Gammaray Bursts (all you ever wanted to know)

[marcus]

A new review article on GRB has come out

http://arxiv.org/abs/astro-ph/0605208
Gamma-Ray Bursts
P. Meszaros
To appear in Rep. Prog. Phys., 74 pages, 11 figures
"Gamma-ray bursts are the most luminous explosions in the Universe, and their origin and mechanism are the focus of intense research and debate. More than three decades after their discovery, and after pioneering breakthroughs from space and ground experiments, their study is entering a new phase with the recently launched Swift satellite. The interplay between these observations and theoretical models of the prompt gamma ray burst and its afterglow is reviewed."GRBs are a furnace where it may be possible to test quantum gravity at some extremes where its predictions differ from those of ordinary gravity theory

"Gamma-ray bursts are the most luminous explosions in the Universe." That says it. They are worth knowing about and this review article covers both what has been observed so far and what people's ideas are about how GRBs are caused.
The article will likely be out of date soon.

This is the venue, in case anyone is interested in where it will be published:
http://www.iop.org/EJ/journal/RoPP (Reports on Progress in Physics)

 

[marcus]

===sample quote===
...These proved that they were at cosmological distances, comparable to those of the most distant galaxies and quasars known in the Universe. Since even at these extreme distances (up to Gigaparsecs, or around 10^28 cm) they outshine galaxies and quasars by a very large factor, albeit briefly, their energy needs must be far greater. Their electromagnetic energy output during tens of seconds is comparable to that of the Sun over a few 10^10 years, the approximate age of the universe, or to that of our entire Milky Way over a few years. The current interpretation of how this prodigious energy release is produced is that a correspondingly large amount of gravitational energy (roughly a solar rest mass) is released in a very short time (seconds or less) in a very small region (tens of kilometers or so) by a cataclysmic stellar event (the collapse of the core of a massive star, or the subsequent mergers of two remnant compact cores). Most of the energy would escape in the first few seconds as thermal neutrinos, while another substantial fraction may be emitted as gravitational waves. This sudden energy liberation would result in a very high temperature fireball expanding at highly relativistic speeds, which undergoes internal dissipation leading to gamma-rays, and it would later develop into a blast wave as it decelerates against the external medium, producing an afterglow which gets progressively weaker. The resulting electromagnetic energy emitted appears to be of the order of a percent or less of the total energy output, but even this photon output (in gamma-rays) is comparable to the total kinetic energy output leading to optical photons by a supernova over weeks. The remarkable thing about this theoretical scenario is that it successfully predicts many of the observed properties of the bursts. This fireball shock scenario and the blast wave model of the ensuing afterglow have been extensively tested against observations,...
===endquote===

BTW this is part of a discussion assuming the burst is isotropic same in all directions, there is another model he discusses where there is less energy required because some mechanism beams the energy and we just happen to be in the way of a beam----so then the estimates are different

 

[Chronos]

In my mind GRB's must be pop III events. The distance factor alone is very suspicious. I also think GRB's play a significant role in reionization and metalization of the early universe.

 

[Garth]

In my mind GRB's must be pop III events. The distance factor alone is very suspicious. I also think GRB's play a significant role in reionization and metalization of the early universe.

I agree, except it is the long GRB's that I think are PopIII events. Short GRBs seem to be the mergers of two BH's.

But of course these two could be connected.

Hypothesis: Early in the universe's history Pop III's go hyper-nova and produce long GRB's leaving behind IMBHs. This leaves a population of IMBHs in the present universe, some of which are gravitationally bound to each other. Eventually through orbital decay these occasionally coalesce and produce short GRBs.

A plausible scenario?

Garth

 

[Chronos]

If GRB's are jets [which I think they must be], the difference between long and short GRB's might be a matter of alignment.

 

[marcus]

If GRB's are jets [which I think they must be], the difference between long and short GRB's might be a matter of alignment.

the picture I'm getting from your words is that the jet "rakes" across us and it can rake quickly (short burst received) or rake more slowly (long burst received). is that what you have in mind?

=================

BTW I have no reason to form a personal opinion about the different mechanisms underlying long and short but I have read quite a bit favoring what Garth said----so i will repeat it.
Namely LONGIES are produced by collapse of a BIG STAR
while on the other hand
SHORTIES are the result of NEUTRONSTAR MERGER

for some reason there seems to be widespread support for this idea, at least at present
===================

ooops, I have misquoted Garth, what he actually said in post #4 was
**Short GRBs seem to be the mergers of two BH's.**

If I remember what I have heard is shorties are surmised to be mergers of neutronstars, not mergers of BHs. But it could be six of one and halfdozen of the other, or I could be misremembering. In any case there is this widespread idea that shorties come from the merger of SMALL THINGS whatever they might be.

 

[marcus]

since we seem to believe or else to have read slightly different things, I will quote this review article (maybe more to focus discussion than as authority---I doubt there is any real authority on this as yet)

"...A GRB emission which is concentrated in a jet, rather than isotropically, alleviates significantly the energy requirements. There is now extensive observational evidence for such collimated emission from GRBs, provided by breaks in the optical/IR light curves of their afterglows [244, 140, 62]. The inferred total amount of radiant and kinetic energy involved in the explosion is in this case comparable to that of supernovae (except that in GRBs the energy is mostly emitted in a jet in gammarays over tens of seconds, whereas in supernovae it is emitted isotropically in the optical over weeks). While the luminous (electromagnetic) energy output of a GRB is thus “only” of the same order of magnitude as that of supernovae, the explosion is much more concentrated, both in time and in direction, so its specific brightness for an observer aligned with the jet is many orders of magnitude more intense, and appears at much higher characteristic photon energies. Including the collimation correction, the GRB electromagnetic emission is energetically quite compatible with an origin in, say, either compact mergers of neutron star-neutron star (NS-NS) or black hole-neutron star (BHNS) binaries [343, 105, 331, 299], or with a core collapse (hypernova or collapsar) model of a massive stellar progenitor [514, 346, 380, 283, 513], which would be related to but much rarer than core-collapse supernovae..."the reason people classify into two groups is the observed hard gamma DURATION TIMES are in a roughly BIMODAL DISTRIBUTION (i.e. with roughly speaking two peaks).

one peak is somewheres less than 2 seconds and the the other is somewheres greater than 2 seconds

"...The gamma-ray durations range from 0.001 s to about 1000 s, with a roughly bimodal distribution of long bursts greater than 2 s and short bursts of less than 2s [237], and substructure sometimes down to milliseconds..."

the paper shows LIGHTCURVES of various events and the SUBSTRUCTURE made of little SPIKES which sometimes look like "microbursts" a sort of ratatat-tat. I must say that I like seeing the MICROSTRUCTURE pictures----to me it is one of the good things about the article.

it is in this millisecond scale microburst phenomenon that one might be able to look for tiny deviations in the speed of light----predicted by some QG theories----and measure fine enough to possibly exclude deviation thereby falsifying those theories.

 

[Lonewolf]

If I remember what I have heard is shorties are surmised to be mergers of neutronstars, not mergers of BHs.

As far as I know, the best candidate to explain short gamma ray bursts are magnetars. These are neutron stars with surface magnetic fields of the order of 10^{15} G, and interior fields up to 10^{18} G. The shortness essentially comes from the short time the magnetar can retain such a pathological field.

 

[Garth]

ooops, I have misquoted Garth, what he actually said in post #4 was
**Short GRBs seem to be the mergers of two BH's.**

If I remember what I have heard is shorties are surmised to be mergers of neutronstars, not mergers of BHs. But it could be six of one and halfdozen of the other, or I could be misremembering. In any case there is this widespread idea that shorties come from the merger of SMALL THINGS whatever they might be.

Agreed, I could well be wrong!

Is not another difference between Long and Short GRBs is that the short ones are harder, i.e. of higher energy gamma rays?

Garth

 

[Lonewolf]

Is not another difference between Long and Short GRBs is that the short ones are harder, i.e. of higher energy gamma rays?

At least with the short GRBs there is no correlation between length of the bursts and energy of the gamma rays. They come in both soft and hard varities. I'm not sure about the long GRBs. If I was feeling more energetic, I'd dig out some references. I need to get them out of an old, long buried, folder.

 

[Garth]

There is the Wikipedia article Gamma ray burst.

GRBs may be divided into two categories: short bursts (with a duration of less than 2 seconds) and long bursts (with a duration of more than two seconds). Short GRBs tend to emit a larger fraction of their energy as high energy photons (energy above ~100 keV) than low energy photons (it can be said that they have "harder" spectra than long bursts).

There is now almost universal agreement in the astrophysics community that the long-duration bursts (> 2 sec) are associated with the beamed energy from a specific kind of supernova event, known as a collapsar. The collapse of supermassive stars (Zero Age Main Sequence (ZAMS) mass between 40 and 100 solar masses) once their silicon "burning" is complete directly forms a black hole.

The most promising model for the short duration bursts (< 2 sec but typically averaging 0.3 sec) is that developed by Martin Rees in the 1990s. In this model two neutron stars coalesce or a neutron star is devoured by a black hole. This causes an enormous release of gravitational potential energy. The lack of material around such a system means that the energy release stops as soon as the merger is complete (hence the short duration of the burst).

Links to refereed papers would be good...

Garth

 

[Lonewolf]

As far as I know, the best candidate to explain short gamma ray bursts are magnetars. These are neutron stars with surface magnetic fields of the order of 10^{15} G, and interior fields up to 10^{18} G. The shortness essentially comes from the short time the magnetar can retain such a pathological field.

Silly me, there are two main types of short GRB. One is of the type that Marcus alluded to. The other is a soft gamma repeater, which is in essence a magnetar. The soft gamma ray repeater is an example of a short burst (I think, I can't remember the typical duration of a pulse) that emits soft gamma-rays/hard x-rays on a regular basis.

 

[Chronos]

No doubt there are different models for long and short GRB emissions:
Quiescent Burst Evidence for Two Distinct GRB Emission Components
http://arxiv.org/abs/astro-ph/0403360

I also agree the spectral signatures are different. The hard facts, as I see them, is GRB's are remote events. From that perspective, I perceive them as different manifestations of similar events. Since I'm feeling bold at the moment, I think hard GRB's should be strongly polarized compared to short GRB's.