@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 {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 {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.} }