Variability of Extragalactic Radio Sources at Low Frequencies

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Session 109 -- Extragalactic Radio Sources, Jets
Display presentation, Thursday, 12, 1995, 9:20am - 6:30pm

[109.04] Variability of Extragalactic Radio Sources at Low Frequencies

Daniel R. Altschuler (NAIC/Arecibo Observatory), Jose F. Salgado (Department of Astronomy, The University of Michigan), Brian K. Dennison (Department of Physics, Virginia Polytechnic Institute and State University), Tapasi Ghosh (NAIC/Arecibo Observatory)

We have obtained 318 and 430 MHz "light curves" for 33 extragalactic radio sources over a period of 13 yr. The measurements were made with the Arecibo 305-m radio telescope at approximately bimonthly intervals from Jan 1980 to Feb 1989, and less regularly between Oct 1989 and Oct 1993, for a total of 64 observing sessions. Several sources show substantial variability, as presented here. In particular, we briefly discuss the remarkable behavior found for the source 2050+364. Between about 1985.75 and 1987.75, the 430-MHz flux density of this source suddenly decreased from a relatively steady value of 3.3 Jy to a roughly constant level of 2.25 Jy, later returning to its original value. There was no similar behavior at 318 MHz, nor at higher frequencies (4.8, 8.0 and 14.5 GHz) as observed at Michigan during the same period. This optically-unidentified source is the sole example in our sample to be situated at low galactic latitude (l=78.9, b=-5.1) and has previously been extensively studied in relation to the effect of scattering by the ISM on its milliarcsec-scale structure. These investigations have led to the conclusion that the image size of 2050+364 scales approximately as lambda$^2$, as expected if it is scatter-broadened by the intervening plasma turbulence. Indeed, recent VLBI measurements at 327 MHz reveal an angular size of 81 mas and strengthen the case for this effect. The line of sight to this source passes through the extremely-complicated "Cygnus Super-bubble" region, which is known to consists of hot x-ray gas, radio continuum and line sources, H-alpha filaments and high obscuration by dust. We attempt to explain the prolonged flux-density minimum observed in the 430-MHz light curve, and the absence of any significant variation at 318 MHz, as due to refractive effects from some of these intervening components in the ISM.

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