Time domain astronomy
When astronomy began, the celestial sphere was considered to be a very static world. Indeed the typical time scales on which the objects we see are changing are much larger than a human’s life. Some departures from this rule existed. Planets were moving in a strange cosmic ballet, comets seemed to appear cyclicly, asteroids were falling on Earth and more rarely, a sudden bright light appeared followed by a slow fading (now known as a supernova, a famous example is the supernova of the year 1054).
With the observation of the sky at different wavelength (from gamma to radio) and the rapid sensitivity increase of the instruments, a different vision of the universe has emerged. In addition to the quiet universe of peaceful stars and galaxies, the vision of a violent and catastrophic universe has emerged. In this universe, giant stars explode, neutron stars merge at the end of a cataclysmic dance, stars are ripped apart and swallowed by giant black holes; it is the universe of the novae, supernovae, gamma-ray bursts, binary stars pulsars, active galactic nuclei and many more.
What characterizes this new field of astronomy is the transient nature of those events. Time scales extend from less than a second to hundreds of days for the longest. To characterize those events, it is required to observe quickly to catch the event as soon as possible, to revisit it periodically to measure how it is fading, and all this in different wavelengths to characterize the event the better.
Gamma-ray bursts (GRBs) are the perfect example of transient phenomena. Their prompt emission in gamma rays, which outshine all the other gamma-ray sources last from 0.1s to hundreds of seconds. After this prompt emission comes a rapidly fading emission that is observed in all wavelengths (called the afterglow).
Sometimes the localization of the GRB from its prompt emission is not good enough to allow for its careful study by large telescopes. GRANDMA, thanks to its connected robotic telescopes all over the world, will scan the most probable region of the sky and search for the GRB emission. Once the position is accurately measured, GRANDMA’s telescopes’ photometric and spectroscopic capabilities will then allow for a detailed study of its properties. In particular, the measurement of the distance is of primary importance, to access the intrinsic properties of the GRB such as the total energy released.
LIGO and Virgo revealed a new universe using another messenger: gravitational waves. In 2015, the merger of binary black holes was observed for the first time. In 2017, the merger of two neutron stars, and its follow-up by all the instruments available on ground and space, is the most important astrophysical event of the last 30 years. So many discoveries at once: short gamma-ray burst model confirmation, the origin of the heavy elements in the universe, independent measurement of the Hubble constant, and others. Whatever the process, to emit intense gravitational waves, a system has to be changing at extreme speeds, and therefore they are naturally transient.
There is another messenger, an elusive one: the neutrino. Thanks to huge international effort, there is evidence for the existence of neutrinos of astrophysical origin reaching Earth, measured by IceCube. Recently, an association with a source (BLLac) has been claimed (though it still needs to be confirmed).
Unfortunately neutrinos and, even worse, gravitational waves, are not very well localized (from a few square degrees to hundreds-thousands square degrees). As for the GRB, GRANDMA will scan the most probable region of the sky and search for an electromagnetic counterpart. Once the position of the source is known, the detailed characteristics of the source will be measured thanks to GRANDMA’s telescopes photometric and spectroscopic capabilities.
- B. P. Abbott, R. Abbott, T. D. Abbott, F. Acernese, K. Ackley, C. Adams, T. Adams, P. Addesso, R. X. Adhikari, V. B. Adya, and et al. Multimessenger Observations of a Binary Neutron Star Merger, AAS, 848:L12, October 2017.
- B. P. Abbott, R. Abbott, T. D. Abbott, F. Acernese, K. Ackley, C. Adams,T. Adams, P. Addesso, R. X. Adhikari, V. B. Adya, and et al. GW170817 : Observation of Gravitational Waves from a Binary Neutron Star Inspiral. Physical Review Letters, 119(16):161101, October 2017.
- Multimessenger observations of a flaring blazar coincident with high-energy neutrino IceCube-170922A,” The IceCube, Fermi-LAT, MAGIC, AGILE, ASAS-SN, HAWC, H.E.S.S, INTEGRAL, Kanata, Kiso, Kapteyn, Liverpool telescope, Subaru, Swift/NuSTAR, VERITAS, and VLA/17B-403 teams. Science 361, eaat1378 (2018)