Abstract Monitoring absorption lines in the radio spectra of compact extragalactic sources that lie behind local molecular clouds provides a method for exploring the AU-scale structure of the foreground clouds.

10.1017/s0252921100019722

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86, 43, and 22 GH[CLC]z[/CLC] VLBI Observations of 3C 120

The Astrophysical Journal (1999)

José L. Gómez Alan P. Marscher A. Alberdi

We present the first 86 GHz VLBI observations of the radio galaxy 3C 120, together with contemporaneous 43 and 22 GHz polarimetric VLBA observations. The very high angular resolution obtained at 86 GHz provides an upper limit to the size of the core of 54 μas (0.025 h-1 pc). This represents a direct determination of the base of the jet that is independent of variability arguments (which depend on uncertain estimates of the Doppler factor) and places it below approximately 1 lt-month. Comparison with previous VLBA observations after a 1 yr interval shows pronounced changes in the structure and polarization of the jet. Most of the components are found to follow a curved path while undergoing a steepening of their spectra accompanied by a decrease in total and polarized emission. However, at least one component is observed to follow a quasi-ballistic motion, accompanied by a flattening of its spectrum as well as an increase in total and polarized flux. This may be explained by its interaction with the external medium, resulting in a shock that enhances the emission and aligns the magnetic field perpendicular to the component motion, thereby producing an increase of the degree of polarization from undetected values to as high as 15%. A second strong component, with the highest degree of polarization (23%), is found to have experienced a displacement from the ridge line of the structural position angle of the jet as it moved downstream. We have found a mean swing to the south of the position angle of the innermost components of ~6° between late 1996 and 1997, which may be responsible for the jet curvature observed at parsec and kiloparsec scales.

Position angle

Very-long-baseline interferometry

10.1086/312170

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Multiwavelength polarization observations of theγ-ray bright quasar PKS 0420-014

EPJ Web of Conferences (2013)

I. S. Troitskiy D. A. Morozova S. G. Jorstad Alan P. Marscher V. M. Larionov

We analyze total and polarized intensity images of the quasar PKS 0420-014 obtained monthly with the VLBA at 43 GHz during 2008–2012 along with γ-ray data provided by the Fermi Large Area Telescope and multi-color photometric and polarimetric measurements collected by different optical telescopes. During this period the quasar underwent a number of optical flares, which were accompanied by rapid rotation of polarization angle, an increase of activity in γ-rays, and the appearance of new superluminal knots in the parsec-scale jet. We investigate the fine structure of the flares at different wavelengths and in polarized light, and determine kinematic parameters of the knots. We compare the rapid evolution of the optical polarization with the polarization of the VLBI core and knots. We interpret the multi-wavelength behavior within a model that places the blazar "dissipation zone" at the millimeter-wave core of the parsec-scale jet.

Superluminal motion

Very-long-baseline interferometry

BL Lac object

10.1051/epjconf/20136107008

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5-day photo-polarimetric WEBT Campaign on Blazar S5 0716+714 - a Study of Microvariabiltiy in Blazar

AAS (2015)

Gopal Bhatta Michal Ostrwoski Ł. Stawarz S. Zoła D. Jableka

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X-ray Polarization Observations of BL Lacertae

arXiv (Cornell University) (2022)

R. Middei Ioannis Liodakis M. Perri Simonetta Puccetti E. Cavazzuti

Blazars are a class of jet-dominated active galactic nuclei with a typical double-humped spectral energy distribution. It is of common consensus the Synchrotron emission to be responsible for the low frequency peak, while the origin of the high frequency hump is still debated. The analysis of X-rays and their polarization can provide a valuable tool to understand the physical mechanisms responsible for the origin of high-energy emission of blazars. We report the first observations of BL Lacertae performed with the Imaging X-ray Polarimetry Explorer ({IXPE}), from which an upper limit to the polarization degree $Π_X<$12.6\% was found in the 2-8 keV band. We contemporaneously measured the polarization in radio, infrared, and optical wavelengths. Our multiwavelength polarization analysis disfavors a significant contribution of proton synchrotron radiation to the X-ray emission at these epochs. Instead, it supports a leptonic origin for the X-ray emission in BL Lac.

BL Lac object

Spectral energy distribution

Degree of polarization

10.48550/arxiv.2211.13764

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The awakening of BL Lacertae: observations by Fermi, Swift and the GASP-WEBT★

Monthly Notices of the Royal Astronomical Society (2013)

C. M. Raiteri M. Villata F. D’Ammando V. M. Larionov Mark Gurwell

Since the launch of the Fermi satellite, BL Lacertae has been moderately active at gamma-rays and optical frequencies until May 2011, when the source started a series of strong flares. The exceptional optical sampling achieved by the GLAST-AGILE Support Program (GASP) of the Whole Earth Blazar Telescope (WEBT) in collaboration with the Steward Observatory allows us to perform a detailed comparison with the daily gamma-ray observations by Fermi. Discrete correlation analysis between the optical and gamma-ray emission reveals correlation with a time lag of 0 +- 1 d, which suggests cospatiality of the corresponding jet emitting regions. A better definition of the time lag is hindered by the daily gaps in the sampling of the extremely fast flux variations. In general, optical flares present more structure and develop on longer time scales than corresponding gamma-ray flares. Observations at X-rays and at millimetre wavelengths reveal a common trend, which suggests that the region producing the mm and X-ray radiation is located downstream from the optical and gamma-ray-emitting zone in the jet. The mean optical degree of polarisation slightly decreases over the considered period and in general it is higher when the flux is lower. The optical electric vector polarisation angle (EVPA) shows a preferred orientation of about 15 deg, nearly aligned with the radio core EVPA and mean jet direction. Oscillations around it increase during the 2011-2012 outburst. We investigate the effects of a geometrical interpretation of the long-term flux variability on the polarisation. A helical magnetic field model predicts an evolution of the mean polarisation that is in reasonable agreement with the observations. These can be fully explained by introducing slight variations in the compression factor in a transverse shock waves model.

Flare

10.1093/mnras/stt1672

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Citations (108)

High-Frequency VLBI Observations and Theoretical Thoughts of Gamma-Ray Blazars

Alan P. Marscher

Very-long-baseline interferometry

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Upcoming Kepler monitoring of OJ 287

ATel (2015)

Rick Edelson I. M. McHardy S. G. Jorstad Alan P. Marscher T. Hovatta

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Synchrotron self-Compton interpretation of multiwaveband observations of gamma-ray bright blazars.

Astronomy & Astrophysics Supplement Series (1996)

Alan P. Marscher John Travis

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Highly Significant Detection of X-Ray Polarization from the Brightest Accreting Neutron Star Sco X-1

arXiv (Cornell University) (2023)

Fabio La Monaca Alessandro Di Marco Juri Poutanen Matteo Bachetti S. Motta

The Imaging X-ray Polarimetry Explorer (IXPE) measured with high significance the X-ray polarization of the brightest Z-source Scorpius X-1, resulting in the nominal 2-8 keV energy band in a polarization degree of 1.0(0.2)% and a polarization angle of 8(6){\deg} at 90% of confidence level. This observation was strictly simultaneous with observations performed by NICER, NuSTAR, and Insight-HXMT, which allowed for a precise characterization of its broad-band spectrum from soft to hard X-rays. The source has been observed mainly in its soft state, with short periods of flaring. We also observed low-frequency quasi-periodic oscillations. From a spectro-polarimetric analysis, we associate a polarization to the accretion disk at <3.2% at 90% of confidence level, compatible with expectations for an electron-scattering dominated optically thick atmosphere at the Sco X-1 inclination of 44{\deg}; for the higher-energy Comptonized component, we obtain a polarization of 1.3(0.4)%, in agreement with expectations for a slab of Thomson optical depth of ~7 and an electron temperature of ~3 keV. A polarization rotation with respect to previous observations by OSO-8 and PolarLight, and also with respect to the radio-jet position angle, is observed. This result may indicate a variation of the polarization with the source state that can be related to relativistic precession or to a change in the corona geometry with the accretion flow.

Position angle

Thomson scattering

10.48550/arxiv.2311.06359

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