View Full Version : Discussion: Swift Measures the Distance to ...

2005-Apr-05, 05:20 PM
SUMMARY: Since its launch late last year, NASA's Swift space telescope has already spotted 24 gamma-ray bursts - the most powerful known explosions in the Universe. Now Swift has measured the distance to two of these explosions which occurred on March 18/19; both are more than 9 billion light-years away. Swift should become even more accurate in the next few months as more of its instruments are enabled.

View full article (http://www.universetoday.com/am/publish/swift_measures_two_grbs.html)

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2005-Apr-05, 05:52 PM
What's the point of swift? Are we using swift just to find out where blackholes are coming from/located? Do blackholes cause GRB when they form?

Don Alexander
2005-Apr-05, 06:12 PM
Excuse my much too educated mind, but I don't understand this news release.

While it is true that the two mentioned GRBs have the also mentioned redshifts (1.44 and 3.24), both these redshifts were measured by absorption spectra from ground-based telescopes:

Berger et al., 2005, GCN # 3122 for GRB 050318 with Magellan
Fynbo et al., 2005, GCN # 3136 for GRB 050319 with the mentioned NOT

These are not the first redshifts for Swift-GRBs (1.29 for GRB 050126, Berger et al., 2005, GCN # 3088, with the Keck; 1.949 for GRB 050315, Kelson et al., 2005, GCN # 3100, also with Magellan), and an analysis of the Swift-UVOT data on GRB 050318 (de Pasquale et al., 2005, GCN # 3123) states either high intrinsic reddening or an absorber at a redshift of 2.7.

Whatever is being presented in this news release, it has not been distributed via the GCN.

Furthermore, Swift probably detected XRF 050215B in a summed V band image with UVOT (Mason et al., 2005, GCN # 3037).

And while GRB 050319 was in reach of larger amateur telescopes (unfiltered magnitude of 16.16, Quimby et al., GCN # 3135), this was seconds after the GRB, and it was already at CR = 17.6 a mere eight minutes later (and that was a case of SLOW decay...). Thus, any amateur would have to observe within minutes to catch it - and have at least a 10 inch scope.

Now for something slightly off topic:

THE piece of astronews these days is this:

http://www.space.com/scienceastronomy/0504...planet_pic.html (http://www.space.com/scienceastronomy/050401_first_extrasolarplanet_pic.html)

I hope you are not ignoring this on purpose, Mr. Fraser. I know the people who wrote this paper, as they are at the University of Jena in Germany which is more or less around the corner from my institute. The "uncertain mass estimate" comes from actually using models that are applicable only to older objects, and the people who developed these models (Burrows, Baraffe etc.) are actually stating that this is the case and the models developed by Guenther Wuchterl, one of the Jena scientists, are the only ones that work. Neuhaeuser et al. derived a mass between one and three Jupiter masses with two completely different methods, making it a very reliable result.

And that makes it an absolutely historic result!

So what about it?

David Alexander Kann
PhD student, Gamma-Ray burst Afterglow Collaboration at ESO (GRACE)
Thueringer Landessternwarte Tautenburg, Germany

Don Alexander
2005-Apr-05, 06:28 PM
Concerning Swift, I should maybe add:

Swift does not have a spectrograph, but takes images through six filters, three in the ultraviolet (unaccesible from the ground) and the standard UBV.

In the far ultraviolet, beyond the Lyman alpha line, absoption by neutral hydrogen dims radiation extremely. GRBs, lying at cosmic distance, have this Lyman break redshifted into the near ultraviolet or optical regimes. So if you take images in all of these filters, and the brightness of tha afterglow suddenly drops strongly, you can deduce that this is the Lyman break and thus measure an approximate redshift - a so-called photometric redshift.

The early afterglow of GRB 050319 was detected in BV, but not in U (and especially not in the far UV), making a redshift estimate possible, but it was never published - like I wrote, z = 3.24 comes from a ground-based spectrum.

I hope this clears things up a little bit.

David Alexander Kann
Thueringer Landessternwarte Tautenburg

2005-Apr-05, 08:00 PM
Originally posted by TuTone@Apr 5 2005, 05:52 PM
What's the point of swift? Are we using swift just to find out where blackholes are coming from/located? Do blackholes cause GRB when they form?
We don't know much about GRB's but are learning more through Swift. Swift is making observations of GRBs and thir afterglows to give people physical data to compare their models to. There is a thought that some GRBs are cuased by a massive star forming a black hole. Other GRBs may have other causes. Once we have good data about the bursts and afterglows of hundreds or thousands of GRBs we will make better testable guesses about what they are, and what range of things might cause them. That's the point of Swift.

2005-Apr-06, 12:25 AM
The different spectra associated with the grbs suggets to me that they are originating from different environments. Perhaps some intervening material absorbed the ultraviolet radiation at the source of some of the events. To me this suggests that events not associated with ultraviolet radiation emanate from SMBHs with a halo of dust dense enough to absorb UV radiation. The other events may be collapsing stars or smaller black holes with little surrounding dust.

2005-Apr-06, 04:39 AM
Aahh I see, thanks Anton its clear as mud!