Very Large Array observations of rapid non-periodic variations in OJ 287

Abstract

Time variations in compact extragalactic objects can be used to constrain both their emission mechanisms and models for their geometry. In particular, periodic variations constrain the popular black hole model for these sources^1,2. There have been many reports of the presence (and absence) of both periodic^3–8 and non-periodic⁹ variability in the BL Lacertae object OJ 287 = 0851 + 202, from the optical and infrared through centimetre radio wavelengths (Table 1). The apparent consistency of the reported periodicity near P =15.7 min and the absence of variations in other sources observed in the same way lends some credence to these variations, even though they are only a few per cent of the mean intensity. We observed OJ 287 with the Very Large Array (VLA) at 5, 15 and 22 GHz several times in early 1983. The 5-GHz data, taken coincident with the 22-GHz observations of Valtaoja et al.⁸, show OJ 287 to have decreased in flux by about 1.8% in an irregular way over a span of about 7 h, and to have fluctuated by about 0.5% on a timescale of about 15 min. Brightness temperatures derived from causality constraints range between 10¹⁶ and 10²⁰ K. Fourier spectra of the time series were analysed using a novel one-dimensional complex CLEAN algorithm to remove the effects of the data sampling. At 5 GHz there was no harmonic component to the intensity of OJ 287 which exceeded 0.1% of the mean for any period between 80 and 5,000 s. The limits at 15 and 22 GHz were 2% and 0.8%, respectively.