What is the true origin of gamma ray bursts?

[RAD4921]

Last night I was watching a show on the science channel about astronomy. The show was very interesting but it was after my bed time and I fell asleep when they were talking about the mysterious origin of gamma ray burst. Last I remember they were talking about the burst coming from the early far away part of the universe.
What are causing these enormous explosions? What is the latest news on gamma ray burst?
rad

 

[ray b]

http://www.nasa.gov/mission_pages/swift/bursts/short_burst_oct5.html

 

[SpaceTiger]

To summarize, there are (as far as we know) two types of bursts: short and long. Several of the long ones have been associated with known supernova remnants, so we suspect that they are coming from explosions of very massive stars (called hypernovae). The short ones have recently been associated with collisions of compact objects (black holes or neutron stars). I would call the latter identification somewhat more tentative, but it's probably correct, since we have suspected these explanations for a long time now.

 

[Chronos]

Short and long GRB's are the long and short of it. These highly energetic bursts are believed to originate from different mechanisms. Hypernova is a popular explanation. The events are highly redshifted, suggesting they are ancient. Check arxiv for discussion. References available upon request. The link given by ray b is very good.

 

[Garth]

The short GRB's, which have been modeled by neutron star or BH mergers, are not only shorter in duration but of higher energy, 'harder', however, because of the significant energy loss by the GZK (Greisen-Zatsepin-Kuzmin 1966) mechanism, the present universe is not transparent to the highest energy cosmic rays (10²⁰ eV), http://prola.aps.org/abstract/PRL/v73/i26/p3491_1.
Therefore any sources contributing to the bulk of these cosmic rays should be within 500 Mpc of Earth for 10¹⁹ eV CR particles and a few ten's of Mpc for 10²⁰ eV CR particles.

Therefore these short duration bursters are probably more 'local' (local super cluster) than the long duration ones that are at cosmological distances (z ~/> 1).

However, they may be of the same source, viz: the end products of Pop III stars widely thought to pre-exist the formation of galaxies and Pop II and Pop I stars.

In this scenario these large primordial stars would quickly consume their nuclear fuel and go hyper-nova, subsequently observed as long duration GRBs. They would leave behind a population of IMBHs, or less massive neutron stars, which may or may not be in gravitational association with each other. A sub-set of this population would eventually merge and can be observed relatively locally as short duration GRBs.

The frequency and distribution of short GRBs could then be an indicator for the distribution of 'local' IMBH's.

Garth

 

[SpaceTiger]

The short GRB's, which have been modeled by neutron star or BH mergers, are not only shorter in duration but of higher energy, 'harder', however, because of the significant energy loss by the GZK (Greisen-Zatsepin-Kuzmin 1966) mechanism, the present universe is not transparent to the highest energy cosmic rays (10²⁰ eV), http://prola.aps.org/abstract/PRL/v73/i26/p3491_1.

What does the spectrum of light from GRBs have to do with the GZK cutoff? Gamma-ray observatories can't observe beyond the tens of GeV range, while the GZK cutoff is at 5 x 10^{19} eV. There are theories that suggest GRBs should be the source of high-energy cosmic rays, but I don't know of any observations that confirm this.

 

[Garth]

What does the spectrum of light from GRBs have to do with the GZK cutoff? Gamma-ray observatories can't observe beyond the tens of GeV range, while the GZK cutoff is at 5 x 10^{19} eV. There are theories that suggest GRBs should be the source of high-energy cosmic rays, but I don't know of any observations that confirm this.

Agreed - The hypothesis is that short GRB's, being a source of 'hard' cosmic rays, are also the source of the ultra high energy cosmic rays.

Hypotheses of other possible sources of UHECRs are welcome!

Garth

 

[SpaceTiger]

Agreed - The hypothesis is that short GRB's, being a source of 'hard' cosmic rays, are also the source of the ultra high energy cosmic rays.

To my knowledge, GRBs haven't been confirmed as a source of any cosmic rays. Where are you getting this information?

 

[Garth]

To my knowledge, GRBs haven't been confirmed as a source of any cosmic rays. Where are you getting this information?

Agreed - there haven't been any confirmed sources of UHECR's - its a mystery, however it has been suggested that GRB's and UHECR's might have the same origin. http://arxiv.org/PS_cache/astro-ph/pdf/9506/9506081.pdf

The above UHECR models are the ones most directly associated with clusters, per se. There are, on the other hand, many source models based on phenomena associated with individual galaxies, more or less independent of their residence in clusters. For completeness, I mention briefly a small sampling of these ideas. Long duration gamma-ray bursts (GRBs) are now generally seen to be a consequence of ultrarelativistic fireballs associated with the core collapse of massive stars; that is a hypernova. Since they involve ultra-relativistic shocks, they have been suggested by several authors as possible accelerators of UHECRs. (e.g., Waxman 1995, Vietri 1995). Gallant and Achterberg(1999) pointed out, however, that these shocks decelerate too fast to produce UHECRs unless they take place in a strongly decreasing external density, such as that in a pre-existing stellar wind. A more serious concern comes from the realization that most GRBs are seen at large redshift; that is, they were much more common in the early universe than they are today. Photo pion losses would be enormous for protons reaching us from cosmological GRBs, leading Scully and Stecker (2002), for example, to argue that the full energy requirements to explain observed UHECR would exceed realistic estimates for GRBs by at least two orders of magnitude.
On the other hand, more local GBR sources, being discrete rather than continuous events, should lead to correlated UHECRs, which are not seen.

though not without problems!

Perhaps a ubiquitous population of IMBHs might resolve the last objection?

Garth

 

[Garth]

There are theories that suggest GRBs should be the source of high-energy cosmic rays, but I don't know of any observations that confirm this.

Perhaps this paper published today on the physics arXiv may throw some light on the matter, Gamma Ray Bursts as Possible High Energy Sources.

Evidence for two components in BATSE and EGRET/TASC data suggest that GRBs are sources of high-energy cosmic rays.

Garth

 

[SpaceTiger]

Perhaps this paper published today on the physics arXiv may throw some light on the matter, Gamma Ray Bursts as Possible High Energy Sources.
Garth

They're just making an inference from their particular model of gamma-ray bursts and the observed spectrum of gamma rays. I certainly wouldn't call it observational confirmation of cosmic rays from GRBs.

I'm not saying that I don't believe it as a strong possibility, I'm just saying that this is far from being considered a "fact", observational or otherwise.

 

[Garth]

They're just making an inference from their particular model of gamma-ray bursts and the observed spectrum of gamma rays. I certainly wouldn't call it observational confirmation of cosmic rays from GRBs.

I'm not saying that I don't believe it as a strong possibility, I'm just saying that this is far from being considered a "fact", observational or otherwise.

Agreed; the identification of high energy cosmic rays with any particular source is always going to be problematic.

Garth

 

[Nereid]

Agreed; the identification of high energy cosmic rays with any particular source is always going to be problematic.
Garth

Perhaps not tooo problematic, what with Auger and LOFAR getting under way. Besides, haven't CANGAROO and H.E.S.S. already detected some SNRs and an AGN as clear sources of TeV gammas (so they are also very, very likely to be sources of at least some of the higher energy CRs)?

 

[Nereid]

But back to the OP ... there may very well be at least one other significant class of GRB - (flares/quakes on) magnetars. Indeed, some of the brightest 'GRBs' have been magnetar quakes/flares (e.g. GRB 790305, which was almost certainly a starquake in the magnetar in N49 in the LMC).

 

[Chronos]

I'm pretty nervous about this stuff. I think we might be seeing unrelated events that mimic each other... i.e., some are local and some are cosmological. Local starquakes very well might mimic distant hypernovae. In that sense, I agree with Garth that the GZK cutoff could be an important clue.