@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.} } @article {2012A\&A...537A..93R, title = {{VLBI imaging of M 81* at 43 GHz}}, journal = {Astronomy and Astrophysics}, volume = {537}, year = {2012}, month = {jan}, pages = {A93}, abstract = {Context. The nearby spiral galaxy M 81 harbours in its core a low-luminosity active galactic nucleus (LLAGN), and appears to be closely related to the more distant and powerful active galactic nuclei (AGNs) seen in quasars and radio galaxies. The intrinsic size of this object is unknown because of too much scattering, and it has shown a core-jet morphology with weak extended emission rotating with wavelength.
Aims: The proximity of M 81 (D = 3.63 Mpc) allows for a detailed investigation of its nucleus. The nucleus is four orders of magnitude more luminous than the Galactic centre, and is therefore considered to be a link between Sgr A$^{*}$ and the more powerful nuclei of radio galaxies and quasars. Our main goal was to determine the size of M 81{\l}owast} at a shorter wavelength to directly test whether the frequency-size dependent law {$\Theta$} {\prop} {$ν$}$^{-0.8}$ is still valid for wavelengths shorter than 1 cm. In addition, we also aimed to confirm the rotation of the source as a function of frequency.
Methods: We observed the continuum 7 mm radio emission of M 81{\l}owast} with the Very Long Baseline Array on Sep. 13, 2002, using nearby calibrators to apply their interferometric observables to the target source to increase the chances of detection. The source was detected on all baselines and hybrid mapping was possible.
Results: We present the first 7 mm VLBI image of the core of M 81{\l}owast}, which is the highest resolution image ever of this source. Modelling the interferometric visibilities with two Gaussian functions sets constraints on the angular size of its core down to 38 microarcseconds, corresponding to a maximum (projected) linear size of 138 AU, and shows extended emission towards the NE with a position angle of \~{}50{\deg}. A fit of a one Gaussian elliptical function yields a position angle of 28 {\plusmn} 8 degrees for its elongated, compact structure. Combining the 7 mm size with earlier measurements at other frequencies, we determine a frequency-size dependence of {$\Theta$} {\prop} {$ν$}$^{(-0.88 {\textpm} 0.04)}$.
Conclusions: Our VLBI imaging of M 81{\l}owast} has clearly detected its core-jet structure, and has allowed us to estimate a size for its core, with a minimum size of 138 AU ({\ap}100 Schwarzschild radii). Our work opens the way for future observations of M 81{\l}owast} at high-angular resolution, including the monitoring of its structure, given that much higher bandwidths are currently available on the interferometric networks. In particular, this would allow one to test for possible proper motions of the core or of its components in the inner jet of M 81{\l}owast}, and for the speed of the detected jet components.}, keywords = {galaxies: active, galaxies: individual: M 81, galaxies: nuclei, instrumentation: interferometers, radio continuum: galaxies}, doi = {10.1051/0004-6361/201118399}, url = {http://adsabs.harvard.edu/abs/2012A\%26A...537A..93R}, author = {Ros, E. and M. A. P{\'e}rez-Torres} } @article {2011A\&A...535L..10M, title = {{VLBI observations of SN 2011dh: imaging of the youngest radio supernova}}, journal = {Astronomy and Astrophysics}, volume = {535}, year = {2011}, pages = {L10}, abstract = {We report on the VLBI detection of supernova SN 2011dh at 22 GHz using a subset of the EVN array. The observations took place 14 days after the discovery of the supernova, thus resulting in a VLBI image of the youngest radio-loud supernova ever. We provide revised coordinates for the supernova with milli-arcsecond precision, linked to the ICRF. The recovered flux density is a factor \~{}2 below the EVLA flux density reported by other authors at the same frequency and epoch of our observations. This discrepancy could be due to extended emission detected with the EVLA or to calibration problems in the VLBI and/or EVLA observations.}, keywords = {ISM: supernova remnants, radiation mechanisms: non-thermal, radio continuum: general, supernovae: general, supernovae: individual: SN 2011dh}, doi = {10.1051/0004-6361/201118195}, url = {http://adsabs.harvard.edu/abs/2011A\%26A...535L..10M}, author = {Mart{\'\i}-Vidal, I. and Tudose, V. and Paragi, Z. and Yang, J. and Marcaide, J.~M. and Guirado, J.~C. and Ros, E. and Alberdi, A. and M. A. P{\'e}rez-Torres and Argo, M.~K. and van der Horst, A.~J. and Garrett, M.~A. and Stockdale, C.~J. and Weiler, K.~W.} } @article {2010A\&A...515A..71C, title = {{Vibrationally excited HC$_{3}$N in NGC 4418}}, journal = {Astronomy and Astrophysics}, volume = {515}, year = {2010}, month = {jun}, pages = {A71}, abstract = {
Aims: We investigate the molecular gas properties of the deeply obscured luminous infrared galaxy NGC 4418. We address the excitation of the complex molecule HC$_{3}$N to determine whether its unusually luminous emission is related to the nature of the buried nuclear source.
Methods: We use IRAM 30 m and JCMT observations of rotational and vibrational lines of HC$_{3}$N to model the excitation of the molecule by means of rotational diagrams.
Results: We report the first confirmed extragalactic detection of vibrational lines of HC$_{3}$N. We detect 6 different rotational transitions ranging from J = 10-9 to J = 30-29 in the ground vibrational state and obtain a tentative detection of the J = 38-37 line. We also detect 7 rotational transitions of the vibrationally excited states v$_{6}$ and v$_{7}$, with angular momenta ranging from J = 10-9 to 28-27. The energies of the upper states of the observed transitions range from 20 to 850 K. In the optically thin regime, we find that the rotational transitions of the vibrational ground state can be fitted for two temperatures, 30 K and 260 K, while the vibrationally excited levels can be fitted for a rotational temperature of 90 K and a vibrational temperature of 500 K. In the inner 300 pc of NGC 4418, we estimate a high HC$_{3}$N abundance, of the order of 10$^{-7}$.
Conclusions: The excitation of the HC$_{3}$N molecule responds strongly to the intense radiation field and the presence of warm, dense gas and dust at the center of NGC 4418. The intense HC$_{3}$N line emission is a result of both high abundances and excitation. The properties of the HC$_{3}$N emitting gas are similar to those found for hot cores in Sgr B2, which implies that the nucleus ({\l}t}300 pc) of NGC 4418 is reminiscent of a hot core. The potential presence of a compact, hot component (T = 500 K) is also discussed.}, keywords = {galaxies: active, galaxies: evolution, galaxies: starburst, globular clusters: individual: NGC 4418, ISM: molecules, radio lines: ISM}, doi = {10.1051/0004-6361/200913370}, url = {http://adsabs.harvard.edu/abs/2010A\%26A...515A..71C}, author = {F. Costagliola and Aalto, S.} } @article {2010ApJ...714L.271B, title = {{VLBI Detection of an Active Galactic Nucleus Pair in the Binary Black Hole Candidate SDSS J1536+0441}}, journal = {The Astrophysical Journal Letters}, volume = {714}, year = {2010}, month = {may}, pages = {L271-L274}, abstract = {We present the first pc-scale radio imaging of the radio-quiet candidate binary black hole system SDSS J1536+0441. The observations were carried out by the European VLBI Network at a frequency of 5 GHz, allowing the imaging of SDSS J1536+0441 with a resolution of \~{}10 mas (\~{}50 pc). Two compact radio cores are detected at the position of the kpc-scale components VLA-A and VLA-B, proving the presence of two compact active nuclei with radio luminosity L$_{R}$ \~{} 10$^{40}$ erg s$^{-1}$, thus ruling out the possibility that both radio sources are powered by a 0.1 pc binary black hole. From a comparison with published 8.5 GHz flux densities, we derived an estimate of the radio spectral index of the two pc-scale cores. Both cores have a flat or an inverted spectral index, and at least for the case of VLA-A we can rule out the possibility that synchrotron self-absorption is responsible for the inverted radio spectrum. We suggest that thermal free-free emission from an X-ray-heated disk wind may be powering the radio emission in VLA-A.}, keywords = {galaxies: active, quasars: individual: SDSS J153636.22+044127.0, radio continuum: general}, doi = {10.1088/2041-8205/714/2/L271}, url = {http://adsabs.harvard.edu/abs/2010ApJ...714L.271B}, author = {Bondi, M. and M. A. P{\'e}rez-Torres} } @article {2010MNRAS.407L...6V, title = {{VLT and GTC observations of SDSS J0123+00: a type 2 quasar triggered in a galaxy encounter}}, journal = {Monthly Notices of the Royal Astronomical Society}, volume = {407}, year = {2010}, month = {sep}, pages = {L6-L10}, abstract = {We present long-slit spectroscopy, continuum and [OIII] {${\l}ambda$}5007 imaging data obtained with the Very Large Telescope and the Gran Telescopio Canarias of the type 2 quasar SDSS J0123+00 at z = 0.399. The quasar lies in a complex, gas-rich environment. It appears to be physically connected by a tidal bridge to another galaxy at a projected distance of \~{}100kpc, which suggests this is an interacting system. Ionized gas is detected to a distance of at least \~{}133kpc from the nucleus. The nebula has a total extension of \~{}180kpc. This is one of the largest ionized nebulae ever detected associated with an active galaxy. Based on the environmental properties, we propose that the origin of the nebula is tidal debris from a galactic encounter, which could as well be the triggering mechanism of the nuclear activity. SDSS J0123+00 demonstrates that giant, luminous ionized nebulae can exist associated with type 2 quasars of low radio luminosities, contrary to expectations based on type 1 quasar studies. Based on observations carried out at the European Southern Observatory (Paranal, Chile) with FORS2 on VLT-UT1 [programme 69.A-0123(A)] and at the Observatory Roque de los Muchachos (La Palma, Spain) with OSIRIS on GTC (programme GTC27-09A). E-mail: montse@iaa.es}, keywords = {galaxies: ISM, quasars individual: SDSS J012341.47+004435.9, quasars: emission lines}, doi = {10.1111/j.1745-3933.2010.00890.x}, url = {http://adsabs.harvard.edu/abs/2010MNRAS.407L...6V}, author = {Villar-Mart{\'\i}n, M. and Tadhunter, C. and P{\'e}rez, E. and Humphrey, A. and Mart{\'\i}nez-Sansigre, A. and Delgado, R.~G. and M. A. P{\'e}rez-Torres} }