These objects are very faint in the optical, making their spectroscopic identification extremely challenging. Instead, our near-IR spectroscopic observations have been successful in identifying the redshift of two of them (z=2.08 and z=1.35), and tentatively even of the third one (z=2.13). When combined with the X-ray properties, our results clearly indicate that all these objects host obscured QSOs (4e44 < L(2-10keV) < 1.5e45 erg/s, 2e22 < N_H < 4e23 cm-2) at high redshift. The only object with unresolved morphology in the K band shows broad Halpha emission, but not broad Hbeta, implying a type 1.9 AGN classification. The other two objects are resolved and dominated by the host galaxy light in the K band, and appear relatively quiescent: one of them has a LINER-like emission line spectrum and the other presents only a single, weak emission line which we tentatively identify with Halpha. The galaxy luminosities for the latter two objects are an order of magnitude brighter than typical local L* galaxies and the derived stellar masses are well in excess of 10^11 Msun. For these objects we estimate black hole masses higher than 10^9 Msun and we infer that they are radiating at Eddington ratios L/L_Edd < 0.1. We discuss the implications of these findings for the coevolution of galaxies and black hole growth. Our results provide further support that X-ray sources with high X/O ratios and very red colors tend to host obscured QSO in very massive galaxies at high redshift.

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arXiv2005

The HELLAS2XMM survey. VII. The hard X-ray luminosity function of AGN up to z=4: more absorbed AGN at low luminosities and high redshifts

We have determined the cosmological evolution of the density of AGN and of their Nh distribution as a function of the un-absorbed 2-10 keV luminosity up to redshift 4. We used the HELLAS2XMM sample combined with other published catalogs, yielding a total of 508 AGN. Our best fit is obtained with a luminosity-dependent density evolution (LDDE) model where low luminosity (Lx~10^43 erg s^-1) AGN peak at z~0.7, while high luminosity AGN (Lx>10^45 erg s^-1) peak at z~2.0. A pure luminosity evolution model (PLE) can instead be rejected. There is evidence that the fraction of absorbed (Nh>10^22 cm^-2) AGN decreases with the intrinsic X-ray luminosity, and increases with the redshift. Our best fit solution provides a good fit to the observed counts, the cosmic X-ray background, and to the observed fraction of absorbed AGN as a function of the flux in the 10^-15 10^44 erg s^-1) AGN have a density of 267 deg^-2 at fluxes S(2-10)>10^-15 erg s^-1 cm^-2. Using these results, we estimate a density of supermassive black holes in the local Universe of 3.2 (h^2_70) x 10^5 M_sol Mpc^-3, which is consistent with the recent measurements of the black hole mass function in the local galaxies.

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arXiv2005

XMM-Newton observation of the bright Seyfert 1 galaxy, MCG+8-11-11

We report on the XMM-Newton observation of the bright Seyfert 1 galaxy, MCG+8-11-11. Data from the EPIC/p-n camera, the Reflection Gratings Spectrometers (RGS) and the Optical Monitor (OM) have been analyzed. The p-n spectrum is well fitted by a power law, a spectrally unresolved Fe Kalpha line, a Compton reflection component (whose large value, when compared to the iron line equivalent width, suggests iron underabundance), and absorption by warm material. Absorption lines are apparent in the RGS spectra, but their identification is uncertain and would require large matter velocities. The UV fluxes measured by the OM are well above the extrapolation of the X-ray spectrum, indicating the presence of a UV bump.

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arXiv2005

Clearing up the clouds around NGC 4151: evidence of a highly ionized absorber

The Seyfert 1 galaxy NGC4151 is characterized by complex X-ray absorption, well described by a dual absorber, composed of a uniform mildly ionized gas and a cold system that partially covers the central source. However, in one of the 5 BeppoSAX observations, the spectrum shows two peculiar features. An absorption feature is detected around 8.5-9 keV with a statistical significance of 99.96%. This feature can be fitted either with an absorption edge at E=8.62^{+0.34}_{-0.52} keV with optical depth τ=0.06\pm0.03 or with an absorption line with 9.5^{+1.3}_{-0.6} keV, width σ=0.95^{+1.2}_{-0.7} keV and EW= 200 eV. In the first case, we associate the feature to highly ionized iron at rest, like FeXXII-FeXXIII (E_{rest}=8.4-8.5 keV). In the second case the feature could be identified with a blend of FeXXV and FeXXVI lines, with an outflow velocity v \approx (0.09-0.26)c. This spectrum is also characterized by a substantial reduction of the absorption column density and the covering fraction of the dual absorber. In particular the column density of the mildly ionized and cold absorbers is \approx 3-5 times lower than observed in the other states, and the covering fraction is reduced by \approx 40 per cent. We propose a possible explanation linking the two properties in terms of a multi-phase ionized absorber.

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arXiv2005

X-ray obscuration and obscured AGN in the local universe

In this paper we discuss the X-ray properties of 49 local (z<0.035) Seyfert 2 galaxies with HST/WFC2 high-resolution optical coverage. It includes the results of 26 still unpublished Chandra and XMM-Newton observations, which yield 25 (22) new X-ray detections in the 0.5-2 keV (2-10 keV) energy band. Our sample covers a range in the 2-10 keV observed flux from 3x10^{-11} to 6x10^{-15} erg cm$^{-2}$ s$^{-1}$. The percentage of the objects which are likely obscured by Compton-thick matter (column density, N_H>1.6x10^{24} atoms/cm/cm) is ~50%, and reaches ~80% for log(F_{2-10})<12.3. Hence, K-alpha fluorescent iron lines with large Equivalent Width (EW > 0.6 keV) are common in our sample (6 new detections at a confidence level >2 sigma). They are explained as due to reflection off the illuminated side of optically thick material. We confirm a correlation between the presence of a ~100-pc scale nuclear dust in the WFC2 images and Compton-thin obscuration. We interpret this correlation as due to the large covering fraction of gas associated with the dust lanes following an idea originally proposed by Malkan et al. 1998, and Matt 2000). The X-ray spectra of highly obscured AGN invariably present a prominent soft excess emission above the extrapolation of the hard X-ray component. This soft component can account for a very large fraction of the overall X-ray energy budget. As this component is generally unobscured - and therefore likely produced in extended gas structures - it may lead to a severe underestimation of the nuclear obscuration in z~1 absorbed AGN, if standard X-ray colors are used to classify them. As a by-product of our study, we report the discovery of a soft X-ray, luminous (~7x10^{40} erg/s) halo embedding the interacting galaxy pair Mkn266.

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arXiv2005

Where does the hard X-ray diffuse emission in clusters of galaxies come from?

The surface brightness produced by synchrotron radiation in Clusters of Galaxies with a radio-halo sets a degenerate constraint on the magnetic field strength, the relativistic electron density and their spatial distributions. Using the Coma radio-halo as a case-study, predictions are made for the brightness profile expected in the 20-80 keV band due to ICS by the relativistic electrons on the CMB, for a range of central values of the magnetic field B_0 and models of its radial dependence. We show that the presence of B-field scalar fluctuations on small scales tends to systematically depress the electron density required by the radio data, hence to decrease the ICS brightness expected. These predictions are useful to evaluate the sensitivity required in future imaging HXR instruments, in order to obtain direct information on the spatial distribution and content of relativistic electrons, hence on the magnetic field properties. If compared with the flux in the Coma HXR tail - interpreted as ICS from within the radius R_h - the predictions lead to values of B_0 which are lower than those obtained from Faraday Rotation measurements. The discrepancy is somewhat reduced if the radio-halo profile is extrapolated out to R_{vir}, i.e. about 3 R_h, or if it is assumed that B(r) \propto n_{th}(r) (Dolag et al. 2002). Note that in the latter case, n_{rel}(r) has its minimum value at the center of the cluster. If real and from ICS, the bulk of the HXR tail should then be contributed by electrons other than those responsible for the bulk of the radio-halo emission. This case illustrates the need for spatially resolved spectroscopy in the HXR, in order to obtain solid information on the non-thermal content of Clusters of Galaxies.

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arXiv2004

HEXIT-SAT: a mission concept for X-ray grazing incidence telescopes from 0.5 to 70 keV

While the energy density of the Cosmic X-ray Background (CXB) provides a statistical estimate of the super massive black hole (SMBH) growth and mass density in the Universe, the lack, so far, of focusing instrument in the 20-60 keV (where the CXB energy density peaks), frustrates our effort to obtain a comprehensive picture of the SMBH evolutionary properties. HEXIT-SAT (High Energy X-ray Imaging Telescope SATellite) is a mission concept capable of exploring the hard X-ray sky with focusing/imaging instrumentation, to obtain an unbiased census of accreting SMBH up to the redshifts where galaxy formation peaks, and on extremely wide luminosity ranges. This will represent a leap forward comparable to that achieved in the soft X-rays by the Einstein Observatory in the late 70'. In addition to accreting SMBH, and very much like the Einstein Observatory, this mission would also have the capabilities of investigating almost any type of the celestial X-ray sources. HEXIT-SAT is based on high throughput (>400 cm2 @ 30 keV; >1200 cm2 @ 1 keV), high quality (15 arcsec Half Power Diameter) multi-layer optics, coupled with focal plane detectors with high efficiency in the full 0.5-70keV range. Building on the BeppoSAX experience, a low-Earth, equatorial orbit, will assure a low and stable particle background, and thus an extremely good sensitivity for faint hard X-ray sources. At the flux limits of 1/10 microCrab (10-30 keV) and 1/3 microCrab (20-40 keV) (reachable in one Msec observation) we should detect ~100 and ~40 sources in the 15 arcmin FWHM Field of View respectively, thus resolving >80% and ~65% of the CXB where its energy density peaks.

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arXiv2004

X-ray reprocessing in Seyfert Galaxies: simultaneous XMM-Newton/BeppoSAX observations

We selected a sample of eight bright unobscured (at least at the iron line energy) Seyfert Galaxies observed simultaneously by XMM-Newton and BeppoSAX, taking advantage of the complementary characteristics of the two missions. The main results of our analysis can be summarized as follows: narrow neutral iron lines are confirmed to be an ubiquitous component in Seyfert spectra; none of the analyzed sources shows unambiguously a broad relativistic iron line; all the sources of our sample (with a single exception) show the presence of a Compton reflection component; emission lines from ionized iron are observed in some sources; peculiar weak features around 5-6 keV (possibly arising from rotating spots on the accretion disk) are detected in two sources. The scenario emerging from these results strongly requires some corrections for the classical model of reprocessing from the accretion disk. As for materials farther away from the Black Hole, our results represent a positive test for the Unification Model, suggesting the presence of the torus in (almost) all sources, even if unobscured.

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arXiv2004

The HELLAS2XMM survey: VI. X-ray absorption in the 1dfAGN sample through a spectral analysis

The spectroscopic analysis of 117 serendipitous sources in the HELLAS2XMM 1df (1 degree field) survey is described. Of these, 106 sources, of which 86% have a spectroscopic redshift, are used to evaluate the fraction of X-ray absorbed (log NH>22) Active Galactic Nuclei (AGN) in the 2--10 keV flux range 0.8-20E-14 erg/cm2/s. This fraction turns out lower than what is predicted by two well known Cosmic X-Ray Background synthesis models, and the discrepancy is significant at the 99.999% level. This result consolidates the findings recently obtained by other authors. In the flux interval explored, the data are consistent with an intrinsic distribution of the absorbing columns (flat per decade above logNH>21) independent of luminosity and redshift, together with an AGN luminosity function evolving purely in luminosity. It is shown that, on the other hand, extrapolation to lower fluxes fails to reproduce the results inferred from the Chandra Deep Field North survey. It is found that about 40% of the high luminosity sources in our sample have best fit logNH>22, and the surface density of these X-ray obscured QSOs can then be estimated at about 48 per square degree, at the flux limit of ~1E-14} erg/cm2/s of the HELLAS2XMM 1df survey. As a side issue, 5 or 6 out of 60 sources, that is about 10%, identified with broad line AGN, turn out to be affected by logNH>22 absorption.

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arXiv2004

The HELLAS2XMM survey. V. Near-Infrared observations of X-ray sources with extreme X/O ratios

We present the results of deep near-infrared observations of 11 hard X-ray selected sources in the Hellas2XMM survey, with faint optical magnitude (R>24) and high X-ray-to-optical flux ratio. All but one of the sources have been detected in the Ks band, with bright counterparts (Ks<19) and red colors (R-K>5), and therefore belong to the ERO population. A detailed analysis of the surface brightness profiles allows us to classify all of the NIR counterparts. There are 2 point-like objects, 7 elliptical galaxies and one source with a disky profile. None of the extended sources shows any evidence for the presence of a central unresolved object tracing the putative X-ray emitting AGN. Using both the R-K colors and the morphological information, we have estimated for all the sources a ``minimum photometric redshift'', ranging between 0.8 and 2.4; the elliptical hosts have zmin=0.9-1.4. We computed the X-ray properties using these redshifts: most of the sources have NH>10^{22}, with unabsorbed X-ray luminosities up to 10^{45}erg s^{-1}. These objects therefore belong to the population of obscured (Type II) quasars and, from a statistical point of view, they turn out to be a non-negligible fraction (~10%) of the most luminous AGN. Selecting the high X/O sources for a follow-up study in the NIR is therefore a powerful technique aimed at studying at high redshift the hosts of Type II AGN. Overall, our results seem to indicate that the hosts are mostly elliptical galaxies at z~1, and that these near-IR bright objects would be among the most massive spheroids at these epochs.

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arXiv2004

X-Ray Evolution of Active Galactic Nuclei and Hierarchical Galaxy Formation

We have incorporated the description of the X-ray properties of Active Galactic Nuclei (AGNs) into a semi-analytic model of galaxy formation, adopting physically motivated scaling laws for accretion triggered by galaxy encounters. Our model reproduces the level of the cosmic X-ray background at 30 keV; we predict that the largest contribution (around 2/3) comes from sources with intermediate X-ray luminosity 10^{43.5}< L_X/erg/s <10^{44.5}, with 50 % of the total specific intensity produced at z<2. The predicted number density of luminous X-ray AGNs (L_X>10^{44.5} erg/s in the 2-10 keV band) peaks at z around 2 with a decline of around 3 dex to z=0; for the low luminosity sources (10^{43}<L_X/erg/s <10^{44}) it has a broaderand less pronounced maximum around z 1.5. The comparison with the data shows a generally good agreement. The model predictions slightly exceed the observed number of low-luminosity AGNs at z around 1.5, with the discrepancy progressively extending to intermediate-luminosity objects at higher redshifts; we discuss possible origins for the mismatch. Finally, we predict the source counts and the flux distribution at different redshifts in the hard (20-100 keV) X-ray band for the sources contributing to the X- ray background.

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arXiv2003

Rapid N_H Changes in NGC 4151

We have analyzed two long BeppoSAX observations of the bright Seyfert galaxy NGC 4151, searching for short timescale (10-200 ksec) X-ray spectral variability. The light curve of a softness ratio, chosen as most sensitive to pinpoint changes of the column density of the absorbing gas along the line of sight, shows significant variations. We try to model these variations by performing a detailed, time resolved, spectral analysis. We find significant, large (factors of 1.5-6) variations of the absorber column densities on time scales of 40-200 ksec. These values are 10-100 times shorter than those found by Risaliti et al. 2002 in a sample of Seyfert 2 galaxies, and provide strong constraints on the geometry of the obscuring medium.

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