@article {2013A\&A...559A..75B, title = {{The TeV blazar Markarian 421 at the highest spatial resolution}}, journal = {Astronomy and Astrophysics}, volume = {559}, year = {2013}, month = {nov}, pages = {A75}, abstract = {Context. High-resolution radio observations allow us to directly image the innermost region of active galactic nuclei. The Very Long Baseline Array (VLBA) data analyzed in this paper were obtained during a multiwavelength (MWL) campaign, carried out in 2011, from radio to very high energy gamma rays, on the TeV blazar Markarian 421 (Mrk 421).
Aims: Our aim was to obtain information on the jet structure in Mrk 421 during the MWL campaign at the highest possible angular resolution and with high temporal frequency observations, in order to compare structural and flux density evolution with higher energy variations.
Methods: We consider data obtained with the VLBA at 43 GHz through two sets of observations: one is part of a dedicated multi-frequency monitoring campaign, in which we observed Mrk 421 once a month from January to December 2011 at three frequencies; the other is extracted from the Boston University monitoring program, which observes 34 blazars at 43 GHz about once per month. We model-fit the data in the visibility plane, study the proper motion of jet components, the light curve, and the spectral index of the jet features. We compare the radio data with optical light curves obtained at the Steward Observatory, considering also the optical polarization information.
Results: Mrk 421 has a bright nucleus and a one-sided jet extending towards the north-west for a few parsecs. The model-fits show that brightness distribution is well described using 6-7 circular Gaussian components, four of which are reliably identified at all epochs; all components are effectively stationary except for component D, at \~{}0.4 mas from the core, whose motion is, however, subluminal. Analysis of the light curve shows two different states, with the source being brighter and more variable in the first half of 2011 than in the second half. The highest flux density is reached in February. A comparison with the optical data reveals an increase of the V magnitude and of the fractional polarization simultaneous with the enhancement of the radio activity. Table 3 is available in electronic form at http://www.aanda.orgVLBA images are only available as FITS files at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/559/A75}, keywords = {BL Lacertae objects: individual: Markarian 421, galaxies: active, galaxies: jets}, doi = {10.1051/0004-6361/201321858}, url = {http://adsabs.harvard.edu/abs/2013A\%26A...559A..75B}, author = {Blasi, M.~G. and Lico, R. and Giroletti, M. and Orienti, M. and Giovannini, G. and Cotton, W. and Edwards, P.~G. and Fuhrmann, L. and Krichbaum, T.~P. and Kovalev, Y.~Y. and Jorstad, S. and Marscher, A. and Kino, M. and Paneque, D. and M. A. P{\'e}rez-Torres and Piner, B.~G. and Sokolovsky, K.~V.} } @article {2012A\&A...545A.117L, title = {{VLBA monitoring of Mrk 421 at 15 GHz and 24 GHz during 2011}}, journal = {Astronomy and Astrophysics}, volume = {545}, year = {2012}, month = {sep}, pages = {A117}, abstract = {Context. High-resolution radio observations are ideal for constraining the value of physical parameters in the inner regions of active-galactic-nucleus jets and complement results on multiwavelength (MWL) observations. This study is part of a wider multifrequency campaign targeting the nearby TeV blazar Markarian 421 (z = 0.031), with observations in the sub-mm (SMA), optical/IR (GASP), UV/X-ray (Swift, RXTE, MAXI), and {$\gamma$} rays (Fermi-LAT, MAGIC, VERITAS).
Aims: We investigate the jet{\textquoteright}s morphology and any proper motions, and the time evolution of physical parameters such as flux densities and spectral index. The aim of our wider multifrequency campaign is to try to shed light on questions such as the nature of the radiating particles, the connection between the radio and {$\gamma$}-ray emission, the location of the emitting regions and the origin of the flux variability.
Methods: We consider data obtained with the Very Long Baseline Array (VLBA) over twelve epochs (one observation per month from January to December 2011) at 15 GHz and 24 GHz. We investigate the inner jet structure on parsec scales through the study of model-fit components for each epoch.
Results: The structure of Mrk 421 is dominated by a compact (\~{}0.13 mas) and bright component, with a one-sided jet detected out to \~{}10 mas. We identify 5-6 components in the jet that are consistent with being stationary during the 12-month period studied here. Measurements of the spectral index agree with those of other works: they are fairly flat in the core region and steepen along the jet length. Significant flux-density variations are detected for the core component.
Conclusions: From our results, we draw an overall scenario in which we estimate a viewing angle 2{\deg} {\l}t} {\th}etas} {\l}t} 5{\deg} and a different jet velocity for the radio and the high-energy emission regions, such that the respective Doppler factors are {$δ$}$_{r}$ \~{} 3 and {$δ$}$_{h.e.}$ \~{} 14. Table 2 is available in electronic form at http://www.aanda.orgFITS files are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/545/A117}, keywords = {BL Lacertae objects: individual: Mrk 421, galaxies: active, galaxies: jets}, doi = {10.1051/0004-6361/201219870}, url = {http://adsabs.harvard.edu/abs/2012A\%26A...545A.117L}, author = {Lico, R. and Giroletti, M. and Orienti, M. and Giovannini, G. and Cotton, W. and Edwards, P.~G. and Fuhrmann, L. and Krichbaum, T.~P. and Sokolovsky, K.~V. and Kovalev, Y.~Y. and Jorstad, S. and Marscher, A. and Kino, M. and Paneque, D. and M. A. P{\'e}rez-Torres and Piner, G.} }