Watching Gamma Rays from the Safety of Earth

Two of the four H.E.S.S. telescopes in Namibia. Image credit: HESS. Click to enlarge.
Our planet is exposed to almost four dozen octaves of electro-magnetic radiation from the Universe around us. Of those, half-a-dozen octaves can be detected from the Earth’s surface. During the 1990’s several extraordinary new octaves were added with the advent of high-sensitivity CCD imagers and modern computing systems. Today we can track super-high energy gamma rays back to their sources in space ? even while safely ensconced in the Earth?s protective mantle of air.

Well before the turn of the third millennium, it was realized that high-energy photons penetrating the air causes a secondary form of light known as Cherenkov Radiation (CR). CR was first observed by Pierre and Marie Curie when investigating radioactivity at the turn of the 20th century. It wasn’t until the mid-1930s that the hauntingly beautiful “blue-white” glow given off by glass in the presence of radioactivity was studied in detail.

CR was first fully investigated by the Russian experimentalist P. A. Cherenkov in 1936. Cherenkov found that whenever high-energy photons (or particles) pass through a transparent gas, liquid, or glass at velocities greater than the speed of light for that substance, a shower of secondary light is created. In terms of the Earth’s atmosphere, such showers typically occur as gamma rays approach within 10 km’s of sea level and the resulting luminosity projects a light cone (or “light pool”) roughly 250ms in diameter.

Enter the Max Plank Institute of Physics (MPIK) of Heidelberg, Germany in the early 1990s.

In 1992 MPIK tested the first in a series of prototypes intended to develop a full scale IACT (Imaging Atmospheric Cherenkov Telescopes) system. That instrument (CT1) proved that CR showers could be detected using CCDs. It also showed that computers could accurately log a CR shower’s time and position in the sky. A later instrument (CT2) increased CR sensitivity and resolution by adding aperture. Meanwhile improvements were made to associated imaging, data processing, and sky sensing components. By combining four CT2-class instruments together, the first full IACT system was developed in 1995 (CT3). Because of this progress, MPIK’s own website could later say that “Ground-based Imaging Atmospheric Cherenkov Telescopes have become the most efficient experimental technique for the observation of cosmic gamma rays in the TeV energy range.”

IACT systems monitor for CR showers using two or more widely spaced light-collecting mirror assemblies pointed at the same part of the sky. Because CR originates in the Earth’s atmosphere – not well-off in the Universe itself – each mirror sees a shower from a different perspective. The resulting “stereoscopic vision” works like eye and brain to precisely determine the path a gamma ray takes after entering the atmosphere. Based on that data – along with laws governing the way photons move – computers calculate the location of gamma ray source in space. Each ray effectively acts like a luminous finger pointing back toward a distant cosmic source.

By 1998 the first purely astronomical IACT (HEGRA – High Energy Gamma Ray Astronomy) was put into service by MPIK on La Palma in the Canary Islands. HEGRA confirmed dozens of high energy gamma ray sources – many hurling photons of more than 1 terra-electron-volts of energy (the amount of force stored in a single electron accelerated by a trillion volts of electricity). Among them were the Crab Nebula pulsar in Taurus and the giant elliptical galaxy M87 – regent of the Coma-Virgo galaxy cluster.

Today even more advanced IACT systems collect CR. One of the most sophisticated instruments (H.E.S.S – High Energy Stereoscopic System) was developed by MPIK along with a consortium of European scientific and educational organizations. Currently HESS consists of four separate 12m diameter IACTS gathering faint CR light in the dark skies above the 1.8km high Khomas Highlands of Namibia, Africa.

Named after Nobel Prize winning physicist Victor Hess (who discovered cosmic rays in 1912), HESS uses an array of four IACT mirror systems. Each spherical IACT mirror consists of 382, 60cm diameter individually-adjustable sub-mirrors reflecting CR light into a large electronic “camera”. Light focused on the camera is detected by a honeycomb of 960 “smartpixel” photo-multiplier tubes (PMTs). The four IACTs are placed in a square and spaced by 120 meters to give an optimally stereoscopic view of the sky within the 250m light pool caused by a CR event.

Each HESS IACT is ten times more sensitive than its corresponding HEGRA unit – and has to be, for the total amount of CR light in the sky is 10 stellar magnitudes fainter than starlight. HESS IACTS can resolve CR showers caused by photons as “weak” as .1 TeV while discriminating between high-energy particles and photons. Using a pair of IACTS, gamma ray sources can be isolated to less than 5 arc-minutes of angular resolution – roughly 1/6th the apparent size of the full moon. To simplify detection, HESS IACTS can scan 5 degrees of the sky at a time.

One of the fundamental questions before astrophysicists is to determine just how nature manages to pack so much punch into those mass-less, charge-less photons. Currently no terra-electron-volt particle accelerators are on line – and such devices only work with charged particles – not photons. It may fall to IACTS like HESS to lead the way.

In a paper entitled “Observation of the giant radio galaxy M87 at TeV energies using H.E.S.S”, M Beilicke of the Institute for Expermental Physics, Hamburg Germany and associates have used HESS to determine that the giant elliptical galaxy M87 is a strong and possibly periodically variable source of high-energy gamma ray photons.

According to the paper, “M87 is of particular interest for observations of TeV energies. The large jet angle makes it different from the so far observed TeV emitting AGN of the blazar type.” Using HESS, the team determined that high-energy photons originate from a point source centered in the midst of M87 – precisely where it’s AGN is thought to be. Unlike blazars however, M87’s relativistic jets do not point at the Earth.

Meanwhile the team may have also discovered that gamma ray output from M87’s AGN is variable “on time scales of years.” According to M. Bielecke et al, “Such a result would be very important since various models for the TeV gamma-ray production in M87 could be ruled out.” The team goes on to say that “Mechanisms correlated with cosmic rays, large scale jet structures, and exotic dark matter particle annihilation could not explain variability in the TeV gamma ray emission on these time-scales.”

As in many areas of contemporary astronomical investigation, observing M87 across a wide-range of the em band may be essential to understanding how those tiny mass-free wave-particles of light can carry so much ?weight?. There is no doubt that capturing the ‘blue-white” glow of Cherenkov radiation put off by our Earth’s very own atmosphere will play a critical role in making this possible.

Written by Jeff Barbour