A. J. Kus

AGN and starbursts at high redshift: High resolution EVN radio observations of the Hubble Deep Field

We present deep, wide-field European VLBI Network (EVN) 1.6 GHz observations of the Hubble Deep Field (HDF) region with a resolution of 0.025 arcseconds. Above the 210 μ Jy/beam (

The multifrequency variability of Mrk 421

5\sigma

RoboPol: optical polarization-plane rotations and flaring activity in blazars

) detection level, the EVN clearly detects two radio sources in a field that encompasses the HDF and part of the Hubble Flanking Fields (HFF). The sources detected are: VLA J123644+621133 (a

RoboPol: the optical polarization of gamma-ray-loud and gamma-ray-quiet blazars

z=1.013

Hybrid morphology radio sources from the FIRST survey

, low-luminosity FR-I radio source located within the HDF itself) and VLA J123642+621331 (a dust enshrouded, optically faint,

12.2 GHz survey towards 6.7 GHz methanol masers: A comparison of 12.2 GHz and 6.7 GHz spectra

z=4.424

30 GHz flux density measurements of the Caltech-Jodrell flat-spectrum sources with OCRA-p

starburst system). A third radio source, VLA J123646+621404, is detected at the

OCRA: a one-centimeter receiver array

4\sigma

30 GHz observations of sources in the Very Small Array fields

level. The VLBI detections of all three sources suggest that most of the radio emission of these particular sources (including the dusty starburst) is generated by an embedded AGN.

One Centimetre Receiver Array-prototype observations of the CRATES sources at 30 GHz

We present a model of the multifrequency variability of the blazar Mrk 421. The model explains correlated variability observed from Very High Energy (VHE) gamma rays to radio frequencies. We assume that the dominant part of the stationary emission from the radio frequencies to the X-rays is generated by the synchrotron radiation of relativistic electrons ejected from the central engine. The particles move from the center of the source with relativistic velocities and form an inhomogeneous jet. We perform detailed calculations of the radiation transfer and calculate evolution of the electron energy spectrum along the jet. We explain the observed variability by the evolving synchrotron and Inverse-Compton (IC) radiation of a compact component (a blob) which travels along the jet. Two scenarios have been considered as mechanisms to generate VHE flares. The first scenario assumes that the high energy electrons, necessary for generation of the VHE flares, are injected into the jet, directly from the central engine or from an acceleration zone (e.g., a shock wave). The second scheme assumes that the high energy electrons are generated in situ by acceleration, for example by diffusive shock waves or a localized turbulence inside the jet. The particles evolve along the jet. They are cooled by the radiative processes and by the adiabatic expansion which compete with the acceleration process and the injection of high energy electrons. We present new observations we obtained in the radio domain for Mrk 421. The radio data gathered in February-April 2001 show a well defined radio outburst which corresponds to an X-ray outburst observed by RXTE-ASM and a gamma-ray flare detected by HEGRA in the TeV range. The best of our knowledge, this is the first direct observational evidence for a flare observed simultaneously in the radio range and at very high energies. Our scenario with acceleration of electrons in the middle part of the jet describes well the temporal evolution of such multispectral flare.