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Przejdź do strony domowej biblioteki Pedagogiczna Biblioteka Wojewódzka im. Komisji Edukacji Narodowej w Lublinie Link otwiera się w nowym oknie Logo biblioteki Prolib Integro - strona głównaPedagogiczna Biblioteka Wojewódzka im. Komisji Edukacji Narodowej w Lublinie
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Wyświetlanie 1-10 z 19
Tytuł:
AGN duty cycle estimates for the ultra-steep spectrum radio relic VLSS J1431.8+1331
Autorzy:
Brienza, M.
Shulevski, A.
Barthel, P. D.
Vilchez, N.
Miley, G.
Rafferty, D. A.
White, G. J.
Morganti, R.
Bîrzan, L.
Mahony, E.
Harwood, J. J.
Brüggen, M.
Jamrozy, Marek
Chyży, Krzysztof
Brunetti, G.
Röttgering, H. J. A.
van Weeren, R. J.
Wise, M. W.
Kunert-Bajraszewska, M.
de Gasperin, F.
Horellou, C.
Conway, J.
Opis:
Context. Steep spectrum radio sources associated with active galactic nuclei (AGN) may contain remnants of past AGN activity episodes. Studying these sources gives us insight into the AGN activity history. Novel instruments like the LOw Frequency ARray (LOFAR) are enabling studies of these fascinating structures to be made at tens to hundreds of MHz with sufficient resolution to analyse their complex morphology. Aims: Our goal is to characterize the integrated and resolved spectral properties of VLSS J1431+1331 and estimate source ages based on synchrotron radio emission models, thus putting constraints on the AGN duty cycle. Methods: Using a broad spectral coverage, we have derived spectral and curvature maps, and used synchrotron ageing models to determine the time elapsed from the last time the source plasma was energized. We used LOFAR, Giant Metrewave Radio Telescope (GMRT) and Jansky Very Large Array (VLA) data. Results: We confirm the morphology and the spectral index values found in previous studies of this object. Based on our ageing analysis, we infer that the AGN that created this source currently has very low levels of activity or that it is switched off. The derived ages for the larger source component range from around 60 to 130 Myr, hinting that the AGN activity decreased or stopped around 60 Myr ago. We observe that the area around the faint radio core located in the larger source component is the youngest, while the overall age of the smaller source component shows it to be the oldest part of the source. Conclusions: Our analysis suggests that VLSS J1431.8+1331 is an intriguing, two-component source. The larger component seems to host a faint radio core, suggesting that the source may be an AGN radio relic. The spectral index we observe from the smaller component is distinctly flatter at lower frequencies than the spectral index of the larger component, suggesting the possibility that the smaller component may be a shocked plasma bubble. From the integrated source spectrum, we deduce that its shape and slope can be used as tracers of the activity history of this type of steep spectrum radio source. We discuss the implications this conclusion has for future studies of radio sources having similar characteristics.
Dostawca treści:
Repozytorium Uniwersytetu Jagiellońskiego
Artykuł
Tytuł:
Subarcsecond international LOFAR radio images of the M82 nucleus at 118 MHz and 154 MHz
Autorzy:
Jackson, N.
Wucknitz, O.
Beswick, R.
Haverkorn, M.
Miley, G. K.
Deller, A. T.
Jarvis, M. J.
Moldón, J.
Heald, G.
Toribio, M. C.
White, G. J.
Beck, R.
Carozzi, T. D.
Adebahr, B.
Brüggen, M.
Chyży, Krzysztof
Aalto, S.
Brunetti, G.
Röttgering, H. J. A.
Pérez-Torres, M. A.
Dettmar, R.-J.
Morabito, L. K.
Horellou, C.
Martí-Vidal, I.
Varenius, E.
Conway, J. E.
Best, P.
van Driel, W.
Opis:
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 (${0.3}′′$) and sensitivity ($\sigma = 0.15 mJy/beam$) at low frequencies (<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.
Dostawca treści:
Repozytorium Uniwersytetu Jagiellońskiego
Artykuł
Tytuł:
The LOFAR LBA Sky Survey : II. First data release
Autorzy:
Offringa, A.
Mingo, B.
Norden, M. J.
Bonafede, A.
Asabere, B.
Miley, G.
Zarka, P.
Dijkema, T. J.
Callingham, J. R.
Iacobelli, M.
Schwarz, D. J.
Brüggen, M.
Oei, M. S. S. L.
Chyży, Krzysztof
Brunetti, G.
Prandoni, I.
Röttgering, H. J. A.
Cuciti, V.
van Weeren, R. J.
Williams, W. L.
Bomans, D. J.
de Gasperin, F.
Kadler, M.
Hardcastle, M. J.
Shimwell, T.
Cassano, R.
Edler, H. W.
Emig, K. L.
Botteon, A.
Tasse, C.
Opis:
Context. The Low Frequency Array (LOFAR) is the only existing radio interferometer able to observe at ultra-low frequencies (<100 MHz) with high resolution (<15") and high sensitivity (<1 mJy beam$^{-1}$). To exploit these capabilities, the LOFAR Surveys Key Science Project is using the LOFAR Low Band Antenna (LBA) to carry out a sensitive wide-area survey at 41–66 MHz named the LOFAR LBA Sky Survey (LoLSS). Aims. LoLSS is covering the whole northern sky above declination 24° with a resolution of 15" and a sensitivity of 1–2 mJy beam$^{-1}$ (1σ) depending on declination, field properties, and observing conditions. Here we present the first data release, including a discussion of the calibration strategy and the properties of the released images and catalogues. Methods. A fully automated pipeline was used to reduce the 95 fields included in this data release. The data reduction procedures developed for this project have a general application and are currently being used to process almost all LOFAR LBA interferometric observations. Compared to the preliminary release, direction-dependent errors have been derived and corrected for during the calibration process. This results in a typical sensitivity of 1.55 mJy beam$^{-1}$, which is four times better than for the preliminary release, at the target resolution of 15". Results. The first data release of the LOFAR LBA Sky Survey covers 650 deg$^{2}$ in the HETDEX spring field. The resultant data products released to the community include mosaic images (I and V Stokes) of the region, and a catalogue of 42 463 detected sources and related Gaussian components used to describe the sources' morphologies. Separate catalogues for the six in-band frequencies of 44, 48, 52, 56, 60, and 64 MHz are also released. Conclusions. The first data release of LoLSS shows that, despite the influences of the ionosphere and radio frequency interference, LOFAR can conduct large-scale surveys in the frequency window 42-66 MHz with unprecedentedly high sensitivity and resolution. The data can be used to derive unique information on the low-frequency spectral properties of many thousands of sources with a wide range of applications in extragalactic and galactic astronomy.
Dostawca treści:
Repozytorium Uniwersytetu Jagiellońskiego
Artykuł
Tytuł:
LOFAR Facet Calibration
Autorzy:
Best, P. N.
Bonafede, A.
Croston, J. H.
Orrú, E.
Miley, G. K.
Jarvis, M. J.
Rafferty, D. A.
Heald, G.
Ogrean, G. A.
Sridhar, S. S.
White, G. J.
Bîrzan, L.
Dijkema, T. J.
Murray, S. S.
Andrade-Santos, F.
Jones, C.
Brüggen, M.
van der Tol, S.
Sarazin, C. L.
Forman, W. R.
Chyży, Krzysztof
Sabater, J.
Intema, H. T.
Brunetti, G.
Dawson, W. A.
Ensslin, T.
Röttgering, H. J. A.
Simionescu, A.
van Weeren, R. J.
Rudnick, L.
Williams, W. L.
Ferrari, C.
de Gasperin, F.
Horellou, C.
Shimwell, T. W.
Hoeft, M.
Hardcastle, M. J.
Mevius, M.
Pizzo, R.
Kraft, R. P.
Cassano, R.
Stroe, A.
Opis:
LOFAR, the Low-Frequency Array, is a powerful new radio telescope operating between 10 and 240 MHz. LOFAR allows detailed sensitive high-resolution studies of the low-frequency radio sky. At the same time LOFAR also provides excellent short baseline coverage to map diffuse extended emission. However, producing high-quality deep images is challenging due to the presence of direction-dependent calibration errors, caused by imperfect knowledge of the station beam shapes and the ionosphere. Furthermore, the large data volume and presence of station clock errors present additional difficulties. In this paper we present a new calibration scheme, which we name facet calibration, to obtain deep high-resolution LOFAR High Band Antenna images using the Dutch part of the array. This scheme solves and corrects the direction-dependent errors in a number of facets that cover the observed field of view. Facet calibration provides close to thermal noise limited images for a typical 8 hr observing run at ∼ 5'' resolution, meeting the specifications of the LOFAR Tier-1 northern survey.
Dostawca treści:
Repozytorium Uniwersytetu Jagiellońskiego
Artykuł
Wyświetlanie 1-10 z 19

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