An extensive-air-shower-like event registered with the TUS orbital detector

Abstract

TUS (Tracking Ultraviolet Set-up) is the world's first orbital detector of ultra-high-energy cosmic rays (UHECRs). It was launched into orbit on 28th April 2016 as a part of the scientific payload of the Lomonosov satellite. The main aim of the mission was to test the technique of measuring the ultraviolet fluorescence and Cherenkov radiation of extensive air showers generated by primary cosmic rays with energies above ∼100 EeV in the Earth atmosphere from space. During its operation for 1.5 years, TUS registered almost 80,000 events with a few of them satisfying conditions anticipated for extensive air showers (EASs) initiated by UHECRs. Here we discuss an event registered on 3rd October 2016. The event was measured in perfect observation conditions as an ultraviolet track in the nocturnal atmosphere of the Earth, with the kinematics and the light curve similar to those expected from an EAS. A reconstruction of parameters of a primary particle gave the zenith angle around 44̂ but an extreme energy not compatible with the cosmic ray energy spectrum obtained with ground-based experiments. We discuss in details all conditions of registering the event, explain the reconstruction procedure and its limitations and comment on possible sources of the signal, both of anthropogenic and astrophysical origin. We believe this detection represents a significant milestone in the space-based observation of UHECRs because it proves the capability of an orbital telescope to detect light signals with the apparent motion and light shape similar to what are expected from EASs. This is important for the on-going development of the future missions KLYPVE-EUSO and POEMMA, aimed for studying UHECRs from space.

An overview of the JEM-EUSO program and results

“…The JEM-EUSO Program M. Bertaina deeply scrutinized. Its phenomenology and the possible interpretations are reported in detail in [27]. Similar searches performed with Mini-EUSO support the interpretation of an anthropogenic origin of this event.…”

Main results of the TUS experiment on board the Lomonosov satellite

“…The most interesting of these events, recorded on 3 October 2016 above Minnesota, was analyzed in detail in paper [6] in an assumption of its EAS origin. In work [7], the event is considered in the context of a scenario of so-called relativistic dust grains.…”

“…It is difficult to interpret them as an EAS fluorescence due to their significant signal amplitudes: they vary in the range from 120 to 450 photons per m 2 per s, which corresponds to a primary energy of a proton the order of 1 ZeV and higher. The probability of registering such an UHECR is of the order of 10 −3 -10 −5 , see [6].…”

Estimation of the exposure of the TUS space-based cosmic ray observatory

“…The estimate of the trigger performance depends on a number of factors, among them the sensitivity of the photodetector, the level of the background illumination and software parameters of the trigger. During an incident described in [3], 20% of the PMTs were destroyed and sensitivities of the remaining PMTs changed in comparison with pre-flight measurements. A number of attempts of in-flight calibration have been performed but none of them is fully reliable yet.…”