## Overview of Mult-messenger Astrophysics/Astronomy ##
Multi-messenger studies means studying at least two or more cosmic messenger particles to study the transient phenomena in our universe. Using multiple messengers greatly extends our understanding of the universe. The cosmic messengers include the electromagnetic waves, cosmic rays, gravitational waves, and neutrinos.
Some of the most important open questions in astrophysics are the origin of astrophysical neutrinos, acceleration mechanics of high energy cosmic rays. Multi-messenger studies can help answer these questions.
Up to now, there are three successful multi-messenger detections:1) In 1987, the observation of the supernova 1987A, where neutrinos are observed in neutrino experiments about 2 or 3 hours before the visual observations. 2) 30 years later in 2017, the observation of the gravitational wave and the electromagnetic observations of the gamma ray burst observed by Fermi and Integral. 3) TXS 0506, where for the first time, blazars are identified as one neutrino source.
## Importance of Neutrinos for the Multi-Messenger Studies ##
Among those multiple messengers, neutrinos are an important type of messenger. Neutrinos are neutral and only interact via gravity and weak interactions. So they can provide good positioning for sources that may not be easy via the electromagnetic method.
For example, cosmic rays are charged particles, so their origins are obscured by the galactic magnetic fields, so cosmic ray observatories can detect them but their observed arrival directions do not point back to their source. During the propagation of cosmic rays, neutrinos are produced during the interaction of cosmic rays and the extragalactic background light. Since neutrinos are not bent by magnetic fields, they can act as good tracers.
The coincidences of neutrinos and electromagnetic or GW counterparts, and can reveal subthreshold events that may not generate interest within one single observatory, or even reveal new sources.
As neutrinos travel with nearly the speed of the light, a real time or near real time alert system based on neutrinos is possible - this is vital for follow-ups for those high energy transient sources that are time-dependent with the flux quickly varying.
## KM3NeT Multi-Messenger Alerts ##
KM3NeT will be integrated a part of the global multi-messenger community, we will (soon) be able to:1) to receive external alerts (from GCN https://gcn.gsfc.nasa.gov/ which ) and search for correlated neutrinos in KM3NeT. 2) to send neutrino alerts that we observe in KM3NeT and send them out to external experiments for external follow-up.
# Alert Types
The planned alert types include:
* Multiplet alerts:Multiple neutrinos from one source within some time window, suggesting a potential neutrino source)
* High-Energy Neutrino alerts:potential neutrinos from astrophysical sources. (The higher the energy, the higher the probablity of it being of astrophysical origin)
*Other alerts to be defined
# Alert short description use, data description, interfaces
