@article {2011A\&A...533A.111M,
	title = {{Detection of jet precession in the active nucleus of M 81}},
	journal = {Astronomy and Astrophysics},
	volume = {533},
	year = {2011},
	month = {sep},
	pages = {A111},
	abstract = {We report on very-long-baseline-interferometry (VLBI) monitoring observations of the low-luminosity active galactic nucleus (LLAGN) in the galaxy M 81 at the frequencies of 1.7, 2.3, 5.0, and 8.4 GHz. The observations reported here are phase-referenced to the supernova SN 1993J (located in the same galaxy) and cover from late 1993 to late 2005. The large amount of available observations allows us to study the stability of the AGN position in the frame of its host galaxy at different frequencies and chromatic effects in the jet morphology, together with their time evolution. The source consists at all frequencies of a slightly resolved core and a small jet extension towards the northeast direction (position angle of \~{}65 degrees) in agreement with previous publications. We find that the position of the intensity peak in the images at 8.4 GHz is very stable in the galactic frame of M 81 (proper motion upper limit about 10 {$μ$}as per year). We confirm previous reports that the peaks at all frequencies are systematically shifted among them, possibly due to opacity effects in the jet as predicted by the standard relativistic jet model. We use this model, under plausible assumptions, to estimate the magnetic field in the jet close to the jet base and the mass of the central black hole. We obtain a black-hole mass of \~{}2 {\times} 10$^{7}$ M$_{\&sun;}$, comparable to estimates previously reported using different approaches, but the magnetic fields obtained are 10$^{3}$-10$^{4}$ times lower than previous estimates. We find that the positions of the cores at 1.7, 2.3, and 5.0 GHz are less stable than that at 8.4 GHz and evolve systematically, shifting southward at a rate of several tens of {$μ$}as per year. The evolution in the jet orientation seems to be related to changes in the inclination of the cores at all frequencies. These results can be interpreted as due to a precessing jet. The evolving jet orientation also seems to be related to a flare in the peak flux densities at 5.0 and 8.4 GHz, which lasts \~{}4 years (from mid 1997 to mid 2001). An increase in the accretion rate of the black hole, and its correlation with the jet luminosity via the disk-jet connection model, seems insufficient to explain this long flare and the simultaneous evolution in the jet orientation. A continued monitoring of the flux density and the jet structure evolution in this LLAGN will be necessary to further confirm our jet precession model.},
	keywords = {astrometry, galaxies: active, galaxies: individual: M 81, galaxies: jets, radiation mechanisms: non-thermal, radio continuum: galaxies},
	doi = {10.1051/0004-6361/201117211},
	url = {http://adsabs.harvard.edu/abs/2011A\%26A...533A.111M},
	author = {Mart{\'\i}-Vidal, I. and Marcaide, J.~M. and Alberdi, A. and M. A. P{\'e}rez-Torres and Ros, E. and Guirado, J.~C.}
}
@article {2011A\&A...526A.142M,
	title = {{Radio emission of SN1993J: the complete picture. I. Re-analysis of all the available VLBI data}},
	journal = {Astronomy and Astrophysics},
	volume = {526},
	year = {2011},
	month = {feb},
	pages = {A142},
	abstract = {We have performed a complete re-calibration and re-analysis of all the available Very-Long-Baseline-Interferometry (VLBI) observations of supernova SN1993J, following an homogeneous and well-defined methodology. VLBI observations of SN1993J at 69 epochs, spanning 13 years, were performed by two teams, which used different strategies and analysis tools. The results obtained by each group are similar, but their conclusions on the supernova expansion and the shape and evolution of the emitting region differ significantly. From our analysis of the combined set of observations, we have obtained an expansion curve with unprecedented time resolution and coverage. We find that the data from both teams are compatible when analyzed with the same methodology. One expansion index (m$_{3}$ = 0.87 {\plusmn} 0.02) is enough to model the expansion observed at 1.7 GHz, while two expansion indices (m$_{1}$ = 0.933 {\plusmn} 0.010 and m$_{2}$ = 0.796 {\plusmn} 0.005), separated by a break time, t$_{br}$ = 390 {\plusmn} 30 days, are needed to model the data, at frequencies higher than 1.7 GHz, up to day \~{}4000 after explosion. We thus confirm the wavelength dependence of the size of the emitting region reported by one of the groups. We also find that all sizes measured at epochs later than day \~{}4000 after explosion are systematically smaller than our model predictions (i.e., an additional expansion index might be needed to properly model these data). We also estimate the fractional shell width (0.31 {\plusmn} 0.02, average of all epochs and frequencies) and the level of opacity to the radio emission by the ejecta. We find evidence of a spectral-index radial gradient in the supernova shell, which is indicative of a frequency-dependent ejecta opacity. Finally, we study the distribution and evolution of the azimuthal anisotropies (hot spots) found around the radio shell during the expansion. These anisotropies have intensities of \~{}20\% of the mean flux density of the shell, and appear to systematically evolve during the expansion.},
	keywords = {galaxies: individual: M 81, radio continuum: stars, supernovae: general, supernovae: individual: SN1993J},
	doi = {10.1051/0004-6361/200913831},
	url = {http://adsabs.harvard.edu/abs/2011A\%26A...526A.142M},
	author = {Mart{\'\i}-Vidal, I. and Marcaide, J.~M. and Alberdi, A. and Guirado, J.~C. and M. A. P{\'e}rez-Torres and Ros, E.}
}
@article {2011A\&A...526A.143M,
	title = {{Radio emission of SN1993J: the complete picture. II. Simultaneous fit of expansion and radio light curves}},
	journal = {Astronomy and Astrophysics},
	volume = {526},
	year = {2011},
	pages = {A143},
	abstract = {We report on a simultaneous modelling of the expansion and radio light curves of the supernova SN1993J. We developed a simulation code capable of generating synthetic expansion and radio light curves of supernovae by taking into consideration the evolution of the expanding shock, magnetic fields, and relativistic electrons, as well as the finite sensitivity of the interferometric arrays used in the observations. Our software successfully fits all the available radio data of SN 1993J with a standard emission model for supernovae, which is extended with some physical considerations, such as an evolution in the opacity of the ejecta material, a radial decline in the magnetic fields within the radiating region, and a changing radial density profile for the circumstellar medium starting from day 3100 after the explosion.},
	keywords = {acceleration of particles, galaxies: individual: M 81, radiation mechanisms: non-thermal, radio continuum: stars, supernovae: general, supernovae: individual: SN1993J},
	doi = {10.1051/0004-6361/201014517},
	url = {http://adsabs.harvard.edu/abs/2011A\%26A...526A.143M},
	author = {Mart{\'\i}-Vidal, I. and Marcaide, J.~M. and Alberdi, A. and Guirado, J.~C. and M. A. P{\'e}rez-Torres and Ros, E.}
}
