Search for the most energetic photon – Gamma rays open a window
For the first time, the Gamma-ray sky at PeV energies was opened to observations. The Chinese experiment Lhaaso (Large High Altitude Air Shower Observatory), after almost a year of observations, found 12 cosmic accelerators in our Galaxy. The recorded photon energy exceeds the “psychological threshold” of the 1 PeV (Petaelettronvolt = 10^15 eV, that is, a quadrillion of electronvolt, or ~1000 erg), reaching the value of 1.4 PeV. The first results, published a few days ago in Nature, are a resounding testimony of the potential of this research infrastructure.
“Until now it was thought that very few celestial objects could emit Gamma-ray radiation at such extreme energies.” explains Martina Cardillo of INAF. “The sources must in fact be able to accelerate particles to energies of 1 PeV (hence the name PeVatrons), that is energies up to 100 times greater than those reached by the most powerful accelerators ever built on Earth. The Lhaaso experiment has shown that they are not rare.”
The importance of finding these accelerators lies in the fact that we know that the spectrum of Galactic cosmic rays extends at least to the energies of the PeV. Direct observation of sources where such particles are present can then help researchers to identify which astrophysical phenomena are capable of accelerating particles so efficiently. The candidate sources proposed so far are supernova remnants, “pulsar wind nebulae” and young stellar associations. In fact, some of these objects are found in the uncertainty regions of the Lhaaso sources.
Lhaaso is the most important experiment for the study of galactic cosmic rays currently underway. It can study both charged particles (determining their energy distribution, chemical composition and anisotropy with an unprecedented resolution) and photons from the most disparate galactic and extra-galactic sources.
“Detection of an astonishing 1.4 PeV Gamma-ray emission from the Cygnus Cocoon not only makes the last unexplored window of the electromagnetic spectrum experimentally accessible, but also unquestionably demonstrates the existence of the PeVatrons.” adds Giuseppe di Sciascio of INFN, and visiting professor of the Chinese Academy of Sciences.
The current paradigm that wants the supernova remnants responsible for the production and acceleration of most galactic cosmic rays is undermined. In fact, the Cygnus Cocoon is a superbubble, containing young and massive stars (newly formed OB2 star associations), sources of cosmic rays alternative to supernova remnants.
These northern hemisphere sources are a perfect target for observation with the Cherenkov telescopes of the ASTRI Mini-Array, that will study the emission and morphology of the sources in depth and will improve on the identification of the PeV emission counterparts.
In fact, both the ASTRI Mini-Array and the future CTA, currently under construction by an international collaboration of which INAF is one of the main partners, will have a better angular resolution than Lhaaso and will allow us to clearly pinpoint the object responsible for the PeV emissions. Observations in the next years will also make it possible the identification of the different classes of cosmic ray sources as a function of their energy.
“These results are crucial also for the detection of astrophysical neutrinos. If protons (i.e. an adronic mechanisms) are involved in the production of such energetic photons, at least the three brightest sources could be, in fact, within range of the neutrino detectors currently in operation or under design” concludes Giuseppe di Sciascio.
Articolo scientifico: Ultrahigh-energy photons up to 1.4 petaelectronvolts from 12 γ-ray Galactic sources Cao et al. 2021, Nature
LHAASO (PI Zhen Cao) is a complex instrument: at 4410 m it is one of the instruments at higher altitude ever built: the lower thickness of atmosphere allows “a little better” view of the sky, in particular in some energy bands. Its size (> 2 Km), the detector density for measuring the different secondary components of cosmic radiation and its sensitivity, make it the undisputed protagonist to finally solve the major problems still open in the physics of galactic cosmic rays, more than a century after their discovery.
ASTRI (P.I. G. Pareschi) is a Gamma-ray experiment led by INAF with the collaboration of other international partners with the aim of observing the Gamma-ray sky from a few to hundreds of TeV by building a Mini-Array of 9 telescopes in Tenerife (Canary Islands, Spain – thanks to the Instituto de Astrofisica de Canaria, IAC hospitality). The Mini-Array telescopes are similar to those that will be constructed for the CTA project in the south.
Social: Facebook ASTRIgamma; Instagram @astrigamma
Translation by Giuseppe Fiasconaro and Anna Wolter
This post is also available in: Italian
