Gamma Waves

“Gamma wave” is not, strictly speaking, a standard scientific term … at least not in physics, and this is rather curious (the standard physics term is “gamma ray”).

The part of the electromagnetic spectrum ‘to the left’ (high energy/short wavelength/high frequency) is called the gamma ray region; the word ‘ray’ was in common use at the time of the discovery of this form of radiation (‘cathode rays’, ‘x-rays’, and so on); by the time it was discovered that gamma rays (and x-rays) are electromagnetic radiation (and that cathode rays, beta radiation, and alpha radiation, is not), the word ‘ray’ was well-entrenched. On the other hand, radio waves were discovered as a result of a new theory of electromagnetism … Maxwell’s equations predict the existence of electromagnetic waves (and that’s exactly what Hertz discovered, in 1886).

Paul Villard is credited with having discovered gamma radiation, in 1900, though it was Rutherford who gave them the name “gamma rays”, in 1903 (Rutherford had discovered alpha and beta rays in 1899). So when, and how, was it discovered that gamma rays are, in fact, gamma waves (just like radio waves, only with much, much, much shorter wavelengths)? In 1914; Ernest Rutherford and Edward Andrade used crystal diffraction to measure the wavelength of gamma rays emitted by Radium B (which is the radioactive isotope of lead, 214Pb) and Radium C (which is the radioactive isotope of bismuth, 214Bi).

We usually think of electromagnetic radiation in terms of photons, a term which arises from quantum physics; for astronomy (which is almost entirely based on electromagnetic radiation/photons), however, instruments and detectors are nearly always more easily understood in terms of whether they detect waves (e.g. radio receivers) or particles (e.g. scintillators). In gamma ray astronomy, in all instruments used to date, the particle nature of gamma rays is key (for direct detection anyway; Cherenkov telescopes work quite differently!). Can the circle be closed? Is it possible to use crystal diffraction (or something similar) – as Rutherford and Andrade did – and the wave nature of gamma rays, to build gamma ray astronomical instruments? Yes … and the next generation of gamma ray observatories might include just such instruments!

NASA has some good background material on gamma rays as electromagnetic radiation, and gamma ray astronomy: for example, Gamma Rays, and Electromagnetic Spectrum.

Universe Today has a few stories related to the wave nature of gamma rays; for example INTEGRAL Dissects Super-Bright Gamma Ray Burst, and Watching Gamma Rays from the Safety of Earth. Here’s some information on alpha radiation.

Astronomy Cast episodes Gamma Ray Astronomy, Detectors, and Electromagnetism give good background too.


Einstein Still Rules, Says Fermi Telescope Team

In this illustration, one photon (purple) carries a million times the energy of another (yellow). Some theorists predict travel delays for higher-energy photons, which interact more strongly with the proposed frothy nature of space-time. Yet Fermi data on two photons from a gamma-ray burst fail to show this effect. The animation below shows the delay scientists had expected to observe. Credit: NASA/Sonoma State University/Aurore Simonnet


While the Fermi Space Telescope has mapped the gamma ray sky with unprecedented resolution and sensitivity, it now has been able to take a measurement that has provided rare experimental evidence about the very structure of space and time, unified as space-time. Einstein’s theory of relativity states that all electromagnetic radiation travels through a vacuum at the same speed. Fermi detected two gamma ray photons which varied widely in energy; yet even after traveling 7 billion years, the two different photons arrived almost simultaneously.

On May 10, 2009, Fermi and other satellites detected a so-called short gamma ray burst, designated GRB 090510. Astronomers think this type of explosion happens when neutron stars collide. Ground-based studies show the event took place in a galaxy 7.3 billion light-years away. Of the many gamma ray photons Fermi’s LAT detected from the 2.1-second burst, two possessed energies differing by a million times. Yet after traveling some seven billion years, the pair arrived just nine-tenths of a second apart.

“This measurement eliminates any approach to a new theory of gravity that predicts a strong energy dependent change in the speed of light,” Michelson said. “To one part in 100 million billion, these two photons traveled at the same speed. Einstein still rules.”

“Physicists would like to replace Einstein’s vision of gravity — as expressed in his relativity theories — with something that handles all fundamental forces,” said Peter Michelson, principal investigator of Fermi’s Large Area Telescope, or LAT, at Stanford University in Palo Alto, Calif. “There are many ideas, but few ways to test them.”

Artist concept of Fermi in space. Credit: NASA
Artist concept of Fermi in space. Credit: NASA

Many approaches to new theories of gravity picture space-time as having a shifting, frothy structure at physical scales trillions of times smaller than an electron. Some models predict that the foamy aspect of space-time will cause higher-energy gamma rays to move slightly more slowly than photons at lower energy.

GRB 090510 displayed the fastest observed motions, with ejected matter moving at 99.99995 percent of light speed. The highest energy gamma ray yet seen from a burst — 33.4 billion electron volts or about 13 billion times the energy of visible light — came from September’s GRB 090902B. Last year’s GRB 080916C produced the greatest total energy, equivalent to 9,000 typical supernovae.

More images and videos about the Fermi Space Telescope.

Lead image caption: In this illustration, one photon (purple) carries a million times the energy of another (yellow). Some theorists predict travel delays for higher-energy photons, which interact more strongly with the proposed frothy nature of space-time. Yet Fermi data on two photons from a gamma-ray burst fail to show this effect. The animation below shows the delay scientists had expected to observe. Credit: NASA/Sonoma State University/Aurore Simonnet

Source: NASA

What is Cherenkov Radiation?

How the CANGAROO Imaging Cherenkov Air Telescope works

Cherenkov radiation is named after the Russian physicist who first worked it out in detail, in 1934, Pavel Alekseyevich Cherenkov (he got a Nobel for his work, in 1958; because he’s Russian, it’s also sometimes called Cerenkov radiation).

Nothing’s faster than c, the speed of light … in a vacuum. In the air or water (or glass), the speed of light is slower than c. So what happens when something like a cosmic ray proton – which is moving way faster than the speed of light in air or water – hits the Earth’s atmosphere? It emits a cone of light, like the sonic boom of a supersonic plane; that light is Cherenkov radiation.

The Cherenkov radiation spectrum is continuous, and its intensity increases with frequency (up to a cutoff); that’s what gives it the eerie blue color you see in pictures of ‘swimming pool’ reactors.

Perhaps the best known astronomical use of Cherenkov radiation is in ICATs such CANGAROO (you guessed it, it’s in Australia!), H.E.S.S. (astronomers love this sort of thing, that’s a ‘tribute’ to Victor Hess, pioneer of cosmic rays studies), and VERITAS (see if you can explain the pun in that!). As a high energy gamma ray, above a few GeV, enters the atmosphere, it creates electron-positron pairs, which initiate an air shower. The shower creates a burst of Cherenkov radiation lasting a few nanoseconds, which the ICAT detects. Because Cherenkov radiation is well-understood, the bursts caused by gamma rays can be distinguished from those caused by protons; and by using several telescopes, the source ‘on the sky’ can be pinned down much better (that’s what one of the Ss in H.E.S.S. stands for, stereoscopic).

The more energetic a cosmic ray particle, the bigger the air shower it creates … so to study really energetic cosmic rays – those with energies above 10^18 ev (which is 100 million times as energetic as what the LHC will produce), which are called UHECRs (see if you can guess) – you need cosmic ray detectors spread over a huge area. That’s just what the Pierre Auger Cosmic Ray Observatory is; and its workhorse detectors are tanks of water with photomultiplier tubes in the dark (to detect the Cherenkov radiation of air shower particles).

However I think the coolest use of Cherenkov radiation in astronomy is IceCube, which detects the Cherenkov radiation produced by muons in Antarctic ice … traveling upward. These muons are produced by rare interactions of muon neutrinos with hydrogen or oxygen nuclei (in the ice), after they have traveled through the whole Earth, from the Artic (and before that perhaps a few hundred megaparsecs from some distant blazer).

ICAT: imaging Cherenkov Air Telescope
CANGAROO: Collaboration of Australia and Nippon (Japan) for a Gamma Ray Observatory in the Outback
H.E.S.S.: High Energy Stereoscopic System
VERITAS: Very Energetic Imaging Telescope Array System
UHECR: ultra-high-energy cosmic ray

This NASA webpage gives more details of how ICATs work.

Quite a few Universe Today stories are about Cherenkov radiation; for example Astronomers Observe Bizarre Blazar with Battery of Telescopes, and High Energy Gamma Rays Go Slower Than the Speed of Light?.

Examples of Astronomy Casts which include this topic: Cosmic Rays, and Gamma Ray Astronomy.


Top Ten Gamma Ray Sources from the Fermi Telescope

This view from NASA's Fermi Gamma-ray Space Telescope is the deepest and best-resolved portrait of the gamma-ray sky to date. The image shows how the sky appears at energies more than 150 million times greater than that of visible light. Among the signatures of bright pulsars and active galaxies is something familiar -- a faint path traced by the sun. Credit: NASA/DOE/Fermi LAT Collaboration


The Fermi Telescope is seeing a Universe ablaze with Gamma Rays! A new map combining nearly three months of data from the Fermi Gamma-ray Space Telescope is giving astronomers an unprecedented look at the high-energy cosmos.

“Fermi has given us a deeper and better-resolved view of the gamma-ray sky than any previous space mission,” said Peter Michelson, the lead scientist for the spacecraft’s Large Area Telescope (LAT) at Stanford University. “We’re watching flares from supermassive black holes in distant galaxies and seeing pulsars, high-mass binary systems, and even a globular cluster in our own.”

The sources of these gamma rays come from within our solar system to galaxies billions of light-years away. To show the variety of the objects the LAT is seeing, the Fermi team created a “top ten” list comprising five sources within the Milky Way and five beyond our galaxy.

The top five sources within our galaxy are:

The Sun. Now near the minimum of its activity cycle, the sun would not be a particularly notable source except for one thing: It’s the only one that moves across the sky. The sun’s annual motion against the background sky is a reflection of Earth’s orbit around the sun.

“The gamma rays Fermi now sees from the sun actually come from high-speed particles colliding with the sun’s gas and light,” Thompson notes. “The sun is only a gamma-ray source when there’s a solar flare.” During the next few years, as solar activity increases, scientists expect the sun to produce growing numbers of high-energy flares, and no other instrument will be able to observe them in the LAT’s energy range.

LSI +61 303. This is a high-mass X-ray binary located 6,500 light-years away in Cassiopeia. This unusual system contains a hot B-type star and a neutron star and produces radio outbursts that recur every 26.5 days. Astronomers cannot yet account for the energy that powers these emissions.

PSR J1836+5925. This is a pulsar — a type of spinning neutron star that emits beams of radiation — located in the constellation Draco. It’s one of the new breed of pulsars discovered by Fermi that pulse only in gamma rays.

47 Tucanae. Also known as NGC 104, this is a sphere of ancient stars called a globular cluster. It lies 15,000 light-years away in the southern constellation Tucana.

The Large Area Telescope (LAT) on Fermi detects gamma-rays through matter (electrons) and antimatter (positrons) they produce after striking layers of tungsten. Credit: NASA/Goddard Space Flight Center Conceptual Image Lab
The Large Area Telescope (LAT) on Fermi detects gamma-rays through matter (electrons) and antimatter (positrons) they produce after striking layers of tungsten. Credit: NASA/Goddard Space Flight Center Conceptual Image Lab

Click here to view an animation of the LAT

Unidentified. More than 30 of the brightest gamma-ray sources Fermi sees have no obvious counterparts at other wavelengths. This one, designated 0FGL J1813.5-1248, was not seen by previous missions, and Fermi’s LAT sees it as variable. The source lies near the plane of the Milky Way in the constellation Serpens Cauda. As a result, it’s likely within our galaxy — but right now, astronomers don’t know much more than that.

The top five sources beyond our galaxy are:

NGC 1275. Also known as Perseus A, this galaxy at the heart of the Perseus Galaxy Cluster is known for its intense radio emissions. It lies 233 million light-years away.

Hubble Space Telescope image of a blazar galaxy.  Credit: NASA
Hubble Space Telescope image of a blazar galaxy. Credit: NASA

3C 454.3. This is a type of active galaxy called a “blazar.” Like many active galaxies, a blazar emits oppositely directed jets of particles traveling near the speed of light as matter falls into a central supermassive black hole. For blazars, the galaxy happens to be oriented so that one jet is aimed right at us. Over the time period represented in this image, 3C 454.3 was the brightest blazar in the gamma-ray sky. It flares and fades, but for Fermi it’s never out of sight. The galaxy lies 7.2 billion light-years away in the constellation Pegasus.

PKS 1502+106. This blazar is located 10.1 billion light-years away in the constellation Boötes. It appeared suddenly, briefly outshone 3C 454.3, and then faded away.

PKS 0727-115. This object’s location in the plane of the Milky Way would lead one to expect that it’s a member of our galaxy, but it isn’t. Astronomers believe this source is a type of active galaxy called a quasar. It’s located 9.6 billion light-years away in the constellation Puppis.

Unidentified. This source, located in the southern constellation Columba, is designated 0FGL J0614.3-3330 and probably lies outside the Milky Way. “It was seen by the EGRET instrument on NASA’s earlier Compton Gamma Ray Observatory, which operated throughout the 1990s, but the nature of this source remains a mystery,” Thompson says.

The LAT scans the entire sky every three hours when operating in survey mode, which is occupying most of the telescope’s observing time during Fermi’s first year of operations. These snapshots let scientists monitor rapidly changing sources.

The all-sky image released today shows us how the cosmos would look if our eyes could detect radiation 150 million times more energetic than visible light. The view merges LAT observations spanning 87 days, from August 4 to October 30, 2008.

Source: NASA