Optical Monitoring and Intraday Variabilities of the BL Lac Object PKS 0735+178

Abstract

Within a monitored duration of more than 7 yr, we obtained 1358 pairs of gri observations (514 at the g band, 393 at the r band, and 451 at the i band) for PKS0735+178 with a 1.26 m telescope in 20 nights from 2015 February 24 to 2016 April 28, and 98 VRI observations (27 at the V band, 35 at the R band, and 36 at the I band) with a 1.56 m telescope in 8 nights from 2009 January 13 to 2014 March 17. Based on the observations and analyses, we obtained the following results. (1) The maximum variation was 2.69 mag at the g band, and the largest intraday variations were Δm g = 0.89 ± 0.08 mag at the g band, Δm r = 0.38 ± 0.04 mag at the r band, and Δm i = 0.78 ± 0.15 mag at the i band. (2) We analyzed the intraday observations and find intraday optical variabilities with minimum timescales of ΔT g = 10.08 minutes, ΔT r = 10.08 minutes, and ΔT i = 10.08 minutes. (3) The variability rates of the magnitudes with time were consistent at three bands not only in the dimming stages but also in the brightening stages. (4) On 2016 January 19, we found intraday periodic oscillations, with a period P = 66.9 ± 4.1 minutes, based on which we calculated the mass to be in the approximate range of 1.5 × 107 M ⊙ to ∼8.9 × 107 M ⊙. (5) Based on our data and observations from the literature, we built long-term lightcurves of more than 100 yr and obtained two quasi-periods of P = 0.9 ± 0.4 yr and 12.4 ± 0.6 yr.