WSRT Observations of hourly flux variations in OJ287 at 6cm wavelength
The Impact of VLBI on Astrophysics and Geophysics
Observations of short term flux variations provide one of the few means of determining the smallest scale sizes in compact radio sources. The fastest variations are generally found in BL Lac-type objects, 0J287 being one of the best studied. In 1985 Valtaoja et al. (Nature 31_4, 148) found evidence for variations at a level of (typically) a few % with a period of 15^7 in OJ287 at frequencies of 22 and 37 GHz. [Such periodic variability was, however, not detected at 5 GHz during one of their
... ng one of their observations by Dreher et al. (Nature 3_2_0, 239, 1 986)]. In order to confirm and possibly improve on the reported results we have made two series of observations of 0J287, in the fall of 1 985 and the fall of 1 986, using the WSRT at 4.87 1 * GHz. The September 1985 observations revealed a very dramatic drop in brightness and the 1986 observations were set up with sufficient calibration to ascertain the limiting factors of the WSRT for flux monitoring work. Contacts with D. Roberts (Brandeis) and J. Dreher (MIT) led to simultaneous coverage at the VLA for as much overlap as was possible. ^ In 1985 OJ287 was observed for three periods of 12 on September 30 and November 10 and 16. The 1986 observations of OJ287 formed part of an 8x12 project involving the sources 3C84, OJ287, CTD93 and BL Lac. Each source was observed twice with the^expectation that, all things being equal, a division of successive 12 lightcurves would eliminate all systematic, stable, system imperfections. The observations on 3C84, a supposedly nonvariable source, shows that th^s is indeed the case to an rms accuracy of 0.2-0.3? on scales of 1 -12 . A total band of 80 MHz wide, in 8x10 MHz sub-bands, centered at se c 4874 MHz was recorded with a time resolution of 20 . The final sec resolution used was 60 . All combinations between two dipoles oriented at 0° and 90° position angle, were correlated. System temperature and gain calibration was done by firing a noise source every 10 . The theoretical noise level, for 1 integration, was about 2 mJy, or 0.04? of the source flux density. The actual accuracy achieved was about 5 times worse for reasons still not understood. We wished to obtain a continuous record of data on OJ287, and split-array observing being not (yet) possible, we could not switch to a flux reference at rapid intervals. [Because suitable reference sources 97 M. J. Reid and J. M. Moron (eds.), The Impact of VLBI on Astrophysics and Geophysics, 97-98. ©1988 by the IAU. available at https://www.cambridge.org/core/terms. https://doi.