The VLBA-BU-BLAZAR Multi-Wavelength Monitoring Program

Jorstad, Svetlana G.; Marscher, A. P.

doi:10.3390/galaxies4040047

Cited by 78 publications

(63 citation statements)

References 22 publications

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“…The images that we used come mostly from the analysis of the VLBA Calibrator Survey (VCS; Beasley et al 2002;Fomalont et al 2003;Petrov et al 2005Petrov et al , 2006Kovalev et al 2007;Petrov et al 2008) and regular geodesy VLBI program (Petrov et al 2009;Piner et al 2012) at 2 and 8 GHz. Additionally, we made some use of images from the VLBI Imaging and Polarimetry Survey at 5 GHz (Helmboldt et al 2007;Petrov & Taylor 2011), the VCS releases 7, 8, and 9 (Petrov 2016) at 7.4 GHz; VLBI observing programs for Fermi-AGN associations (e.g., Schinzel et al 2015) at 8 GHz; the 15 GHz Monitoring Of Jets in active galactic Nuclei with VLBA Experiments program (MOJAVE, Lister et al 2009), 24 and 43 GHz images from the K/Q survey (Charlot et al 2010) and the VLBA-BU Blazar Monitoring Program (Jorstad & Marscher 2016). The jet direction j is determined from the inner direction of the jet ridge line calculated directly from the VLBI images.…”

Section: Observational Data and Basic Analysismentioning

confidence: 99%

VLBI-Gaiaoffsets favor parsec-scale jet direction in active galactic nuclei

Kovalev

Petrov

Plavin

2017

A&A

107

102

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Context. The data release 1 (DR1) of milliarcsecond-scale accurate optical positions of stars and galaxies was recently published by the space mission Gaia. Aims. We study the offsets of highly accurate absolute radio (very long baseline interferometry, VLBI) and optical positions of active galactic nuclei (AGN) to see whether or not a signature of wavelength-dependent parsec-scale structure can be seen. Methods. We analyzed VLBI and Gaia positions and determined the direction of jets in 2957 AGNs from their VLBI images. Results. We find that there is a statistically significant excess of sources with VLBI-to-Gaia position offset in directions along and opposite to the jet. Offsets along the jet vary from 0 to tens of mas. Offsets in the opposite direction do not exceed 3 mas. Conclusions. The presense of strong, extended parsec-scale optical jet structures in many AGNs is required to explain all observed VLBI-Gaia offsets along the jet direction. The offsets in the opposite direction shorter than 1 mas can be explained either by a non-point-like VLBI jet structure or a "core-shift" effect due to synchrotron opacity.

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Section: Observational Data and Basic Analysismentioning

confidence: 99%

VLBI-Gaiaoffsets favor parsec-scale jet direction in active galactic nuclei

Kovalev

Petrov

Plavin

2017

A&A

107

102

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“…Results of over eight years of monthly monitoring of a sample of blazars (the most luminous and variable BL Lac objects and flat-spectrum radio quasars) with the VLBA at 7 mm by the Boston University blazar group (Jorstad & Marscher 2016) 1 show that most γ-ray flares are simultaneous (within errors) with the appearance of a new superluminal component or a major outburst in the VLBI core of the jet, defined as the bright, compact feature at the upstream end of the jet (see Marscher et al 2008Marscher et al , 2010Jorstad et al 2013;. A burst in particle and magnetic energy density is therefore required when jet disturbances cross the radio core in order to produce γ-ray flares, which can naturally be explained by identifying the radio core with a recollimation shock (e.g., Daly & Marscher 1988;Gómez et al 1995Gómez et al , 1997Marscher 2009Marscher , 2012Mizuno et al 2015;Martí et al 2016).…”

Section: Introductionmentioning

confidence: 99%

High-Precision Astrometric Millimeter Very Long Baseline Interferometry Using a New Method for Multi-Frequency Calibration

Dodson

Rioja

Molina

et al. 2017

ApJ

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In this paper we describe a new approach for mm-VLBI calibration that provides bona-fide astrometric alignment of the mm-wavelength images from a single source, for the measurement of frequency dependent effects, such as 'coreshifts' near the black hole of AGN jets. We achieve our astrometric alignment by solving firstly for the ionospheric (dispersive) contributions using wide-band cmwavelength observations. Secondly we solve for the tropospheric (non-dispersive) contributions by using fast frequency-switching at the target mm-wavelengths. These solutions can be scaled and transferred from the low frequency to the high frequency. To complete the calibration chain one additional step was required to remove a residual constant phase offset on each antenna. The result is an astrometric calibration and the measurement of the core-shift between 22 and 43 GHz for the jet in BL Lacertae to be -8±5, 20±6 µas, in RA and Declination, respectively. By comparison to conventional phase referencing at cm-wavelengths we are able to show that this core shift at mm-wavelengths is significantly less than what would be predicted by extrapolating the low frequency result, which closely followed the predictions of the Blandford & Königl conical jet model. As such it would be the first demonstration for the association of the VLBI core with a recollimation shock, normally hidden at low frequencies due to the optical depth, which could be responsible for the γ-ray production in blazar jets.-2 -

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“…BL Lac was observed throughout the period of interest at 43 GHz with the VLBA under the VLBA-BU-BLAZAR monitoring program [8] and at 15.4 GHz under the Monitoring Of Jets in Active galactic nuclei with VLBA Experiments (MOJAVE) program [9], the data calibration and imaging procedures of which were identical to those described by [10] and [9], respectively. • between the jet axis and line of sight [11].…”

Section: Vlbamentioning

confidence: 99%

Multiwavelength observations of the blazar BL Lacertae: a new fast TeV γ-ray flare

Feng¹,

Jorstad²,

Marscher³

et al. 2017

Proceedings of 35th International Cosmic Ray Conference — PoS(ICRC2017)

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Observations of fast TeV γ-ray flares from blazars reveal the extreme compactness of emitting regions in blazar jets. Combined with very-long-baseline radio interferometry measurements, they probe the structure and emission mechanism of the jet. We report on a fast TeV γ-ray flare from BL Lacertae observed by VERITAS, with a rise time of about 2.3 hours and a decay time of about 36 minutes. The peak flux at >200 GeV measured with the 4-minute binned light curve is (4.2 ± 0.6) × 10 −6 photons m −2 s −1 , or ∼180% the Crab Nebula flux. Variability in GeV γ-ray, X-ray, and optical flux, as well as in optical and radio polarization was observed around the time of the TeV γ-ray flare. A possible superluminal knot was identified in the VLBA observations at 43 GHz. The flare constrains the size of the emitting region, and is consistent with several theoretical models with stationary shocks.

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The VLBA-BU-BLAZAR Multi-Wavelength Monitoring Program

Cited by 78 publications

References 22 publications

VLBI-Gaiaoffsets favor parsec-scale jet direction in active galactic nuclei

VLBI-Gaiaoffsets favor parsec-scale jet direction in active galactic nuclei

High-Precision Astrometric Millimeter Very Long Baseline Interferometry Using a New Method for Multi-Frequency Calibration

Multiwavelength observations of the blazar BL Lacertae: a new fast TeV γ-ray flare

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