A 15.7-min periodicity in OJ287

The BL Lac-type object OJ287, with a redshift z = 0.306 and a luminosity exceeding 1047 erg s−1 at maximum light, is among the brightest and most violently variable objects of its class. Rapid variations, with timescales of a few hours or less, have been reported over a wide range of wavelengths1,2 and also in polarization3, imposing strong constraints on the size and dynamics of the source4,5. The detection of periodic variations in OJ287, or in any other compact extragalactic source, would be even more important, as these could be related to rotation or orbital motion in the source. So far the only searches sensitive to periodic variations shorter than 1 h are the photometric observations of Visvanathan and Elliot6, Frohlich et al.7 and Kiplinger8, in which a 40-min period in OJ287 was respectively found, confirmed and not detected. We report here the detection of a 15.7-min periodic flux variation in OJ287 in April 1981 at 37 GHz. The source has been subsequently monitored for 265 h at 22 and 37 GHz, as well as in optical photometry, and the same period has been seen in simultaneous observations. The peak–peak amplitude of the variation is 1–10% of the total intensity. Other periods may also exist.