Follow-up of transients in the 2030s

Followup observations of transients has been a rapidly evolving field during the life time of the VLT. The ESO community has played a prominent role in this field, not the least by utilising X-shooter. New opportunities through space missions continue to emerge, and in the 2030 Gravity Wave observatories will change the field dramatically. Yet, the ELT will not for the first many years of operation be equipped with instrumentation well suited for contributing in this field. Notably, a visible spectrograph with high throughput will not be available. Detector technologies, higher efficiency optical coatings and AO assisted operation will allow to push the limits of the VLT further than what was achieved with X-shooter. Particularly, lower read noise/photon counting detectors, will allow to push towards medium-high resolution (∼30.000), where an instrument will also become relevant for stellar spectroscopy. We propose to study an instrument concept with similar wavelength coverage as X-shooter, but with a single slit, higher resolution and with a goal of reaching 50% efficiency. A limited multi-object capability, allowing observing of a handful of objects, using periscopes, over the ∼20' FoV of a VLT UT will allow pushing the limits of stellar spectroscopy. Specifically, it will become possible to do transmission spectroscopy of exoplanets, while simultaneously observing similar bright comparison stars, and without any modal noise, which limits spectroscopy through fibers.