Hunting for elusive multi-messenger transients with INTEGRAL

Recent years have been marked by the accelerated development of time domain astronomy, owing to the evolution of observational techniques in wide-field rapid optical and radio surveys, as well as the development of the computing resources and methods necessary to promptly interpret overwhelmingly rich observational data. In addition, new kinds of observatories dedicated to gravitational waves and neutrinos have reached a degree of maturity that has enabled breakthrough multi-messenger observations of previously undetected fundamental physical processes in space. Short and energetic events are generally easier to separate from the background than persistent sources, and this is why the first truly multi-messenger signals were also detected as transient events. The properties or even the very nature of many of the new classes of transient sources remain obscure: they are typically associated with peculiar supernovae, mergers of compact objects, or tidal disruption events. In order to decipher the complex physical processes at play in these exceptional systems, it is vital to collect exhaustive and diverse observations of the source, and in particular, observations in different energy bands. Among the most interesting, and yet the most challenging is the observation of the hard X-ray and gamma-ray emission: it often reveals a distinct emission component that points to the most dense and energetic regions at the heart of the source. The INTEGRAL spacecraft is especially well-equipped to observe unpredictable, short-lived, and energetic hard X-ray and gamma-ray transients. It carries a collection of detectors that monitor the entire sky with over 80% duty cycle and simultaneously perform deep and sensitive observations of a large sky region spanning 30x30 degrees. Interpreting the observations collected by these instruments is challenging, especially when the analysis has to be done with real-time efficiency, a critical element needed for the community follow-up of these often quickly fading transients. I will highlight recent pioneering observations of short energetic transients made with INTEGRAL. I will begin by discussing the observations of gamma-ray bursts, in particular in association with the gravitational wave events and high-energy neutrinos. Then, I will review how INTEGRAL observations of fast hard X-ray transients helps to reveal mechanisms at the core of the some of short energetic transients likely associated with the deaths of the massive stars. Finally, I will discuss how the recent discoveries in the domain of multi-messenger transients were made possible by a global effort to achieve a new degree of automation and interoperability.