@book {181, title = {{The Spanish Square Kilometre Array White Book}}, series = {ArXiv e-prints}, year = {2015}, keywords = {Astrophysics - Instrumentation and Methods for Astrophysics}, url = {http://adsabs.harvard.edu/abs/2015arXiv150603474A}, author = {Acosta-Pulido, J.~A. and Agudo, I. and Alberdi, A. and Alcolea, J. and Alfaro, E.~J. and Alonso-Herrero, A. and Anglada, G. and Arnalte-Mur, P. and Ascasibar, Y. and Ascaso, B. and Azulay, R. and Bachiller, R. and Baez-Rubio, A. and Battaner, E. and Blasco, J. and Brook, C.~B. and Bujarrabal, V. and Busquet, G. and Caballero-Garcia, M.~D. and Carrasco-Gonzalez, C. and Casares, J. and Castro-Tirado, A.~J. and Colina, L. and Colomer, F. and de Gregorio-Monsalvo, I. and del Olmo, A. and Desmurs, J.-F. and Diego, J.~M. and Dominguez-Tenreiro, R. and Estalella, R. and Fernandez-Soto, A. and Florido, E. and Font, J. and Font, J.~A. and Fuente, A. and Garc{\'\i}a-Benito, R. and Garcia-Burillo, S. and Garc{\'\i}a-Lorenzo, B. and Gil de Paz, A. and Girart, J.~M. and Goicoechea, J.~R. and G{\'o}mez, J.~F. and Gonzalez-Garcia, M. and Gonz{\'a}lez-Mart{\'\i}n, O. and Gonzalez-Serrano, J.~I. and Gorgas, J. and Gorosabel, J. and Guijarro, A. and Guirado, J.~C. and Hern{\'a}ndez-Garc{\'\i}a, L. and Hernandez-Monteagudo, C. and Herranz, D. and R. Herrero-Illana and Hu, Y.-D. and Huelamo, N. and Huertas-Company, M. and Iglesias-Paramo, J. and Jeong, S. and Jimenez-Serra, I. and Knapen, J.~H. and Lineros, R.~A. and Lisenfeld, U. and Marcaide, J.~M. and Marquez, I. and Marti, J. and Marti, J.~M. and Mart{\'\i}-Vidal, I. and Martinez-Gonzalez, E. and Martin-Pintado, J. and Masegosa, J. and Mayen-Gijon, J.~M. and Mezcua, M. and Migliari, S. and Mimica, P. and Mold{\'o}n, J. and Morata, O. and Negueruela, I. and Oates, S.~R. and Osorio, M. and Palau, A. and Paredes, J.~M. and Perea, J. and P{\'e}rez-Gonz{\'a}lez, P.~G. and Perez-Montero, E. and M. A. P{\'e}rez-Torres and Perucho, M. and Planelles, S. and Pons, J.~A. and Prieto, A. and Quilis, V. and Ramirez-Moreta, P. and Ramos Almeida, C. and Rea, N. and Rib{\'o}, M. and Rioja, M.~J. and Rodriguez Espinosa, J.~M. and Ros, E. and Rubi{\~n}o-Martin, J.~A. and Ruiz-Granados, B. and Sabater, J. and Sanchez, S. and Sanchez-Contreras, C. and Sanchez-Monge, A. and Sanchez-Ramirez, R. and Sintes, A.~M. and Solanes, J.~M. and Sopuerta, C.~F. and Tafalla, M. and Tello, J.~C. and Tercero, B. and Toribio, M.~C. and Torrelles, J.~M. and Torres, M.~A.~P. and Usero, A. and Verdes-Montenegro, L. and Vidal-Garcia, A. and Vielva, P. and Vilchez, J. and Zhang, B.-B.} } @article {2015A\&A...574A.114V, title = {{Subarcsecond international LOFAR radio images of the M82 nucleus at 118 MHz and 154 MHz}}, journal = {Astronomy and Astrophysics}, volume = {574}, year = {2015}, month = {feb}, pages = {A114}, abstract = {Context. The nuclear starburst in the nearby galaxy M82 provides an excellent laboratory for understanding the physics of star formation. This galaxy has been extensively observed in the past, revealing tens of radio-bright compact objects embedded in a diffuse free-free absorbing medium. Our understanding of the structure and physics of this medium in M82 can be greatly improved by high-resolution images at low frequencies where the effects of free-free absorption are most prominent.
Aims: The aims of this study are, firstly, to demonstrate imaging using international baselines of the Low Frequency Array (LOFAR), and secondly, to constrain low-frequency spectra of compact and diffuse emission in the central starburst region of M82 via high-resolution radio imaging at low frequencies.
Methods: The international LOFAR telescope was used to observe M82 at 110-126 MHz and 146-162 MHz. Images were obtained using standard techniques from very long baseline interferometry. images were obtained at each frequency range: one only using international baselines, and one only using the longest Dutch (remote) baselines.
Results: The 154 MHz image obtained using international baselines is a new imaging record in terms of combined image resolution (0.3{\textquoteright}{\textquoteright}) and sensitivity ({$\sigma$} = 0.15 mJy/beam) at low frequencies ({\l}t}327 MHz). We detected 16 objects at 154 MHz, six of these also at 118 MHz. Seven objects detected at 154 MHz have not been catalogued previously. For the nine objects previously detected, we obtained spectral indices and emission measures by fitting models to spectra (combining LOFAR with literature data). Four weaker but resolved features are also found: a linear (50 pc) filament and three other resolved objects, of which two show a clear shell structure. We do not detect any emission from either supernova 2008iz or from the radio transient source 43.78+59.3. The images obtained using remote baselines show diffuse emission, associated with the outflow in M82, with reduced brightness in the region of the edge-on star-forming disk.}, keywords = {galaxies: individual: M82, galaxies: star formation, galaxies: starburst, supernovae: individual: 2008iz, techniques: high angular resolution, techniques: interferometric}, doi = {10.1051/0004-6361/201425089}, url = {http://adsabs.harvard.edu/abs/2015A\%26A...574A.114V}, author = {Varenius, E. and Conway, J.~E. and Mart{\'\i}-Vidal, I. and Beswick, R. and Deller, A.~T. and Wucknitz, O. and Jackson, N. and Adebahr, B. and M. A. P{\'e}rez-Torres and Chy{\.z}y, K.~T. and Carozzi, T.~D. and Mold{\'o}n, J. and Aalto, S. and Beck, R. and Best, P. and Dettmar, R.-J. and van Driel, W. and Brunetti, G. and Br{\"u}ggen, M. and Haverkorn, M. and Heald, G. and Horellou, C. and Jarvis, M.~J. and Morabito, L.~K. and Miley, G.~K. and R{\"o}ttgering, H.~J.~A. and Toribio, M.~C. and White, G.~J.} } @article {2014ApJ...792...38P, title = {{Constraints on the Progenitor System and the Environs of SN 2014J from Deep Radio Observations}}, journal = {The Astrophysical Journal}, volume = {792}, year = {2014}, pages = {38}, abstract = {We report deep EVN and eMERLIN observations of the Type Ia SN 2014J in the nearby galaxy M82. Our observations represent, together with JVLA observations of SNe 2011fe and 2014J, the most sensitive radio studies of Type Ia SNe ever. By combining data and a proper modeling of the radio emission, we constrain the mass-loss rate from the progenitor system of SN 2014J to $\backslash$dot$\{$M$\}$ $\backslash$lesssim 7.0$\backslash$times 10\^{}$\{$-10$\}$ $\{$$\backslash$,$\{$M\_$\{$$\backslash$odot $\}$ $\backslash$,yr\^{}$\{$-1$\}$$\}$$\}$ (for a wind speed of 100 km s$^{-1}$). If the medium around the supernova is uniform, then n $_{ISM}$ {\l}t}\~{} 1.3 cm$^{-3}$, which is the most stringent limit for the (uniform) density around a Type Ia SN. Our deep upper limits favor a double-degenerate (DD) scenario{\mdash}involving two WD stars{\mdash}for the progenitor system of SN 2014J, as such systems have less circumstellar gas than our upper limits. By contrast, most single-degenerate (SD) scenarios, i.e., the wide family of progenitor systems where a red giant, main-sequence, or sub-giant star donates mass to an exploding WD, are ruled out by our observations. (While completing our work, we noticed that a paper by Margutti et al. was submitted to The Astrophysical Journal. From a non-detection of X-ray emission from SN 2014J, the authors obtain limits of $\backslash$dot$\{$M$\}$ $\backslash$lesssim 1.2 $\backslash$times 10\^{}$\{$-9$\}$ M $_{\&sun;}$ yr$^{-1}$ (for a wind speed of 100 km s$^{-1}$) and n $_{ISM}$ {\l}t}\~{} 3.5 cm$^{-3}$, for the {$\rho$}vpropr $^{-2}$ wind and constant density cases, respectively. As these limits are less constraining than ours, the findings by Margutti et al. do not alter our conclusions. The X-ray results are, however, important to rule out free-free and synchrotron self-absorption as a reason for the radio non-detections.) Our estimates on the limits on the gas density surrounding SN2011fe, using the flux density limits from Chomiuk et al., agree well with their results. Although we discuss the possibilities of an SD scenario passing observational tests, as well as uncertainties in the modeling of the radio emission, the evidence from SNe 2011fe and 2014J points in the direction of a DD scenario for both.}, keywords = {stars: mass-loss, supernovae: individual: SN2011fe SN2014J}, doi = {10.1088/0004-637X/792/1/38}, url = {http://adsabs.harvard.edu/abs/2014ApJ...792...38P}, author = {M. A. P{\'e}rez-Torres and Lundqvist, P. and Beswick, R.~J. and Bj{\"o}rnsson, C.~I. and Muxlow, T.~W.~B. and Paragi, Z. and Ryder, S. and Alberdi, A. and Fransson, C. and Marcaide, J.~M. and Mart{\'\i}-Vidal, I. and Ros, E. and Argo, M.~K. and Guirado, J.~C.} } @article {219, title = {{The radio core structure of the luminous infrared galaxy NGC 4418. A young clustered starburst revealed?}}, journal = {\aap}, volume = {566}, year = {2014}, pages = {A15}, keywords = {galaxies: individual: NGC 4418, galaxies: Seyfert, galaxies: star formation}, doi = {10.1051/0004-6361/201323303}, author = {Varenius, E. and Conway, J.~E. and Mart{\'\i}-Vidal, I. and Aalto, S. and Beswick, R. and Costagliola, F. and Kl{\"o}ckner, H.-R.} } @article {2014A\&A...566A..15V, title = {{The radio core structure of the luminous infrared galaxy NGC 4418. A young clustered starburst revealed?}}, journal = {Astronomy and Astrophysics}, volume = {566}, year = {2014}, month = {jun}, pages = {A15}, abstract = {Context. The galaxy NGC 4418 contains one of the most compact obscured nuclei within a luminous infrared galaxy (LIRG) in the nearby Universe. This nucleus contains a rich molecular gas environment and an unusually high ratio of infrared-to-radio luminosity (q-factor). The compact nucleus is powered by either a compact starburst or an active galactic nucleus (AGN).
Aims: The aim of this study is to constrain the nature of the nuclear region (starburst or AGN) within NGC 4418 via very-high-resolution radio imaging.
Methods: Archival data from radio observations using the European Very Long Baseline Interferometry Network (EVN) and Multi-Element Radio Linked Interferometer Network (MERLIN) interferometers are imaged. Sizes and flux densities are obtained by fitting Gaussian intensity distributions to the image. The average spectral index of the compact radio emission is estimated from measurements at 1.4 GHz and 5.0 GHz.
Results: The nuclear structure of NGC 4418 visible with EVN and MERLIN consists of eight compact ({\l}t}49 mas i.e. {\l}t}8 pc) features spread within a region of 250 mas, i.e. 41 pc. We derive an inverted spectral index {$\alpha$} {\ge} 0.7 (S$_{nu}$ {\prop} {$ν$}$^{alpha}$) for the compact radio emission.
Conclusions: Brightness temperatures {\gt}10$^{4.8}$ K indicate that these compact features cannot be HII-regions. The complex morphology and inverted spectrum of the eight detected compact features is evidence against the hypothesis that an AGN alone is powering the nucleus of NGC 4418. The compact features could be super star clusters with intense star formation, and their associated free-free absorption could then naturally explain both their inverted radio spectrum and the low radio-to-IR ratio of the nucleus. The required star formation area density is extreme, however, and close to the limit of what can be observed in a well-mixed thermal/non-thermal plasma produced by star formation, and is also close to the limit of what can be physically sustained.}, keywords = {galaxies: individual: NGC 4418, galaxies: Seyfert, galaxies: star formation}, doi = {10.1051/0004-6361/201323303}, url = {http://adsabs.harvard.edu/abs/2014A\%26A...566A..15V}, author = {Varenius, E. and Conway, J.~E. and Mart{\'\i}-Vidal, I. and Aalto, S. and Beswick, R. and F. Costagliola and Kl{\"o}ckner, H.-R.} } @article {2012A\&A...541A.135M, title = {{Over-resolution of compact sources in interferometric observations}}, journal = {Astronomy and Astrophysics}, volume = {541}, year = {2012}, month = {may}, pages = {A135}, abstract = {We review the effects of source size in interferometric observations and focus on the cases of very compact sources. If a source is extremely compact and/or weak (such that it is impossible to detect signatures of source structure in the visibilities), we describe a test of hypothesis that can be used to set a strong upper limit to the size of the source. We also estimate the minimum possible size of a source whose structure can still be detected by an interferometer (i.e., the maximum theoretical over-resolution power of an interferometer), which depends on the overall observing time, the compactness of the array distribution, and the sensitivity of the receivers. As a result, and depending on the observing frequency, the over-resolution power of forthcoming ultra-sensitive arrays, such as the Square Kilometer Array (SKA), may allow us to study the details of sources at angular scales as small as a few {$μ$}as.}, keywords = {instrumentation: interferometers, techniques: high angular resolution, techniques: interferometric}, doi = {10.1051/0004-6361/201118334}, url = {http://adsabs.harvard.edu/abs/2012A\%26A...541A.135M}, author = {Mart{\'\i}-Vidal, I. and M. A. P{\'e}rez-Torres and Lobanov, A.~P.} } @article {2011A\&A...533C...1M, title = {{Coherence loss in phase-referenced VLBI observations (Corrigendum)}}, journal = {Astronomy and Astrophysics}, volume = {533}, year = {2011}, month = {sep}, pages = {C1}, abstract = {We report an erratum in an equation that models the relation of the limiting dynamic range to the angular distance between target and calibrator.}, keywords = {addenda, atmospheric effects, errata, instrumentation: interferometers, techniques: interferometric}, doi = {10.1051/0004-6361/201014203e}, url = {http://adsabs.harvard.edu/abs/2011A\%26A...533C...1M}, author = {Mart{\'\i}-Vidal, I. and Ros, E. and M. A. P{\'e}rez-Torres and Guirado, J.~C. and Jim{\'e}nez-Monferrer, S. and Marcaide, J.~M.} } @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...529A..47M, title = {{Electron cooling and the connection between expansion and flux-density evolution in radio supernovae}}, journal = {Astronomy and Astrophysics}, volume = {529}, year = {2011}, month = {may}, pages = {A47}, abstract = {{Radio supernovae (RSNe) are weak and rare events. Their typical maximum radio luminosities are of the order of only 10$^{27}$ erg s$^{-1}$ Hz$^{-1}$. There are, however, very few cases of relatively bright (and/or close) RSNe, from which the expansion of the shock and the radio light curves at several frequencies have been monitored covering several years. Applying the standard model of radio emission from supernovae, it is possible to relate the defining parameters of the modelled expansion curve to those of the modelled light curves in a simple algebraic way by assuming an evolution law for the magnetic field and for the energy density of the population of synchrotron-emitting electrons. However, cooling mechanisms of the electrons may considerably affect this connection between light curves and expansion curve, and lead to wrong conclusions on the details of the electron acceleration and/or on the CSM radial density profile. We study how electron cooling modifies the flux-density decay rate of RSNe for a set of plausible/realistic values of the magnetic field and for different expansion regimes. We use these results to estimate the magnetic fields of different RSNe observed to date and compare them to those obtained by assuming energy equipartition between particles and magnetic fields. For some of the best monitored RSNe, for which deceleration measurements, optically thin spectral index, and power-law time decay have been observed (SN 1979C, SN 1986J, SN 1993J, and SN 2008iz), we find self-consistent solutions for the index of the power-law circumstellar density profile (s = 2 for all cases), the index of the power-law relativistic electron population (rather steep values}, keywords = {acceleration of particles, radiation mechanisms: nonthermal, radio continuum: stars, supernovae: general}, doi = {10.1051/0004-6361/201016105}, url = {http://adsabs.harvard.edu/abs/2011A\%26A...529A..47M}, author = {Mart{\'\i}-Vidal, I. and M. A. P{\'e}rez-Torres and Brunthaler, A.} } @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.} } @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..53M, title = {{Coherence loss in phase-referenced VLBI observations}}, journal = {Astronomy and Astrophysics}, volume = {515}, year = {2010}, month = {jun}, pages = {A53}, abstract = {Context. Phase-referencing is a standard calibration technique in radio interferometry, particularly suited for the detection of weak sources close to the sensitivity limits of the interferometers. However, effects from a changing atmosphere and inaccuracies in the correlator model may affect the phase-referenced images, and lead to wrong estimates of source flux densities and positions. A systematic observational study of signal decoherence in phase-referencing and its effects in the image plane has not been performed yet.
Aims: We systematically studied how the signal coherence in Very-Long-Baseline-Interferometry (VLBI) observations is affected by a phase-reference calibration at different frequencies and for different calibrator-to-target separations. The results obtained should be of interest for a correct interpretation of many phase-referenced observations with VLBI.
Methods: We observed a set of 13 strong sources (the S5 polar cap sample) at 8.4 and 15 GHz in phase-reference mode with 32 different calibrator/target combinations spanning angular separations between 1.5 and 20.5 degrees. We obtained phase-referenced images and studied how the dynamic range and peak flux-density depend on observing frequency and source separation.
Results: We obtained dynamic ranges and peak flux densities of the phase-referenced images as a function of frequency and separation from the calibrator. We compared our results with models and phenomenological equations previously reported.
Conclusions: The dynamic range of the phase-referenced images is strongly limited by the atmosphere at all frequencies and for all source separations. The limiting dynamic range is inversely proportional to the sine of the calibrator-to-target separation. Not surpriseingly, we also find that the peak flux densities decrease with source separation, relative to those obtained from the self-calibrated images.}, keywords = {atmospheric effects, instrumentation: interferometers, techniques: interferometric}, doi = {10.1051/0004-6361/201014203}, url = {http://adsabs.harvard.edu/abs/2010A\%26A...515A..53M}, author = {Mart{\'\i}-Vidal, I. and Ros, E. and M. A. P{\'e}rez-Torres and Guirado, J.~C. and Jim{\'e}nez-Monferrer, S. and Marcaide, J.~M.} }