Big Bright Burst

This is hot off the stellar press: as NASA announced today (with cool pictures), a brilliant, long, and rather nearby GRB, or “gamma-ray burster”, was observed on April 27th, initially by the Fermi and Swift satellites.  Gamma-rays are just an old name for photons (i.e. particles of light) which have lots more energy (per photon) than the photons of visible light.   And a GRB is a distant astronomical explosion that produces an enormously bright flare of these high-energy photons, typically for a short time (seconds or minutes), though this one lasted for hours.   It is believed that a narrow jet of high-energy particles produced in a supernova (a powerful explosion of a star) is behind these flares, but they are still poorly understood and are under active study.

Everything about last week’s GRB is on the exceptional side.  The most energetic photon detected had somewhat more energy than the photons produced in the decays of Higgs particles, a bit less than the energy of the photons that Fermi might be seeing from dark matter, and more than three times more energy than any GRB photon previously detected by Fermi. Its gamma rays were produced for many hours, setting another record.  It lasted so long that several other types of telescopes were able to observe it, including those that look at visible light (it was even seen by an amateur astronomer), and those that look at radio waves (which are made from photons with vastly lower energy).  And it was relatively close… well, relatively compared to most GRB’s.  It occured in a galaxy 3.6 billion light years away.  Now that is still a good fraction of the distance across the visible part of our universe, but still, it puts this GRB in the top 5% as far as proximity to Earth.

With such a vast amount of data to work with, it seems very likely that astronomers will learn qualitatively new things about GRBs by studying this blast.  In astronomy, it sometimes takes just one spectacular event to change the scientific landscape!  The next phase of the process will involve directly detecting the lesser (but still intense) glow from the (presumed) supernova that produced the GRB flare.  Stay tuned!  It should be a matter of a week or so…

85 responses to “Big Bright Burst

  1. I know all the neutrino guys have procedures in place to take supernova data (a friend works on T2K and had the practice alarm go off on him once). I know you’re not an astrophysicist, but should we have expected the large neutrino detectors to have gone off as well if the burs did come from a supernova?

    • Good question: 3.6 billion light years away is (presumably) way too far. (It wasn’t so easy to detect the supernova in the Large Magellanic Cloud, less than 200,000 light years away, using neutrinos.) The reason we can detect the photons from so far away is simply that photons are much easy to detect; if one goes into your detector, it will generally hit something. Most neutrinos that enter your detector will go right through it — in fact they’ll go right through the earth without banging into anything. So the number of neutrinos that must pass through your detector, for even one of them to leave a trace, is enormous.

      The GRB flare itself involves particles that have (presumably) been turned into a beam, by the star’s magnetic fields. That’s why it’s so bright. The neutrinos aren’t affected by the magnetic fields, so we would not expect many extra-energetic neutrinos in this beam.

      So if this is really a supernova (and not something truly exotic) I would not expect any neutrinos to be detected. Of course I’d be thrilled to be wrong!


      • The neutrinos produced in the core-collapse of the star are unlikely to be observed (they have energy in the range of ~MeV, and are emitted isotropically; the flux at the earth from very-extra-galactic sources will be far too small for detectors like Super-K to observe them), but it’s expected that neutrinos produced in the collimated jet via interaction of protons with photons and subsequent pion decay could be observed at the earth. These neutrinos would be high-energy (>TeV) and beamed, making their flux at the earth large enough that detectors like IceCube or Antares could find a small number.

        Last year IceCube published an upper limit on high-energy neutrinos from GRBs which constrainted some of the parameters for the collimated jet that erupts from the dying star:

        • Thanks for the correction, Dan. I had thought these secondarily-produced neutrinos would be too few for an observation. But I had forgotten about the fact that Ice Cube was actively looking for this phenomenon.

          For the latest GRB, can one roughly estimate the number and spectrum of neutrinos based on the flux of photons measured by Fermi? Should we *expect* that they will make a detection, just based on what Fermi has observed already? Or are you telling me that the uncertainties on these collimated jets are far too large for any such estimate?

        • JollyJoker

          How long would the flight time of a MeV neutrino be compared to light from a supernova that far away? I realize we don’t have a lower mass limit for electron neutrinos, but assuming 1 meV or something?

          In a related question, is there any chance of the larger detectors seeing neutrinos with low enough energy that they would arrive noticeably later than the higher-energy ones?

          • Let’s give the neutrino a mass of 1ev ((A possible mass by many accounts.) This would give it a speed of 0.999999999999c (Roughly) This means that a 1MeV neutrino emitted at the same time as a photon and traveling 3.6 billion light years would arrive on earth about 0.0036 years later or about 30 hours later.

          • Kudzu’s calculation is right (up to a factor of 2); but notice the time delay goes like m^2/E^2. So if the neutrinos have a mass of 0.1 eV (rather than 1) and if you see them at 10 MeV rather than 1 MeV (note: energetic neutrinos interact more strongly with matter and are more easily detected), then the delay is just a few seconds, much shorter than the length of the main GRB. And for neutrinos in the many GeV to TeV range, there will be no observable delay.

          • JollyJoker

            …and Ice Cube isn’t sensitive to energies much less than 100 GeV, so that in itself lowers the time delay by ten orders of magnitude from 30 hours. Six more if the mass is a millielectronvolt instead of an ev. About 10^-11 seconds then.

            Apparently we’re far from measuring time delays. It’s almost absurd that they’re so light that a significant fraction of the age of the universe is too short a time.

  2. As the EM spectrum extends with no limit , i always wondered ; what lies beyond the gama energy …..what powerful kinds of photons we expect ?

    • We call everything about 1 MeV or so a “gamma ray”. The nomenclature is historical and not so useful anymore for particle physicists. You might just as well call it a “high-energy photon”.

  3. Why would the GRB flare arrive before the glow from the supernova?

    • The GRB flare is believed to be caused by jets of material produced by the explosion itself, somehow punching though the material around the star at the time of the explosion itself. Once the jets die out, so does the GRB flare.

      The glow is caused by a more complex series of processes, as lower-energy photons from the explosion heat the gas that was kicked off the dying star, long before the explosion. The glow becomes brighter for about a week or two as the star’s exploding fireball expands, temporarily overcompensating for cooling of the fireball. You can learn a bit more here:

  4. is there a paper on how the 125 higgs affects supernova models, or is it too early for that ? Also just wondering if there is a new Higgs twist on the m in E = mc^2 .

    • If supernova models were much affected by the Higgs mass, then we could have used supernovas to learn something about the Higgs mass before it was measured directly. Since no one tried to measure the Higgs mass that way, you can guess: there’s no effect.

      No, there’s no change in the m in E = mc^2. The Higgs field affects how large the masses of certain particles are; but it does not affect the general Einstein relation between energy, momentum and mass.

  5. Then in theory does the high energy extend to infinite values or ther is some mechanism to taper it ?

    • We don’t know of any mechanism to cut it off. However, we also don’t know anything about such energies experimentally.

      • There is a well known limit for ultra high energy cosmic rays, it is the GZK limit, about 5x 10^19 eV, at least for charged cosmic rays. It comes from the interaction of such high energy cosmic rays wiyh the photons of the cosmic background (the relic of the big bang, recently studied by Planck Mission). At such energies, in the “center of mass”, the interaction of particles is sufficient to produce pions, then they loose energy all along the away till they come under 5x 10^9 eV.
        GZK comes from his discoverers, one american and separatly two russian, Greisen, Zatsepin andKuzmin, in the 1960 (after discovery of the CMB).
        I think photons can interact with cosmic background photons to produce various particles (why not a Higgs…).

        • The center-of-mass energy of two colliding massless (or nearly massless) particles is equal (or nearly equal) to the square root of 2 E1 E2, where E1 and E2 are the energies of the two particles. This formula is true in any (or almost any) frame; it requires only that both particles be traveling at (or near) light speed.

          To make a Higgs boson off cosmic microwave background photons (with energies about 0.1 meV = 10^(-4) eV) would require the more energetic of the two photons to have energy of order 10^26 eV = 10^17 GeV, close to the Planck scale. We’ve never seen a cosmic ray anywhere up that high.

          Even to make an electron-positron pair in a collision of an energetic gamma-ray with a cosmic microwave background photon would require it to have energy of 10^16 eV = 10^7 GeV = 10 million GeV.

          The highest-energy photon observed during this GRB had energy of about 10^11 eV = 100 GeV. Not even close. Such photons have essentially no interaction with the cosmic microwave background.

          It’s very important to put numbers to work!

          • Yes, numbers are important, and I did not did the calculation, and I was a littel bit exagerating (or joking) that such collisions could produce a Higgs. But indeed, there is a limit, they should loose energy above this limit, even if it is very high!

          • That is certainly correct.

          • This is an interesting topic in itself. It might be nice to have a (short) post on it sometime. Possibly relating to your excellent article on particle ‘annihilation’.

        • Marshall Eubanks

          Note that about 20 or so cosmic ray particles have been detected with energies > 5 x 10^19 ev. I know of no solid determination as to the distances of their sources (and thus whether they are inside or outside the GZK horizon).

        • I am curious, why does this limit not relate to lighter particles, positron-electron pairs? Must the particles produced be electrically neutral (as opposed to a pair of opposite charges)?

  6. Can you suggest a paper that would give an idea of the hydrodynamics involved in producing these jets in a supernova explosion? If I understand correctly they are not present in all supernovae.

    • I’d have to do a web search, just like you. These are very complicated processes, and the various types of supernova explosions are still not fully understood.

  7. Fair enough, I’ll do a web search.

  8. I had read that the photon burst from a supernova which outshines the galaxy it contained is only 1% and the 99% of the explosive output is in the form of neutrinos. They said neutrinos also have an important feature in that they escape the exploding star BEFORE the photon which enable astronomers to anticipate the photon stream from a supernova. And that there exists a network of neutrino detectors around the world that belongs to SNEWS (Supernova Early Warning System) that in the event a burst of neutrinos is detected, SNEWS alerts the astronomy community and the amateur astronomer community as well.

    • **”The SuperNova Early Warning System (SNEWS) is a network of neutrino detectors designed to give early warning to astronomers in the event of a supernova in the Milky Way galaxy or a nearby galaxy such as the Large Magellanic Cloud or the Canis Major Dwarf Galaxy. Enormous numbers of neutrinos are produced in the core of a red giant star as it collapses on itself. In the current model the neutrinos are emitted well before the light from the supernova peaks, so in principle neutrino detectors could give advance warning to astronomers that a supernova has occurred and may soon be visible. The neutrino pulse from supernova 1987A was detected 3 hours before the associated photons (although SNEWS was not yet active).

      The current members of SNEWS are Borexino, Super-Kamiokande, LVD, SNO and IceCube. SNO is not currently active as it is being upgraded to its successor program SNO+.

      As of June 2011[needs update], SNEWS has not issued any SN alerts.”**

      • See the discussion with Dan above. The host galaxy’s much too far away for the basic neutrinos from the supernova; they are too few and too low-energy to be detected. But neutrinos created by particles in the jet interacting with material outside the star might be high-enough energy, and sufficiently beamed toward us, to be detected by a detector like Ice Cube. I’m looking into the details of this more carefully.

  9. veeramohan

    So heavy neutrinos are nearly equal to high-energy photons (from above GRB) in it’s speed ? – they could be detected and also equal in escaping dark matter gravitational pull – outside 3D spacetime ?

    • They could be detected in principle, the problem being they are not focused like the photons are and also are much harder to detect. Right now the position is ‘We might be able to detect them, but don’t count on it.’

      • That’s not correct; there are unfocused ones that we have no hope of detecting, but there is a focused source as well, as I’ve been reminding myself as I read the literature.

        • Aaah, very true. Thanks for the correction.

        • veeramohan

          Thank you Mr Kudzu, Proff Strassler,
          The unique high energy (above 135 GeV?) of photons from above GRB can tell, they came from dark matter annihilation – from the regions of galactic center.
          This uniqueness is the capacity (high energ) to escape dark matter gravitational pull – in outside 3D spacetime – which is not the realm of less energy photons.

          • veeramohan

            Sorry question mark at the end.

          • There’s no connection between GRB photons and dark matter. Nor is there a “cutoff” in the energies of photons from the recent GRB; there was just a dribbling of high-energy photons, and the last one happens to have been nearly 100 GeV. The physics of an exploding star is completely unrelated to the annihilation of dark matter in the center of the galaxy. See the new post on neutrinos from the GRB.

  10. kashyap vasavada

    What is the range of energies of these gamma rays? Do they come from fusion of heavy nuclei in supernovas in such a short time? Is there any chance that some of them could be due to particle-antiparticle annihilation?

  11. I read that shortest wave-length is 10 ^-15 but could be shorter with no limit , it is very stimulating for imagination just to imagine what might exist beyond that !!

  12. What is the wave length of that burst ?

    • ??? Bursts don’t have wavelength; they produce a vast number of photons, and each photon has a different wavelength. The wavelengths extend from ultramicroscopic to macroscopic.

  13. Iwas thinking about what Alain explained , the GZK limit which smolin wrote that violating it would be the most important discovery changing the 20 th century physics………thanks Alain

    • Well, Smolin is (of course) referring to his own theories; but indeed, were the GZK cutoff violated, that would open up a new area of research, one which might not have anything to do with Smolin’s work. And it might not be the most important discovery changing 20th century physics — that’s hype.

  14. I meant burst`s photon energy hence wavelength.

    • Same answer: the burst does not have a photon energy. It produces photons with a huge range of energies. What are you asking? I told you the energy of the most energetic photon observed. That of course is probably not the energy of the most energetic photon produced; only a teensy teensy fraction of the photons go into the Fermi detector.

  15. What about Marshall`s comment of events beyond the GZK ? are we to anticipate some exotic physics ?

    • No,

      The AGASSA experiment in Japan had indeed reported the observation of more events than was expected (10 times more), but it was not confirmed either by Auger observatory in Argentina nor HiRes in Utah. There should have been systematic errors not considered in AGASSA.

      In fact, it is possible to see ultra high energy cosmic rays above the GZK limit (or better cutoff), that is 5x 10^19 eV. These (charged) cosmic rays, mostly protons it is believed, interact with the photons of the cosmic background (2.7 K) emitting pions. It is a slow process, it takes time to loose energy so that they come under the threshold. It has been calculated taht it requires some 50 Mpc (163 millions of light years) to achieve this. So if these cosmic rays have been produced inside or near our galaxy, they still could have an energy beyond the GZK treshold.

      50 Mpc is a great distance, but it is meanwhile the neighbouring of our galaxy. The observed gamma ray burst, even one of the closest observed, is still 3.6 billions light years…

      See :

  16. Mike Anthis

    Might the Sun (and other stars) be sensitive to interstellar signals, such as GRBs or neutrino bursts? If it is, some of the effects would take time to reach the photosphere, and would show up later. Might we find some gamma ray or neutrino history by looking for delayed signals in the solar radiation? Might the 22 year cycle be interstellar?

    • There are a few problems with a mechanism like you propose. Firstly the 22 year cycle is a *cycle* meaning that it is quite regular. It would be difficult, if not impossible, to think of a stellar phenomena that could in any way produce such a regular cycle over the long (to us at least) period it has been observed as opposed to a random and chaotic result.

      The second problem is the sheer energy of the sun. While GRBs are powerful where they occur, at this distance even the most energetic adds only an infinitesimal fraction of its energy to our solar system. The sun produces a massive amount of energy, the oft-cited factoid being 4 tons of mass into energy every second.

      This relates to the third problem, the fact that such energy causes the sun to radiate energy and mass in all directions. The solar wind and radiation strip mass constantly from the planets (Mostly as gases.) and we have been hoping for an increase in background radiation to tell us when the Voyager probe leaves the solar system’s protective magnetic field.

      Finally the sun’s photosphere is far, far less than our atmosphere relative to the sun, a faint corona of soon-to-be-ejected plasma and it takes an incredibly long and turbulent time for even light to get *out* of the sun, let alone into it. It is possible GRBs might have some exotic effect, but it is hard to imagine what it could be.

    • The time for neutrinos from the sun’s interior to reach the photosphere is just light-travel time. The time for photons to reach the photosphere (more precisely, the time for changes in the sun’s interior to reach the surface via photons) is a million years. So the timing’s a bit off…

      Moreover, I don’t see a way (with known or imaginable physics) that any interstellar signal could measurably affect the sun, yet be difficult for us to detect on earth using modern scientific technology. (I might think differently if our understanding of the sun was as weak as it was 40 years ago — but we know, via helioseismology, that we understand it very well now.) So — without having thought about it carefully — I don’t immediately see this line of reasoning as being very promising.

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