arXiv subjects

Gabriele Ponti

Publications and source records attributed to Gabriele Ponti.

102 records · Page 6Linked to original sources

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

Variable X-ray absorption in the mini-BAL QSO PG 1126-041

X-ray studies of active galactic nuclei (AGN) with powerful nuclear winds are important for constraining the physics of the inner accretion/ejection flow around supermassive black holes (SMBHs) and for understanding the impact of such winds on the AGN environment. Our main scientific goal is to constrain the properties of the circum-nuclear matter close to the SMBH in the mini-broad absorption line quasar (mini-BAL QSO) PG 1126-041 using a multi-epoch observational campaign with XMM-Newton. We performed temporally resolved X-ray spectroscopy and simultaneous UV and X-ray photometry on the most complete set of observations and on the deepest X-ray exposure of a mini-BAL QSO ever. We found complex X-ray spectral variability on time scales of both months and hours, which is best reproduced by means of variable massive ionized absorbers along the line of sight. As a consequence, the observed optical-to-X-ray spectral index is found to be variable with time. In the highest signal-to-noise observation we detected highly ionized X-ray absorbing material outflowing much faster (v ~ 16500 km/s) than the UV absorbing one (v ~ 5000 km/s). This highly ionized absorber is found to be variable on very short (a few kiloseconds) time scales. Our findings are qualitatively consistent with line-driven accretion disk winds scenarios. Our observations have opened the time-resolved X-ray spectral analysis field for mini-BAL QSOs. Only with future deep studies will we be able to map the dynamics of the inner flow and understand the physics of AGN winds and their impact on the environment.

astro-ph.CO↗

arXiv2011

Extended hard X-ray emission from the Vela pulsar wind nebula

The nebula powered by the Vela pulsar is one of the best examples of an evolved pulsar wind nebula, allowing to access the particle injection history and the interaction with the supernova ejecta. We report on the INTEGRAL discovery of extended emission above 18 keV from the Vela nebula. The northern side has no known counterparts and it appears larger and more significant than the southern one, which is in turn partially coincident with the cocoon, the soft X-ray and TeV filament towards the center of the remnant. We also present the spectrum of the Vela nebula in the 18-400 keV energy range as measured by IBIS/ISGRI and SPI on board the INTEGRAL satellite. The apparent discrepancy between IBIS/ISGRI, SPI, and previous measurements is understood in terms of point spread function, supporting the hypothesis of a nebula more diffuse than previously thought. A break at ~25 keV is found in the spectrum within 6' from the pulsar after including the Suzaku XIS data. Interpreted as a cooling break, this points out that the inner nebula is composed by electrons injected in the last ~2000 years. Broad-band modeling also implies a magnetic field higher than 10 microGauss in this region. Finally, we discuss the nature of the northern emission, which might be due to fresh particles injected after the passage of the reverse shock.

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↗

arXiv2011

Multiwavelength campaign on Mrk 509. IV. Optical-UV-X-ray variability and the nature of the soft X-ray excess

We present the analysis of XMM-Newton and Swift optical-UV and X-ray observations of the Seyfert-1/QSO Mrk 509, part of an unprecedented multi-wavelength campaign, investigating the nuclear environment of this AGN. The XMM-Newton data are from a series of 10 observations of about 60 ks each, spaced from each other by about 4 days, taken in Oct-Nov 2009. During our campaign, Mrk 509 was also observed with Swift for a period of about 100 days, monitoring the behaviour of the source before and after the XMM-Newton observations. With these data we have established the continuum spectrum in the optical-UV and X-ray bands and investigated its variability on the timescale of our campaign with a resolution time of a few days. In order to measure and model the continuum as far as possible into the UV, we also made use of HST/COS observations of Mrk 509 (part of our coordinated campaign) and of an archival FUSE observation. We have found that in addition to an X-ray power-law, the spectrum displays soft X-ray excess emission below 2 keV, which interestingly varies in association with the thermal optical-UV emission from the accretion disc. The change in the X-ray power-law component flux (albeit smaller than that of the soft excess), on the other hand, is uncorrelated to the flux variability of the soft X-ray excess and the disc component on the probed timescale. The results of our simultaneous broad-band spectral and timing analysis suggest that, on a resolution time of a few days, the soft X-ray excess of Mrk 509 is produced by the Comptonisation of the thermal optical-UV photons from the accretion disc by a warm (0.2 keV) optically thick (tau ~ 17) corona surrounding the inner regions of the disc. This makes Mrk 509, with a black hole mass of about 1-3 x 10^8 solar masses, the highest mass known system to display such behaviour and origin for the soft X-ray excess.

astro-ph.CO↗

arXiv2010

Molecular clouds: X-ray mirrors of the Galactic nuclear activity

We present the result of a study of the X-ray emission from the Galactic Centre Molecular Clouds (MC), within 15 arcmin from Sgr A*. We use XMM-Newton data spanning about 8 years. We observe an apparent super-luminal motion of a light front illuminating a MC. This might be due to a source outside the MC (such as Sgr A* or a bright and long outburst of a X-ray binary), while it can not be due to low energy cosmic rays or a source located inside the cloud. We also observe a decrease of the X-ray emission from G0.11-0.11, behaviour similar to the one of Sgr B2. The line intensities, clouds dimensions, columns densities and positions with respect to Sgr A*, are consistent with being produced by the same Sgr A* flare. The required high luminosity (about 1.5 10^39 erg s-1) can hardly be produced by a binary system, while it is in agreement with a flare of Sgr A* fading about 100 years ago.

astro-ph.HE↗

arXiv2010

Discovery of a superluminal Fe K echo at the Galactic Center: The glorious past of Sgr A* preserved by molecular clouds

We present the result of a study of the X-ray emission from the Galactic Centre (GC) Molecular Clouds (MC) within 15 arcmin from Sgr A*. We use XMM-Newton data (about 1.2 Ms of observation time) spanning about 8 years. The MC spectra show all the features characteristic of reflection: i) intense Fe Kalpha, with EW of about 0.7-1 keV, and the associated Kbeta line; ii) flat power law continuum and iii) a significant Fe K edge (tau~0.1-0.3). The diffuse low ionisation Fe K emission follows the MC distribution, nevertheless not all MC are Fe K emitters. The long baseline monitoring allows the characterisation of the temporal evolution of the MC emission. A complex pattern of variations is shown by the different MC, with some having constant Fe K emission, some increasing and some decreasing. In particular, we observe an apparent super-luminal motion of a light front illuminating a Molecular nebula. This might be due to a source outside the MC (such as Sgr A* or a bright and long outburst of a X-ray binary), while it cannot be due to low energy cosmic rays or a source located inside the cloud. We also observe a decrease of the X-ray emission from G0.11-0.11, behaviour similar to the one of Sgr B2. The line intensities, clouds dimensions, columns densities and positions with respect to Sgr A*, are consistent with being produced by the same Sgr A* flare. The required high luminosity (about 1.5~10^39 erg/s) can hardly be produced by a binary system, while it is in agreement with a flare of Sgr A* fading about 100 years ago. The low intensity of the Fe K emission coming from the 50 and the 20 km/s MC places an upper limit of 10^36 erg/s to the mean luminosity of Sgr A* in the last 60-90 years. The Fe K emission and variations from these MC might have been produced by a single flare of Sgr A*.

astro-ph.HE↗

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.

astro-ph.GA↗

arXiv2009

CAIXA: a Catalogue of AGN In the XMM-Newton Archive - Correlations

We presented CAIXA, a Catalogue of AGN in the XMM-Newton Archive, in Bianchi et al. (2009). It consists of all the radio-quiet X-ray unobscured (N$_\mathrm{H}<2\times10^{22}$ cm$^{-2}$) active galactic nuclei (AGN) observed by XMM-Newton in targeted observations, whose data are public as of March 2007. With its 156 sources, this is the largest catalogue of high signal-to-noise X-ray spectra of AGN. All the EPIC pn spectra of the sources in CAIXA were extracted homogeneously and a baseline model was applied in order to derive their basic X-ray properties. These data are complemented by multiwavelength data found in the literature: Black Hole masses, Full Width Half Maximum (FWHM) of Hbeta, radio and optical fluxes. A systematic search for correlations between the X-ray spectral properties and the multiwavelength data was performed for the sources in CAIXA. We discuss here some of the significant (>99.9 % confidence level) correlations.

astro-ph.GA↗

arXiv2008

CAIXA: a Catalogue of AGN In the XMM-Newton Archive I. Spectral analysis

We present CAIXA, a Catalogue of AGN In the XMM-Newton Archive. It consists of all the radio-quiet X-ray unobscured ($\mathrm{N_H}<2\times10^{22}$ cm$^{-2}$) Active Galactic Nuclei (AGN) observed by XMM-Newton in targeted observations, whose data are public as of March 2007. With its 156 sources, this is the largest catalogue of high signal-to-noise X-ray spectra of AGN. All the EPIC pn spectra of the sources in CAIXA were extracted homogeneously and a baseline model was applied in order to derive their basic X-ray properties. These data are complemented by multiwavelength data found in the literature: Black Hole masses, Full Width Half Maximum (FWHM) of H$β$, radio and optical fluxes. Here we describe our homogeneous spectral analysis of the X-ray data in CAIXA and present all the results on the parameters adopted in our best-fit models.

astro-ph↗

arXiv2006

The XMM-Newton view of GRS1915+105 during a "plateau"

Two XMM-Newton observations of the black-hole binary GRS1915+105 were triggered in 2004 (April 17 and 21), during a long "plateau" state of the source. We analyzed the data collected with EPIC-pn in Timing and Burst modes, respectively. No thermal disc emission is required by the data; the spectrum is well fitted by four components: a primary component (either a simple power law or thermal Comptonization) absorbed by cold matter with abundances different than those of standard ISM; reprocessing from an ionized disc; emission and absorption lines; and a soft X-ray excess around 1 keV. The latter is not confirmed by RGS (which were used in the second observation only); if real, the excess could be due to reflection from the optically thin, photoionized plasma of a disc wind, in which case it may provide a way to disentangle intrinsic from interstellar absorption. Indeed, the former is best traced by the higher abundances of heavier elements, while an independent estimate of the latter may be given by the value we get for the disc wind component only, which roughly coincides with what is found for lower-Z species.

astro-ph↗

arXiv2006

The XMM-Newton View of GRS1915+105

(abridged) Two XMM-Newton observations of the black-hole binary GRS1915+105 were triggered in 2004, during a long "plateau" state of the source. (...) While the light curves show just small amplitude variations (a few percent) at timescales longer than a few seconds, a QPO is seen at about 0.6 Hz (...). The pn spectrum is well fitted without invoking thermal disk emission, on the base of four main components: a primary one (...), absorbed by cold matter with abundances different than those of standard ISM; reprocessing from an ionized disk; emission and absorption lines; and a soft X-ray excess around 1 keV. However, the latter is not confirmed by the RGS spectra, whose difference from the EPIC-pn ones actually lacks of a fully satisfactory explanation. If real, the soft X-ray excess may be due to reflection from an optically thin, photoionized disk wind; in this case it may yield a way to disentangle intrinsic from interstellar absorption.

astro-ph↗