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1521. Blazars in unidentified {gamma}-ray sources
ID:
ivo://CDS.VizieR/J/ApJ/752/61
Title:
Blazars in unidentified {gamma}-ray sources
Short Name:
J/ApJ/752/61
Date:
21 Oct 2021
Publisher:
CDS
Description:
One of the main scientific objectives of the ongoing Fermi mission is unveiling the nature of unidentified {gamma}-ray sources (UGSs). Despite the major improvements of Fermi in the localization of {gamma}-ray sources with respect to the past {gamma}-ray missions, about one-third of the Fermi-detected objects are still not associated with low-energy counterparts. Recently, using the Wide-field Infrared Survey Explorer survey, we discovered that blazars, the rarest class of active galactic nuclei and the largest population of {gamma}-ray sources, can be recognized and separated from other extragalactic sources on the basis of their infrared (IR) colors. Based on this result, we designed an association method for the {gamma}-ray sources to recognize if there is a blazar candidate within the positional uncertainty region of a generic {gamma}-ray source. With this new IR diagnostic tool, we searched for {gamma}-ray blazar candidates associated with the UGS sample of the second Fermi {gamma}-ray LAT catalog (2FGL). We found that our method associates at least one {gamma}-ray blazar candidate as a counterpart to each of 156 out of 313 UGSs analyzed. These new low-energy candidates have the same IR properties as the blazars associated with {gamma}-ray sources in the 2FGL catalog.
1522. Blazar (sub-)mm & {gamma}-ray luminosities
ID:
ivo://CDS.VizieR/J/ApJ/754/23
Title:
Blazar (sub-)mm & {gamma}-ray luminosities
Short Name:
J/ApJ/754/23
Date:
21 Oct 2021
Publisher:
CDS
Description:
The coexistence of Planck and Fermi satellites in orbit has enabled the exploration of the connection between the (sub-)millimeter and {gamma}-ray emission in a large sample of blazars. We find that the {gamma}-ray emission and the (sub-)mm luminosities are correlated over five orders of magnitude, L_{gamma}_{prop.to}L_(sub-)mm_. However, this correlation is not significant at some frequency bands when simultaneous observations are considered. The most significant statistical correlations, on the other hand, arise when observations are quasi-simultaneous within two months. Moreover, we find that sources with an approximate spectral turnover in the middle of the mm-wave regime are more likely to be strong {gamma}-ray emitters. These results suggest a physical relation between the newly injected plasma components in the jet and the high levels of {gamma}-ray emission.
1523. Blazars with {gamma}-ray counterparts. I.
ID:
ivo://CDS.VizieR/J/ApJS/206/12
Title:
Blazars with {gamma}-ray counterparts. I.
Short Name:
J/ApJS/206/12
Date:
21 Oct 2021
Publisher:
CDS
Description:
We present a new method for identifying blazar candidates by examining the locus, i.e., the region occupied by the Fermi {gamma}-ray blazars in the three-dimensional color space defined by the WISE infrared colors. This method is a refinement of our previous approach that made use of the two-dimensional projection of the distribution of WISE {gamma}-ray-emitting blazars (the Strip) in the three WISE color-color planes. In this paper, we define the three-dimensional locus by means of a principal component analysis of the color distribution of a large sample of blazars composed of all the ROMA-BZCAT sources with counterparts in the WISE All-Sky Catalog associated with {gamma}-ray sources in the second Fermi-LAT catalog (2FGL; the WISE Fermi blazars sample, WFB). Our new procedure yields a total completeness of c_tot_~81% and a total efficiency of e_tot_~97%. We also obtain local estimates of the efficiency and completeness as functions of the WISE colors and galactic coordinates of the candidate blazars. The catalog of all WISE candidate blazars associated with the WFB sample is also presented, complemented by archival multi-frequency information for the alternative associations. Finally, we apply the new association procedure to all {gamma}-ray blazars in the 2FGL and provide a catalog containing all the {gamma}-ray candidate blazars selected according to our procedure.
1524. Blazars with {gamma}-ray counterparts. II.
ID:
ivo://CDS.VizieR/J/ApJS/206/13
Title:
Blazars with {gamma}-ray counterparts. II.
Short Name:
J/ApJS/206/13
Date:
21 Oct 2021
Publisher:
CDS
Description:
A significant fraction (~30%) of the high-energy gamma-ray sources listed in the second Fermi Large Area Telescope catalog (2FGL) are still of unknown origin, being not yet associated with counterparts at low energies. We recently developed a new association method to identify if there is a {gamma}-ray blazar candidate within the positional uncertainty region of a generic 2FGL source. This method is entirely based on the discovery that blazars have distinct infrared colors with respect to other extragalactic sources found, thanks to the Wide-field Infrared Survey Explorer (WISE) all-sky observations. Several improvements have also been performed to increase the efficiency of our method in recognizing {gamma}-ray blazar candidates. In this paper we applied our method to two different samples, the first constituted by unidentified {gamma}-ray sources (UGSs), and the second by active galaxies of uncertain type, both listed in the 2FGL. We present a catalog of IR counterparts for ~20% of the UGSs investigated. Then, we also compare our results for the associated sources with those present in the literature. In addition, we illustrate the extensive archival research carried out to identify the radio, infrared, optical, and X-ray counterparts of the WISE-selected, {gamma}-ray blazar candidates. Finally, we discuss the future developments of our method based on ground-based follow-up observations.
1525. Blazhko effect from 4yr of Kepler data
ID:
ivo://CDS.VizieR/J/ApJS/213/31
Title:
Blazhko effect from 4yr of Kepler data
Short Name:
J/ApJS/213/31
Date:
21 Oct 2021
Publisher:
CDS
Description:
In order to benefit from the four year unprecedented precision of the Kepler data, we extracted light curves from the pixel photometric data of the Kepler space telescope for 15 Blazhko RR Lyrae stars. To collect all the flux from a given target as accurately as possible, we defined tailor-made apertures for each star and quarter. In some cases, the aperture finding process yielded sub-optimal results, because some flux have been lost even if the aperture contains all available pixels around the star. This fact stresses the importance of those methods that rely on the whole light curve instead of focusing on the extrema (O-C diagrams and other amplitude independent methods). We carried out detailed Fourier analysis of the light curves and the amplitude independent O-C diagram. We found 12 (80%) multiperiodically modulated stars in our sample. This ratio is much higher than previously found. Resonant coupling between radial modes, a recent theory for explaining the Blazhko effect, allows single, multiperiodic, or even chaotic modulations. Among the stars with two modulations, we found three stars (V355 Lyr, V366 Lyr, and V450 Lyr) where one of the periods dominates in amplitude modulation, but the other period has a larger frequency modulation amplitude. The ratio between the primary and secondary modulation periods is almost always very close to the ratios of small integer numbers. It may indicate the effect of undiscovered resonances. Furthermore, we detected the excitation of the second radial overtone mode f_2_ for three stars where this feature was formerly unknown. Our data set comprises the longest continuous, most precise observations of Blazhko RR Lyrae stars ever published. These data are made publicly available and will be unrivaled for years to come.
1526. Blazhko effect in Galactic RR Lyrae
ID:
ivo://CDS.VizieR/J/MNRAS/466/2602
Title:
Blazhko effect in Galactic RR Lyrae
Short Name:
J/MNRAS/466/2602
Date:
21 Oct 2021
Publisher:
CDS
Description:
We present the first paper of a series focused on the Blazhko effect in RR Lyrae type stars pulsating in the fundamental mode that are located in the Galactic bulge. A comprehensive overview of the incidence rate and light-curve characteristics of the Blazhko stars is given. We analysed 8282 stars having the best quality data in the OGLE-IV survey, and found that at least 40.3 per cent of the stars show modulation of their light curves. The number of Blazhko stars we identified is 3341, which is the largest sample ever studied, implying that these are the most relevant statistical results currently available. Using combined data sets with OGLE-III observations, we found that 50 per cent of the stars that show unresolved peaks close to the main component in OGLE-IV are actually Blazhko stars with extremely long periods. Blazhko stars with modulation occur preferentially among RR Lyrae stars with shorter pulsation periods in the Galactic bulge. Fourier amplitude and phase coefficients based on the mean light curves appear to be substantially lower for Blazhko stars than for stars with an unmodulated light curve on average. We derived new relations for the compatibility parameter D_m_ in the I passband and relations that allow for differentiating modulated and non-modulated stars easily based on R_31_, {phi}_21_ and {phi}_31_. Photometric metallicities, intrinsic colours and absolute magnitudes computed using empirical relations are the same for Blazhko and non-modulated stars in the Galactic bulge, suggesting there is no correlation between the occurrence of the Blazhko effect and these parameters.
1527. Blind photometric study of NGC 2264 region
ID:
ivo://CDS.VizieR/J/A+A/621/A14
Title:
Blind photometric study of NGC 2264 region
Short Name:
J/A+A/621/A14
Date:
21 Oct 2021
Publisher:
CDS
Description:
Thanks to their extensive and homogeneous sky coverage, deep, large-scale, multi-wavelength surveys are uniquely suited to statistically identify and map young star clusters in our Galaxy. Such studies are crucial to address themes like the initial mass function, or the modes and dynamics of star cluster formation and evolution. We aim to test a purely photometric approach to statistically identify a young clustered population embedded in a large population of field stars, with no prior knowledge on the nature of stars in the field. We conducted our blind test study on the NGC 2264 region, which hosts a well-known, richly populated young cluster (~3Myr-old) and several active star-forming sites. We selected a large (4 deg^2^) area around the NGC 2264 cluster, and assembled an extensive r, i, J catalog of the field from pre-existing large-scale surveys, notably Pan-STARRS1 and UKIDSS. We then mapped the stellar color locus on the (i-J, r-i) diagram to select M-type stars, which offer the following observational advantages with respect to more massive stars: i) they comprise a significant fraction of the Galactic stellar population; ii) their pre-main sequence phase lasts significantly longer than for higher-mass stars; iii) they exhibit the strongest luminosity evolution from the pre-main sequence to the main sequence; iv) their observed r, i, J colors provide a direct and empirical estimate of AV. A comparative analysis of the photometric and spatial properties of M-type stars as a function of AV enabled us to probe the structure and stellar content of our field. Using only r, i, J photometry, we could identify two distinct populations in our field: a diffuse field population and a clustered population in the center of the field. The presence of a concentration of occulting material, spatially associated with the clustered population, allowed us to derive an estimate of its distance (800-900pc) and age (0.5-5Myr); these values are overall consistent with the literature parameters for the NGC 2264 star-forming region. The extracted clustered population exhibits a hierarchical structure, with two main clumps and peaks in number density of objects around the most extincted locations within the field. An excellent agreement is found between the observed substructures for the clustered population and a map of the NGC 2264 subregions reported in the literature. Our selection of clustered members is coherent with the literature census of the NGC 2264 cluster for about 95% of the objects located in the inner regions of the field, where the estimated contamination rate by field stars in our sample is only 2%. In addition, the availability of a uniform dataset for a large area around the NGC 2264 region enabled us to discover a population of about a hundred stars with indications of statistical membership to the cluster, therefore extending the low-mass population census of NGC 2264 to distances of 10-15pc from the cluster cores. By making use solely of deep, multi-band (r, i, J) photometry, without assuming any further knowledge on the stellar population of our field, we were able to statistically identify and reconstruct the structure of a very young cluster that has been a prime target for star formation studies over several decades. The method tested here can be readily applied to surveys such as Pan-STARRS and the future LSST to undertake a first complete census of low-mass, young stellar populations down to distances of several kpc across the Galactic plane.
1528. BL Lac candidates for TeV observations
ID:
ivo://CDS.VizieR/J/ApJS/207/16
Title:
BL Lac candidates for TeV observations
Short Name:
J/ApJS/207/16
Date:
21 Oct 2021
Publisher:
CDS
Description:
BL Lac objects are the most numerous class of extragalactic TeV-detected sources. One of the biggest difficulties in investigating their TeV emission is due to their limited number, since only 47 BL Lac objects are known to be TeV emitters. In this paper, we propose new criteria to select TeV BL Lac candidates based on infrared and X-ray observations. We apply our selection criteria to the BL Lac objects listed in the ROMA-BZCAT catalog (Cat. J/A+A/495/691), thereby identifying 41 potential TeV emitters. We then perform a search over a more extended sample combining the ROSAT bright source catalog (Cat. IX/10) and the WISE all-sky survey (Cat. II/311), revealing 54 additional candidates for TeV observations. Our investigation also led to a tentative classification of 16 unidentified X-ray sources as BL Lac candidates. This analysis provides new interesting BL Lac targets for future observations with ground-based Cherenkov telescopes.
1529. BL Lac from SDSS
ID:
ivo://CDS.VizieR/J/AJ/129/2542
Title:
BL Lac from SDSS
Short Name:
J/AJ/129/2542
Date:
21 Oct 2021
Publisher:
CDS
Description:
We present a sample of 386 BL Lacertae candidates identified from 2860deg^2^ of the Sloan Digital Sky Survey (SDSS) spectroscopic database. The candidates are primarily selected to have quasi-featureless optical spectra and low proper motions as measured from SDSS and USNO-B positions; however, our ability to separate Galactic from extragalactic quasi-featureless objects (QFOs) on the basis of proper motion alone is limited by the lack of reliable proper motion measurements for faint objects. Based largely on this remarkably clean color separation, we subdivide the sample into 240 probable candidates and 146 additional less probable (likely stellar) candidates.
1530. BL Lac 86GHz images
ID:
ivo://CDS.VizieR/J/A+A/649/A153
Title:
BL Lac 86GHz images
Short Name:
J/A+A/649/A153
Date:
22 Feb 2022
Publisher:
CDS
Description:
Controversial studies on the jet collimation profile of BL Lacertae (BL Lac), the eponymous blazar of the BL Lac objects class, complicate the scenario in this already puzzling class of objects. Understanding the jet geometry in connection with the jet kinematics and the physical conditions in the surrounding medium is fundamental for better constraining the formation, acceleration, and collimation mechanisms in extragalactic jets. With the aim of investigating the jet geometry in the innermost regions of the BL Lac jet, and resolving the controversy, we explore the radio jet in this source using high-resolution millimeter-wave VLBI data. We collect 86GHz GMVA and 43 GHz VLBA data to obtain stacked images that we use to infer the jet collimation profile by means of two comparable methods. We analyze the kinematics at 86GHz, and we discuss it in the context of the jet expansion. Finally, we consider a possible implication of the Bondi sphere in shaping the jet of BL Lac. The jet in BL Lac expands with an overall conical geometry. A higher expanding rate region is observed between ~5 and 10pc (de-projected) from the black hole. Such a region is associated with the decrease in brightness usually observed in high frequency VLBI images of BL Lac. The jet retrieves the original jet expansion around 17pc, where the presence of a recollimation shock is supported by both the jet profile and the 15GHz kinematics (MOJAVE survey). The change in the jet expansion profile occurring at ~5pc could be associated with a change in the external pressure at the location of the Bondi radius (~3.3x10^5^R_S_)/

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