6 thoughts on “Cosmic retrospective — gamma ray bursts”

1. Regarding the origins of bursts and how the energy of gamma rays can vary tremendously, this Physics World article “Cosmic gamma-ray energy record shattered by high-altitude observatory” (06 Jul 2019) highlights that range.

   Cosmic gamma rays with energies as high as 450 TeV (1012 eV) have been observed by the ASgamma observatory in Tibet … This shatters the previous record of 75 TeV, which was set by the High-Energy-Gamma-Ray Astronomy observatory on the Canary Islands.

   ASgamma detected 24 gamma rays with energies in the 100-450 TeV range. The particles appear to originate in the Crab Nebula, which is a supernova remnant about 6000 light-years away. It is home to a pulsar – a rapidly rotating neutron star that broadcasts a bright beam of electromagnetic radiation.

   … the ASgamma researchers describe the Crab Nebula pulsar as “the most powerful natural electron accelerator known so far in our galaxy”.

2. Here’s an interesting article about the speed of gamma-ray jets from cosmic beacons: Forbes > “Ask Ethan: Can Gamma-Ray Jets Really Travel Faster Than The Speed Of Light?” by Ethan Siegel, Senior Contributor (Oct 5, 2019).

   Remember that experiment (or video of a demonstration) in physics lab showing what happens to the speed of light when a laser beam is split into two paths, one through air (or vacuum) and one through water? (See this Fermilab YouTube video for an explanation of the actual physics.)

   … why, … was there a recent story claiming that gamma-ray jets, where gamma-rays themselves are a high-energy form of light, can travel faster-than-light?

   When you pass light through a medium, … its electric and magnetic fields interact with the particles in the medium, and the light is forced to move at a slower speed: the speed of light in that particular medium. … the amount that light slows down by depends on the light’s wavelength.

   [The radiation in the story] is not moving faster than c, the speed of light in a vacuum, but v, the speed of light in the particle-filled medium [matter-rich environment] surrounding the source of these gamma rays.

   What the researchers did was introduce a new, simple model that would explain the bizarre properties seen in pulsing gamma-ray bursts. They model the gamma-ray emissions as originating from a jet of fast-moving particles, which is consistent with what we know. But they then introduce a fast-moving impactor wave that runs into this expanding jet, and as the density (and other properties) of the medium changes, that wave then accelerates from moving slower-than-light to moving faster-than-light in that medium.

   [The radiation is] slower than light through the medium for one part of the journey and faster than light through the medium for another part of the journey [and is observed as separate pulses] …

   References

   Cherenkov radiation

   Compton radiation

   UCLA Physics > YouTube video (2013) “How to Measure the Speed of Light”

3. A multinational global project to build a new generation of ground-based gamma-ray telescopes moved forward this year with successful operation of a prototype telescope.

   Science Daily > “Scientists detect crab nebula using innovative gamma-ray telescope” – First-of-its-kind telescope promises to shed new light on the physics of high-energy phenomena, from supernovae to dark matter by Columbia University (June 2, 2020).

   Scientists have detected gamma rays from the Crab Nebula, the most famous of supernova remnants, using a next-generation [prototype] telescope that opens the door for astrophysicists to study some of the most energetic and unusual objects in the universe.

   Over time the light from the supernova faded away, leaving behind the remains of a powerful, rapidly spinning neutron star, or pulsar, that can still be seen within a cloud of gas, dust and highly energetic subatomic particles, which emit radiation across the electromagnetic spectrum. The most energetic of those particles radiate gamma rays.

   “The camera triggers upon bursts of light that occur when a gamma ray collides with an air molecule, and records these signals at a rate of a billion frames per second,” said Humensky, who collaborated with colleagues at Barnard College to build major components of SCT’s mirror alignment system and develop its control software. “This allows us to reconstruct the gamma rays with extraordinary precision.”

   The sighting of the Crab Nebula, announced at the 236th meeting of the American Astronomical Society June 1, lays the groundwork for the use of the SCT in the future Cherenkov Telescope Array observatory. Slated for completion in 2026, the observatory, with its configuration of 120 telescopes of varying sizes split between Chile and Spain’s Canary Islands, will detect sources of gamma rays 100 times faster than current instruments.

4. More insight into (and a new model of) the sources of gamma rays.

   • SciTechDaily > “Mysterious ‘Empty Sky’ Gamma-Ray Puzzle Solved – May Lead Astrophysicists To Unravel Dark Matter” by Australian National University (September 20, 2021)

   (quote) Star-forming galaxies are responsible for creating gamma-rays that until now had not been associated with a known origin, researchers from The Australian National University (ANU) have confirmed.

   Lead author Dr. Matt Roth, from the ANU Research School of Astronomy and Astrophysics, said until now [since the 1960s] it has been unclear what created gamma-rays — one of the most energetic forms of light in the Universe — that appear in patches of seemingly ’empty sky’.

   .. Cosmic rays are important because they create large amounts of gamma-ray emission in star-forming galaxies when they collide with the interstellar gas.

   1. when gas falls into supermassive black holes.

   2. via star formation in the disks of galaxies (the majority of this diffuse gamma-ray radiation).

5. A cosmic flash from a powerful type of supernova? Multi-wavelength observations may answer the question.

   • Space.com > “Astronomers just spotted the most powerful flash of light ever seen” by Tereza Pultarova (October 14, 2022) – The gamma-ray burst [GRB221009A] was also the nearest [2.4 billion light-years away] ever detected.

   The so-called gamma-ray burst, the most energetic type of electromagnetic explosion known to exist in the universe, was first spotted by telescopes Sunday (Oct. 9). … these explosions unleash as much energy as the sun will emit during its entire 10-billion-year lifetime.

   The flash detected Sunday was the strongest one ever observed, releasing 18 teraelectronvolts of [aggregate] energy.

   [Astronomer Gemma Anderson, Curtin University in Australia] “… the radio, optical, X-ray and gamma-ray light it produces is extremely bright and therefore easy to observe. We can therefore study this [“long”] gamma ray burst with lots of big and small telescopes around the world and collect very comprehensive datasets as it first brightens and then fades away.”

   Here’s a visualization.

   • NASA > APOD > GRB 221009A (2022 October 15 )

   
   Image Credit: NASA, DOE, Fermi LAT Collaboration

   Explanation: Gamma-ray burst GRB 221009A likely signals the birth of a new black hole, formed at the core of a collapsing star long ago in the distant universe. The extremely powerful blast is depicted in this animated gif constructed using data from the Fermi Gamma Ray Space Telescope.

   Fermi captured the data at gamma-ray energies, detecting photons with over 100 million electron volts. In comparison visible light photons have energies of about 2 electron volts.

   A steady, high energy gamma-ray glow from the plane of our Milky Way galaxy runs diagonally through the 20 degree wide frame at the left, while the transient gamma-ray flash from GRB 221009A appears at center and then fades.

   One of the brightest gamma-ray bursts ever detected GRB 221009A is also close as far as gamma-ray bursts go, but still lies about 2 billion light-years away.

   In low Earth orbit Fermi’s Large Area Telescope recorded gamma-ray photons from the burst for more than 10 hours as high-energy radiation from GRB 221009A swept over planet Earth on Sunday, October 9, 2022.

   Here’s another visualization.

   • NASA > APOD > X-Ray Rings Around a Gamma Ray Burst [GRB 221009A] (2022 October 17)

   

   Image Credit: NASA Swift Obs.; Data: B. Cenko (NASA’s GSFC), A. Beardmore (U. Leicester) et al.; Processing: J. Miller (U. Michigan)

   Explanation: Why would x-ray rings appear around a gamma-ray burst? The surprising answer has little to do with the explosion itself but rather with light reflected off areas of dust-laden gas in our own Milky Way Galaxy.

   GRB 221009A was a tremendous explosion – a very bright gamma-ray burst (GRB) that occurred far across the universe with radiation just arriving in our Solar System last week.

   Since GRBs can also emit copious amounts of x-rays, a bright flash of x-rays arrived nearly simultaneously with the gamma-radiation.

   In this case, the X-rays also bounced off regions high in dust right here in our Milky Way Galaxy, creating the unusual reflections. The greater the angle between reflecting Milky Way dust and the GRB, the greater the radius of the X-ray rings [How about a geometric diagram?], and, typically, the longer it takes for these light-echoes to arrive.

6. 

   This article is a useful recap of gamma rays. It discusses where these high-energy photons fit in the electromagnetic spectrum (compared to visible light); their energy levels, production, health hazards (protection measures), history of detection; cosmic bursts and current space observatories. As well as some useful references.

   However, with a degree in physics and astrophysics, Cooper still introduces his article by using the popsci legacy characterization of photons as particles which can behave as waves. Rather than, as in quantum field theory, that all elementary so-called particles are localized excitations in fields. Packets of energy modeled as waves. (As physicist Art Hobson wrote in 2017, do radio telescopes look like they’re capturing particles? – see Point particles RIP).

   • Space.com > “Gamma rays: Everything you need to know about these powerful packets of energy” by Keith Cooper (December 28, 2022) – Gamma rays can only be detected by sensors made of dense metals and takes over six feet (1.8 meters) of concrete to block.

   (quote) Photons of light are massless particles that are essentially packets of energy. Because of a quantum-mechanical phenomenon known as wave-particle duality, particles can behave like waves, and photons are no different. Photons have wavelengths, and the amplitude of their wavelength determines where they sit on the electromagnetic spectrum.

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