Abstract

Gaia is an ESA cornerstone mission, delivering precision photometry, astrometry, spectrophotometry, and spectroscopy, all based on regular image-scanning the sky through two telescopes and the largest focal-plane array (1Gpixel) yet launched. This, by design, makes the time-domain the heart of Gaia’s scientific requirements and capabilities. Gaia is monitoring the sky with cadences of seconds (between CCDs), hours (between fields-of-view) and weeks (between visits), with real-time source detection implemented by a fixed, available, and well understood on-board algorithm. Every Gaia source obtains near-simultaneous spectrophotometry.

The GaiaAlerts system has been running routinely and reliably since January 2016, publishing 6 transients per day (see http://gsaweb.ast.cam.ac.uk/alerts/home), using well-defined selection criteria. We scan the whole sky exploring into the Galactic plane and crowded regions which are typically hard to do from the ground. I will describe the challenges we face in searching through half a billion CCD measurements every day to identify and publish Gaia’s transient events. I will discuss the properties of the alerts published to date and highlight some of our most interesting discoveries. I will examine the completeness and biases in our selection criteria, and look ahead to ongoing updates to the system, including making the most of Gaia’s spectrophotometry and astrometry.