Optical monitoring and intra-day variabilities of BL Lac Objects OJ 287

Abstract

Abstract Using the 1.26 m National Astronomical Observatory-Guangzhou University Infrared/Optical Telescope (NGT), we monitor one BL Lac object, OJ 287. For this source, we obtain 15 094 gri observations (4900 at g band, 5184 at r band and 5010 at i band) in 155 nights from 2014 December 13 to 2019 March 15. Based on the upper observations, we obtain the following results. (1) The total variation amplitude is ∼2.3 mag. (2) There are intra-day variabilities (IDVs). The IDV timescales (Δ T) are in the range from 7.69 min (Δ m = 0.06 ± 0.02 mag) to 371.09 min (Δ m = 0.26 ± 0.04 mag). (3) There are strong correlations between Δ T and Δ m, Δ m = (2.91 ± 0.66) × 10−4ΔT + (0.08 ± 0.009), with r = 0.52, p = 5.33 × 10−5. (4) There are intra-day periods in this source, with the period P ≈ 94 min on 2017 December 10. When we supplement the observations from the literature, we can obtain that the long-term period is about 12.02 ± 0.41 yr. (5) The spectral properties of OJ 287 show the bluer-when-brighter behavior, whatever state the source is at.