An optical transient candidate of ≲2 s duration captured by wide-field video observations

Abstract Recent time-domain surveys in the optical have revealed rapid transients that evolve on timescales of ${\lesssim }10$ d, expanding the population of transients toward the short-duration regime. The transient search on even shorter timescales, particularly those lasting only seconds, or less, remains a largely unexplored frontier, offering the significant potential for discovering objects from unexpected populations. Very short-duration optical transients could be potential counterparts to millisecond-duration fast radio bursts (FRBs), providing clues about their origins. However, the optical search for transients on such short timescales has been limited primarily due to instrumental constraints. Here, we report the discovery of an optical transient candidate (TMG20200322) with a duration of ${\lesssim }2\ {\rm s}$ by wide-field video observations in the direction of the Earth’s shadow. TMG20200322 was detected in just two consecutive images of 1-s exposure time, with its shape becoming elongated in the second frame. PSF shape variability analysis of the field stars reveals such an elongated PSF cannot be explained by atmospheric fluctuations. We investigate the potential origins of TMG20200322 in two scenarios: meteoroid impact flashes on near-Earth asteroids (NEAs) and head-on meteors in the Earth’s atmosphere. None of the scenarios provides a satisfactory explanation for this transient. We derive a sky-projected rate of the TMG20200322 event to be $R_{\rm trans} = (3.4 \times 10^{-2})^{+0.13}_{-0.028} \ {\rm deg^{-2}\ d^{-1}}$ and an upper limit of second-timescale transients with durations of $1 \rm \ s \le \tau \lesssim 15 \rm \ s$ to be $R_{\rm trans} \lesssim 0.10\ {\rm deg^{-2}\ d^{-1}}$ for the non-detection case. We highlight that continuous monitoring observations in the direction of the Earth’s shadow could be a key strategy to unveiling a new population of optical transients on timescales of seconds or less.