Probing the extragalactic fast transient sky at minute time-scales with DECam
Lien, A.; Cucchiara, A.; Wolf, C.; Rest, A.; Möller, A.; Abbott, T. M. C.; Onken, C. A.; Flynn, C.; Moriya, T. J.; Bhandari, S.; Ravasio, M. E.; Jankowski, F.; Andreoni, I.; Caleb, M.; Ryder, S.; Cooke, J.; Pritchard, T.; Petroff, E.; Keane, E. F.; Parthasarathy, A.; Vohl, D.; Farah, W.; Chang, S. -W.; Price, D. C.; Webb, S.
Australia, United States, United Kingdom, Italy, Chile, US Virgin Islands, Japan, Netherlands
Abstract
Searches for optical transients are usually performed with a cadence of days to weeks, optimized for supernova discovery. The optical fast transient sky is still largely unexplored, with only a few surveys to date having placed meaningful constraints on the detection of extragalactic transients evolving at sub-hour time-scales. Here, we present the results of deep searches for dim, minute-time-scale extragalactic fast transients using the Dark Energy Camera, a core facility of our all-wavelength and all-messenger Deeper, Wider, Faster programme. We used continuous 20 s exposures to systematically probe time-scales down to 1.17 min at magnitude limits g > 23 (AB), detecting hundreds of transient and variable sources. Nine candidates passed our strict criteria on duration and non-stellarity, all of which could be classified as flare stars based on deep multiband imaging. Searches for fast radio burst and gamma-ray counterparts during simultaneous multifacility observations yielded no counterparts to the optical transients. Also, no long-term variability was detected with pre-imaging and follow-up observations using the SkyMapper optical telescope. We place upper limits for minute-time-scale fast optical transient rates for a range of depths and time-scales. Finally, we demonstrate that optical g-band light-curve behaviour alone cannot discriminate between confirmed extragalactic fast transients such as prompt GRB flashes and Galactic stellar flares.