The 1960’s was the height of the Cold War. It was an era of mutual and, some say, healthy mistrust between superpowers. The nuclear test ban treaty had been signed as a way to deescalate tensions. But as with any treaty its success depended on each party doing its part to honor the terms of the agreement. So each side closely monitored the other. The United States was alert to the possibility that the Soviet Union would try to skirt the terms of the treaty by testing nuclear weapons in space. Ground based testing was easy to monitor. Just look for the distinctive seismic signatures of a nuclear detonation. Space-based blasts were more of a challenge to spot. In anticipation of a treaty violation the U.S. Air Force launched the Vela series of military satellites in hopes of detecting the distinctive gamma radiation that is produced by the triggering of a nuke.
On July 2nd, 1967 an event occurred that raised eyebrows among the Vela monitoring team. It seemed that two of the Vela satellites had detected an unusual flash of gamma radiation. It was unusual because the flash did not resemble that of any known type of nuclear weapons signature. It turned out that this was the first detection of what later became known as gamma ray bursts. Their source was unknown but one thing became obvious, they had an extraterrestrial, extrasolar origin.[i]
So what kind of event could produce such fantastic amounts of energetic radiation? The search began for culprits. Most were attributed to intergalactic sources. I remember one particularly interesting, but highly speculative, possibility in the popular science press. Could we have been witnessing an interstellar war taking place between space-faring civilizations? Keep the context in mind. These were the years of the first Star Wars trilogy. These were also the years of the United State’s Strategic Defense Initiative or SDI which later became popularly known as “Star Wars.” In retrospect, it didn’t seem like such a kooky idea. What really did seem preposterous was still to be discovered.
In 1991 NASA launched it Compton Gamma Ray Observatory on a nine year mission to uncover the sources of these enigmatic GRB’s among other things.[ii] The data that the observatory streamed back only deepened the GRB mystery.
Figure 1. Isotropic distribution of GRB's
Astronomers were expecting to see a map of sources that roughly conformed to the galactic plane since most had assumed that the GRB’s had a “local origin.” Compton’s map showed an isotropic distribution of GRB sources instead.[iii] What this implied was that GRB’s were extragalactic in origin and this was truly startling. The reason that astronomers did a collective double-take was that an extragalactic origin with an isotropic distribution coupled with their observed redshift values meant that distances to the GRB’s had to be measured in mega to gigaparsecs![iv] The first to be reliably distanced was GRB 970508 at just under 2 Gigaparsecs. [v] That meant that GRB’s had to be bright… very bright. In order to shine as brightly as they do in highly energetic gamma radiation there had to be something radically different at their hearts to power these flashes. Calculations of the required energy flux for some sources revealed something quite extraordinary. In fact it had to be so extraordinary that it was thought to be “un-physical.” At those distances even the most massive star would not be able to emit the required energy (bolometric flux) for some observed GRB’s at even a 100% mass to energy conversion. So either we had to get a better understanding of what was going on or admit that this was a new kind of physics. Clearly further study was required.
GAMMA RAY OBSERVATION
Figure 2. Focusing X-rays. Ineffective for gamma rays.There are two enormous challenges to observing gamma ray bursts. First is that gamma rays are difficult (if not impossible) to “focus.” Gamma rays are so energetic that they pass though lenses and mirrors or are absorbed. So instruments of a design that are similar to those used by optical astronomers won’t work.
Even instruments that are designed to image energetic X-ray sources by focusing the (x-ray) light using grazing incidence mirrors are ineffective with gamma rays. [vi] [Fig. 2.]
The second problem is that GRB’s are relatively short duration events lasting from just over one second to only a few minutes.[vii] [Fig. 3]
Figure 3. Light curves of GRB's showing event durations
So in order to get an observation you either needed to be extraordinarily lucky to have your telescope pointed in the right part of the sky at the right time or you needed to have a very responsive collection of instruments. One of these instruments was NASA’s SWIFT[viii].
Swift is a satellite (and collection of instruments) designed to rapidly detect, locate and classify GRB’s and then alert ground based teams so that the afterglow of the GRB could be studied. This mission has observed and classified well over 500 GRB’s. Something else was needed though… an observatory that could quickly detect GRB’s and image them at resolutions that were previously unattainable.
FERMI
Last February 9th, I was fortunate enough to attend a talk by Dr. Lynn Cominsky here in Sonoma County (CA) on GRB’s. Dr. Cominsky is one of the scientific co-investigators for the Fermi observatory as well as Swift (and the education and public outreach lead for Fermi, Swift, XMM-Newton and Nustar).[ix] Dr. Cominsky’s principal area of interest is high-energy astronomy.Fermi (TOFKAG)[x] brought exciting new capabilities to the imaging of gamma rays. Dr. Cominsky explained that Fermi uses two separate instruments to sense and image a GRB. First, is the GLAST Burst Monitor… it is sensitive to (softer) gamma rays and x-rays with energies in the range of 8 KeV to 30 MeV. GRB photons have energies in the neighborhood of 100 MeV[xi] so the GBM triggers an “alert” that a high energy event is going on but it can’t be specific that it’s a GRB. Further, the GBM doesn’t discriminate by direction as it is an omnidirectional detector… except that the Earth acts as an effective shield. So if the GBM is triggered, the event must be in the direction away from the Earth. When the GBM detects a bright gamma ray source it “alerts” the 3-ton LAT instrument in addition to numerous ground based manned and robotic telescopes.
LARGE AREA TELESCOPE
When Dr. Cominsky began describing the LAT she did so with equal measures of pride and giddy enthusiasm. LAT solves the problem of how to image the highest energy photons (20 KeV to 300MeV) by converting them directly into matter then watching how that matter scintillates in the detector arrays. It’s actually pretty cool!A gamma ray barrels into the LAT and passes through an anti-coincidence detector to reduce background noise. It then shoots through an array of tungsten foils. When our gamma ray strikes (interacts with) a tungsten atom its energy is converted directly into mass as a particle-antiparticle pair (the pair are an electron and a positron to conserve charge). [Fig 4.]
Figure 4. LAT instrument.
The newly minted electron-positron pair cascade down the detector passing silicon strip detectors that track their position and progress so that the direction can be evaluated. At the base of the detector the electron-positron pair slam into a cesium iodide calorimeter that then evaluates the energy of not only the scintillating pair but also of the energy of the original gamma ray photon. LAT is an orbiting telescope that’s constructed like a detector in a sophisticated particle accelerator. [xii] And it’s surprisingly capable as an imaging device. [Fig. 5]
Figure 5. Fermi’s whole sky view showing gamma ray sources in the galactic plane and beyond.
SO WHAT ARE THEY?
Gamma rays are produced thermally (at as much as billions Kelvin), and by the relativistic acceleration of charged particles colliding with the interstellar medium as well as by the annihilation of particle pairs. They are associated with high energy bodies and events such as pulsars, rapidly spinning black hole accretion discs, compact body mergers, active galactic nuclei, blazars, collapsars (hypernovas), magnetars and yes, even terrestrial thunderstorms.[xiii]We have learned that gamma ray bursts come in two main flavors… long duration gamma-ray bursts and those of short duration. [Fig. 3] But it’s important to remember that much like fingerprints no two are alike.
So what are the clues? First, we can detect an optical afterglow from some GRB’s. If an afterglow is visible then we can analyze its spectrum. The long duration GRB030329 emitted an afterglow that, on analysis, correlated pretty closely with the spectral signature of a Type 1c core-collapse supernova. Later an expanding shell was detected adding further evidence to its identity. [xiv]
A NEW PHYSICS?
Some of these long duration bursts have been detected at EXTREME distances… as in 13 GLY. How is that possible? For a star to radiate that much energy from that kind of distance it would require more than the most massive star known to convert every kilogram of its mass directly into energy (E=mc2) in just a couple of minutes! Wait a minute… even that’s not close to enough? Did we need to find a new physics to explain how a body could radiate that amount of energy in such a brief amount of time to be so brightly visible at high energy wavelengths at such stupendous distances?No. The old physics works fine.
Imagine if you will that you are standing 5m next to a sportsman with a rifle. You are wearing your hearing and eye protection so that when he fires a round downrange the most that happens to you is maybe you flinch at the sound of the gun’s report. Now let’s try something different (but don’t do this at home!!).
Stand the same 5m away from the rifle but this time look right down the bore. Stop!!! You get it. Same rifle, same distance, same ammunition, dramatically different consequences! These core-collapse supernovas, also known as collapsars or hypernovas, produce radiation in twin narrow beams, jets if you will. Core collapse supernovae have different energetics. The doomed star has no hydrogen envelope, no helium envelope and it is rotating so there is a preferred axis of ejection.[xv] If the observer on Earth happens to be looking directly down the axis of one of the beams it’s like looking down the gun barrel.
Short duration GRB’s are not explained by hypernovas. What are they? Is an area of ongoing research because we still don’t know for sure. There are some interesting models, however. According to Dr. Cominsky the leading candidates are neutron star mergers, or neutron star-black hole mergers and the dramatic “rearrangement” or unraveling of the tightly coiled intense magnetic field flux of a magnetar. Both would produce the particle accelerations necessary to radiate EM strongly (and briefly) in the gamma. Only one thing is for certain… we still have much to learn about these high energy phenomena.
References and endnotes:
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[i] "Observations of Gamma-Ray Bursts of Cosmic Origin" Klebesadel R.W., Strong I.B., and Olson R.A. 1973, Ap.J.(Letters) 182, L85
[ii] http://heasarc.gsfc.nasa.gov/docs/cgro/
[iii] http://heasarc.gsfc.nasa.gov/docs/cgro/images/epo/gallery/grbs/2704_grbs_fluence.jpg
[iv] http://ucp.uchicago.edu/cgi-bin/resolve?id=doi:10.1086/133674
[v] Reichart, Daniel E. (1998). "The Redshift of GRB 970508". Astrophysical Journal Letters 495: L99. doi:10.1086/311222.
[vi] http://imagine.gsfc.nasa.gov/docs/science/how_l1/xray_telescopes.html
[vii] http://upload.wikimedia.org/wikipedia/commons/e/ef/GRB_BATSE_12lightcurves.png
[viii] http://www.nasa.gov/mission_pages/swift/main/index.html
[ix] http://www-glast.sonoma.edu/~lynnc/
[x] The Observatory Formerly Known As GLAST :-)
[xi] Lynn Cominsky 2/27/11
[xii] http://www-glast.stanford.edu/
[xiii] Lynn Cominsky 2/27/11
[xiv] Freedman, R. Universe 9th Ed. 2010 P. 589
[xv] Filippenko, A. 2007 Astro C10 lecture 30
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