2019MNRAS.483.3036O

For three radio-loud γ-ray detected narrow-line Seyfert 1 (γ-ray NLSy1) galaxies, we report optical variability on intra-night and/or week-like time-scales, based on five >= 3 h long monitoring sessions for each galaxy. The radio-loudness factors (R_1.4 GHz_)¹ for these galaxies, namely 1H 0323+342 (z = 0.0629), PKS J1222+0413 (z = 0.966), and PKS J1505+0326 (z = 0.408) are ∼318, ∼1534, and ∼3364 at 1.4 GHz, respectively. For the most distant γ-ray NLSy1, PKS J1222+0413, intra-night optical variability (INOV) characterization is presented for the first time. The blazar-like behaviour of the nearest γ-ray NLSy1, 1H 0323+342, which showed strong INOV on four of the five nights, was unexpected in view of its recent reclassification as radio intermediate (R_ 5 GHz_ <= 25). Its particularly violent INOV is manifested by two optical outbursts lasting ∼1 h, whose rapid brightening phase is shown to imply a doubling time of ∼1 h for the optical synchrotron flux, after (conservatively) deducting the thermal optical emission contributed by the host galaxy and the Seyfert nucleus. A more realistic decontamination could well reduce substantially the flux doubling time, bringing it still closer in rapidity to the ultra-fast VHE (>100 GeV) flares reported for the blazars PKS 1222+216 and PKS 2155-304. A large contamination by thermal optical emission may, in fact, be common for NLSy1s as they are high Eddington rate accretors. This study further suggests that superluminal motion in the radio jet could be a robust diagnostic of INOV.