Changing-look active galactic nuclei candidates at z ≤ 0.02 in the 105-month swift-BAT catalogue

Changing-look active galactic nuclei (CL AGN) are objects that exhibit dramatic spectral and flux changes within a timescale of years or decades. In this paper, we study a sample of AGN located at z ≤ 0.0200 in the 105-month Swift-BAT catalogue to identify CL AGN within the volume using their hard X-ray lightcurves. Among the 174 AGN (Seyfert 1 & 2) within the sample, we find that 27 i.e., (15.51 ± 0.03) %, show statistically significant flux changes suggesting that they are candidates for CL AGN. We then investigate the possible factors that could cause the changes seen in these AGN, based on the pattern of their X-ray lightcurves, and past studies. We find for one object, tidal disruption event is likely the cause. For three other candidates, patchy torus distribution could be the factor, which is in contradiction with the AGN unified model as this model suggests that AGN has a smooth and uniform distribution of torus. However, more data especially multiwavelength data are required in order to confirm all of these CL AGN candidates are indeed CL AGN, and in order to study the variation factors in more detail.