F. Panessa

The XMM-Newton serendipitous survey - IV. Optical identification of the XMM-Newton medium sensitivity survey (XMS)

Aims. X-ray sources at intermediate fluxes (a few x 10(-14) erg cm(-2) s(-1)) with a sky density of similar to 100 deg(-2) are responsible for a significant fraction of the cosmic X-ray background at various energies below 10 keV. The aim of this paper is to provide an unbiased and quantitative description of the X-ray source population at these fluxes and in various X-ray energy bands. Methods. We present the XMM-Newton Medium sensitivity Survey (XMS), including a total of 318 X-ray sources found among the serendipitous content of 25 XMM-Newton target fields. The XMS comprises four largely overlapping source samples selected at soft (0.5-2 keV), intermediate (0.5-4.5 keV), hard (2-10 keV) and ultra-hard (4.5-7.5 keV) bands, the first three of them being flux-limited. Results. We report on the optical identification of the XMS samples, complete to 85-95%. At the flux levels sampled by the XMS we find that the X-ray sky is largely dominated by Active Galactic Nuclei. The fraction of stars in soft X-ray selected samples is below 10%, and only a few per cent for hard selected samples. We find that the fraction of optically obscured objects in the AGN population stays constant at around 15-20% for soft and intermediate band selected X-ray sources, over 2 decades of flux. The fraction of obscured objects amongst the AGN population is larger (similar to 35-45%) in the hard or ultra-hard selected samples, and constant across a similarly wide flux range. The distribution in X-ray-to-optical flux ratio is a strong function of the selection band, with a larger fraction of sources with high values in hard selected samples. Sources with X-ray-to-optical flux ratios in excess of 10 are dominated by obscured AGN, but with a significant contribution from unobscured AGN.

The X-ray and radio connection in low-luminosity active nuclei

We present the results of the correlation between the nuclear 2–10 keV X-ray and radio (at 2 cm, 6 cm, and 20 cm) luminosities for a well-defined sample of local Seyfert galaxies. We use a sample of low luminosity radio galaxies (LLRGs) for comparison. In both Seyfert and LLRG samples, X-ray and radio luminosities are significantly correlated over 8 orders of magnitude, indicating that the X-ray and radio emission sources are strongly coupled. Moreover, both samples show a similar regression slope, LX ∝ L 0.97 R , but Seyfert galaxies are three orders of magnitude less luminous in the radio band than LLRGs. This suggests that either similar physical mechanisms are responsible for the observed emission or a combination of different mechanisms ends up producing a similar correlation slope. Indeed, the common belief for LLRG is that both the X-ray and radio emission are likely dominated by a relativistic jet component, while in Seyfert galaxies the X-ray emission probably arises from a disk-corona system and the radio emission is attributed to a jet/outflow component. We investigate the radio loudness issue in the two samples and find that the Seyfert galaxies and the LLRGs show a different distribution of the radio loudness parameters. No correlation is found between the luminosity and the radio loudness; however, the latter is related to the black hole mass and anti-correlated with the Eddington ratio. The dichotomy in the radio loudness between Seyfert and LLRG observed down to low Eddington ratios, L2−10 keV/LEdd ∼ 10 −8 , does not support the idea that the origin of the radio loudness is due to a switch in the accretion mode.

THE FAINTEST SEYFERT RADIO CORES REVEALED BY VLBI

In this Letter, we report on dual-frequency European VLBI Network observations of the faintest and least luminous radio cores in Seyfert nuclei, going to sub-millijansky flux densities and radio luminosities around 1019 W Hz–1. We detect radio emission from the nuclear region of four galaxies (NGC 4051, NGC 4388, NGC 4501, and NGC 5033), while one (NGC 5273) is undetected at the level of ~100 μJy. The detected compact nuclei have rather different radio properties: spectral indices range from steep (α>0.7) to slightly inverted (α = –0.1), brightness temperatures vary from TB = 105 K to larger than 107 K, and cores are either extended or unresolved, in one case accompanied by lobe-like features (NGC 4051). In this sense, diverse underlying physical mechanisms can be at work in these objects: jet-base or outflow solutions are the most natural explanations in several cases; in the case of the undetected NGC 5273 nucleus, the presence of an advection-dominated accretion flow is consistent with the radio luminosity upper limit.

An intermediate black hole spin in the NLS1 galaxy SWIFT J2127.4+5654: chaotic accretion or spin energy extraction?

We have observed the hard X-ray selected Narrow-Line Seyfert 1 galaxy SWIFT J2127.4+5654 with Suzaku. We report the detection of a broad relativistic iron emission line from the inner accretion disc. Partial covering by either neutral or partially ionized gas cannot reproduce the spectral shape and, even if its presence is assumed, it does not significantly change the inferred broad-line parameters. By assuming that the inner edge of the accretion disc corresponds to the innermost stable circular orbit of the black hole space–time, the line profile enables us to measure a black hole spin a = 0.6 ± 0.2. However, a non-rotating Schwarzschild space–time is excluded at just above the 3σ level, while a maximally spinning Kerr black hole is excluded at the ∼5σ level. The intermediate spin we measure may indicate a recent merger, or that accretion-driven black hole growth in this source proceeds through short-lived episodes with chaotic angular momentum alignment between the disc and the hole rather than via prolonged accretion. The measured steep emissivity index (q � 5) constrains the irradiating X-ray source to be very centrally concentrated. Light bending may help focus the primary emission towards the innermost accretion disc, thus steepening the irradiation profile. On the other hand, steep profiles can also be reached if magnetic extraction of the hole rotational energy is at work. If this is the case, the interplay between accretion (spinning up the black hole) and rotational energy extraction (spinning it down) forces the hole to reach an equilibrium spin value which, under standard assumptions, is remarkably consistent with our measurement. Rotational energy extraction would then be able to simultaneously account for the intermediate spin and steep emissivity profile we infer from our spectral analysis without the need to invoke chaotic accretion episodes. We also report tentative evidence for short timescale line profile variability. The relatively low statistical significance of the variability (about 98 per cent confidence level) prevents us from drawing any firm conclusions which must be deferred to future observations.

The INTEGRAL Complete Sample of Type 1 AGN

In this paper we discuss the broad-band X-ray characteristics of a complete sample of 36 type 1 active galactic nuclei (AGN), detected by INTEGRAL in the 20–40 keV band above the 5.5? level. We present, for all the objects in the sample, the broad-band (1–110 keV) spectral analysis obtained by using INTEGRAL/Swift/Burst Alert Telescope observations together with XMM–Newton, Chandra, ASCA and Swift/X-Ray Telescope data. We also present the general average properties of the sample, i.e. the distribution of photon indices, high-energy cut-offs, reflection fractions and absorption properties, together with an in-depth analysis of their parameter space. We find that the average Seyfert 1 power law has an index of 1.7 with a dispersion of 0.2. The mean cut-off energy is at around 100 keV, with most objects displaying Ec in the range 50–150 keV; the average amount of Compton reflection is 1.5 with a typical dispersion of 0.7. We do not find any convincing correlation between the various parameters, an indication that our analysis is not strongly dependent by the interplay between them. Finally, we investigate how the results presented in this work fit into current frameworks for AGN spectral modelling and cosmic diffuse X-ray background synthesis models

First high-energy observations of narrow-line Seyfert 1s with INTEGRAL/IBIS

Narrow-line Seyfert 1 (NLS1) galaxies are very interesting objects which display peculiar properties when compared to their broad-line analogues (BLS1). Although well studied in many wavebands, their behaviour at >10 keV is poorly studied and yet important to discriminate between models invoked to explain the complexity observed in the X-ray band. Here, we present for the first time high-energy observations (17–100 keV) of five NLS1 galaxies (three bona fide and two candidates) detected by INTEGRAL/Imager on Board INTEGRAL Satellite (IBIS) and provide for all of them a broad-band spectral analysis using data obtained by Swift/XRT below 10 keV. The combined INTEGRAL spectrum is found to be steeper (?= 2.6 ± 0.3) than those of classical Seyfert 1 objects. This is due to a high-energy cut-off, which is required in some individual fits as in the average broad-band spectrum. The location of this high-energy cut-off is at lower energies (E? 60 keV) than typically seen in classical type 1 active galactic nuclei (AGNs); a reflection component may also be present but its value (R < 0.8) is compatible with those seen in standard Seyfert 1s. We do not detect a soft excess in individual objects but only in their cumulative spectrum. Our results suggest a lower plasma temperature for the accreting plasma which combined to the high accretion rates (close to the Eddington rate) points to different nuclear conditions in broad and NLS1 galaxies, likely related to different evolutionary stages.

Broad-band study of hard X-ray-selected absorbed active galactic nuclei

[Abridged]We report on the broadband X-ray properties of a complete sample of absorbed Seyfert galaxies hard X-ray selected with INTEGRAL. The sample is composed of 33 sources: 15 are newly discovered above 20 keV while 18 are already known AGN. For 17 sources we have performed a broadband analysis with XMM and INTEGRAL data. We have complemented the analysis of the 16 remaining sources with existing broadband studies. The spectra are well reproduced with an absorbed primary emission with a high energy cutoff and its scattered fraction below 2-3 keV, plus the Compton reflection features. A high energy cut-off is found in 30% of the sample, with an average value below 150 keV. The hard X-ray selection favours the detection of more obscured sources, with the log NH average value of 23.15. The diagnostic plot NH vs F(corr)(2-10keV)/F(20-100keV) allowed the isolation of the Compton thick objects and may represent a useful tool for future hard X-ray observations of newly discovered AGN. We are unable to associate the reflection components (continuum and Fe line) with the absorbing gas as a torus, a more complex scenario being necessary. In the Compton thin sources, a fraction (but not all) of the Fe line needs to be produced in a gas located closer to the BH than the thick torus, and this is possibly associated with the optical BLR, responsible also for the absorption. We still need a Compton thick medium (not intercepting the line of sight) likely associated to a torus, which contributes to the Fe line and produces the observed reflection continuum above 10 keV. The Iwasawa-Taniguchi effect can not be confirmed with our data. Finally, the comparison with a sample of unobscured AGN shows that, type 1 and type 2 (once corrected for absorption) Seyfert are characterized by the same nuclear/accretion properties (luminosity, bolometric luminosity, Eddington ratio), supporting the unified view.

The INTEGRAL/IBIS AGN catalogue - I. X-ray absorption properties versus optical classification

In this work we present the most comprehensive INTEGRAL active galactic nucleus (AGN) sample. It lists 272 AGN for which we have secure optical identifications, precise optical spectroscopy and measured redshift values plus X-ray spectral information, i.e. 2–10 and 20– 100 keV fluxes plus column density. Here we mainly use this sample to study the absorption properties of active galaxies, to probe new AGN classes and to test the AGN unification scheme. We find that half (48 per cent) of the sample is absorbed, while the fraction of Compton-thick AGN is small (∼7 per cent). In line with our previous analysis, we have however shown that when the bias towards heavily absorbed objects which are lost if weak and at large distance is removed, as it is possible in the local Universe, the above fractions increase to become 80 and 17 per cent. We also find that absorption is a function of source luminosity, which implies some evolution in the obscuration properties of AGN. A few peculiar classes, so far poorly studied in the hard X-ray band, have been detected and studied for the first time such as 5 X-ray bright optically normal galaxies, 5 type 2 QSOs and 11 low-ionization nuclear emission regions. In terms of optical classification, our sample contains 57 per cent of type 1 and 43 per cent of type 2 AGN; this subdivision is similar to that found in X-rays if unabsorbed versus absorbed objects are considered, suggesting that the match between optical and X-ray classifications is overall good. Only a small percentage of sources (12 per cent) does not fulfil the expectation of the unified theory as we find 22 type 1 AGN which are absorbed and 10 type 2 AGN which are unabsorbed. Studying in depth these outliers we found that most of the absorbed type 1 AGN have X-ray spectra characterized by either complex or warm/ionized absorption more likely due to ionized gas located in an accretion disc wind or in the biconical structure associated with the central nucleus, therefore unrelated to the toroidal structure. Among 10 type 2 AGN which resulted to be unabsorbed, at most 3–4 per cent are still eligible to be classified as ‘true’ type 2A GN.

Simultaneous X-ray and optical observations of true type 2 Seyfert galaxies

We present the results of a campaign of simultaneous X-ray and optical observations of ‘true’ type 2 Seyfert galaxies candidates, i.e. active galactic nuclei without a broad-line region (BLR). Out of the initial sample composed of eight sources, one object, IC 1631, was found to be a misclassified starburst galaxy, another, Q2130−431, does show broad optical lines, while other two, IRAS 01428−0404 and NGC 4698, are very likely absorbed by Compton-thick gas along the line of sight. Therefore, these four sources are not unabsorbed Seyfert 2s as previously suggested in the literature. On the other hand, we confirm that NGC 3147, NGC 3660 and Q2131−427 belong to the class of true type 2 Seyfert galaxies, since they do not show any evidence for a broad component of the optical lines nor for obscuration in their X-ray spectra. These three sources have low accretion rates ( u m = Lbol/LEdd 0.01), in agreement with theoretical models which predict that the BLR disappears below a critical value of Lbol/LEdd. The last source, Mrk 273x, would represent an exception even of these accretion-dependent versions of the Unification Models, due to its high X-ray luminosity and accretion rate, and no evidence for obscuration. However, its optical classification as a Seyfert 2 is only based on the absence of a broad component of Hβ, due to the lack of optical spectra encompassing the Hα band.

The broad-band XMM-Newton and INTEGRAL spectra of bright type 1 Seyfert galaxies

Aims. The 0.5–150 keV broad-band spectra of a sample of nine bright type 1 Seyfert galaxies are analyzed here. These sources have been discovered/detected by INTEGRAL and subsequently observed with XMM-Newton for the first time with high sensitivity below 10 keV. The sample, although small, is representative of the population of type 1 AGN which are now being observed above 20 keV. Methods. The intrinsic continuum has been modeled using three different parameterizations: a power-law model, an exponential cutoff power-law and an exponential cut-off power-law with a Compton reflection component. In each model the presence of intrinsic absorption, a soft component and emission line reprocessing features has also been tested. Results. A simple power-law model is a statistically good description of most of the spectra presented here; an FeK line, fully and/or partial covering absorption and a soft spectral component are detected in the majority of the sample sources. The average photon index (� Γ� = 1.7 ± 0.2) is consistent, within errors, with the canonical spectral slope often observed in AGN although the photon index distribution peaks in our case at flat Γ (∼1.5) values. For four sources, we find a significantly improved fit when the power-law is exponentially cut-off at an energy which is constrained to be below ∼150 keV. The Compton reflection parameter could be estimated in only two objects of the sample and in both cases is found to be R > 1.