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>> A powerful outburst in a distant galaxy produced photons with high enough energies to be detected by ground-based telescopes for the first time First senten

by tejtm 7y ago

>> A powerful outburst in a distant galaxy produced photons with high enough energies to be detected by ground-based telescopes for the first time First sentence makes no sense. several galaxies produce photons with high enough energies to be detected by ground-based mark-1 eyeballs every night. Staggering numbers of galaxies produce photons with high enough energies to be detected by ground-based telescopes every night. They need to include the unusually short wavelength of the photons to convey their message.

_ph_ 7y ago
Well, the sentence has to be understood in the context of the headline, which specifically talks about gamma radiation, which so far was not possible to observe from earth as the atmosphere absorbs it. So this must have been a really scary event, sending photons over 4 billion light years, energetic enough to pass through the absorbing atmosphere. We can only hope to never be hit by a GRB from our cosmic neighbourhood, that would be a life-ending event.
- maxnoe 7y ago
  No gamma rays at these energies are able to penetrate the atmosphere and reach the ground. We only detect the particles indirectly. When a high energy particle enters the atmosphere, it creates a cascade of particles called an air shower. In that cascade, charged particles, mainly electrons and positrons, travel faster than the speed of light in air and thus produce Cherenkov radiation. This is light in the UV to blue visible range, you may know it from pictures of nuclear power plants. The telescope measure the Cherenkov light and from that reconstruct the properties of the particle that created the air shower. This event was not the first time we measured high energy photons on the ground, we do each night. But it was the first time we measured them from a GRB.
Voloskaya 7y ago
The subject of the sentence is "a powerful outburst" not "a distant galaxy". The powerful outburst produced photons with high enough energies to be detected by ground-based telescopes for the first time.
- ncmncm 7y ago
  It was part of a distant galaxy. The relevant part, as it turns out. Anything else there that might previously have been potentially relevant probably isn't anymore.
sandworm101 7y ago
If you can look up and see gamma rays with your eyeballs, marvel has a movie deal for you.
- Aperocky 7y ago
  The wording of the article made it sound like it went thru the atmosphere only because it’s got high energy. Which is wrong, it should be despite of.
  - strainer 7y ago
    Its not really wrong. The atmosphere absorbed most of it but some made it through because there was so much of it. The atmosphere happens to absorb most of the high energy wavelengths involved, it also absorbs many low energy wavelengths. There are 'windows' of wavelengths which get to pass through to different degrees. The details are intricately grasped by quantum electrodynamics, but don't really lend themselves to accessible wording.
    - salawat 7y ago
      I think it does actually. A gamma ray is just light, we would expect the signal to get attenuated early on in the atmosphere. This particular one had enough oomph to punch down to ground level instruments and actually generate a response. Either it lucked out statistically and had a momentary straight shot to the ground (I mean, it could happen), or the thing had so much energy it ploughed straight through anything that got in its way before dumping the rest of its energy in the detector/ground beneath it. Regardless, that is one hell of a signal. The worst case of starburn imaginable. Short of death by neutrino flux in proximity to a supernova any way.
      - maxnoe 7y ago
        This is not really how things work. We are talking about single particles of light having energies between ~ 50 GeV and up to 10 TeV (for this GRB, other sources go even higher). At these energies, the atmosphere is completely opaque and the gamma rays are absorbed at around 20 km height. However, the process kicks of a cascade of particles, the gamma ray is absorbed by creating an electron-positron pair, each with half the energy. These particles then create new gamma rays via bremsstrahlung and those gamma rays again are absorbed via pair production creating a cascade of high energy particles. The charged part of this cascade (electrons and positrons) are moving faster than the speed of light in air and are thus producing Cherenkov radiation, which is in the UV to the visible range. It is this light our telescopes detect and from this light we reconstruct the original properties of the first gamma ray that entered the atmosphere.
8bitsrule 7y ago
The visible light we see with our eyeballs - produced by electron transitions in atoms - has an energy of 2-3 eV; the atmosphere is transparent to them. It's mostly opaque to the reported energies, produced by the decay of the the nuclei of atoms. The universe is also opaque to these energies, because they interact with 'background photons' (and possibly electrons) in their travel. We're lucky that only a few 'make it through' (and require sensitive scopes to detect). https://science.sciencemag.org/content/320/5884/1752 https://science.sciencemag.org/content/320/5884/1752 (The energies in the new report are 10 times higher than those in the cited paper from 2008.)
perl4ever 7y ago
Maybe they mean it was the first time for that particular galaxy?
- ncmncm 7y ago
  First time for any galaxy, that we know of.
  - maxnoe 7y ago
    No, not really. We are talking here of the first time a short burst of only a few minutes was detected from a source not previously known. Distant galaxies with super massive black holes in their center are a well known source of this kind of gamma radiation. Known as active galactic nuclei and especially blazars
    - ncmncm 7y ago
      First time this energetic, by an order of magnitude. The suggestion is that they were produced by a different process that concentrated energy more than the usual sources. It's not hard to get more total energy released, just by being bigger. To concentrate more energy into individual photons requires more intense processes. A black hole crashing into a magnetar would cause unusually intense activity for some while.

by tejtm 7y ago

_ph_ 7y ago

Well, the sentence has to be understood in the context of the headline, which specifically talks about gamma radiation, which so far was not possible to observe from earth as the atmosphere absorbs it. So this must have been a really scary event, sending photons over 4 billion light years, energetic enough to pass through the absorbing atmosphere. We can only hope to never be hit by a GRB from our cosmic neighbourhood, that would be a life-ending event.

maxnoe 7y ago
No gamma rays at these energies are able to penetrate the atmosphere and reach the ground. We only detect the particles indirectly. When a high energy particle enters the atmosphere, it creates a cascade of particles called an air shower. In that cascade, charged particles, mainly electrons and positrons, travel faster than the speed of light in air and thus produce Cherenkov radiation. This is light in the UV to blue visible range, you may know it from pictures of nuclear power plants. The telescope measure the Cherenkov light and from that reconstruct the properties of the particle that created the air shower. This event was not the first time we measured high energy photons on the ground, we do each night. But it was the first time we measured them from a GRB.

Voloskaya 7y ago

The subject of the sentence is "a powerful outburst" not "a distant galaxy". The powerful outburst produced photons with high enough energies to be detected by ground-based telescopes for the first time.

ncmncm 7y ago
It was part of a distant galaxy. The relevant part, as it turns out. Anything else there that might previously have been potentially relevant probably isn't anymore.

sandworm101 7y ago

If you can look up and see gamma rays with your eyeballs, marvel has a movie deal for you.

Aperocky 7y ago
The wording of the article made it sound like it went thru the atmosphere only because it’s got high energy. Which is wrong, it should be despite of.
- strainer 7y ago
  Its not really wrong. The atmosphere absorbed most of it but some made it through because there was so much of it. The atmosphere happens to absorb most of the high energy wavelengths involved, it also absorbs many low energy wavelengths. There are 'windows' of wavelengths which get to pass through to different degrees. The details are intricately grasped by quantum electrodynamics, but don't really lend themselves to accessible wording.
  - salawat 7y ago
    I think it does actually. A gamma ray is just light, we would expect the signal to get attenuated early on in the atmosphere. This particular one had enough oomph to punch down to ground level instruments and actually generate a response. Either it lucked out statistically and had a momentary straight shot to the ground (I mean, it could happen), or the thing had so much energy it ploughed straight through anything that got in its way before dumping the rest of its energy in the detector/ground beneath it. Regardless, that is one hell of a signal. The worst case of starburn imaginable. Short of death by neutrino flux in proximity to a supernova any way.
    - maxnoe 7y ago
      This is not really how things work. We are talking about single particles of light having energies between ~ 50 GeV and up to 10 TeV (for this GRB, other sources go even higher). At these energies, the atmosphere is completely opaque and the gamma rays are absorbed at around 20 km height. However, the process kicks of a cascade of particles, the gamma ray is absorbed by creating an electron-positron pair, each with half the energy. These particles then create new gamma rays via bremsstrahlung and those gamma rays again are absorbed via pair production creating a cascade of high energy particles. The charged part of this cascade (electrons and positrons) are moving faster than the speed of light in air and are thus producing Cherenkov radiation, which is in the UV to the visible range. It is this light our telescopes detect and from this light we reconstruct the original properties of the first gamma ray that entered the atmosphere.

8bitsrule 7y ago

The visible light we see with our eyeballs - produced by electron transitions in atoms - has an energy of 2-3 eV; the atmosphere is transparent to them. It's mostly opaque to the reported energies, produced by the decay of the the nuclei of atoms. The universe is also opaque to these energies, because they interact with 'background photons' (and possibly electrons) in their travel. We're lucky that only a few 'make it through' (and require sensitive scopes to detect). https://science.sciencemag.org/content/320/5884/1752 https://science.sciencemag.org/content/320/5884/1752 (The energies in the new report are 10 times higher than those in the cited paper from 2008.)

perl4ever 7y ago

Maybe they mean it was the first time for that particular galaxy?

ncmncm 7y ago
First time for any galaxy, that we know of.
- maxnoe 7y ago
  No, not really. We are talking here of the first time a short burst of only a few minutes was detected from a source not previously known. Distant galaxies with super massive black holes in their center are a well known source of this kind of gamma radiation. Known as active galactic nuclei and especially blazars
  - ncmncm 7y ago
    First time this energetic, by an order of magnitude. The suggestion is that they were produced by a different process that concentrated energy more than the usual sources. It's not hard to get more total energy released, just by being bigger. To concentrate more energy into individual photons requires more intense processes. A black hole crashing into a magnetar would cause unusually intense activity for some while.