MSV-2035 Astronomy Document - Inside Design - FINAL - FINAL

Astronomy & Astrophysics 20 2.9 Transients andTime-domainMulti-messengerAstronomy A sizable information about our understanding of the Universe is associated with multi-wavelength observations of different types of astrophysical transients. For example, observations of supernovae (SNe), gamma-ray bursts (GRBs) and Cepheid variables are essential to build the so called “distance ladder” and in turn helpful towards constraining observational cosmology. Studies of transients of various categories are required to understand extreme physics, nucleosynthesis of heavy elements, nature of relativistic shocks, particle accelerations, cosmic-ray production, effects of gravity etc. Recent advancements in observational facilities beyond the electromagnetic spectrum have enabled to generalize the area covering a variety of new astrophysical sources at diverse time scales including interstellar asteroids and comets, exoplanets, accreting black holes, merging compact objects etc. This larger scope along with an increased use of sophisticated software, computation tools and astronomical instrumentation using cutting edge technology have opened a new window to understand transients even beyond electromagnetic spectrum in a much broader perspective i.e. multi-messenger, in terms of resolvingmany open questions in the field ofA&A. Over the last century, systematic searches have led to discoveries of transients from a few to thousands in number. Recent, automated survey programmes have led to the discovery of new types of transients such as the fast blue optical transients (FBOTs) and fast radio bursts (FRBs). Large scale surveys in the coming decade are expected to increase these discoveries by a factor of more than 100, and also to discover a number of unknown types of transients as we start probing the yet unexplored parts of the parameter space described by wavelength, flux, change in flux and time. Recent history has also shown that some of the most interesting and often unexpected breakthroughs come from these discoveries, especiallywhen the large scale surveys are combinedwith good followup andmonitoring capabilities. Transients that are a result of explosive events related to the end points of stellar evolution produce essentially all of the heavy elements in the Universe, thus influencing the chemical evolution of a galaxy, star formation and the origin of life. Energetic transients like core-collapse SNe lead to the formation of compact objects like neutron star or black hole. The most extreme stellar explosions like different types of GRBs and their association with very energetic supernovae have a great potential to decipher the underlying extreme physics. Electromagnetic observations of these transients not only probe the newly synthesised elements and their distribution in the ejecta, but also place strong constraints on the explosion mechanisms and environments. Compact binary interactions such as mergers of neutron stars, or white dwarfs also lead to explosive events and observational evidences indicate towards their associationwith GRBs, SNe and gravitational wave (GW) events. Detailed theoretical modeling and numerical simulations along with enormous amount of multi-wavelength observational evidences have opened a new window for such time-critical transients to be studied in amore systematic way. Some of the unresolved questions are: what kind of progenitors are responsible to produce thermonuclear supernovae (SNe Ia) - does a white dwarf explode due to a merger with another white dwarf, or due to accretion from a non- degenerate companion? Are some classes of recurrent novae single-degenerate progenitors of SNe Ia? In the case of core-collapse SNe the dominant mechanisms responsible for the removal of the outer layers of the progenitors are still very unclear as well as the understanding of how ubiquitous the presence of a circumstellar matter around supernovae is. Also, understanding which stars explode as supernovae leaving behind a neutron star remnant and which stars collapse forming black holes remains a fundamental astrophysical problem. The prompt emission mechanism and the role of magnetic fields in jet formation, energy dissipation and shock acceleration in GRBs are still not clearly MEGA SCIENCE VISION-2035