Aims.
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.
Methods.
We used X-ray images from
Chandra
and radio images from Very Large Array to explore the morphology of the central area. We also studied the spectra of the nucleus and the surrounding region using observations from
Chandra
and
XMM-Newton
.
Results.
We find diffuse soft X-ray radiation and hot-spots 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 Γ ∼ 2.3. The nucleus shows a hard power law (Γ ∼ 1.4) modified by complex absorption. A narrow iron K
α
line is also clearly detected in all observations, but there is no evidence for relativistic reflection.
Conclusions.
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.
ABSTRACT
We present a machine learning model to classify active galactic nuclei (AGNs) and galaxies (AGN-galaxy classifier) and a model to identify type 1 (optically unabsorbed) and type 2 (optically ...absorbed) AGN (type 1/2 classifier). We test tree-based algorithms, using training samples built from the X-ray Multi-Mirror Mission–Newton (XMM–Newton) catalogue and the Sloan Digital Sky Survey (SDSS), with labels derived from the SDSS survey. The performance was tested making use of simulations and of cross-validation techniques. With a set of features including spectroscopic redshifts and X-ray parameters connected to source properties (e.g. fluxes and extension), as well as features related to X-ray instrumental conditions, the precision and recall for AGN identification are 94 and 93 per cent, while the type 1/2 classifier has a precision of 74 per cent and a recall of 80 per cent for type 2 AGNs. The performance obtained with photometric redshifts is very similar to that achieved with spectroscopic redshifts in both test cases, while there is a decrease in performance when excluding redshifts. Our machine learning model trained on X-ray features can accurately identify AGN in extragalactic surveys. The type 1/2 classifier has a valuable performance for type 2 AGNs, but its ability to generalize without redshifts is hampered by the limited census of absorbed AGN at high redshift.
Abstract
A small fraction of Narrow Line Seyfert 1s (NLSy1s) are observed to be γ-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 multifrequency analysis of FBQS J1644+2619, one of the most recently discovered γ-ray emitting NLSy1s. We analyse an ∼80 ks XMM–Newton observation obtained in 2017, as well as quasi-simultaneous multiwavelength observations covering the radio–γ-ray range. The spectral energy distribution of the source is similar to the other γ-ray NLSy1s, confirming its blazar-like nature. The X-ray spectrum is characterized by a hard photon index (Γ = 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 Comptonization in a warm, optically thick corona. We discuss our results in the context of the other γ-ray NLSy1s and note that the majority of them have similar X-ray spectra, with properties intermediate between blazars and radio-quiet NLSy1s.
Context. The analysis of the variability of active galactic nuclei (AGN) at different wavelengths and the study of possible correlations of different spectral windows are a current main field of ...inquiry. Optical variability has been largely used to identify AGN in multivisit surveys. The strength of a selection based on optical variability lies in the opportunity of analyzing 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 that is expected from next-generation surveys. Aims. 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 more than three years. This dataset is more than three times larger than the dataset presented in our previous analysis, and the observing baseline is about eight times longer. Methods. We push toward deeper magnitudes (r(AB) ∼ 23.5 mag) than were reached in past studies. We made wide use of ancillary multiwavelength catalogs in order to confirm the nature of our AGN candidates, and constrained the accuracy of the method based on spectroscopic and photometric diagnostics. We also performed tests aimed at assessing the relevance of dense sampling in view of future wide-field surveys. Results. 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 AGN (59% vs. ∼15% in our previous work), as well as in the completeness of unobscured and obscured AGN. The effectiveness of the method confirms the importance of developing future more refined techniques for the automated analysis of larger datasets.
Context. Active galaxies are characterized by variability at every wavelength, with timescales from hours to years depending on the observing window. Optical variability has proven to be an effective ...way of detecting AGNs in imaging surveys, lasting from weeks to years. Aims. In the present work we test the use of optical variability as a tool to identify active galactic nuclei in the VST multiepoch survey of the COSMOS field, originally tailored to detect supernova events. Methods. We make use of the multiwavelength data provided by other COSMOS surveys to discuss the reliability of the method and the nature of our AGN candidates. Results. The selection on the basis of optical variability returns a sample of 83 AGN candidates; based on a number of diagnostics, we conclude that 67 of them are confirmed AGNs (81% purity), 12 are classified as supernovae, while the nature of the remaining 4 is unknown. For the subsample of AGNs with some spectroscopic classification, we find that Type 1 are prevalent (89%) compared to Type 2 AGNs (11%). Overall, our approach is able to retrieve on average 15% of all AGNs in the field identified by means of spectroscopic or X-ray classification, with a strong dependence on the source apparent magnitude (completeness ranging from 26% to 5%). In particular, the completeness for Type 1 AGNs is 25%, while it drops to 6% for Type 2 AGNs. The rest of the X-ray selected AGN population presents on average a larger rms variability than the bulk of non-variable sources, indicating that variability detection for at least some of these objects is prevented only by the photometric accuracy of the data. The low completeness is in part due to the short observing span: we show that increasing the temporal baseline results in larger samples as expected for sources with a red-noise power spectrum. Our results allow us to assess the usefulness of this AGN selection technique in view of future wide-field surveys.
Context.
Variability has proven to be a powerful tool to detect active galactic nuclei (AGN) in multi-epoch surveys. The new-generation facilities expected to become operational in the next few years ...will mark a new era in time-domain astronomy and their wide-field multi-epoch campaigns will favor extensive variability studies.
Aims.
We present our analysis of AGN variability in the second half of the VST survey of the Wide
Chandra
Deep Field South, performed in the
r
band and covering a 2 sq. deg area. The analysis complements a previous work, in which the first half of the area was investigated. We provide a reliable catalog of variable AGN candidates, which will be critical targets in future variability studies.
Methods.
We selected a sample of optically variable sources and made use of infrared data from the
Spitzer
mission to validate their nature by means of color-based diagnostics.
Results.
We obtain a sample of 782 AGN candidates among which 12 are classified as supernovae, 54 as stars, and 232 as AGN. We estimate a contamination ≲20% and a completeness ∼38% with respect to mid-infrared selected samples.
Context. The X-ray spectra of active galactic nuclei (AGN) unveil properties of matter around the super-massive black hole (SMBH). Aims. We investigate the X-ray spectra of AGN focusing on Compton ...reflection and fluorescence. These are two of the most important processes of interaction between primary radiation and circumnuclear material that is located far away from the SMBH, as indicated by the unresolved spectral emission lines (most notably the Fe line) in the X-ray spectra of AGN. Contributions from the inner accretion disk, affected by relativistic effects as expected, have also been detected in several cases. Methods. We studied the average X-ray spectrum of a sample of 263 X-ray unabsorbed AGN that yield 419 023 counts in the 2−12 keV rest-frame band distributed among 388 XMM-Newton spectra. Results. We fitted the average spectrum using a (basically) unabsorbed power law (representing the primary radiation). From a second model that represents the interaction (through Compton reflection and fluorescence) of this primary radiation with matter located far away from the central engine (e.g. the putative torus), 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 a 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 significant ionisation of the accretion disk. Conclusions. We detect distant neutral reflection associated with a narrow Fe line 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 a 6 sigma confidence level, suggesting that both reflection components are present. The disk-reflection component accounts for about 70% of the total reflected flux.
The XMM deep survey in the CDF-S Falocco, S.; Paolillo, M.; Comastri, A. ...
Astronomy and astrophysics (Berlin),
12/2017, Letnik:
608
Journal Article
Recenzirano
Odprti dostop
Aims. We aim to study the variability properties of bright hard X-ray selected active galactic nuclei (AGN) in the redshift range between 0.3 and 1.6 detected in the Chandra Deep Field South ...(XMM-CDFS) by a long (~3 Ms) XMM observation. Methods. Taking advantage of the good count statistics in the XMM CDFS, we search for flux and spectral variability using the hardness ratio (HR) techniques. We also investigate the spectral variability of different spectral components (photon index of the power law, column density of the local absorber, and reflection intensity). 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. Results. The flux variability is significant in all the sources investigated. The HRs in general are not as variable as the fluxes, in line with previous results on deep fields. 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 at 99% confidence level. 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. Conclusions. X-ray flux fluctuations are ubiquitous in AGN, though in some cases the data quality does not allow for their detection. In general, the significant flux variations are not associated with spectral variability: photon index and column densities are not significantly variable in nine out of the ten AGN over long timescales (from three to six and a half years). Photon index variability is found only in one source (which is steeper when brighter) out of seven unabsorbed AGN. The percentage of spectrally variable objects is consistent, within the limited statistics of sources studied here, with previous deep samples.
The XMM Deep survey in the CDF-S Falocco, S; Carrera, F J; Corral, A ...
Astronomy and astrophysics (Berlin),
07/2013, Letnik:
555
Journal Article
Recenzirano
Odprti dostop
X-ray spectroscopy of active galactic nuclei (AGN) offers the opportunity to directly probe the inner regions of the accretion disk. Reflection of the primary continuum on the circumnuclear accreting ...matter produces features in the X-ray spectrum that help to explore the physics and the geometry of the innermost region, close to the central black hole. We present the results of our analysis of average AGN XMM-Newton X-ray spectra in the Chandra Deep Field South observation (hereafter, XMM CDFS), in order to explore the Fe line features in distant AGN up to z~3.5. Thanks to the high quality of the XMM CDFS spectra and to the detailed modelling of the continuum and instrumental effects, we have shown that the most distant AGN exhibit a highly significant iron emission feature. The EW of both features are similar to those observed in individual nearby AGN, hence they must be a widespread characteristic of AGN, since otherwise the average values would be smaller than observed.
PDF
