C. M. V. Impellizzeri — Page 3

arXiv2008

6.7 GHz methanol absorption toward the Seyfert 2 galaxy NGC 3079

The detection of the 6.7 GHz line of methanol (CH3OH) is reported for the first time toward an object beyond the Magellanic Clouds. Using the Effelsberg 100 m telescope, two absorption features were identified toward the Seyfert 2 galaxy NGC 3079. Both components probably originated on lines-of-sight toward the central region, presumably absorbing the radio continuum of the nuclear sources A, B, and E of NGC 3079. One absorption feature, at the systemic velocity, is narrow and may arise from gas not related to the nuclear environment of the galaxy. The weaker blue-shifted component is wider and may trace outflowing gas. Total A-type CH3OH column densities are estimated to be between a few times 10^13 and a few times 10^15 cm^-2. Because of a highly frequency-dependent continuum background, the overall similarity of HI, OH, and CH3OH absorption profiles hints at molecular clouds that cover the entire area occupied by the nuclear radio continuum sources ~ 4 pc.

astro-ph↗

arXiv2008

Space VLBI polarimetry of IDV sources: Lessons from VSOP and prospects for VSOP2

To locate and image the compact emission regions in quasars, which are closely connected to the phenomenon of IntraDay Variability (IDV), space VLBI observations are of prime importance. Here we report on VSOP observations of two prominent IDV sources, the BL Lac objects S5 0716+714. To monitor their short term variability, these sources were observed with VSOP at 5 GHz in several polarisation sensitive experiments, separated in time by one day to six days, in autumn 2000. Contemporaneous flux density measurements with the Effelsberg 100m radio telescope were used to directly compare the single dish IDV with changes of the VLBI images. A clear IDV behaviour in total intensity and linear polarization was observed in 0716+714. Analysis of the VLBI data shows that the variations are located inside the VLBI core component of 0716+714. In good agreement with the single-dish measurements, the VLBI ground array images and the VSOP images, both show a decrease in the total flux density of ~20 mJy and a drop of ~5 mJy in the linear polarization of the VLBI core. No variability was found in the jet. From the variability timescales we estimate a source size of a few micro-arcseconds and brightness temperatures exceeding 10^15 K. Independent of whether the interpretation of the IDV seen in the VLBI core is source intrinsic or extrinsic a lower limit of T_B > 2x10^12 K is obtained by model fitting of the VLBI-core. Our results show that future VSOP2 observations should be accompanied by a single dish monitoring not only to discriminate between source-extrinsic and source-intrinsic effects but to allow also a proper calibration and interpretation of ultra-high resolution VSOP2 images.

astro-ph↗