Probing Millisecond Optical Timescales with Continuous-Readout Mode Observations from ZTF
The subminute-timescale phase space is an underexplored regime of the optical sky. Sub-second optical observations are difficult to accomplish with traditional CCD exposure-readout cycles, and alternative technologies like EMCCDs and CMOS have historically limited surveys to targeted observation. Continuous-readout mode enables millisecond-timescale observations on traditional cameras by leaving the shutter open during readout, enabling high-cadence observations on all-sky surveys. We present the first results from continuous-readout mode data from the Zwicky Transient Facility. We developed a deep-learning-based data analysis pipeline and report results on 141 square degrees, or 1128 star-hours, of data, sampled at 300x/second. Our Convolutional Neural Network can retrieve optical transients of duration 9.7-29.2 milliseconds with a precision of 66% and recall of 78% for brightness changes of 10% and greater, and our subsequent light curve-based analysis has an efficiency of >94.6%. We retrieve a total of ~300,000 candidates on sources of magnitude G_{RP} < 12, of which 100 are above SNR=1.55 and were visually inspected. After accounting for noise and filtering contaminants both known and unique to this mode of observation, these 100 candidates were eliminated as being astrophysical in nature, leading to upper limits on the observed rate of ~10-ms transient events associated with point sources as well as specifically with white dwarfs and quasars. This first survey-scale characterization of the bright millisecond optical transient and variable sky establishes the rarity of short-timescale variability in the optical regime.