Exploring the Limits of GRBs: A Look at the Integrated Sachs-Wolfe Effect

[wolram]

http://www.astrobio.net/news/article642.html

About 1,000 GRBs occur daily, but Earth-based telescopes only are able to see perhaps one a day Because SWIFT will be outside the Earth's atmosphere, it will see the GRB events even if the sky is cloudy or it's the middle of the day.
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that is a huge amount of energy

 

[Ivan Seeking]

Originally posted by wolram
http://www.astrobio.net/news/article642.html

About 1,000 GRBs occur daily

WOW! I would normally say more but that's all that comes to mind right now. WOW!

 

[outandbeyond2004]

Another wow is that so few are observable on the ground. How come? The atmosphere, yes, but - oh, yeah, most GRBs are faint, right?

 

[Nereid]

While quite a few GRBs have been seen from the ground, in light, radio, or even http://lheawww.gsfc.nasa.gov/docs/gamcosray/legr/bacodine/gcn3/2176.gcn3 , AFAIK, none have been initially detected in any region of the EM spectrum other than gammas (or almost coincidently, X-rays). It's not that long-duration* GRBs are necessarily faint in the optical (>20 mag, for example), but that there are enormous numbers of brigther objects (mostly stars), so finding one is very difficult. In the gamma sky, the situation couldn't be more different - the 'one GRB per day' which BATSE detected were the brightest objects in the sky - a bit like a star as bright as Sirius - or brighter - suddenly appearing approx once a day.

SWIFT will help a lot by having greater sensitivity, faster and better localisation (BATSE couldn't say where the GRBs it 'saw' were, to better than a few degrees), and faster communication with the astronomical community (HETE is OK, but far from efficient).

*short-duration GRBs are a complete mystery; AFAIK, they've never been 'seen' in anything but gamma (maybe some X-rays), never been well localised, no afterglow detected, etc, etc. They may be the first peak of a magnetar quake, but it's hard to say.

 

[cragwolf]

How did they calculate 1000 per day? And if we live in a spatially infinite universe, shouldn't that be a number density instead of a number?

 

[wolram]

cragwolf,
How did they calculate 1000 per day?
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In 1991, the Burst and Transient Source Experiment (BATSE) aboard NASA's Compton Gamma-Ray Observatory discovered that this gamma-ray burst radiation was "isotropic," or uniformly coming from everywhere. BATSE could monitor nearly the entire sky for gamma-ray transient sources (i.e., sources that suddenly gave off a large amount of gamma-rays and then fade). In addition, BATSE could localize where in the sky the burst was coming from.
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this is a snip from origonal posted site.

 

[marcus]

beginnings of reply to cragwolf

Originally posted by cragwolf
How did they calculate 1000 per day? And if we live in a spatially infinite universe, shouldn't that be a number density instead of a number?

I don't know how they estimate that and I'm interested. I hope someone finds an article that gives a clue as to how they arrive at that figure.

The usual "particle horizon" estimate is 47 Gly
that is the present distance from us of the most distant object that we could have gotten light from up til now.

that would correspond to a certain volume measured in cubic Gly
and there are estimates of the presentday density of matter
and probably someone can find an estimate of the number of galaxies per cubic Gly at the present moment

these are all estimates it would be interesting to know

Cragwolf, I am guessing that the rough order-of-magnitude guess of 1000 per day is judged from how many GRBs they've seen and how insensitive they think their instruments are! In other words it is the wildest of wild stabs. But I certainly don't KNOW how they got that estimate and it would be neat if one of us could find out.

Like, if we are currently receiving light from 1000 billion galaxies and if we are getting 1000 GRBs per day then that wouild indicate that in an average galaxy a GRB occurs at the rate of once every billion days. It would be nice to know the actual answer, how often a GRB occurs in an average galaxy.

Is there some technical article about GRBs that someone has a link to that might give a clue about this?

 

[clicky]

I wonder

I wonder if the GRBs will retain their isotorpic distribution at rate of 1000s per day?

I can't find the link to this stranger than strange observation.

 

[Nereid]

The "1000 per day" estimate surely comes from charts like the http://swift.gsfc.nasa.gov/science/objectives/cosmology.html with models of the sources.

Since next to nothing is known about the short-duration GRBs, a fair bit of speculation must have gone into the estimate.

The main reason why we couldn't ever 'see' more than ~ 1 GRB per day is that most don't have beams which point towards us. Although we now have at least some confidence of the beam angle for long GRBs, AFAIK there's nothing to even hint what the beam angle (if any) might be for short GRBs.

clicky: I wonder if the GRBs will retain their isotorpic distribution at rate of 1000s per day?

It'd be a BIG surprise if there was any significant deviation from isotropy for the long GRBs - except for such things as correlations with distant, rich clusters.

 

[clicky]

Can you show ?

Can you show other phomena that show anisotropies at the sale discovered for the CMB?

http://www.jb.man.ac.uk/news/vsa2/

Do the GRBs show some anisotropy at that scale?

I guess they probably do.

 

[Nereid]

Originally posted by clicky
Can you show other phomena that show anisotropies at the sale discovered for the CMB?

http://www.jb.man.ac.uk/news/vsa2/

Do the GRBs show some anisotropy at that scale?

I guess they probably do.

The CMB is z ~1100; from the lower diagram on the SWIFT webpage I provided a link to (my earlier post), you'll see that GRBs will likely be detected only out to z ~5-10 (max). As long-duration GRBs originate in star-forming regions, they are expected to have much the same sky distribution as (blue) galaxies. The space density of galaxies shows some interesting patterns; however, http://www.nottingham.ac.uk/~ppzsjm/apm/apm.html is more even.

However, the key is large numbers. In the 2dF survey, there are >200,000 galaxies; in the APM, >3 million (anyone want to give the equivalent number for COBE or WMAP?). BATSE detected only ~3,000 GRBs; SWIFT will detect only ~1,000.

How much CMB-sized anisotropy could be seen in only ~1,000 data points?

 

[wolram]

http://arxiv.org/PS_cache/astro-ph/pdf/0401/0401166.pdf

this is the wrong url sorry i will try and find the correct one.

 

[Nereid]

wolfram,

I'm not sure many PF members and guests would be able to understand the point you are making re the Integrated Sachs-Wolfe Effect (I'm not sure I understand ). Would you care to elaborate?

Nereid