arXiv subjects

Stefano Bianchi

Publications and source records attributed to Stefano Bianchi.

At least 181 records · Page 10Linked to original sources

arXiv2013

The NGC 3341 minor merger: a panchromatic view of the active galactic nucleus in a dwarf companion

We present X-ray (Chandra), radio (EVLA and EVN), and archival optical data of the triple-merging system in NGC3341. Our panchromatic analysis confirms the presence of a Seyfert 2 AGN in NGC3341B, one of the secondary dwarf companions. On the other hand, the nucleus of the primary galaxy, consistent with a star-forming region of a few solar masses per year, and NGC3341C are very unlikely to host an AGN. We therefore suggest that NGC3341 is an exceptional example of an AGN triggered in the satellite galaxy of a minor-merging system. The existence of such a system can have important implications in the models of hierarchical growth of structures. Further observational and theoretical efforts on NGC3341 and potentially similar sources are needed in order to understand the role of minor mergers on the onset of AGN activity, and in the evolution of massive galaxies.

astro-ph.CO↗

arXiv2013

The Hot and Energetic Universe: A White Paper presenting the science theme motivating the Athena+ mission

This White Paper, submitted to the recent ESA call for science themes to define its future large missions, advocates the need for a transformational leap in our understanding of two key questions in astrophysics: 1) How does ordinary matter assemble into the large scale structures that we see today? 2) How do black holes grow and shape the Universe? Hot gas in clusters, groups and the intergalactic medium dominates the baryonic content of the local Universe. To understand the astrophysical processes responsible for the formation and assembly of these large structures, it is necessary to measure their physical properties and evolution. This requires spatially resolved X-ray spectroscopy with a factor 10 increase in both telescope throughput and spatial resolving power compared to currently planned facilities. Feedback from supermassive black holes is an essential ingredient in this process and in most galaxy evolution models, but it is not well understood. X-ray observations can uniquely reveal the mechanisms launching winds close to black holes and determine the coupling of the energy and matter flows on larger scales. Due to the effects of feedback, a complete understanding of galaxy evolution requires knowledge of the obscured growth of supermassive black holes through cosmic time, out to the redshifts where the first galaxies form. X-ray emission is the most reliable way to reveal accreting black holes, but deep survey speed must improve by a factor ~100 over current facilities to perform a full census into the early Universe. The Advanced Telescope for High Energy Astrophysics (Athena+) mission provides the necessary performance (e.g. angular resolution, spectral resolution, survey grasp) to address these questions and revolutionize our understanding of the Hot and Energetic Universe. These capabilities will also provide a powerful observatory to be used in all areas of astrophysics.

astro-ph.HE↗

arXiv2013

The Hot and Energetic Universe: The close environments of supermassive black holes

Most of the action in Active Galactic Nuclei (AGN) occurs within a few tens of gravitational radii from the supermassive black hole, where matter in the accretion disk may lose up to almost half of its energy with a copious production of X-rays, emitted via Comptonization of the disk photons by hot electrons in a corona and partly reflected by the accretion disk. Thanks to its large effective area and excellent energy resolution, Athena+ contributions in the understanding of the physics of accretion in AGN will be fundamental - and unique - in many respects. It will allow us to map the disk-corona system - which is crucial to understand the mechanism of energy extraction and the relation of the corona with winds and jets - by studying the time lags between reflected and primary photons. These lags have been recently discovered by XMM-Newton, but only Athena+ will have the sensitivity required to fully exploit this technique. Athena+ will also be able e.g. to determine robustly the spin of the black hole in nearby sources (and to extend these measurements beyond the local Universe), to establish the nature of the soft X-ray components, and to map the circumnuclear matter within the AGN inner parsec with unprecedented details.

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arXiv2012

X-ray absorption variability in NGC 4507

We present a complete spectral analysis of an XMM-Newton and Chandra campaign of the obscured AGN in NGC 4507, consisting of six observations spanning a period of six months, ranging from June 2010 to December 2010. We detect strong absorption variability on time scales between 1.5 and 4 months, suggesting that the obscuring material consists of gas clouds at parsec-scale distance. The lack of significant variability on shorter time scales suggests that this event is not due to absorption by broad line region clouds, which was instead found in other studies of similar sources. This shows that a single, universal structure of the absorber (either BLR clouds, or the parsec-scale torus) is not enough to reproduce the observed complexity of the X-ray absorption features of this AGN.

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arXiv2012

Accretion and outflow of gas in Markarian 509

A major uncertainty in models for photoionised outflows in AGN is the distance of the gas to the central black hole. We present the results of a massive multiwavelength monitoring campaign on the bright Seyfert 1 galaxy Mrk 509 to constrain the location of the outflow components dominating the soft X-ray band. Mrk 509 was monitored by XMM-Newton, Integral, Chandra, HST/COS and Swift in 2009. We have studied the response of the photoionised gas to the changes in the ionising flux produced by the central regions. We were able to put tight constraints on the variability of the absorbers from day to year time scales. This allowed us to develop a model for the time-dependent photoionisation in this source. We find that the more highly ionised gas producing most X-ray line opacity is at least 5 pc away from the core; upper limits to the distance of various absorbing components range between 20 pc up to a few kpc. The more lowly ionised gas producing most UV line opacity is at least 100 pc away from the nucleus. These results point to an origin of the dominant, slow (v<1000 km/s) outflow components in the NLR or torus-region of Mrk 509. We find that while the kinetic luminosity of the outflow is small, the mass carried away is likely larger than the 0.5 Solar mass per year accreting onto the black hole. We also determined the chemical composition of the outflow as well as valuable constraints on the different emission regions. We find for instance that the resolved component of the Fe-K line originates from a region 40-1000 gravitational radii from the black hole, and that the soft excess is produced by Comptonisation in a warm (0.2-1 keV), optically thick (tau~10-20) corona near the inner part of the disk.

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arXiv2012

Active galaxy 4U 1344-60: did the relativistic line disappear?

X-ray bright active galactic nuclei represent a unique astrophysical laboratory for studying accretion physics around super-massive black holes. 4U 1344-60 is a bright Seyfert galaxy which revealed relativistic reflection features in the archival XMM-Newton observation. We present the spectroscopic results of new data obtained with the Suzaku satellite and compare them with the previous XMM-Newton observation. The X-ray continuum of 4U 1344-60 can be well described by a power-law component with the photon index ~ 1.7 modified by a fully and a partially covering local absorbers. We measured a substantial decrease of the fraction of the partially absorbed radiation from around 45% in the XMM-Newton observation to less than 10% in the Suzaku observation while the power-law slope remains constant within uncertainties. The iron line in the Suzaku spectrum is relatively narrow, $σ=(0.08 \pm 0.02)$ keV, without any suggestion for relativistic broadening. Regarding this, we interpret the iron line in the archival XMM-Newton spectrum as a narrow line of the same width plus an additional red-shifted emission around 6.1 keV. No evidence of the relativistic reflection is present in the Suzaku spectra. The detected red-shifted iron line during the XMM-Newton observation could be a temporary feature either due to locally enhanced emission or decreased ionisation in the innermost accretion flow.

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arXiv2012

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. AGN without a Broad Line Region (BLR). Out of the initial sample composed by 8 sources, one object, IC1631, was found to be a misclassified starburst galaxy, another, Q2130-431, does show broad optical lines, while other two, IRAS01428-0404 and NGC4698, 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 NGC3147, NGC3660, 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 ($\dot{m}= L_{bol}/L_{Edd} \la0.01$), in agreement with theoretical models which predict that the BLR disappears below a critical value of $L_{bol}/L_{Edd}$. The last source, Mrk273x, would represent an exception even of this 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 the Hβ, due to the lack of optical spectra encompassing the Hα band.

astro-ph.GA↗

arXiv2012

AGN Obscuration and the Unified Model

Unification Models of Active Galactic Nuclei postulate that all the observed differences between Type 1 and Type 2 objects are due to orientation effects with respect to the line-of-sight to the observer. The key ingredient of these models is the obscuring medium, historically envisaged as a toroidal structure on a parsec scale. However, many results obtained in the last few years are clearly showing the need for a more complex geometrical distribution of the absorbing media. In this paper we review the various pieces of evidence for obscuring media on different scales, from the vicinity of the black hole to the host galaxy, in order to picture an updated unification scenario explaining the complex observed phenomenology. We conclude by mentioning some of the open issues.

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arXiv2012

The Link Between the Hidden Broad Line Region and the Accretion Rate in Seyfert 2 Galaxies

In the past few years more and more pieces of evidence have been presented for a revision of the widely accepted Unified Model of Active Galactic Nuclei. A model based solely on orientation cannot explain all the observed phenomenology. In the following, we will present evidence that accretion rate is also a key parameter for the presence of Hidden Broad Line Regions in Seyfert 2 galaxies. Our sample consists of 21 sources with polarized Hidden Broad Lines and 18 sources without Hidden Broad Lines. We use stellar velocity dispersions from several studies on the CaII and Mg b triplets in Seyfert 2 galaxies, to estimate the mass of the central black holes via the Mbh-σ\ast relation. The ratio between the bolometric luminosity, derived from the intrinsic (i.e. unabsorbed) X-ray luminosity, and the Eddington luminosity is a measure of the rate at which matter accretes onto the central supermassive black hole. A separation between Compton-thin HBLR and non-HBLR sources is clear, both in accretion rate (log Lbol/LEdd = -1.9) and in luminosity (log Lbol = 43.90). When, properly luminosity-corrected, Compton-thick sources are included, the separation between HBLR and non-HBLR is less sharp but no HBLR source falls below the Eddington ratio threshold. We speculate that non-HBLR Compton-thick sources with accretion rate higher than the threshold, do possess a BLR, but something, probably related to their heavy absorption, is preventing us from observing it even in polarized light. Our results for Compton-thin sources support theoretical expectations. In a model presented by Nicastro (2000), the presence of broad emission lines is intrinsically connected with disk instabilities occuring in proximity of a transition radius, which is a function of the accretion rate, becoming smaller than the innermost stable orbit for very low accretion rates and therefore luminosities.

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arXiv2011

CAIXA: a catalogue of AGN in the XMM-Newton archive III. Excess Variance Analysis

We report on the results of the first XMM systematic "excess variance" study of all the radio quiet, X-ray un-obscured AGN. The entire sample consist of 161 sources observed by XMM for more than 10 ks in pointed observations which is the largest sample used so far to study AGN X-ray variability on time scales less than a day. We compute the excess variance for all AGN, on different time-scales (10, 20, 40 and 80 ks) and in different energy bands (0.3-0.7, 0.7-2 and 2-10 keV). We observe a highly significant and tight (~0.7 dex) correlation between excess variance and MBH. The subsample of reverberation mapped AGN shows an even smaller scatter (~0.45 dex) comparable to the one induced by the MBH uncertainties. This implies that X-ray variability can be used as an accurate tool to measure MBH and this method is more accurate than the ones based on single epoch optical spectra. The excess variance vs. accretion rate dependence is weaker than expected based on the PSD break frequency scaling, suggesting that both the PSD high frequency break and the normalisation depend on accretion rate in such a way that they almost completely counterbalance each other. A highly significant correlation between excess variance and 2-10 keV spectral index is observed. Both the variability vs. LBol and FWHM_Hbeta correlations are consistent with being just by-products of the correlation with MBH. The soft and medium variability is very well correlated with the hard variability, suggesting that the additional soft components (i.e. soft excess, warm absorber) add a minor contribution to the total variability. Once the variability is rescaled for MBH and mdot, no significant difference between narrow-line and broad-line Seyfert 1 is observed. The results are in agreement with a picture where, to first approximation, all local AGN have the same variability properties once rescaled for MBH and accretion rate.

astro-ph.HE↗

arXiv2011

Extreme Warm Absorber variability in the Seyfert Galaxy Mrk 704

In about half of Seyfert galaxies, the X-ray emission is absorbed by an optically thin, ionized medium, the so-called "Warm Absorber", whose origin and location is still a matter of debate. The aims of this paper is to put more constraints on the warm absorber by studying its variability. We analyzed the X-ray spectra of a Seyfert 1 galaxy, Mrk 704, which was observed twice, three years apart, by XMM-Newton. The spectra were well fitted with a two zones absorber, possibly covering only partially the source. The parameters of the absorbing matter - column density, ionization state, covering factor - changed significantly between the two observations. Possible explanations for the more ionized absorber are a torus wind (the source is a polar scattering one) or, in the partial covering scenario, an accretion disk wind. The less ionized absorber may be composed of orbiting clouds in the surroundings of the nucleus, similarly to what already found in other sources, most notably NGC 1365.

astro-ph.CO↗

arXiv2010

Radio Loud AGN in the 2XMMi catalogue

We are carrying out a search for all radio loud Active Galactic Nuclei observed with XMM-Newton, including targeted and field sources to perform a multi-wavelength study of these objects. We have cross-correlated the Veron-Cetty & Veron (2010) catalogue with the XMM-Newton Serendipitous Source Catalogue (2XMMi) and found about 4000 matched sources. A literature search provided radio, optical, and X-ray data for 403 sources. Here we summarize the first results of our study.

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arXiv2010

The broad-band X-ray spectrum of the Seyfert 1 galaxy, MCG+8-11-11

We present a long (100 ks) Suzaku observation of one of the X-ray brightest AGN, MCG+8-11-11. These data were complemented with the 54-month Swift BAT spectrum, allowing us to perform a broad-band fit in the 0.6-150 keV range. The fits performed in the 0.6-10 keV band give consistent results with respect to a previous XMM-Newton observation, i.e. the lack of a soft excess, warm absorption along the line of sight, a large Compton reflection component (R~1) and the absence of a relativistic component of the neutral iron K$α$ emission line. However, when the PIN and Swift BAT data are included, the reflection amount drops significantly (R~0.2-0.3), and a relativistic iron line is required, the latter confirmed by a phenomenological analysis in a restricted energy band (3-10 keV). When a self-consistent model is applied to the whole broadband data, the observed reflection component appears to be all associated to the relativistic component of the iron K$α$ line. The resulting scenario, though strongly model-dependent, requires that all the reprocessing spectral components from Compton-thick material must be associated to the accretion disc, and no evidence for the classical pc-scale torus is found. The narrow core of the neutral iron K$α$ line is therefore produced in a Compton-thin material, like the BLR, similarly to what found in another Seyfert galaxy, NGC7213, but with the notable difference that MCG+8-11-11 presents spectral signatures from an accretion disc. The very low accretion rate of NGC7213 could explain the lack of relativistic signatures in its spectrum, but the absence of the torus in both sources is more difficult to explain, since their luminosities are comparable, and their accretion rates are completely different.

astro-ph.GA↗

arXiv2010

High resolution X-ray spectroscopy and imaging of Mrk573

We present a detailed analysis of the XMM-Newton RGS high resolution X-ray spectra of the Seyfert 2 galaxy, Mrk573. This analysis is complemented by the study of the Chandra image, and its comparison to optical (HST) and radio (VLA) data. The soft X-ray emission is mainly due to gas photoionised by the central AGN, as indicated by the detection of radiative recombination continua from OVII and OVIII, as well as by the prominence of the OVII forbidden line. This result is confirmed by the best fit obtained with a self-consistent CLOUDY photoionisation model. However, a collisionally excited component is also required, in order to reproduce the FeXVII lines, accounting for about 1/3 of the total luminosity in the 15-26 A band. Once adopted the same model in the Chandra ACIS data, another photoionised component, with higher ionisation parameter, is needed to take into account emission from higher Z metals. The broadband ACIS spectrum also confirms the Compton-thick nature of the source. The imaging analysis shows the close morphological correspondence between the soft X-ray and the [OIII] emission. The radio emission appears much more compact, although clearly aligned with the narrow line region. The collisional phase of the soft X-ray emission may be due to starburst, requiring a star formation rate of $\simeq5-9$ M$_\odot$ yr$^{-1}$, but there is no clear evidence of this kind of activity from other wavelengths. On the other hand, it may be related to the radio ejecta, responsible for the heating of the plasma interacting with the outflow, but the estimated pressure of the hot gas is much larger than the pressure of the radio jets, assuming equipartition and under reasonable physical parameters.

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arXiv2009

Spatially Resolved Chandra HETG Spectroscopy of the NLR Ionization Cone in NGC 1068

We present initial results from a new 440-ks Chandra HETG GTO observation of the canonical Seyfert 2 galaxy NGC 1068. The proximity of NGC 1068, together with Chandra's superb spatial and spectral resolution, allow an unprecedented view of its nucleus and circumnuclear NLR. We perform the first spatially resolved high-resolution X-ray spectroscopy of the `ionization cone' in any AGN, and use the sensitive line diagnostics offered by the HETG to measure the ionization state, density, and temperature at discrete points along the ionized NLR. We argue that the NLR takes the form of outflowing photoionized gas, rather than gas that has been collisionally ionized by the small-scale radio jet in NGC 1068. We investigate evidence for any velocity gradients in the outflow, and describe our next steps in modeling the spatially resolved spectra as a function of distance from the nucleus.

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arXiv2009

Searching for AGN Outflows: Spatially Resolved Chandra HETG Spectroscopy of the NLR Ionization Cone in NGC 1068

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arXiv2009

The soft X-ray polarization in obscured AGN

The soft X-ray emission in obscured active galactic nuclei (AGN) is dominated by emission lines, produced in a gas photoionized by the nuclear continuum and likely spatially coincident with the optical narrow line region (NLR). However, a fraction of the observed soft X-ray flux appears like a featureless power law continuum. If the continuum underlying the soft X-ray emission lines is due to Thomson scattering of the nuclear radiation, it should be very highly polarized. We calculated the expected amount of polarization assuming a simple conical geometry for the NLR, combining these results with the observed fraction of the reflected continuum in bright obscured AGN.

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arXiv2009

CAIXA: a Catalogue of AGN In the XMM-Newton Archive II. Multiwavelength correlations

We presented CAIXA, a Catalogue of AGN in the XMM-Newton Archive, in a companion paper. Here, a systematic search for correlations between the X-ray spectral properties and the multiwavelength data was performed for the sources in CAIXA. All the significant (>99.9% confidence level) correlations are discussed along with their physical implications on current models of AGN. Two main correlations are discussed in this paper: a) a very strong anti-correlation between the FWHM of the H$β$ optical line and the ratio between the soft and the hard X-ray luminosity. Although similar anti-correlations between optical line width and X-ray spectral steepness have already been discussed in the literature (see e.g., Laor et al. 1994, Boller et al. 1996, Brandt et al. 1997), we consider the formulation we present in this paper is more fundamental, as it links model-independent quantities. Coupled with a strong anti-correlation between the V to hard X-ray flux ratio and the H$β$ FHWM, it supports scenarios for the origin of the soft excess in AGN, which require strong suppression of the hard X-ray emission; b) a strong (and expected) correlation between the X-ray luminosity and the black hole mass. Its slope, flatter than 1, is consistent with Eddington ratio-dependent bolometric corrections, such as that recently proposed by Vasudevan & Fabian (2009). Moreover, we critically review through various statistical tests the role that distance biases play in the strong radio to X-ray luminosity correlation found in CAIXA and elsewhere; we conclude that only complete, unbiased samples (such as that recently published by Behar & Laor, 2008) should be used to draw observational constraints on the origin of radio emission in radio-quiet AGN.

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