# Probing the extragalactic fast transient sky at minute timescales with DECam

## TL;DR

This study conducted deep, minute-timescale optical transient searches using DECam, detecting hundreds of sources and constraining the rates of extragalactic fast transients, while finding most candidates to be stellar flares and no counterparts to other wavelengths.

## Contribution

It presents the first systematic deep search for dim, minute-timescale extragalactic optical transients with DECam, setting upper limits on their occurrence rates.

## Key findings

- Detected hundreds of variable sources, mostly stellar flares.
- Found no optical counterparts to fast radio bursts or gamma-ray events.
- Placed upper limits on the rates of minute-timescale extragalactic transients.

## Abstract

Searches for optical transients are usually performed with a cadence of days to weeks, optimised 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 timescales. Here, we present the results of deep searches for dim, minute-timescale 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 20s exposures to systematically probe timescales down to 1.17 minutes 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 multi-band imaging. Searches for fast radio burst and gamma-ray counterparts during simultaneous multi-facility 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-timescale fast optical transient rates for a range of depths and timescales. 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.