44 research outputs found
Relativistic reflection in the average X-ray spectrum of AGN in the V\'eron-Cetty & V\'eron catalogue
The X-ray spectra of active galactic nuclei (AGN) unveil properties of matter
around the super massive black hole (SMBH). We investigate the X-ray spectra of
AGN focusing on Compton reflection and fluorescence, important processes of
interaction between primary radiation and circum-nuclear material. Unresolved
emission lines (most notably the Fe line) in the X-ray spectra of AGN indicate
that this material is located far away from the SMBH. Contributions from the
inner accretion disk, affected by relativistic effects, have also been detected
in several cases.
We studied the average X-ray spectrum of a sample of 263 X-ray unabsorbed AGN
that yield 419023 counts in the 2-12 keV rest-frame band distributed among 388
XMM-Newton spectra. We fitted the average spectrum using a (basically)
unabsorbed power law (primary radiation). From second model that represents the
interaction of the primary radiation with matter located far away from the
SMBH, we found that it was very significantly detected. Finally, we added a
contribution from interaction with neutral material in the accretion disk close
to the central SMBH, which is therefore smeared by relativistic effects, which
improved the fit at 6 sigma. The reflection factors are 0.65 for the accretion
disk and 0.25 for the torus. Replacing the neutral disk-reflection with
low-ionisation disk reflection, also relativistically smeared, fits the data
equally well, suggesting that we do not find evidence for a significant
ionisation of the accretion disk.
We detect distant neutral reflection in the average spectrum of unabsorbed
AGN with z=0.8. Adding the disk-reflection component associated with a
relativistic Fe line improves the data description at 6 sigma confidence level,
suggesting that both reflection components are present. The disk-reflection
component accounts for about 70 % of the total reflected flux.Comment: Accepted by A&A. 10 pages, 7 figure
Accretion and jets in a low luminosity AGN: the nucleus of NGC 1052
We aim to determine the properties of the central region of NGC 1052 using
X-ray and radio data. NGC 1052 (z=0.005) has been investigated for decades in
different energy bands and shows radio lobes and a low luminosity active
galactic nucleus (LLAGN). We use X-ray images from Chandra and radio images
from Very Large Array (VLA) to explore the morphology of the central area. We
also study the spectra of the nucleus and the surrounding region using
observations from Chandra and XMM-Newton. We find diffuse soft X-ray radiation
and hotspots along the radio lobes. The spectrum of the circum-nuclear region
is well described by a thermal plasma (T~0.6 keV) and a power law with photon
index Gamma~2.3. The nucleus shows a hard power law (Gamma~1.4) modified by
complex absorption. A narrow iron K-alpha line is also clearly detected in all
observations, but there is no evidence for relativistic reflection. The
extended emission is consistent with originating from extended jets and from
jet-triggered shocks in the surrounding medium. The hard power-law emission
from the nucleus and the lack of relativistic reflection supports the scenario
of inefficient accretion in an Advection Dominated Accretion Flow (ADAF).Comment: Accepted in A&A. 17 pages, 5 figure
A changing inner radius in the accretion disc of Q0056-363?
Q0056-363 is the most powerful X-ray quasar known to exhibit a broad, likely
relativistic iron line (Porquet & Reeves 2003). It has been observed twice by
XMM-, three and half years apart (July 2000 and December 2003). In the
second observation, the UV and soft X-ray fluxes were fainter, the hard X-ray
power law flatter, and the iron line equivalent width (EW) smaller than in the
2000 observation. These variations can all be explained, at least
qualitatively, if the disc is truncated in the second observation. We report
also on the possible detection of a transient, redshifted iron absorption line
during the 2003 observation.Comment: Accepted for publication in A&
The XMM deep survey in the CDF-S. X. X-ray variability of bright sources
We aim to study the variability properties of bright hard X-ray selected
Active Galactic Nuclei (AGN) with redshift between 0.3 and 1.6 detected in the
Chandra Deep Field South (XMM-CDFS) by a long XMM observation. Taking advantage
of the good count statistics in the XMM CDFS we search for flux and spectral
variability using the hardness ratio techniques. We also investigated spectral
variability of different spectral components. The spectra were merged in six
epochs (defined as adjacent observations) and in high and low flux states to
understand whether the flux transitions are accompanied by spectral changes.
The flux variability is significant in all the sources investigated. The
hardness ratios in general are not as variable as the fluxes. Only one source
displays a variable HR, anti-correlated with the flux (source 337). The
spectral analysis in the available epochs confirms the steeper when brighter
trend consistent with Comptonisation models only in this source. Finding this
trend in one out of seven unabsorbed sources is consistent, within the
statistical limits, with the 15 % of unabsorbed AGN in previous deep surveys.
No significant variability in the column densities, nor in the Compton
reflection component, has been detected across the epochs considered. The high
and low states display in general different normalisations but consistent
spectral properties. X-ray flux fluctuations are ubiquitous in AGN. In general,
the significant flux variations are not associated with a spectral variability:
photon index and column densities are not significantly variable in nine out of
the ten AGN over long timescales (from 3 to 6.5 years). The photon index
variability is found only in one source (which is steeper when brighter) out of
seven unabsorbed AGN. These results are consistent with previous deep samples.Comment: 14 pages, 11 figures. Accepted in A&
FBQS J1644+2619: multiwavelength properties and its place in the class of gamma-ray emitting Narrow Line Seyfert 1s
A small fraction of Narrow Line Seyfert 1s (NLSy1s) are observed to be
gamma-ray emitters. Understanding the properties of these sources is of
interest since the majority of NLSy1s are very different from typical blazars.
Here, we present a multi-frequency analysis of FBQS J1644+2619, one of the most
recently discovered gamma-ray emitting NLSy1s. We analyse an ~80 ks XMM-Newton
observation obtained in 2017, as well as quasi-simultaneous multi-wavelength
observations covering the radio - gamma-ray range. The spectral energy
distribution of the source is similar to the other gamma-ray NLSy1s, confirming
its blazar-like nature. The X-ray spectrum is characterised by a hard photon
index (Gamma = 1.66) above 2 keV and a soft excess at lower energies.The hard
photon index provides clear evidence that inverse Compton emission from the jet
dominates the spectrum, while the soft excess can be explained by a
contribution from the underlying Seyfert emission. This contribution can be
fitted by reflection of emission from the base of the jet, as well as by
Comptonisation in a warm, optically thick corona. We discuss our results in the
context of the other gamma-ray NLSy1s and note that the majority of them have
similar X-ray spectra, with properties intermediate between blazars and
radio-quiet NLSy1s.Comment: Accepted for publication in MNRA
The VOICE Survey : VST Optical Imaging of the CDFS and ES1 Fields
Indexación: Scopus.We present the VST Optical Imaging of the CDFS and ES1 Fields (VOICE) Survey, a VST INAF Guaranteed Time program designed to provide optical coverage of two 4 deg2 cosmic windows in the Southern hemisphere. VOICE provides the first, multi-band deep optical imaging of these sky regions, thus complementing and enhancing the rich legacy of longer-wavelength surveys with VISTA, Spitzer, Herschel and ATCA available in these areas and paving the way for upcoming observations with facilities such as the LSST, MeerKAT and the SKA. VOICE exploits VST's OmegaCAM optical imaging capabilities and completes the reduction of WFI data available within the ES1 fields as part of the ESO-Spitzer Imaging Extragalactic Survey (ESIS) program providing ugri and uBVR coverage of 4 and 4 deg2 areas within the CDFS and ES1 field respectively. We present the survey's science rationale and observing strategy, the data reduction and multi-wavelength data fusion pipeline. Survey data products and their future updates will be released at http://www.mattiavaccari.net/voice/ and on CDS/VizieR.https://pos.sissa.it/275/026/pd
The XMM deep survey in the CDF-S VIII. X-ray properties of the two brightest sources
We present results from the deep XMM-Newton observations of the two brightest
X-ray sources in the Chandra Deep Field South (CDFS), PID 203 (z=0.544) and PID
319 (z=0.742). The long exposure of 2.5 Ms over a 10 year period (net 4 yr with
a 6 yr gap) makes it possible to obtain high quality X-ray spectra of these two
Type I AGN with X-ray luminosity of 10^44 erg/s, typical luminosity for
low-redshift PG quasars, track their X-ray variability both in flux and
spectral shape. Both sources showed X-ray flux variability of ~10-20 per cent
in rms which is similar in the soft (0.5-2 keV) and hard (2-7 keV) bands. PID
203, which has evidence for optical extinction, shows modest amount of
absorption (nH~1e21cm^-2) in the X-ray spectrum. Fe K emission is strongly
detected in both objects with EW~0.2 keV. The lines in both objects are
moderately broad and exhibit marginal evidence for variability in shape and
flux, indicating that the bulk of the line emission come from their accretion
disks rather than distant tori.Comment: 12 pages, 19 figures, accepted for publication in A&
SUDARE-VOICE variability-selection of Active Galaxies in the Chandra Deep Field South and the SERVS/SWIRE region
One of the most peculiar characteristics of Active Galactic Nuclei (AGN) is
their variability over all wavelengths. This property has been used in the past
to select AGN samples and is foreseen to be one of the detection techniques
applied in future multi-epoch surveys, complementing photometric and
spectroscopic methods.
In this paper, we aim to construct and characterise an AGN sample using a
multi-epoch dataset in the r band from the SUDARE-VOICE survey.
Our work makes use of the VST monitoring program of an area surrounding the
Chandra Deep Field South to select variable sources. We use data spanning a six
month period over an area of 2 square degrees, to identify AGN based on their
photometric variability.
The selected sample includes 175 AGN candidates with magnitude r < 23 mag. We
distinguish different classes of variable sources through their lightcurves, as
well as X-ray, spectroscopic, SED, optical and IR information overlapping with
our survey.
We find that 12% of the sample (21/175) is represented by SN. Of the
remaining sources, 4% (6/154) are stars, while 66% (102/154) are likely AGNs
based on the available diagnostics. We estimate an upper limit to the
contamination of the variability selected AGN sample of about 34%, but we point
out that restricting the analysis to the sources with available
multi-wavelength ancillary information, the purity of our sample is close to
80% (102 AGN out of 128 non-SN sources with multi-wavelength diagnostics). Our
work thus confirms the efficiency of the variability selection method in
agreement with our previous work on the COSMOS field; in addition we show that
the variability approach is roughly consistent with the infrared selection.Comment: Published in A & A, 15 pages, 6 figure
Optically variable active galactic nuclei in the 3 yr VST survey of the COSMOS field
The analysis of the variability of active galactic nuclei (AGNs) at different
wavelengths and the study of possible correlations among different spectral
windows are nowadays a major field of inquiry. Optical variability has been
largely used to identify AGNs in multivisit surveys. The strength of a
selection based on optical variability lies in the chance to analyze data from
surveys of large sky areas by ground-based telescopes. However the
effectiveness of optical variability selection, with respect to other
multiwavelength techniques, has been poorly studied down to the depth expected
from next generation surveys. Here we present the results of our r-band
analysis of a sample of 299 optically variable AGN candidates in the VST survey
of the COSMOS field, counting 54 visits spread over three observing seasons
spanning > 3 yr. This dataset is > 3 times larger in size than the one
presented in our previous analysis (De Cicco et al. 2015), and the observing
baseline is ~8 times longer. We push towards deeper magnitudes (r(AB) ~23.5
mag) compared to past studies; we make wide use of ancillary multiwavelength
catalogs in order to confirm the nature of our AGN candidates, and constrain
the accuracy of the method based on spectroscopic and photometric diagnostics.
We also perform tests aimed at assessing the relevance of dense sampling in
view of future wide-field surveys. We demonstrate that the method allows the
selection of high-purity (> 86%) samples. We take advantage of the longer
observing baseline to achieve great improvement in the completeness of our
sample with respect to X-ray and spectroscopically confirmed samples of AGNs
(59%, vs. ~15% in our previous work), as well as in the completeness of
unobscured and obscured AGNs. The effectiveness of the method confirms the
importance to develop future, more refined techniques for the automated
analysis of larger datasets.Comment: 21 pages, 10 figures; accepted for publication in A&
Supernova rates from the SUDARE VST-Omegacam search II. Rates in a galaxy sample
This is the second paper of a series in which we present measurements of the
Supernova (SN) rates from the SUDARE survey. In this paper, we study the trend
of the SN rates with the intrinsic colours, the star formation activity and the
mass of the parent galaxies. We have considered a sample of about 130000
galaxies and a SN sample of about 50 events. We found that the SN Ia rate per
unit mass is higher by a factor of six in the star-forming galaxies with
respect to the passive galaxies. The SN Ia rate per unit mass is also higher in
the less massive galaxies that are also younger. These results suggest a
distribution of the delay times (DTD) less populated at long delay times than
at short delays. The CC SN rate per unit mass is proportional to both the sSFR
and the galaxy mass. The trends of the Type Ia and CC SN rates as a function of
the sSFR and the galaxy mass that we observed from SUDARE data are in agreement
with literature results at different redshifts. The expected number of SNe Ia
is in agreement with the observed one for all four DTD models considered both
in passive and star-forming galaxies so we can not discriminate between
different progenitor scenarios. The expected number of CC SNe is higher than
the observed one, suggesting a higher limit for the minimum progenitor mass. We
also compare the expected and observed trends of the SN Ia rate with the
intrinsic U - J colour of the parent galaxy, assumed as a tracer of the age
distribution. While the slope of the relation between the SN Ia rate and the U
- J color in star-forming galaxies can be reproduced well by all four DTD
models considered, only the steepest of them is able to account for the rates
and colour in star-forming and passive galaxies with the same value of the SN
Ia production efficiency.Comment: A& A accepte