Very High Energy Cosmic Gamma Rays Detected

China’s Large High Altitude Air Shower Observatory (LHAASO) has detected two photons with energies exceeding 1 peta-electron-volt (quadrillion electron-volts or PeV).  The photon at 1.4 PeV is the highest energy photon ever observed. The largest accelerator on Earth (LHC at CERN) can only accelerate particles to 0.01 PeV.

Aerial photograph of LHAASO

When a very high-energy particle strikes Earth’s atmosphere, it triggers a cascade of secondary particles. Ground-based detectors such as LHAASO record these air shower events and can then reconstruct the type, energy and trajectory of the primary particles.  Two of the photons detected from the Crab Nebular by LHAASO are the highest energy photons ever detected from the Crab Nebula: one at 0.88 PeV other at 1.1 PeV.

The crab nebula is the remnant of a supernova that exploded in 1034 AD.  A rapidly spinning pulsar at the centre of the nebula is the ultra-dense core of the exploded star.  The pulsar is about 20 km diameter, spins at 30 revolutions per second and has a mass between 1.4 and 2 solar masses. The crab pulsar is highly magnetised and the spinning magnetic field creates high energy electrons which interact with gases in the nebula creating X-rays and high energy gamma rays.

1.4 PeV  = 1.4 x 1015 eV = 0.000224 joules.

One joule equals the work done (or energy expended) by a force of one newton (N) acting over a distance of one meter (m). One newton equals a force that produces an acceleration of one meter per second (s) per second on a one kilogram (kg) mass. Therefore, one joule equals one newton•meter.

The small calorie or gram calorie (usually denoted cal) is the amount of heat needed to raise the temperature of one gram of water by one degree Celsius (or one kelvin).  One calorie is defined as exactly 4.184 J.  4.184 joules will raise one gram of water by 1 degC.

Hence 0.000224 joules could

raise the temperature 1 mg water by 0.005 degC

kinetic energy of a table tennis ball weights 2.7 g, dropped from 8.5  mm  [h = 0.000224/(0.0027 x 9.8)]

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