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.
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.
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.
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.
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.
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.
Sweet. Figured someone would be along with a more reasonable explanation shortly.
Thank you for that. I figured there were processes in there I wasn't aware of,and that fits with my mental model a lot better than something I kicked out while falling asleep.
Ah so effectively none of the Hev photons reach the ground. I see the papers include the "Major Atmospheric Gamma Imaging Cherenkov (MAGIC) telescopes" data. I wouldn't have expected imaging to be practical because cherenkov is kind of scattering, but they seem to be able to triangulate individual collisions. Thanks for the correction.
We basically see a blob of light from which we can deduce origin, energy and particle type of the particle that created the air shower.
If you have multiple telescopes observing the air showers this gets much better. MAGIC uses two 17m telescopes, VERITAS 4 12 m telescopes and HESS 4 12m and one 28 meter telescope.
CTA is currently in the planning and construction phase and will build around a hundred telescopes at two sites, Chile and La Palma.
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).
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
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.
Depends on the GRB. A supernova 50 light years away is supposed to eliminate all large animal life (including us). Fortunately any stars capable of it are more than 100 Ly away, where they would only eliminate, e.g., civilization.
A magnetar quake 500 light years away would sterilize the planet (but not harm the magnetar at all). Fortunately, the nearest magnetar we know about is 9000 light years away, so its blast would be ~300 times weaker, just enough for a garden-variety mass extinction, and end of civilization. But there might be one closer.
All the ones we detect happen millions of times farther away. So they are only terrifying if you think about how we can even tell something happened that far away.
Are these threats so sparse in the whole universe, or is our region special?
Is it unlikely that we are far enough to be mostly safe from these threats?
Or maybe it's one filter for Fermi paradox (on a planet that isn't far from these kinds of threats life wouldn't survive long enough to develop civilization).
There are myriad^n places in the universe where such threats are infrequent enough for civilization to operate for millions, even billions of years.
The Fermi paradox needs another filter. Some people think it is that interstellar civilizations that arise come into contact with one another, destroying both.
“ Gamma rays are the highest-energy form of radiation, with wavelengths that can be smaller than the nucleus of an atom. (Radio waves, for comparison, have wavelengths ranging between about a millimeter to hundreds of kilometers.)”
This is so insane to me. I knew the wavelengths were small but not this small. Nature is so cool!
I'm still not grasping it either, to the point that I feel we may eventually learn they are neither, something else that looks to us sometimes like a particle and sometimes like a wave.
Astrophysicist here, I'm currently finishing my PhD, mainly focused on data analysis for the kind of telescopes that measured this GRB, while my main focus is on a smaller prototype telescope (FACT), I also work for the one that detected this GRB (MAGIC).
For the satellites all data is publicly available as required by NASA.
Unfortunately all ground based telescopes until now keep their data private but nothing is thrown away ;)
We have several Million air showers recorded.
This is going to change with the next gen observatory, which will operate as an open observatory.
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[ 3.1 ms ] story [ 96.5 ms ] threadFirst 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.
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.
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.
The powerful outburst produced photons with high enough energies to be detected by ground-based telescopes for the first time.
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.
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.
Thank you for that. I figured there were processes in there I wasn't aware of,and that fits with my mental model a lot better than something I kicked out while falling asleep.
If you have multiple telescopes observing the air showers this gets much better. MAGIC uses two 17m telescopes, VERITAS 4 12 m telescopes and HESS 4 12m and one 28 meter telescope.
CTA is currently in the planning and construction phase and will build around a hundred telescopes at two sites, Chile and La Palma.
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
(The energies in the new report are 10 times higher than those in the cited paper from 2008.)
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
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.
A magnetar quake 500 light years away would sterilize the planet (but not harm the magnetar at all). Fortunately, the nearest magnetar we know about is 9000 light years away, so its blast would be ~300 times weaker, just enough for a garden-variety mass extinction, and end of civilization. But there might be one closer.
All the ones we detect happen millions of times farther away. So they are only terrifying if you think about how we can even tell something happened that far away.
Is it unlikely that we are far enough to be mostly safe from these threats?
Or maybe it's one filter for Fermi paradox (on a planet that isn't far from these kinds of threats life wouldn't survive long enough to develop civilization).
The Fermi paradox needs another filter. Some people think it is that interstellar civilizations that arise come into contact with one another, destroying both.
This is so insane to me. I knew the wavelengths were small but not this small. Nature is so cool!
The energies we detect go into the hundreds of TeVs and charged cosmic rays have been detected to much, much higher energies.
There is just now word that maps that to ordinary terms.
AMA ;)
Unfortunately all ground based telescopes until now keep their data private but nothing is thrown away ;) We have several Million air showers recorded.
This is going to change with the next gen observatory, which will operate as an open observatory.