## Overview of Multi-messenger Astrophysics/Astronomy
## Overview of Multi-Messenger Astrophysics
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.
Multi-messenger studies means looking for at least two or more cosmic messenger particles to study the transient phenomena in our universe,such as gamma-ray burst, outburst of active galactic nuclei, fast radio burst, supernova explosion, etc. Using multiple messengers greatly extends our understanding of the universe compared to using one single channel. 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.
Some of the most important open questions in astrophysics are the origin of astrophysical neutrinos, origin of cosmic rays, acceleration mechanics of high energy cosmic rays, etc. 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 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.
Among those multiple messengers, neutrinos are an important type of messenger. Neutrinos are neutral and only interact via gravity and weak interactions. Neutrinos point back to their sources where they were created.
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 may not point back to their sources. 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.
For example, cosmic rays are charged particles, thus they are deflected by the galactic magnetic fields. Cosmic ray observatories are able to detect them but their observed arrival directions do not point back to their sources. 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 for studying the propagation of cosmic rays.
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.
Looking for coincidences of neutrinos and electromagnetic or GW counterparts may also reveal subthreshold events that do not generate interest within each single observatory, or even reveal new sources.
Because neutrinos travel with nearly the speed of the light, a real time or near real time alert system based on neutrinos (with a good angular resolution) is possible. It is vital for follow-ups for some high energy transient sources that are time-dependent with the flux quickly varying. For example, a real time neutrino alert will be able to point to a direction for space eletromagnetic observatories that have a small sky coverage (e.g. Fermi-LAT) to conduct search in their channel.
Because neutrinos travel with nearly the speed of the light, a real time or near real time alert system based on neutrinos (with a good angular resolution) is possible. It is vital for follow-ups for some high energy transient sources that are time-dependent with the flux quickly varying. For example, a real time neutrino alert will be able to point to a direction for space eletromagnetic observatories that have a small sky coverage (e.g. Fermi-LAT) to conduct search in a timely fashion.
## KM3NeT Multi-Messenger Alerts
## KM3NeT Multi-Messenger Neutrino Alerts
* For MeV core-collapse supernova neutrinos, each KM3NeT optical module acts as a detector, each comprising of 31 PMTs. KM3NeT is already connected to SNEWS, the SuperNova Early Warning System (https://snews.bnl.gov/). A CCSN neutrino interaction leads to higher counting rates of individual PMTs and the coincident hit PMTs in the same optical module.
* For the search of MeV core-collapse supernova neutrinos, each KM3NeT optical module acts as a detector, each comprising of 31 PMTs. A CCSN neutrino interaction leads to higher counting rates of individual PMTs and an increase of the number of coincident hit PMTs in the same optical module. Its search entails a different method from the usual neutrino event reconstruction route, and it has a separate alert system called the SuperNova Early Warning System (https://snews.bnl.gov/), so the KM3NeT supernova alerts is not discussed in here. KM3NeT is already connected to SNEWS.
* For the high energy astrophysical fields, KM3NeT will be integrated a part of the global multi-messenger community, we will (soon) be able to:
- 1) to receive external alerts, i.e. alerts generated by external experiments (e.g. gravitatinal waves alerts from LIGO/Virgo, neutrino alerts from other neutrino experiments) via GCN https://gcn.gsfc.nasa.gov/) and conduct search for correlated neutrinos in KM3NeT.
- 2) to send neutrino alerts (to GCN) that we observe in KM3NeT, including multiplet alerts, possible astrophysical neutrinos, any correlated neutrinos we find in the above correlation search. The alerts will be used for external experiments to conduct their correlation search/follow-up.
* For the search of higher energy neutrinos, such as astrophysical neutrinos, KM3NeT will be able to send/receive alerts from/to the multi-messenger community:
1. to receive external alerts, i.e. alerts generated by external experiments (e.g. gravitatinal waves alerts from LIGO/Virgo, neutrino alerts from other neutrino experiments) via GCN https://gcn.gsfc.nasa.gov/) and conduct search for correlated neutrinos in KM3NeT.
2. to send neutrino alerts (to GCN) that we observe in KM3NeT, including multiplet alerts, possible astrophysical neutrinos, any correlated neutrinos we find in the above correlation search. The alerts will be used for external experiments to conduct their correlation search/follow-up.
## KM3NeT Alert Types
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@@ -41,12 +39,34 @@ The planned alert types include:
* Any neutrinos correlated with external alerts
* Other alerts to be defined, or more subcategories divided from the High-Energy Neutrino alerts if necessary (e.g. track HE, cascade HE alerts)
## Alert Data/Interfaces
## Alert Interfaces
The Alert data will be the neutrino candidates in VOEvent format, which is the standard data format for experiments to report and communicate their observed transient celestial events facilitating for follow-ups.
The Alert receiving/sending is via the GCN (https://gcn.gsfc.nasa.gov/). The Alert data will be the neutrino candidates in VOEvent format, which is the standard data format for experiments to report and communicate their observed transient celestial events facilitating for follow-ups.
