How reliable are the redshift measurements from GRB 050904?

[wolram]

http://arxiv.org/abs/astro-ph/0510381

How accurate are these red shift figures ?

Authors: Michel Boër (OHP), J.L. Atteia (LAOMP), Y. Damerdji (OHP, Cesr), B. Gendre (IASF), A. Klotz (OHP, Cesr), G. Stratta (CESR)
Comments: 13 pages, 3 figures. Submitted to Nature, posted for scientific use only

The event of September 4th, 2005 (GRB 050904) was detected by the SWIFT/BAT experiment. The source was found to be at a redshift z = 6.29, corresponding to an age of the Universe which is only 7% of the present epoch. The 25 cm TAROT robotic telescope3 was able to catch the bright flare emitted by GRB 050904 at the time of the prompt high-energy event. In this letter we discuss the flux and the behaviour of the optical emission during the prompt high-energy emission and the early afterglow. We combine our data with simultaneous observations performed in X-rays and we analyze the broad-band spectrum. We show that the optical emission is too bright to have the same origin as the high energy photons. Both the temporal and spectral behaviour of the event are difficult to explain within the current internal or reverse shock models. These observations lead us to emphasize the similarity of GRB 050904 with GRB 990123, a remarkable gamma-ray burst whose optical emission reached 9th magnitude4. While GRB 990123 was, until now, considered as a unique event, our observations suggest the existence of a population of GRBs which have very large isotropic equivalent energies and extremely bright optical counterparts. The luminosity of these GRBs is such that they are easily detectable through the entire universe. Since we can detect them to very high redshift even with small aperture telescopes like TAROT, they will constitute powerful tools for the exploration of the high-redshift Universe and might be used to probe the first generation of stars.

 

[Chronos]

This should be indicative:

http://www.arxiv.org/abs/astro-ph/0509660
"...Here we report the discovery of the afterglow of GRB 050904 and the identification of GRB 050904 as the first very high redshift GRB. We measure its redshift to be 6.39(+0.11,-0.12), which is consistent with the reported spectroscopic redshift (6.29 +/- 0.01)..."

 

[SpaceTiger]

How accurate are these red shift figures ?

The redshift measurement looks reliable. In addition to Chronos' link, here's a paper that estimates the redshift photometrically:

http://lanl.arxiv.org/abs/astro-ph/0509766"

They get the same value as observers at the SUBARU telescope, who estimate it with absorption features:

http://gcn.gsfc.nasa.gov/gcn/gcn3/3937.gcn3"

Redshift measurements are tricky because their errors have an extremely odd distribution. Correct identification of features in the spectrum will usually give you a redshift estimation that is very precise (in this case, +- 0.01), but if you're wrong about the identity of the absorption feature, your redshift is basically meaningless (it could be anything). In this case, they appear to have identified multiple features, so it's probably safe.

 

[Garth]

The event of September 4th, 2005 (GRB 050904) was detected by the SWIFT/BAT experiment. The source was found to be at a redshift z = 6.29, corresponding to an age of the Universe which is only 7% of the present epoch.

Both the temporal and spectral behaviour of the event are difficult to explain within the current internal or reverse shock models. These observations lead us to emphasize the similarity of GRB 050904 with GRB 990123, a remarkable gamma-ray burst whose optical emission reached 9th magnitude4. While GRB 990123 was, until now, considered as a unique event, our observations suggest the existence of a population of GRBs which have very large isotropic equivalent energies and extremely bright optical counterparts.

The luminosity of these GRBs is such that they are easily detectable through the entire universe. Since we can detect them to very high redshift even with small aperture telescopes like TAROT, they will constitute powerful tools for the exploration of the high-redshift Universe and might be used to probe the first generation of stars.

Observations of the (demise of the) long predicted Pop III stars perhaps?

Garth

 

[Chronos]

That paper refers to GRB's, not Pop III stars.

 

[Garth]

That paper refers to GRB's, not Pop III stars.

Quite, that is why I said "perhaps".

There are two types of GRB.

The short-duration GRBs are generally defined as those lasting less than 2 seconds. These bursts have a duration between a few milliseconds and 2 seconds with an average duration of 300 milliseconds (0.3 seconds). It has been suggested that these are caused by neutron star or BH mergers.

The longer-duration GRBs are a factor 10 or so brighter, the gamma-rays are not so 'hard' and are of a more steady power output. It is generally surmised that these are Super Novae where there has been a channelling of radition towards the Earth.

The OP paper was describing an extremely bright and long lasting GRB, its Figure 2 shows the afterglow tracked for three hours after the triger event. It would seem to be a very distant, early (universe age ~ 900Myr) and very bright supernova from the time of late cosmic re-ionisation. A prime candidate for a Pop III star coming to the end of its brief life. As the abstract in that paper said:

They (these GRBs) will constitute powerful tools for the exploration of the high-redshift Universe and might be used to probe the first generation (Pop III?) of stars.

(Italics my insert.)
Garth

 

[SpaceTiger]

I wouldn't expect this to be a good method of finding Pop III stars because GRBs arise from evolved objects; that is, objects polluted with metals. That's not to say that they can't be initiated from Pop III stars, just that it would be extremely hard to tell if they were.

Nonetheless, it is certainly exciting news and I'm eager to see what else Swift can find.

 

[Garth]

There is a major problem in forming Pop III stars if they have zero metallicity because they find it difficult to radiate away the necessary energy.

This problem is alleviated if they are swiftly rotating. The later PopIII stars may of course already be second generation and have the necessary metallicity that is found in the GRBs.

On the other hand some metallicity might be primordial - but you know where that is going and it is not "Mainstream"!

Garth

 

[wolram]

On the other hand some metallicity might be primordial - but you know where that is going and it is not "Mainstream"!
Garth

Could this question be solved if some metallicity remained from a previous
contraction of the U .
or is this not MS ?

 

[Turbot]

Could this question be solved if some metallicity remained from a previous
contraction of the U .
or is this not MS ?

Definitely not mainstream, since a previous contraction to a singularity would have "homogenized" the matter and energy. Mainstream cosmologies fall apart at the singularities.

 

[SpaceTiger]

Could this question be solved if some metallicity remained from a previous
contraction of the U .
or is this not MS ?

There are mainstream models of the universe that include previous expansions/contractions, but they certainly don't predict any leftover metals. If the universe contracted to a small enough size, heavy elements would be broken apart.

 

[Nereid]

Slightly OT, but to what extent might metals be formed right at the end of the (cosmological) nucleogenesis period, in or near local inhomogeneities (e.g. pockets of gas where that were slightly out of equilibrium)?

IIRC, the nucleogenesis work assumes universal equilibrium, but I could be wrong (it wouldn't be the first time ).

 

[Garth]

Slightly OT, but to what extent might metals be formed right at the end of the (cosmological) nucleogenesis period, in or near local inhomogeneities (e.g. pockets of gas where that were slightly out of equilibrium)?
IIRC, the nucleogenesis work assumes universal equilibrium, but I could be wrong (it wouldn't be the first time ).

I do not think that the system was very far from equilibrium at the time, much later on at the surface of last scattering the variation in density was only one part in 10⁵.

In the standard E-dS expansion the universe cooled too quickly for any substantial metallicity to form.

Garth

 

[SpaceTiger]

Slightly OT, but to what extent might metals be formed right at the end of the (cosmological) nucleogenesis period, in or near local inhomogeneities (e.g. pockets of gas where that were slightly out of equilibrium)?

In the standard model, I think we would expect the universe to be extremely uniform during the epoch of nucleosynthesis. However, some of the more exotic theories of dark matter (like primordial black holes) do present possible seeds for such inhomogeneities. In fact, nucleosynthesis measurements are often used to constrain said theories. I couldn't say very specifically how they alter the final abundances, however.

 

[Chronos]

Nereid watches us wallow through the mud and then comments. Listen to what she says, and I'm not saying your are wrong, ST, but consider her remarks. She is very bright.

 

[SpaceTiger]

Nereid watches us wallow through the mud and then comments. Listen to what she says, and I'm not saying your are wrong, ST, but consider her remarks. She is very bright.

I always take Nereid's comments into consideration...I'm not sure why you think I haven't in this case.

 

[Nereid]

Nereid watches us wallow through the mud and then comments. Listen to what she says, and I'm not saying your are wrong, ST, but consider her remarks. She is very bright.

Nereid (sometimes) makes mistakes; Nereid (sometimes) says stupid things; Nereid (sometimes) doesn't check the literature before writing a post; Nereid (sometimes) regrets not waiting a little longer before replying; Nereid (all too often) drinks (too?) much (usually Australian and New Zealand and Chilean and South African wine); Nereid has much, much, much to learn from (every) PF member; ... (you get the idea, I think).

 

[Chronos]

Is that like casting the first stone, Nereid? I hate to bring this up, but, you may not have: cornered the market on dumb remarks; failed to check the literature; replied in haste, posted under the influence of fine spirits; or have nothing left to learn from anyone.

To ST: I never supposed you didn't consider her views. I was more intrigued by what I perceived was a rare disagreement you had with Nereid.