Frequent Observations of Extragalactic Compact Sources At 24 Ghz (Quasars, Radioastronomy, Bl Lacertae).

Abstract

The opacity of the synchrotron process believed responsible for the radio emission of extragalactic compact sources decreases with frequency, hence observations made at higher frequencies see deeper into these objects and give more information on the physical processes responsible for their emission. The maximum operating frequency of the University of Michigan 26-meter paraboloid radio telescope was increased from 14.5 to 24 GHz to make such higher-frequency observations. To achieve this a radiometer and feed system was developed and the antenna was calibrated. The aperture efficiency of this antenna at 24 GHz was low (7.3%), but gain stability and pointing accuracy were sufficient to carry out measurements with an accuracy of 3 percent. Over a period of 20 months observations were made, at approximately weekly intervals, of 16 variable compact extragalactic radio sources (nine QSOs, four BL Lacertae objects and one Seyfert galaxy). The results ranged from sources exhibiting no evidence of variability to OJ 287 which exhibited a variation of 40% in ten days. In most of the sources observed in this program the flux density variability at 24 GHz was of larger amplitude and shorter timescale than at the next highest frequency, 14.5 GHz, and events clearly defined at 24 GHz were often not evident at all in the 4.8 GHz data. The BL Lacertae objects were generally more variable than the QSOs. The spectra for six slowly-varying sources were generally consistent with the predictions of the homogeneous exp and ing synchrotron source model. The QSO 3C 273 exhibited increasing time delays between flux density peaks and a smearing out of the flux density verses time profiles with decreasing frequency, in qualitative agreement with this model. The BL Lacertid OJ 287 showed a series of short timescale ((tau) (TURN) 14 (+OR-) 5 days) outbursts that are best described by prolonged particle injection into a small region of the source resulting in a particle density decreasing with distance from the region of injection. BL Lacertae showed undulations, simultaneous at all four frequencies, that can be explained by in-situ acceleration of an ensemble of electrons.