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P. Kharb D. V. Lal V. Singh J. Bagchi C. H. Ishwara Chandra A. Hota C. Konar Y. Wadadekar P. Shastri M. Das K. Baliyan B. B. Nath M. Pandey-Pommier 《Journal of Astrophysics and Astronomy》2016,37(4):34
We present detailed science cases that a large fraction of the Indian AGN community is interested in pursuing with the upcoming Square Kilometre Array (SKA). These interests range from understanding low luminosity active galactic nuclei in the nearby Universe to powerful radio galaxies at high redshifts. Important unresolved science questions in AGN physics are discussed. Ongoing low-frequency surveys with the SKA pathfinder telescope GMRT, are highlighted. 相似文献
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Optically violent variable quasars and BL Lac objects, which form a subclass of active galactic nuclei are termed as Blazars.
Blazars show high degree variation in their flux and polarization in a relatively shorter period of time. They are strong
radio sources with a non-stellar continuum. Most of the BL Lac objects do not show any emission lines though in some cases
very weak lines are observed. They harbour in elliptical galaxies. Some BL Lacs show superluminal motion indicating the flow
of matter at relativistic speeds close to the line of sight. It is likely that the enormous energy produced by these objects
may be due to a massive black hole with an accretion disc system. Some Blazars have been observed to detect short time variations
in their flux and polarization in the visual band. These studies are aimed at understanding the physical processes taking
place in the central engine. Detecting shorter period variations assumes significance as they impose restrictions on the total
volume from where the huge amount of energy is comming out. To date the recorded shortest period variation in visual band
is 6.3 min in OJ 287, a BL Lac object. Attempts are being made to detect shorter period variations, which pose observational
challenges. It is generally accepted that the central engine of AGN harbours a massive blackhole with an accretion disc surrounding
it.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
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We made polarimetric observations of comet Hale–Bopp covering awide phase angle range, from 18.8 to 47°. At certain phase angles the heliocentricdistance of the comet was less than 1 AU during its pre and post perihelion passages. Oneof the important findings, based on the data in the visual bands, is the higher polarizationwith stronger wavelength dependence compared to comet Halley, indicating the presenceof much finer grains in comet Hale–Bopp. It may also be noted that comet Hale–Bopphas shown highest degree of polarization known so far for any comet and hence fallsin the class of high polarization comets. Polarimetric observations were made of cometC/2000 WM1 (LINEAR)using narrow band (IHW) filters 4845 Å and 7000 Åand broad bands filters BVR during November 23–26, 2001 when the phase angle rangedfrom 15 to 22°. Some of the results based on these observations are presented anddiscussed. 相似文献
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The continuum spectrum of OJ 287, like most other BL Lac objects, is featureless- no emission or absorption lines are observed. However, OJ 287 shows variations at different timescales in flux and polarization at various wavelength bands. Using the available variability data one can estimate the sizes of the emission regions in the source from light travel time arguments. We assume the emission mechanism to be synchrotron radiation by high energy electrons with single power law energy distribution. Theoretical synchrotron spectrum in the frequency range 1011–1017
H
z
is compared with the observed spectral shape, obtained from new multifrequency quasi-simultaneous observations, to estimate the lower and upper cut off frequencies. These frequencies are used to obtain theoretical values of the variability timescales and magnetic field in the emission region. We obtain a value of 0.93 G for the magnetic field and 5.184×104
sec for the cooling time from the quiescent continuum spectrum. The shock-in-jet model explains the spectrum where shocks accelerate the particles and amplify the magnetic field in the jet. This timescale is compared with the one obtained from observed short timescale variability (20 minutes) with proper beaming correction. The short timescale variations (200 minutes in the source frame), possibly caused by an additional, flaring, component of the source, are also used to calculate compressed magnetic field. The observed and theoretically estimated variability timescales and the shape of the spectrum suggest that there are more than one emission components in OJ 287. 相似文献
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