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Mountain observatory nets PeV gamma rays

2 July 2021
The universe seen with protons > 100TeV

Recent years have seen rapid growth in high-energy gamma-ray astronomy, with the first measurement of TeV photons from gamma-ray bursts by the MAGIC telescope and the first detection of gamma rays with energies above 100 TeV by the HAWC observatory.

Now, the Large High Altitude Air Shower Observatory (LHAASO) in China has increased the energy scale at which the universe has been observed by a further order of magnitude. The recent LHAASO detection provides the first clear evidence of the presence of galactic “pevatrons”: sources in the Milky Way capable of accelerating protons and electrons to PeV energies. Although PeV cosmic rays are known to exist, magnetic fields pervading the universe perturb their direction and therefore do not allow their origin to be traced. The gamma rays produced by such cosmic-rays, on the other hand, point directly to their source.

Wide field of view
LHAASO is located in the mountains of the Sichuan province of China and offers a wide field of view to study both high-energy cosmic and gamma rays. Once completed, the observatory will contain a water Cherenkov detector with a total area of about 78,000 m2, 18 widefield- of-view Cherenkov telescopes and a 1 km2 array of more than 5000 scintillator- based electromagnetic detectors (EDs). Finally, more than 1000 underground water Cherenkov tanks (the MDs) are placed over the grid to detect muons.

The latter two detectors, of which only half were finished during data-taking for this study, are used to directly detect the showers produced when high-energy particles interact with the Earth’s atmosphere. The EDs detect the shower profile and incoming angle, using charge and timing information of the detector array, while the MDs are used to distinguish hadronic showers from the electromagnetic showers produced by high-energy gamma rays. Thanks to both its large size and the MDs, LHAASO will ultimately be two orders of magnitude more sensitive at 100 TeV than the HAWC facility in Mexico, the previous most sensitive detector of this type.

The measurements reported by the Chinese-led international LHAASO collaboration reveal a total of 12 sources Astrowatch Mountain observatory nets PeV gamma rays located across the galactic plane (see image above). This distribution is expected, since gamma rays at such energies have a high cross-section for pair production with the cosmic microwave background and therefore the universe starts to become opaque at energies exceeding tens to hundreds of TeV, leaving only sources within our galaxy visible. Of the 12 presented sources, only the Crab nebula can be directly confirmed. This substantiates the pulsar-wind nebulae as a source in which electrons are accelerated beyond PeV energies, which in turn are responsible for the gamma rays through inverse Compton scattering.

Of specific interest is the source responsible for the photon with the highest energy, 1.4 PeV

The origin of the other photons remains unknown as the observed emission regions contain several possible sources within them. The sizes of the emission regions exceed the angular resolution of LHAASO, however, indicating that emission takes place over large scales. Of specific interest is the source responsible for the photon with the highest energy, 1.4 PeV. This came from a region containing both a supernova remnant as well as a star-forming cluster, both of which are prime theoretical candidates for hadronic pevatrons.

Tip of the iceberg
More detailed spectrometry as well as morphological measurements, in which the differences in emission intensity throughout the sources are measured, could allow the sources of > 100 TeV gamma rays to be identified in the next one or two years, say the authors. Furthermore, as the current 12 sources were visible using only one year of data from half the detector, it is clear that LHAASO is only seeing the tip of iceberg when it comes to high-energy gamma rays.

Further reading

Z Cao et al. 2021 Nature 594 33.

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