Naked eye gamma-ray burster (!!)

[Cancer]

Last March, within a few hours of Arthur C Clarke's death, there was a gamma-ray burster (GRB) that was, for a few seconds, visible to the naked eye. GRB080319B was measured at R = 5.76 ... that's above naked-eye threshold in the red ... in a ten-second integration captured by a Polish robotic instrument in Chile. I would be willing to bet that its peak was rather brighter than that, but that peak was very short, probably less than a second, and the ten-second exposure smeared out the peak and blunted it. They have strung their images into a brief "movie" posted at the "Pi of the Sky" site.

 

The redshift of the object is 0.936, which corresponds to a distance of about seven billion light-years. Yes, billion with a B. There's some hyperbole in the popular press about "the brightest thing ever seen" ... more about that later.

 

It's hard to be certain how much power the thing radiated: GRB's seem to be very tightly beamed, so the estimate for the total amount of energy radiated depends on how wide that beam opening angle actually is. Models of the GRB phenomenon have energy yields of about 10^51 ergs, which is in the supernova ballpark, and depending on assumptions this burst might be more or less than that for total energy release. The rate of release, though, is stupefying: at its (very brief!) peak, GRB 080319b was about a thousand times more luminous than the most luminous known quasar.

 

I quote the preprint (from the arXiv preprint server) about how bright the thing might have been had it been closer, and how likely something like that would be:

 

At z = 0.17, the distance of the nearest non-underluminous GRB to date (GRB 030329), this event would peak at R ≈ 1 mag, nearly as bright as the brightest stars in the sky. Or, to carry the comparison to its greatest extreme, we might envision a situation in which a GRB similar to GRB 080319B were to occur in our own Galaxy. At a distance of 1 kpc (and neglecting the probable substantial extinction along Galactic lines of sight at optical wavelengths), the optical flash would peak at magnitude about −28.5, several times the brightness of the Sun [emphasis added]! Such an event must assuredly be extremely unusual — the Galactic GRB rate is probably no greater than 1 per 10^5 to 10^6 years ..., likely only 1% of such bursts are collimated toward Earth, and this is among the brightest 0.1% of bursts ever observed. Altogether, the rate is probably less than 1 per 10^10 − 10^11 yr: unlikely to have ever happened even over the long timescale of geological history, and certainly not a spectacle we can expect to witness anytime soon.

 

The preprint is in techspeak, but you might download it for grins and see what you can make of it. If you know what to make of them, the figures of the light curves speak volumes.

 

They don't provide an estimate for the hard radiation peak flux that would have come a minute or two before that visible-light peak. I might be able to scratch up such an estimate, but it won't happen quickly. I suspect it'd be a planet-sterilizing flash, and whether people on that side of the planet would live the 90 seconds needed to reflexively turn and see what had killed them I don't know.

[Lord Liaden]

Re: Naked eye gamma-ray burster (!!)

 

My guess is that Prof. Clarke finally passed through the Monolith and toward the next stage in his evolution.

 

(Maybe we should start scanning the heavens for floating cosmic fetuses.)

[Nyrath]

Re: Naked eye gamma-ray burster (!!)

 

This also reminds one of Sir Arthur C. Clarke's classic short story "The Star"

[Cancer]

Re: Naked eye gamma-ray burster (!!)

 

When you take the observed fluence of the burst and scale it up by the same factor as they applied to the visible light (about a factor of 5 * 10^13), that results in burst of about 30 MegaJoules/square meter at the top of the atmosphere in the energy range 20 keV to 7 MeV, delivered over about a minute of time.

 

That doesn't mean enough to me to say much more (though it sounds large), but a friend of mine works in the radiation safety office for the state, and I ran that number past him. His response: "Someone exposed to 30 Megajoules/m^2 would be well and truly f***ed." Sounds like the time to die is hours to days, though, so people on that half of the planet would have time to turn around and see the thing that had killed them. That's actually less lethal than I had initially guessed.

[Nyrath]

Re: Naked eye gamma-ray burster (!!)

 

When you take the observed fluence of the burst and scale it up by the same factor as they applied to the visible light (about a factor of 5 * 10^13), that results in burst of about 30 MegaJoules/square meter at the top of the atmosphere in the energy range 20 keV to 7 MeV, delivered over about a minute of time.

 

That doesn't mean enough to me to say much more (though it sounds large), but a friend of mine works in the radiation safety office for the state, and I ran that number past him. His response: "Someone exposed to 30 Megajoules/m^2 would be well and truly f***ed." Sounds like the time to die is hours to days, though, so people on that half of the planet would have time to turn around and see the thing that had killed them. That's actually less lethal than I had initially guessed.

 

I'm suffering from a head-cold right now, so my figures may be suspect. But according to the back of my envelope, the radiation event is as bad as you figure.

 

You say 30 MegaJoules/square meter at the top of the atmosphere. I have no idea how much shielding the atmosphere provides. So I'll just calculate the effect on some unfortunate astronaut above the atmosphere.

http://www.projectrho.com/rocket/rocket3ah.html#effects

 

Radiation flux is 30 MegaJoules/square meter = 3.0 x 10^7 joules/square meter

Radiation exposure is measured in Grays, which are joules per kilogram. So you multiply the radiation flux by the astronaut's cross section, then divide by their body mass.

 

Average person's cross section is 0.445 square meters, average mass is 68 kilograms (see link for details).

 

Grays = (3.0 x 10^7 * 0.445) / 68

Grays = 196,000

 

According to the radiation chart (see link), a dose of a mere 80 Grays is enough to instantly put a person into a coma, and death is certain within 24 hours. 196,000 is probably instant death.

 

Again, this is at the atmosphere's surface. The atmosphere will provide some shielding, but I sort of doubt that it can reduce 196,000 grays to a safe level.

 

Cancer, you may want to run my figures past your friend to double-check. He's the expert, and I have been known to make silly mistakes in arithmetic.

[Cancer]

Re: Naked eye gamma-ray burster (!!)

 

I'll have to wait until I'm home to reread his email, but one of the points he made was that the "certain death" line has been pushed out to larger and larger doses over the last couple decades, mostly on advancing medical care *and* the availability of that advanced medical care. With an exposure that covers half the planet, that assumption breaks down, and you can go back to radiation-mortality relations that were developed back in the 1950s and 60s. He did describe a couple of cases which made me revise my thinking, in the sense that until you hit radiation levels that are visibly cooking meat before your eyes ... and this 30 MJ/m^2 may or may not be at that level, especially since the energy is delivered over about a minute of time ... hard radiation doesn't have that much "stopping power" in terms of very fast killing. Certain death results from much lower exposures than this, but that death could be as much as a few days out.

 

Neither of us worked out how much of that radiation gets stopped by the atmosphere, or having made it through the atmosphere gets stopped in a human body and does damage. He was reacting from his experience without computing, reading an email on a late Friday night. I need to look up a bunch of stuff, but I may be able to figure out how much of that radiation is stopped by the atmosphere (though what happens to the atmosphere is another question, maybe).

[Kristopher]

Re: Naked eye gamma-ray burster (!!)

 

GRBs inside the Milky Way? I was under the impression that they were all far away, and thus long ago.

[Nyrath]

Re: Naked eye gamma-ray burster (!!)

 

GRBs inside the Milky Way? I was under the impression that they were all far away' date=' and thus long ago.[/quote']

Well, as it turns out, astronomers are divided on that topic.

[Kristopher]

Re: Naked eye gamma-ray burster (!!)

 

Well' date=' as it turns out, astronomers are divided on that topic.

 

Sounds like the evidence is stronger for the extragalactic origin.

 

I hope that's the case. We don't need another way to die.

[Nyrath]

Re: Naked eye gamma-ray burster (!!)

 

I'll have to wait until I'm home to reread his email' date=' but one of the points he made was that the "certain death" line has been pushed out to larger and larger doses over the last couple decades, mostly on advancing medical care *and* the [u']availability[/u] of that advanced medical care. With an exposure that covers half the planet, that assumption breaks down, and you can go back to radiation-mortality relations that were developed back in the 1950s and 60s. He did describe a couple of cases which made me revise my thinking, in the sense that until you hit radiation levels that are visibly cooking meat before your eyes ... and this 30 MJ/m^2 may or may not be at that level, especially since the energy is delivered over about a minute of time ... hard radiation doesn't have that much "stopping power" in terms of very fast killing. Certain death results from much lower exposures than this, but that death could be as much as a few days out.

Yes, my data is a bit old.

Go here

http://www.projectrho.com/rocket/rocket3ah.html#effects

and scroll down to "Acute radiation syndrome chart".

 

Radiation exposure first causes the "immediate symptoms." The patient then appears to get better, and feels OK for a period of time called the "latent period." Then the "post-latent symptoms" strike.

 

According to the (old data) on the chart, radiation does not have much stopping power if it is less than 20 Grays (i.e., the immediate symptoms do not include "falling into a coma"). From 20 to 80 Grays, coma occurs in a time span of minutes to seconds. And above 80 Grays, coma is immediate. At that point, I was not talking about "certain death", I meant "instant death", i.e., not unstoppable death coming within 3 weeks, but death coming in a fraction of a second.

 

That last figure is from a US Army study. It seems that they were working on something called an "enhanced radiation weapon" (better know as a "neutron bomb"), and they wanted a dose that would instantly incapacitate the enemy. They came up with a figure of 80 Grays.

 

My (suspect) calculation is that the burster will inflict about 196,000 Grays on somebody outside of the atmosphere, which probably is well above "cooking meat" levels.

 

However, the joke may be on me. On this page, it gives a figure for the radiation shielding provided by the atmosphere to be about 240 grams per square centimeter for radiation coming from straight overhead.

 

For electromagnetic radiation like gamma rays, one "Tenth Value Thickness" is 45 gm/cm^2 (particle radiation has a different formula). This means the atmosphere provides 240/45 = 5.3 TVT worth of protection.

 

The attunation factor is

Af = 0.1 ^TVT

Af = 0.1 ^5.3

Af = 5.0 x 10^-6 or 0.000005

 

G2 = G * Af

G2 = 196,000 * 0.000005

G2 = 0.98 Grays

 

which gives only mild symptoms, and has a 90% survival rate.

 

Again, if I haven't made a math mistake.

[Lawnmower Boy]

Re: Naked eye gamma-ray burster (!!)

 

Have you phoned your atmosphere and told it how much you loved it, lately?

Me, I'm sending flowers.