On Thursday June 28 and Friday June 29, there was a meeting at Radboud University  in the Dutch city of Nijmegen to discuss the potential impact of the emerging field of multi-messenger transient science on Athena. With multi-messenger (MM), we mean here information about cosmic events other than through photons. One should for instance think about gravitational waves, neutrinos and cosmic rays. Many of the multi-messenger signals likely originate from the same astrophysical objects that Athena is studying. Accreting and/or merging black holes of various sizes and masses come to mind. Of course accreting, merging and high magnetic field neutron stars are additional important MM and Athena sources. One of the aspects to consider is the total number of Target of Opportunity observations that Athena is planning to execute. Fortunately, for realistic numbers the expected increase in the number of ToO observations due to the potential follow-up of transient MM events does not drive neither the hardware nor consumables of Athena, but there may be an effect on the mission planning and efficiency. These effects have to be balanced with the new science allowed by these observations and the potential new ways that the existing Athena science goals can be achieved through these observations.

A second aspect that was discussed concerns the possibility to have a system on board at the WFI that allows for an autonomous search for fast X-ray transients through the incoming data. Their existence and rough localisation should then be relayed to Earth. There, follow-up observations in other wavebands can then be quickly initiated. This way Athena would meaningfully contribute to the emerging field of time-domain astronomy. Besides opening up discovery space (unknown unknowns) it would allow the investigation of current known unknowns such as the fast X-ray transients discovered in Jonker et al. (2013), Glennie et al. (2015) and Bauer et al. (2017). However, it was realised that this option should not drive costs up.

Participants: Peter Jonker, Paul O’Brien, Luigi Piro, Elisa Costantini, Eleonora Troja, Arne Rau, David Burrows, Pragati Pradhan, Antonio Martín-Carrillo.