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Start Over Subject active galactic nuclei
191. Circumnuclear structure of NICMOS galaxies
ID:
ivo://CDS.VizieR/J/ApJ/616/707
Title:
Circumnuclear structure of NICMOS galaxies
Short Name:
J/ApJ/616/707
Date:
21 Oct 2021
Publisher:
CDS
Description:
Why are the nuclei of some galaxies more active than others? If most galaxies harbor a central massive black hole, the main difference is probably in how well it is fueled by its surroundings. We investigate the hypothesis that such a difference can be seen in the detailed circumnuclear morphologies of galaxies using several quantitatively defined features, including bars, isophotal twists, boxy and disky isophotes, and strong nonaxisymmetric features in unsharp-masked images. These diagnostics are applied to 250 high-resolution images of galaxy centers obtained in the near-infrared with NICMOS on the Hubble Space Telescope. To guard against the influence of possible biases and selection effects, we have carefully matched samples of Seyfert 1, Seyfert 2, LINER, starburst, and normal galaxies in their basic properties, taking particular care to ensure that each was observed with a similar average scale (10-15pc/pixel).
192. CIV 1549{AA} emission line in AGNs
ID:
ivo://CDS.VizieR/J/ApJ/666/757
Title:
CIV 1549{AA} emission line in AGNs
Short Name:
J/ApJ/666/757
Date:
21 Oct 2021
Publisher:
CDS
Description:
We are exploring a spectroscopic unification for all types of broad-line emitting AGNs. The four-dimensional Eigenvector 1 (4DE1) parameter space organizes quasar diversity in a sequence primarily governed by Eddington ratio. This paper considers the role of CIV {lambda}1549 measures as 4DE1 diagnostics. We use HST archival spectra for 130 sources with S/N high enough to permit reliable CIV {lambda}1549 broad-component measures.
193. CIV and FeK{alpha} Baldwin effects in AGNs
ID:
ivo://CDS.VizieR/J/ApJ/702/767
Title:
CIV and FeK{alpha} Baldwin effects in AGNs
Short Name:
J/ApJ/702/767
Date:
21 Oct 2021
Publisher:
CDS
Description:
We use UV/optical and X-ray observations of 272 radio-quiet Type 1 active galactic nuclei and quasars to investigate the CIV Baldwin Effect (BEff). The UV/optical spectra are drawn from the Hubble Space Telescope, International Ultraviolet Explorer and Sloan Digital Sky Survey archives. The X-ray spectra are from the Chandra and XMM-Newton archives. We apply correlation and partial-correlation analyses to the equivalent widths (EWs), continuum monochromatic luminosities, and {alpha}_ox_, which characterizes the relative X-ray to UV brightness. The EW of the CIV{lambda}1549 emission line is correlated with both {alpha}_ox_ and luminosity. In our sample, narrow Fe K{alpha} 6.4keV emission lines are detected in 50 objects.
194. CIV candidates
ID:
ivo://CDS.VizieR/J/ApJ/708/868
Title:
CIV candidates
Short Name:
J/ApJ/708/868
Date:
21 Oct 2021
Publisher:
CDS
Description:
We surveyed the Hubble Space Telescope UV spectra of 49 low-redshift quasars for z<1 CIV candidates, relying solely on the characteristic wavelength separation of the doublet. After consideration of the defining traits of CIV doublets (e.g., consistent line profiles, other associated transitions, etc.), we defined a sample of 38 definite (group G=1) and five likely (G=2) doublets with rest equivalent widths W_r_ for both lines detected at >=3{sigma}_W_r__. We conducted Monte Carlo completeness tests to measure the unblocked redshift ({Delta}z) and co-moving path length ({Delta}X) over which we were sensitive to CIV doublets of a range of equivalent widths and column densities. Using the power-law model of f(N(C^+3^)), we measured the C^+3^ mass density relative to the critical density: {Omega}_C^+3^_=(6.20^+1.82^_-1.52_)x10^-8^ for 13<=logN(C^+3^)<=15.
195. CIV variability in 105 SDSS quasars
ID:
ivo://CDS.VizieR/J/ApJ/641/78
Title:
CIV variability in 105 SDSS quasars
Short Name:
J/ApJ/641/78
Date:
21 Oct 2021
Publisher:
CDS
Description:
We examine the variability of the high-ionization CIV {lambda}1549 line in a sample of 105 quasars observed at multiple epochs by the Sloan Digital Sky Survey. We find a strong correlation between the change in the CIV line flux and the change in the line width, but no correlations between the change in flux and changes in line center and skewness. The relation between line flux change and line width change is consistent with a model in which a broad line base varies with greater amplitude than the line core. The objects studied here are more luminous and at higher redshift than those normally studied for variability, ranging in redshift from 1.65 to 4.00 and in absolute r-band magnitude from roughly -24 to -28.
196. Classification of Fermi blazar cand. from the 4FGL
ID:
ivo://CDS.VizieR/J/ApJ/887/134
Title:
Classification of Fermi blazar cand. from the 4FGL
Short Name:
J/ApJ/887/134
Date:
21 Oct 2021
Publisher:
CDS
Description:
The recently published fourth Fermi Large Area Telescope source catalog (4FGL) reports 5065 gamma-ray sources in terms of direct observational gamma-ray properties. Among the sources, the largest population is the active galactic nuclei (AGNs), which consists of 3137 blazars, 42 radio galaxies, and 28 other AGNs. The blazar sample comprises 694 flat-spectrum radio quasars (FSRQs), 1131 BL Lac-type objects (BL Lacs), and 1312 blazar candidates of an unknown type (BCUs). The classification of blazars is difficult using optical spectroscopy given the limited knowledge with respect to their intrinsic properties, and the limited availability of astronomical observations. To overcome these challenges, machine-learning algorithms are being investigated as alternative approaches. Using the 4FGL catalog, a sample of 3137 Fermi blazars with 23 parameters is systematically selected. Three established supervised machine-learning algorithms (random forests (RFs), support vector machines (SVMs), artificial neural networks (ANNs)) are employed to general predictive models to classify the BCUs. We analyze the results for all of the different combinations of parameters. Interestingly, a previously reported trend the use of more parameters leading to higher accuracy is not found. Considering the least number of parameters used, combinations of eight, 12 or 10 parameters in the SVM, ANN, or RF generated models achieve the highest accuracy (Accuracy ~91.8%, or ~92.9%). Using the combined classification results from the optimal combinations of parameters, 724 BL Lac type candidates and 332 FSRQ type candidates are predicted; however, 256 remain without a clear prediction.
197. Classification of IR galaxies
ID:
ivo://CDS.VizieR/J/ApJ/646/161
Title:
Classification of IR galaxies
Short Name:
J/ApJ/646/161
Date:
21 Oct 2021
Publisher:
CDS
Description:
Mid-infrared diagnostics are presented for a large portion of the Spitzer Infrared Nearby Galaxies Survey (SINGS) sample plus archival data from ISO and Spitzer. The SINGS data set includes low- and high-resolution spectral maps and broadband imaging in the infrared for over 160 nuclear and extranuclear regions within 75 nearby galaxies spanning a wide range of morphologies, metallicities, luminosities, and star formation rates. Our main result is that these mid-infrared diagnostics effectively constrain a targets dominant power source. The combination of a high-ionization line index and PAH strength serves as an efficient discriminant between AGNs and star-forming nuclei, confirming progress made with ISO spectroscopy on starbursting and ultraluminous infrared galaxies. The sensitivity of Spitzer allows us to probe fainter nuclear and star-forming regions within galaxy disks.
198. Classification of Swift and XMM-Newton sources
ID:
ivo://CDS.VizieR/J/A+A/657/A138
Title:
Classification of Swift and XMM-Newton sources
Short Name:
J/A+A/657/A138
Date:
22 Feb 2022
Publisher:
CDS
Description:
With the advent of very large X-ray surveys, an automated classification of X-ray sources becomes increasingly valuable. This work proposes a revisited naive Bayes classification of the X-ray sources in the Swift-XRT and XMM- Newton catalogs into four classes - AGN, stars, X-ray binaries (XRBs), and cataclysmic variables (CVs) - based on their spatial, spectral, and timing properties and their multiwavelength counterparts. An outlier measure is used to identify objects of other natures. The classifier is optimized to maximize the classification performance of a chosen class (here XRBs), and it is adapted to data mining purposes. We augmented the X-ray catalogs with multiwavelength data, source class, and variability properties. We then built a reference sample of about 25000 X-ray sources of known nature. From this sample, the distribution of each property was carefully estimated and taken as reference to assign probabilities of belonging to each class. The classification was then performed on the whole catalog, combining the information from each property. Using the algorithm on the Swift reference sample, we retrieved 99%, 98%, 92%, and 34% of AGN, stars, XRBs, and CVs, respectively, and the false positive rates are 3%, 1%, 9%, and 15%. Similar results are obtained on XMM sources. When applied to a carefully selected test sample, representing 55% of the X-ray catalog, the classification gives consistent results in terms of distributions of source properties. A substantial fraction of sources not belonging to any class is efficiently retrieved using the outlier measure, as well as AGN and stars with properties deviating from the bulk of their class. Our algorithm is then compared to a random forest method; the two showed similar performances, but the algorithm presented in this paper improved insight into the grounds of each classification. This robust classification method can be tailored to include additional or different source classes and can be applied to other X-ray catalogs. The transparency of the classification compared to other methods makes it a useful tool in the search for homogeneous populations or rare source types, including multi-messenger events. Such a tool will be increasingly valuable with the development of surveys of unprecedented size, such as LSST, SKA, and Athena, and the search for counterparts of multi-messenger events.
199. Classification of X-ray counterparts of 3FGL sources
ID:
ivo://CDS.VizieR/J/ApJ/887/18
Title:
Classification of X-ray counterparts of 3FGL sources
Short Name:
J/ApJ/887/18
Date:
21 Oct 2021
Publisher:
CDS
Description:
Approximately one-third of the gamma-ray sources in the third Fermi-LAT catalog are unidentified or unassociated with objects at other wavelengths. Observations with the X-Ray Telescope on the Neil Gehrels Swift Observatory (Swift-XRT) have yielded possible counterparts in ~30% of these source regions. The objective of this work is to identify the nature of these possible counterparts, utilizing their gamma-ray properties coupled with the Swift derived X-ray properties. The majority of the known sources in the Fermi catalogs are blazars, which constitute the bulk of the extragalactic gamma-ray source population. The galactic population on the other hand is dominated by pulsars. Overall, these two categories constitute the majority of all gamma-ray objects. Blazars and pulsars occupy different parameter space when X-ray fluxes are compared with various gamma-ray properties. In this work, we utilize the X-ray observations performed with the Swift-XRT for the unknown Fermi sources and compare their X-ray and gamma-ray properties to differentiate between the two source classes. We employ two machine-learning algorithms, decision tree and random forest (RF) classifier, to our high signal-to-noise ratio sample of 217 sources, each of which corresponds to Fermi unassociated regions. The accuracy scores for both methods were found to be 97% and 99%, respectively. The RF classifier, which is based on the application of a multitude of decision trees, associated a probability value (P_bzr_) for each source to be a blazar. This yielded 173 blazar candidates from this source sample, with P_bzr_>=90% for each of these sources, and 134 of these possible blazar source associations had P_bzr_>=99%. The results yielded 13 sources with P_bzr_<=10%, which we deemed as reasonable candidates for pulsars, seven of which result with P_bzr_<=1%. There were 31 sources that exhibited intermediate probabilities and were termed ambiguous due to their unclear characterization as a pulsar or a blazar.
200. Classifying 3FGL with ANN
ID:
ivo://CDS.VizieR/J/MNRAS/470/1291
Title:
Classifying 3FGL with ANN
Short Name:
J/MNRAS/470/1291
Date:
21 Oct 2021
Publisher:
CDS
Description:
In its first four years of operation, the Fermi-Large Area Telescope (LAT) detected 3033 {gamma}-ray emitting sources. In the Fermi-LAT Third Source Catalogue (3FGL) about 50 per cent of the sources have no clear association with a likely {gamma}-ray emitter. We use an artificial neural network algorithm aimed at distinguishing BL Lacs from FSRQs to investigate the source subclass of 559 3FGL unassociated sources characterized by {gamma}-ray properties very similar to those of active galactic nuclei. Based on our method, we can classify 271 objects as BL Lac candidates, 185 as FSRQ candidates, leaving only 103 without a clear classification. We suggest a new zoo for {gamma}-ray objects, where the percentage of sources of uncertain type drops from 52 per cent to less than 10 per cent. The result of this study opens up new considerations on the population of the {gamma}-ray sky, and it will facilitate the planning of significant samples for rigorous analyses and multiwavelength observational campaigns.