Written by: Fabian Schüssler
CTA in the Era of Multi-Messenger Astrophysics
After years of preparation, a fundamentally new domain of astronomy and astrophysics has shown its first results: multi-messenger astrophysics. Throughout the past decade, several new astrophysical messengers have provided us with new insights into the most violent phenomena in the Universe: starting with high-energy gamma rays, the detection of an astrophysical flux of high-energy neutrinos and the first direct detection of gravitational waves. Building on these significant breakthroughs, many high-energy astrophysics observatories and groups started to work towards a dream that is now coming true: multi-messenger astrophysics, which, via the exchange and combination of data from very different observatories and messengers, opens new windows and provides unprecedented insights into the most violent phenomena ever observed.
Above: Artist’s illustration of the recent merger of a binary neutron star system emitting gravitational waves and creating a gamma-ray ray burst and a kilonova. Credit: NASA’s Goddard Space Flight Center/CI Lab
The most striking example illustrating the viability of this approach is the detection of electromagnetic signals complementing gravitational waves from the merger of a binary neutron star system. This event, named GW170817, is probably the best-covered astrophysical phenomenon in recent history. Thanks to the huge effort by observatories around the world, it provides for the first time an observational link between binary mergers, short gamma-ray bursts and optical emissions known as kilonovae, and resolves the origin of heavy elements in the Universe . As a hint for the expected CTA performance, H.E.S.S. was the first ground-based instrument to observe the region covering the source region, including the kilonova named SSS17a/AT2017gfo. No high-energy gamma-ray emission could be detected during an extensive observation campaign covering timescales from a few hours to several days after the gravitational wave event .
The image below illustrates the simulated response of CTA follow-up observations of the gravitational wave event GW170817. Thanks to the large field-of-view of CTA, only two individual pointings would be necessary to cover most of the localization region provided by the LIGO and VIRGO gravitational wave interferometers (coloured region). Combined with its high sensitivity, CTA will therefore be able to efficiently search for associated high-energy gamma-ray emission.
Credit: F. Schüssler, IRFU/CEA Paris-Saclay
This very recent example just scratches the surface of the enormous potential of these searches. It is all made possible through the rapid exchange of information across very different instruments and the subsequent joint data analyses. These collaborations build on a long history in astronomy of telescopes at far corners of the Earth jointly monitoring variable objects as the globe rotates. The real-time exchange of information in the study of gamma-ray bursts was introduced in the late 1990s. It is this culture of open data and fast information exchange that will ensure the success of multi-messenger astrophysics.
CTA, with its large field-of-view, extremely fast reaction to alerts and very-high sensitivity, is well suited to lead this revolution. The follow-up observations of gravitational wave events have been assigned the highest priority in CTA’s key science project on transient phenomena. Based on the significant experiences gained with H.E.S.S., MAGIC and VERITAS, preparations for these technically challenging observations are well underway.
 B.P. Abbott, et al., Multi-messenger Observations of a Binary Neutron Star Merger, Astrophys. J. Letters 848 (2017) L12.
 H. Abdalla et al. (H.E.S.S. Collaboration), TeV Gamma-Ray Observations of the Binary Neutron Star Merger GW170817 with H.E.S.S., Astrophys. J. Letters 850 (2017) L22.