Electromagnetic radiation from celestial bodies is still the main source of information to date.
It can be categorized by the wavelength band of the electromagnetic spectrum in which the measurements are taken. As the instruments are sensitive only to a narrow energy band for
Figure 2.1: Distance after which the Universe becomes opaque to electromagnetic radiation of a certain energy. At the highest energies, E >1010 eV, photons are absorbed after traveling short distances, such that the most energetic cosmic events are obscured, as indicated by the black region. This region is, in turn, accessible by other messengers such as neutrinos and gravitational waves. The top panel depicts how the Universe looks in different wave lengths and messengers. Taken from [10].
which they are optimized, the requirements for different telescopes are diverse. Special attention has to be paid to the construction site for ground based telescopes. Popular examples are the Very Large Array (VLA) radio telescope in New Mexico or the Atacama Large Millimeter Array (ALMA) for observations in the infrared in Chile. X-ray observations, for example, must be performed from balloons, rockets or satellites, as the radiation is absorbed in the atmosphere.
For shortest wavelengths, particularly of interest for this work as they are connected with other ultra-high energy messenger particles, measurements are taken with gamma-ray telescopes. A special class of gamma-ray telescopes are the so-called Cherenkov telescopes, such as the High Energy Stereoscopic System (H.E.S.S.) or the Cherenkov Telescope Array (CTA), which do not detect the rays directly, but indirectly via flashes of Cherenkov light produced by gamma-rays interacting in the atmosphere. The combination of observations in different wavelength bands to obtain a more complete picture is called multi-wavelength astronomy.
However, the Universe becomes opaque for highest energy photons because their energy is high enough to interact with low energy photons of the cosmic background. As shown in Fig. 2.1,
lower energy photons with E ≲ 1010 eV can reach us even from large distances. In contrast, high energy photons are attenuated as they travel through space, such that our view of their origin and, with that, the most powerful events in the universe is obscured. To access the information about far away and very energetic sources, other messengers are needed. Cosmic rays are deflected by magnetic fields and, similar to gamma-rays, interact with the extragalactic background light. Neutrinos have a very low cross section such that they travel through the Universe mostly unattenuated, pointing back straight to their sources. Gravitational waves are ripples in space-time which have to be detected with kilometer-sized interferometers at Earth, which makes the spatial reconstruction harder. Indeed, this region of the parameter space is accessible for neutrinos and gravitational waves [10].
Multi-messenger astronomy is a relatively new field of research, which requires sophisticated instruments to detect disparate messengers. By combining these independent measurements, a more complete picture of their production can be drawn, as they are likely correlated with each other already from the creation in the sources. An example is the so-called ∆(1232)-resonance
p+γ →∆+→
which describes an energetic cosmic ray (here: proton) interacting with a photon, for instance in the environment of the source. A ∆-meson is produced which subsequently decays into a baryon plus a charged or neutral pion. The branching ratio is roughly 1/3 for the charged pion channel and 2/3 for producing a neutral pion. These pions decay too, i.e., π+→ µ++νµ or π0 → 2γ, respectively. This shows that as soon as there is acceleration of cosmic rays to high energies and sufficiently dense photon fields to interact with, multi-messenger production is possible.
For low energies, multi-messenger observations happened as early as the 1940s, when cosmic rays were measured coinciding with solar flares, which were also observed electromagnetically [70]. In 1987, supernova SN1987A was detected, first with optical telescopes. A few hours later, neutrinos were detected in Kamiokande-II [71], the Irvine-Michigan-Brookhaven (IMB) experiment [72] and Baksan [73]. Often cited as the beginning of the multi-messenger era, a binary neutron star merger was observed in August 2017, first in gravitational waves and shortly after by electromagnetic radiation. The Laser Interferometer Gravitational-Wave Observatory (LIGO) collaboration reported a gravitational wave signal originating from the galaxy NGC 4993, which was later called GW170817 [62]. A short gamma-ray burst dubbed SGRB170817A was detected by the Fermi Gamma-ray Space Telescope and the INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) 1.7 seconds later [63]. The optical counterpart named AT 2017gfo (originially SSS17a) was detected 11 hours later by the Swope Supernova Survey
(SSS) [74]. In the following, ultraviolet [75], X-ray [76] and radio signals [77] were detected, revealing a brightening of X-ray emissions for about half a year [78]. Strong evidence for a kilonova, in which heavy r-process (rapid neutron capture) nuclei are produced, was reported [79]. Neutrino and cosmic ray production in this event will be reviewed in chapter 6. Only one month later, a very high energy neutrino event with an energy of about 290 TeV named IceCube-170922A was detected by the IceCube collaboration [64]. A few days after, the Fermi-Large Area Telescope (LAT) and the Major Atmospheric Gamma Imaging Cherenkov Telescopes (MAGIC) collaboration reported the detection of gamma-rays from the blazar TXS0506+056, positionally consistent with the neutrino signal [80]. However, it is still controversial if the gamma-rays are correlated to the neutrino event [81]. Note that in multi-messenger astronomy, detection of a messenger and non-detection of another one can constrain production scenarios too.
In order to strengthen the conection between different observatories, networks were created to send out alerts in case of a potential detection, i.e., the observatories share preliminary information on the position of the event, for example. Similarly, archival data is re-investigated to correlate events in different messengers spatially and temporally. The first such network was established in 1999 at Brookhaven National Laboratory and combined multiple neutrino detectors to generate supernova alerts as an early warning system [82]. In 2013, the Astrophysical Multimessenger Observatory Network (AMON) was created, which is a more ambitious project to facilitate multi-messenger observations [83]. Also sub-threshold events can be potentially interesting when looked at with several different instruments. Another automated program to search for astronomical transients is the All Sky Automated Survey for SuperNovae (ASAS-SN).