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Please begin with an informative title:

So few people have questioned the origins of my signature, "big badda boom : GRB 090423 ", and I thought, hey I could write a diary about that. Mysterious astronomical objects with astronomical energy output, what's not to like ?  GRB stands for gamma ray burster.  Follow me across the event horizon to be instantly torn into your constituent atoms, er, I mean to read about why a GRB really is a big badda boom.


You must enter an Intro for your Diary Entry between 300 and 1150 characters long (that's approximately 50-175 words without any html or formatting markup).

First, without further delay, I present GRB 090423 in all it glory:


The Electromagnetic Spectrum

Spectrum is a word you hear a lot, and is the measurement of the strength of a signal as compared to the frequency of that signal, i.e. amplitude/strength on the Y axis and frequency/wavelength/energy on the x-axis.

I thought I might start with a minor digression into the nature of gamma rays.  Gamma rays are electromagnetic radiation.  Many people can be confused by the use of the word "radiation".  Radiation, referring to electromagnetic radiation, includes AM radio signals, TV signals, the signal produced by your cell-phone, heat (infrared), visible light, ultraviolet, x-rays, gamma rays and what comes after gamma rays.  Your cell phone is operating at a frequency of about 2 x 10^9 Hz, gamma rays are on the order of 10^20 Hz (that's a lot of hurts).  They are so energetic that the strength of a gamma ray is generally described in electron volts (eV).  Your dental x-ray is probably less than 100 eV, but the in the emission spectrum for Cobalt 60, there is a gamma ray peak at about 1.2 million eV.

When people get concerned about radiation what they are generall thinking of is ionizing radiation.  Aptly named ionizing radiation has enough energy that it can ionize atoms by knocking electrons loose.  This will generally free that atom from it's current patner and it can then form bonds with other, ionized atoms.  This can be a problem if those atoms are in your DNA and you'd rather not have them making random connections.

Gamma rays are ionizing radiation of the highest order. Nuclear decay often produces gamma rays, and gamma rays tend to go through lead relatively easily, so keeping gamma rays away from you is difficult (a challenge for long term survival in space).  I think I'm digressing too much.  The point is that gamma rays are very energetic which means it takes very energetic events to create them.  That's one clue that a gamma ray burster probably is going to be something kinda crazy.

I thought I saw a flash...

Now astronomer types enjoy looking for things with a lot of energy because the chances are good it's going to be something really interesting.  Interesting astronomical things that might produce lots of gamma rays include colliding neutron stars, matter falling into black holes, colliding black holes, supernovas, and, if you're really lucky, something nobody's thought of before.

You might think the earth's atmosphere should be relatively transparent to gamma rays due to their very high energy, but in fact the gammay ways are stopped by the time they get into the stratosphere.  The other interesting factoid is that the earth's atmosphere has a fairly decent gamma ray glow due to cosmic rays impacting the atmosphere and even lightning as this paper describes.  Regardless you'd generally like to be above the atmosphere for astronomical measurement because you'll get better sensitivity which allows you to observe very faint objects. As with most astronomical observations you're going to want to look for them from the vacuum of space.

A satellite that detects gamma rays would be very useful. Check.  There's been quite a few of them over the years as this is an active area of research.  It turns out the current NASA gamma ray satellite, Fermi, looks for gamma rays at an energy of between 20 MeV to 300 GeV.  Naturally, Fermi's primary purpose in life is to learn more about GRBs.  However satellites are also interested in looking at gamma rays generated from the direction of planet earth because it might uncover interesting things like the fact that lightning bolts can generate gamma rays, or, sadly, because someone just initiated an uncontrolled nuclear reaction.

What I won't talk about is the intricacies of creating an instrument which can detect gamma rays, but clearly this is an important part of any such satellite.

GRBs, What About Them ?

Now that I can look for gamma ray bursters what do I do with the information ?  Well  I can make maps of where gammay ray bursts have occurred and see if there's any pattern to them.  The other useful thing that can be done is to follow the emissions of a gamma ray burster as it "cools off".  Unfortunately I haven't been able to find a good example of the long term record of a GRB (here's one example).  The idea is to observe the spectrum of radiation that it produces and how that spectrum changes over time.  This can provide very important clues as the the kind of event that created the GRB.  generally you would expect that the gamma rays would be generated very close to the start of the event and that the spectrum will fall off in energy as time goes by, such that you will see x-rays, then ultraviolet, visible light, etc...  Seeing the visible light from such an event, or even the x-rays, means you're too late, and you've missed the really good part.  Hence the reason to have gamma ray detectors wandering around in space looking for gamma ray bursts.  Modern communications make it possible to create a trigger such that other satellites and earth bound telescopes can then zoom in on the object and make a detailed record of its afterglow.

How Close is Too Close ?

Diaries are more fun when they've got a good end of the world scenario to introduce. Let's talk about the fun part first, how do we actually know that a gamma ray burster is such a big boom ?
GRB 990123 was a particularly energetic GRB and if you scroll down to the end of the page they describe it:

If the emission is isotropic, then the energy produced in the explosion is about 2 times 10^54 ergs. In other words, in a few seconds this gamma ray burst produced 100,000 more energy than that produced by the Sun in its entire - billion of years - lifetime.
What you are doing is looking at the flux the telescope detected and then integrating over the sphere that encompasses the object whose radius is our distance to the object.  An object 1 billion light years away would have a multiplier of 1.1 x 10^33, based on meters squared.  So if you see a lot of energy in the tiny little aperture of your telescope, the source had a really lot of energy.

That word, "isotropic", is an important part of the this description.  It means that the power output was equivalent in all directions, i.e. spherical.  It's possible that there are mechanisms could produce gamma ray bursts which are very directional, whose energy pours out in cone as opposed to a sphere.  A good analog to this are the jets which are often produced when matter falls into a black hole.  A directional outburst, i.e. non-isotropic would change the estimate of total power quite a bit, but the fact is that the energy output is still, well, astronomical.

So, how close would such an outburst have to be to the earth to give you a really bad sunburn ?

Well one way to figure that out is to know what the current sea level gamma ray flux is, and then think about how close a 10^54 erg isotropic event would have to be to give us maybe 10x or 100x that flux.  S.E. Thorsett has already made the calculations.  Interestingly it turns out you won't get a bad sunburn from the gamma rays directly.  The atmosphere is quite good at absorbing gamma rays. The real problem is that such an event could destroy the ozone layer, which would give you a bad sunburn, and could possibly have caused to mass extinctions in the past, or even high levels of mutation (cambrian explosion ?).  The estimated distance for such an event is about 1500 light years. That's a long way off to cause such a serious amount of damage.  The thing is, the causes of GRBs are not well understood, so it's not at all obvious where a GRB might show up.  There may not be any objects within that distance which could lead to a GRB.  The chances are good though, that such an event could occur somewhere in our galaxy.  Personally, I'm much more concerned about climate change.

So, big, mysterious booms.  And some of them, like GRB 090423 are really, really far away.  What's not to like ?


Gamma Ray Astronomy Team Publications no, really, it's interesting stuff...
Death Star a really fun PBS special about gamma-ray bursters.

Extended (Optional)

Originally posted to squarewheel on Sun Feb 03, 2013 at 11:22 AM PST.

Also republished by Community Spotlight.


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