- ID:
- ivo://CDS.VizieR/J/A+A/634/A112
- Title:
- TXS 2013+370 gamma-ray emitting region
- Short Name:
- J/A+A/634/A112
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The gamma-ray production mechanism and its localization in blazars are still a matter of debate. The main goal of this paper is to constrain the location of the high-energy emission in the blazar TXS 2013+370 and to study the physical and geometrical properties of the inner jet region on sub-pc scales. TXS 2013+370 was monitored during 2002-2013 with VLBI at 15, 22, 43, and 86GHz, which allowed us to image the jet base with an angular resolution of >=0.4pc. By employing CLEAN imaging and Gaussian model-fitting, we performed a thorough kinematic analysis at multiple frequencies, which provided estimates of the jet speed, orientation, and component ejection times. Additionally, we studied the jet expansion profile and used the information on the jet geometry to estimate the location of the jet apex. VLBI data were combined with single-dish measurements to search for correlated activity between the radio, mm, and gamma-ray emission. For this purpose, we employed a cross-correlation analysis, supported by several significance tests. The high-resolution VLBI imaging revealed the existence of a spatially bent jet, described by co-existing moving emission features and stationary features. New jet features, labeled as A1, N, and N1, are observed to emerge from the core, accompanied by flaring activity in radio/mm- bands and rays. The analysis of the transverse jet width profile constrains the location of the mm core to lie <=2pc downstream of the jet apex, and also reveals the existence of a transition from parabolic to conical jet expansion at a distance of ~54pc from the core, corresponding to ~1.5x10^6^ Schwarzschild radii. The cross-correlation analysis of the broad-band variability reveals a strong correlation between the radio-mm and gamma-ray data, with the 1mm emission lagging 49 days behind the rays. Based on this, we infer that the high energy emission is produced at a distance of the order of ~1pc from the VLBI core, suggesting that the seed photon fields for the external Compton mechanism originate either in the dusty torus or in the broad-line region.
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Search Results
- ID:
- ivo://CDS.VizieR/J/MNRAS/385/823
- Title:
- UBVRI light curves of OJ 287
- Short Name:
- J/MNRAS/385/823
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Based on the long-term data in the literature, we present evidence for long-term optical spectral index variability behaviour for BL Lacertae object OJ 287. First we find that the optical spectral index variability period of BL Lacertae object OJ 287 is in agreement with the optical flux variability period of about 11.96yr. It is considered that different processes induce optical variability, and each process should have a definite signature in the spectral index, thus leading to the periodicity of the spectral index. We also find that the time lag between the optical spectral index and the optical flux density is half of the optical flux variability period. This result tends to support the argument that when the brightness of the long-term variations increases, the optical spectra become bluer. These variations can be explained by energy loss of the electrons during the outburst. We show that the spectral index is different during the flares compared with outside the flares; between the first and second flare peaks of the superflares, the spectrum shapes are different. Disparate processes possibly induce the two peaks of OJ 287.
- ID:
- ivo://CDS.VizieR/J/ApJS/209/9
- Title:
- Unidentified gamma-ray sources. IV. X-ray
- Short Name:
- J/ApJS/209/9
- 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 (LAT) catalog are still of unknown origin, having not yet been associated with counterparts at lower energies. To investigate the nature of these enigmatic sources, we present an extensive search of X-ray sources lying in the positional uncertainty region of a selected sample of these unidentified gamma-ray sources (UGSs) that makes use of all available observations performed by the Swift X-ray Telescope before 2013 March 31, available for 205 UGSs. To detect the fainter sources, we merged all the observations covering the Fermi LAT positional uncertainty region at a 95% level of confidence of each UGS. This yields a catalog of 357 X-ray sources, finding candidate X-ray counterparts for ~70% of the selected sample. In particular, 25% of the UGSs feature a single X-ray source within their positional uncertainty region, while 45% have multiple X-ray sources. For each X-ray source, we also looked in the corresponding Swift UVOT merged images for optical and ultraviolet counterparts, also performing source photometry. We found ultraviolet-optical correspondences for ~70% of the X-ray sources. We searched several major radio, infrared, optical, and ultraviolet surveys for possible counterparts within the positional error of the sources in the X-ray catalog to obtain additional information on their nature. Applying the kernel density estimation technique to infrared colors of Wide-Field Infrared Survey Explorer counterparts of our X-ray sources we select six {gamma}-ray blazar candidates. In addition, comparing our results with previous analyses, we select 11 additional {gamma}-ray blazar candidates.
- ID:
- ivo://CDS.VizieR/J/MNRAS/449/2750
- Title:
- V and R lightcurves of BL Lac object PKS 0537-441
- Short Name:
- J/MNRAS/449/2750
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present the colour indices V-R of BL Lacertae object PKS 0537-441 on the basis of the photometric data monitored by the Rapid Eye Mounting (REM) telescope located at the ESO Cerro La Silla observatory (Chile). Nearly 700 quasi-simultaneous data groups were collected during the period from 2004 to 2010. Our colour analysis shows the existence of at least two variability mechanisms: one is the long-term chromatic mechanism in which the variation traces a 0.04+/-0.01 slope in the V-R versus V plot, the other is the chromatic mechanism implying the spectral changes during the flares and on intraday time-scales. A general bluer-when-brighter trend is confirmed in both ascent and descent states during the flares and in the 2010 February flare interval, while a much stronger bluer-when-brighter chromatism is observed in both faint and bright states on intraday time-scales. The discrete correlation function analysis reveals the existence of a time-lag of 13^+5.8^_-1.2_ d between colour indices and magnitudes during the period MJD 54404-54453, whereas no measurable time-lag during other periods is found. Our results suggest that geometric effects may be responsible for the long-term achromatic mechanism, while the intrinsic shock-in-jet mechanism may produce the chromatic variability during the flares and on intraday time-scales.
- ID:
- ivo://CDS.VizieR/J/A+A/553/A107
- Title:
- Variability of 198 extragalactic radio sources
- Short Name:
- J/A+A/553/A107
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Combining measurements taken using the Wilkinson Microwave Anisotropy Probe (WMAP) from 2001 to 2008 with measurements taken using Planck from 2009 to 2010, we investigate the long-term flux density variability of extragalactic radio sources selected from the Planck Early Release Compact Source Catalogue. The single-year, single-frequency WMAP maps are used to estimate yearly-averaged flux densities of the sources in the four WMAP bands: Ka (33GHz), Q (41GHz), V (61GHz), and W (94GHz). We identify 82, 67, 32, and 15 sources respectively as variable at greater than 99% confidence level in these four bands. The amplitudes of variation are comparable between bands, and are not correlated with either the flux densities or the spectral indices of the sources. The number counts of WMAP Ka-band sources are stable from year to year despite the fluctuation caused by individual source variability. Most of our sources show strong correlation in variability between bands. Almost all the sources that show variability are blazars. We have attempted to fit two simple, four-parameter models to the time-series of 32 sources showing correlated variability at multiple frequencies - a long-term flaring model and a rotating-jet model. We find that 19 sources (60%) can be fit with the simple rotating-jet model, and ten of these also fit the simple long-term flaring model. The remaining 13 sources (40%) show more complex variability behaviour that is not consistent with either model. Extended radio galaxies in our sample show no sign of variability, as expected, with the exception of Pictor A for which we report evidence for a millimetre flare lasting between 2002 and 2010.
- ID:
- ivo://CDS.VizieR/J/AJ/137/5022
- Title:
- Variability of radio-bright BL Lac objects
- Short Name:
- J/AJ/137/5022
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Radio-bright BL Lacertae objects (BLOs) are typically very variable and exhibit prominent flaring. We use a sample of 24 BLOs, regularly monitored at Metsahovi Radio Observatory, to get a clear idea of their flaring behavior in the radio domain and to find possible commonalities in their variability patterns. Our goal was to compare the results given by computational timescales and the observed variability parameters determined directly from the flux curves. Also, we wanted to find out if the BLO flares adhere to the generalized shock model, which gives a schematic explanation for the physical process giving rise to the variability. We use long-term monitoring data from 4.8, 8, 14.5, 22, 37, 90, and 230GHz, obtained mainly from the University of Michigan and Metsahovi Radio Observatories. The structure function, discrete correlation function, and Lomb-Scargle periodogram timescales, calculated in a previous study, are analyzed in more detail. Also, we determine flare durations, rise and decay times, and absolute and relative peak fluxes from the monitoring data.
- ID:
- ivo://CDS.VizieR/J/ApJ/833/117
- Title:
- VERITAS gamma-ray TeV LCs of 6 blazars
- Short Name:
- J/ApJ/833/117
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a targeted search for blazar flux-correlated high-energy ({epsilon}_{nu}_>~1TeV) neutrinos from six bright northern blazars, using the public database of northern hemisphere neutrinos detected during "IC40" 40-string operations of the IceCube neutrino observatory (2008 April to 2009 May). Our six targeted blazars are subjects of long-term monitoring campaigns by the VERITAS TeV {gamma}-ray observatory. We use the publicly available VERITAS light curves to identify periods of excess and flaring emission. These predefined intervals serve as our "active temporal windows" in a search for an excess of neutrinos, relative to Poisson fluctuations of the near-isotropic atmospheric neutrino background, which dominates at these energies. After defining the parameters of an optimized search, we confirm the expected Poisson behavior with Monte Carlo simulations prior to testing for excess neutrinos in the actual data. We make two searches: one for excess neutrinos associated with the bright flares of Mrk 421 that occurred during the IC40 run, and one for excess neutrinos associated with the brightest emission periods of five other blazars (Mrk 501, 1ES 0806+524, 1ES 1218+304, 3C 66A, and W Comae), all significantly fainter than the Mrk 421 flares. We find no significant excess of neutrinos from the preselected blazar directions during the selected temporal windows. We derive 90% confidence upper limits on the number of expected flux-associated neutrinos from each search. These limits are consistent with previous point-source searches and Fermi GeV flux-correlated searches. Our upper limits are sufficiently close to the physically interesting regime that we anticipate that future analyses using already-collected data will either constrain models or yield discovery of the first blazar-associated high-energy neutrinos.
- ID:
- ivo://CDS.VizieR/J/ApJ/744/177
- Title:
- VLBA 5GHz observations of Fermi/LAT AGNs
- Short Name:
- J/ApJ/744/177
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The radio properties of blazars detected by the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope have been observed contemporaneously by the Very Long Baseline Array (VLBA). In total, 232 sources were observed with the VLBA. Ninety sources that were previously observed as part of the VLBA Imaging and Polarimetry Survey (VIPS) have been included in the sample, as well as 142 sources not found in VIPS. This very large, 5GHz flux-limited sample of active galactic nuclei (AGNs) provides insights into the mechanism that produces strong {gamma}-ray emission. In particular, we see that {gamma}-ray emission is related to strong, uniform magnetic fields in the cores of the host AGN. Included in this sample are non-blazar AGNs such as 3C84, M82, and NGC 6251. For the blazars, the total VLBA radio flux density at 5GHz correlates strongly with {gamma}-ray flux. The LAT BL Lac objects tend to be similar to the non-LAT BL Lac objects, but the LAT flat-spectrum radio quasars (FSRQs) are significantly different from the non-LAT FSRQs. Strong core polarization is significantly more common among the LAT sources, and core fractional polarization appears to increase during LAT detection.
- ID:
- ivo://CDS.VizieR/J/ApJ/899/141
- Title:
- VLBA observations of the AGN TXS 0128+554
- Short Name:
- J/ApJ/899/141
- Date:
- 14 Mar 2022 07:07:50
- Publisher:
- CDS
- Description:
- We have carried out a Chandra X-ray and multifrequency radio Very Long Baseline Array study of the active galactic nucleus (AGN) TXS0128+554, which is associated with the Fermi {gamma}-ray source 4FGLJ0131.2+5547. The AGN is unresolved in a target 19.3ks Chandra image, and its spectrum is well fit by a simple absorbed power-law model, with no distinguishable spectral features. Its relatively soft X-ray spectrum compared to other compact symmetric objects (CSOs) may be indicative of a thermal emission component, for which we were able to obtain an upper temperature limit of kT=0.08keV. The compact radio morphology and measured advance speed of 0.32c{+/-}0.07c indicate a kinematic age of only 82yr{+/-}17yr, placing TXS0128+554 among the youngest members of the CSO class. The lack of compact, inverted spectrum hotspots and an emission gap between the bright inner jet and outer radio lobe structure indicate that the jets have undergone episodic activity, and were relaunched a decade ago. The predicted {gamma}-ray emission from the lobes, based on an inverse Compton-emitting cocoon model, is three orders of magnitude below the observed Fermi-LAT flux. A comparison to other Fermi-detected and non-Fermi-detected CSOs with redshift z<0.1 indicates that the {gamma}-ray emission likely originates in the inner jet/core region, and that nearby, recently launched AGN jets are primary candidates for detection by the Fermi-LAT instrument.
- ID:
- ivo://CDS.VizieR/J/A+A/520/A113
- Title:
- VLBI detection of 398 extragalactic radio sources
- Short Name:
- J/A+A/520/A113
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The European space astrometry mission Gaia will construct a dense optical QSO-based celestial reference frame. For consistency between optical and radio positions, it will be important to align the Gaia and VLBI frames with the highest possible accuracy. It has been found that only 70 (10%) of the sources from the International Celestial Reference Frame (ICRF) are suitable for establishing this link, either because they are not bright enough at optical wavelengths or because they have significant extended radio emission which precludes reaching the highest astrometric accuracy. In order to improve the situation, we have initiated a VLBI survey dedicated to finding additional suitable radio sources for aligning the two frames. The sample consists of 447 optically- bright (magnitude <=18) extragalactic radio sources, typically 20 times weaker than the ICRF sources, which have been selected by cross-correlating an optical quasar catalog with the NRAO VLA Sky Survey (NVSS, Cat. VIII/65). This paper presents the observing strategy to detect, image, and measure accurate radio positions for these sources. It also provides results on the VLBI detectability of the sources, as derived from initial observations with the European VLBI Network in June and October 2007. Based on these observations, a high detection rate of 89% is found, which is promising for the continuation of this project. This high VLBI detection rate for sources from the NVSS catalog is probably due to the selection process, suggesting that optically-bright quasars have compact radio structures.