Hawc: Are Photons With Monstrous Energies Coming From the Galaxy’s Largest Accelerator?

Photons with an energy of 200 teraelectronvolts are most likely emitted by protons colliding with interstellar gas. The primary source of protons is pulsar HAWC J1825-134 (in the orange circle), the role of the actual accelerator is played by the star cluster [BDS2003] 8 (dark blue).
Credit: HAWC

For years, in the vastness of our galaxy, astrophysicists have been tracking down pevatrons – natural accelerators of particles with monstrous energies. Thanks to the HAWC Observatory for Cosmic Radiation, another probable trace of their existence has just been found: photons with some of the highest energies. However, what is particularly important is that this time the high-energy photons have not only been recorded, but also their probable place of origin has been determined.

We know they exist, we just don’t know where exactly they are or what they look like. Pevatrons – because this is what we are talking about here – are the largest natural particle accelerators in our galaxy, capable of accelerating protons and electrons to energies even many billions of times greater than the energy of photons of visible light. The problem with detecting pevatrons stems from the fact that the particles they accelerate carry an electric charge and are therefore deflected by magnetic fields in the galaxy. The discovery which has just been made thanks to data collected by the High-Altitude Water Cherenkov (HAWC) Gamma-Ray Observatory brings us significantly closer to finding the first cosmic pevatron and understanding its nature.

The HAWC Observatory is located on the slope of the Sierra Negra volcano in Mexico, at an altitude of 4,100 m. It consists of 300 water tanks, each surrounded by sensitive photomultipliers. When a particle of secondary cosmic radiation traveling at a speed faster than the speed of light in water enters a tank, there is an electromagnetic “boom” – a weak flash of radiation (Cherenkov), detected and amplified by the photomultipliers. A careful analysis of flashes observed at the same time in individual tanks makes it possible to extract information about the type, energy and direction of the particle of the primary cosmic radiation which initiated the recorded cascade of secondary particles.

“Based on data collected by the HAWC, we were able to determine the source of photons with energies of about 200 teraelectronvolts. For photons, this is an extreme value, one hundred trillion times greater than the energy typical of photons perceived by our eyes,” says Dr. Sabrina Casanova from the Institute of Nuclear Physics of the Polish Academy of Sciences (IFJ PAN) in Cracow. Together with Dr. Francisco Sales Greus (IFJ PAN & IFIC) and PhD student Dezhi Huang from the Michigan Technological University in Houghton (USA), she is one of the main authors of the analysis published in the excellent astronomical journal The Astrophysical Journal Letters.

Compared to protons and electrons, photons have a pleasant feature: they ignore magnetic fields and run to their target along the shortest path that space-time allows. So, once the direction from which the photons are coming is identified within a galaxy, it is usually possible to determine their source. This is not an easy task, but in this case it was successful. The source of the 200 TeV photons turned out to be a region of the recently discovered pulsar eHWC J1825-134, visible in the southern hemisphere in the background of the Vela constellation, and lying at a distance of about 13 thousand light years from Earth.

Observations of such high-energy photons are rare, and it is even rarer to identify the source. The record currently belongs to photons with energies of 450 TeV, detected with the Sino-Japanese ASgamma detector in Tibet. In that case, the photons came from the vicinity of a pulsar in the famous Crab nebula in the background of the Taurus constellation.

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