GRB Smashes Record for Most Distant Known Object
Written by Nancy Atkinson
A really, really long time ago in a galaxy far away, a massive star exploded. On April 23, 2009, the Swift satellite detected that explosion. This spectacular gamma ray burst was seen 13 billion light years away, with a redshift of 8.2, the highest ever measured. As we hinted yesterday, this object is now the most distant known object, and the burst occurred when the Universe was only 630 million years old, a mere one-twentieth of its current age. This event, called GRB 090423, can tell us much about the early Universe. "We completely smashed the record with this one," said Edo Berger, a professor at Harvard University and a member of the team that first measured the burst’s origin. "This demonstrates for the first time that massive stars existed in the early Universe."
At 3:55 a.m. EDT on April 23, Swift detected a ten-second-long gamma-ray burst of modest brightness, and quickly slewed around to use its Ultraviolet/Optical and X-Ray telescopes on the burst location. Swift saw a fading X-ray afterglow but nothing in visible light. A number of ground based telescopes were alerted to the event and within three hours began to observe the distant GRB.
"This was a pretty amazing event," Berger told Universe Today. "Swift detected this gamma ray burst on April 23 and we immediately followed it up with the Gemini North Telescope in Hawaii, after it was demonstrated it did not have a visible light counterpart. That was the initial hint that this might be a distant object. We observed it in infrared and we found in the different infrared bands that there was a sharp break at a wavelength of about 1.1 microns."
The drop-out corresponds to a redshift of 8.2 and burst distance of about 13 billion light-years.
Other telescopes that made observations were the Very Large Telescope, STFC’s United Kingdom Infrared Telescope (UKIRT), The Telescopio Nazionale Galileo (TNG), the Okayama Astrophysical Observatory, the Fermi Space Telescope and the Plateau de Bure Interferometer.
Subsequent observations the following night from other telescopes confirmed and refined the measurement. Previously, the most distant known object was a galaxy with a redshift of 6.96 discovered in 2006. The most distant GRB found September of 2008 had a redshift of 6.7. "We completely smashed the record with this one,"said Berger. "I think people were thinking it would happen step by step, but we kind of jumped things."
Berger said the burst itself was not unusual; it was a basic a run-of-the–mill GRB. But even that can convey a lot of information. "That might mean that even these early generations of stars are very similar to stars in the local universe, that when they die they seem to produce similar types of gamma ray bursts, but it might be a little early to speculate."
So what does this distant GRB tell us about the early Universe? "This happened a little more than 13 billion years ago," said Berger. "We've essentially been able to find gamma ray bursts throughout the Universe. The nearest ones are only about 100 million light years away, and this most distant one is 13 billion light years away, so it seems that they populate the entire universe. This most distant one demonstrates for the first time that massive stars exist at those very high red shifts. This is something people have suspected for a long time, but there was no direct observational proof. So that is one of the cool results from this observation."
Berger said this event also tells us that perhaps GRBs are the best objects to study which show how the early Universe evolved. "They are extremely bright and very easy to find, comparatively speaking, so they give us hope that this is the right approach. Over the years people have found high redshift quasars and galaxies, but my suspicion is that until the launch of the James Webb Space Telescope the middle of the next decade, this object will remain as the record holder. No other telescope, including the Hubble Space Telescope is capable of finding more distant objects."
Finding this distant object also demonstrates how telescopes around the world can work together. "It’s the combination of Swift pinpointing where these objects are located and the ground-based telescopes immediately responding to these positions and then demonstrating the distance," said Berger. "It's really a great synergy. We've been doing this for a long time now, and I think part of what has been driving this is the desire to find such distant objects.
Berger said astronomers have been speculating about such distant gamma rays bursts for quite some time and there are two missions being proposed to NASA as the next generation gamma ray telescopes. So, now, the fact that we've now found one at such a high distance makes those satellites more attractive for funding because this has now gone from being an idea or gut feeling to real observational proof."
Source: Interview with Edo Berger
Filed under: Gamma Ray Bursts
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April 28th, 2009 at 7:04 am
I wonder about the spacial vs distance distribution of these GRBs. Do they come from everywhere with the same probability and an equal distribution in age (distance), indicating a spherical universe? Or is there an indication of another shape for the universe? Perhaps there aren't enough measurements for statistical analysis.
April 28th, 2009 at 7:58 am
Isn't a redshift of 8.2 much further away than 13 Gly? We see the event as it happened 13 Gy ago, but the expansion of the universe over that time ought to mean it's further away than that, no?
April 28th, 2009 at 8:09 am
@Silenus: For the spatial distribution in the sky check this page from NASAs BATSE. It looks like they are pretty randomly distributed but I haven't read any analysis of it.
http://www.batse.msfc.nasa.gov/batse/grb/skymap/
April 28th, 2009 at 9:28 am
Although the event happened 13 billion years ago, it is INCORRECT to say that the GRB was seen 13 billion light years away. It is a common misconception that size of the universe must be equal to the cosmological time multiplied by the speed of light. That would be true only in a flat universe. On cosmological scale, the universe is highly curved. Due to expansion of the space metric the light from this GRB traveled close to 46.5 billion light years that and was actually heading away from us during the initial part of its journey.
April 28th, 2009 at 10:44 am
Nancy, thanks for this follow-up on the most distant GRB to date. It looks like JWST or Herschel may be needed to provide crucial spectra of the host galaxy, to deduce properties such as metallicity, kinematics, and star-formation rates in the galaxy that contained the observed GRB 090423. In the meantime Swift or other space observatories may detect even more distant GRBs by chance, again helping astrophysicists deduce local conditions in the early universe. As Edo Berger states: "This most distant one demonstrates for the first time that massive stars exist at those very high red shifts. This is something people have suspected for a long time, but there was no direct observational proof. So that is one of the cool results from this observation.” Very cool, indeed!
April 28th, 2009 at 11:00 am
Scibuff,
When you say that initially the light was actually heading away from us, is this because the universe was still inflating fast? That sounds really interesting, but I'm not sure I understand it. Even if space was expanding really fast, isn't the light emitted in a sphere so some of it would still be travelling towards us?
thanks
Becky
April 28th, 2009 at 11:04 am
If the event was 13 billion light years away (or ago) how come we see the gamma rays? Shouldn't they be red shifted to microwave or infra red? The CRB is shifted to microwaves and presumably much of that was originally gamma rays
April 28th, 2009 at 11:36 am
@AstroAl: Let's refresh our memories a bit first, eh?
Redshift (z) is defined as (observed_wavelength – emitted_wavelength)/emitted_wavelength.
So a redshift of 1 means that the observed_wavelength is twice that of the emitted_wavelength, so an x-ray photon of emitted_wavelength 1 nm would be observed to have a wavelength of 2 nm; and if the z were 8.2, then an x-ray photon of emitted_wavelength 1 nm would be observed to have a wavelength of 9.2 nm.
OK so far?
Now the optical, or visual, waveband is very narrow, stretching from only ~340nm to ~700nm. The gamma-ray waveband is much, much, much, much, … wider; in fact, it stretches from ~10 picometres (0.01 nm) to, well, forever (but for practical purposes, approx 1 PeV, which is ~10 billion times smaller, in wavelength).
So gammas emitted by the z = 8.2 GRB will still be received here as gammas, except for those whose wavelengths were near the border with (hard) x-rays … but since the gamma-x-ray border if rather fuzzy, and arbitrary, no one really cares. And, in any case, GRBs emit gammas over a huge range of wavelengths!
The redshift of the CMB (cosmic microwave background) is ~1100, so what we detect, today, as a microwave of wavelength of 1 mm (say) was emitted at ~1 ? (micron), which is in the near-infrared waveband.
Now the CMB is the most perfect, natural, blackbody ever discovered. And the blackbody spectrum has a rapidly falling short wavelength tail … which means that there were essentially no gammas emitted way back then … and even if there were any, even the softest ones, of wavelength 0.01 nm, would today be seen as x-rays, of wavelength ~1 nm.
Does that make sense to you?
April 28th, 2009 at 11:38 am
Looks like the Greek symbol "mu", for micron, did not display!
April 28th, 2009 at 12:18 pm
Dude says: "This demonstrates for the first time that massive stars existed in the early Universe.”
But I thought that we don't really know what causes GRBs? Must it involve a star?
April 28th, 2009 at 12:28 pm
It very nifty that the network is set up so that the big telescopes can cover the afterglow as fast as possible, but it still leaves a gap for communication and orientation.
Are there any (amateur) scopes that follow the pointing of Swift and Fermi continuously? It should be possible to have even just small telescopes that are always up-to-date with where the space telescopes are observing and watching the same part of the sky in the visible.
Obviously this will be a waste of time most of the time, but with just a couple of small scope the investment will be minute and the results gained in the precious minutes between the initial detection and the point where the big scopes come online must be invaluable.
April 28th, 2009 at 12:55 pm
@ Sili: As far as I understand the situation, any amateur astronomer with a suitably equipped telescope can participate with the big boys as far as GRB outbursts are concerned. Alerts can be automatically transmitted to participating amateurs supplying co-ordinate info as soon as it become available. Quite a beneficial pro-am program.
April 28th, 2009 at 2:33 pm
@ Nereid: Thanks – I knew there must be a simple explanation!
April 28th, 2009 at 2:57 pm
@BeckyWS the concept is quite hard to visualize unless you're comfortable thinking about curved spaces and metric expansions … in a nutshell, what happened is that a beam of light we observed was travelling in a "straight line" in a curved expanding space originally heading away from us but the curvature and expansion eventually pointed it at us. There are more details about the ?CDM (cosmological model) describing this http://en.wikipedia.org/wiki/Metric_expansion_of_space
April 28th, 2009 at 4:24 pm
# cipater Says:
April 28th, 2009 at 12:18 pm
"Dude says: “This demonstrates for the first time that massive stars existed in the early Universe.”
But I thought that we don’t really know what causes GRBs? Must it involve a star?"
They are starting to nail down the process a bit. The Source Of All Knowledge (Wikipedia) has a good introduction/survey of the subject under 'Gamma-ray Burst'.
Basically, our understanding in this area is increasing at a dramatic pace – only 10 – 15 years ago, these things were almost a complete mystery. We now know a great deal more. Long duration bursts have been linked with high-mass dying stars. There is now a significant amount of evidence pointing to this as the progenitor mechanism, and they have models that quantitatively agree reasonably well with the emission spectrum etc. However, the details are still pretty sketchy, as one would imagine.
Short duration bursts are slightly more speculative owing to the nature of the burst itself, but astronomers are fairly confident that they originate from merging neutron stars or black holes.
April 29th, 2009 at 12:55 am
@Sili: the AAVSO, which despite its name is international, has a whole section devoted to GRBs (and blazars), its High Energy Network. Quite a few amateurs have observed GRB afterglows, and their observations have been published in many papers.
In fact, the AAVSO now distributes "Swift, INTEGRAL, SuperAGILE, Fermi GBM, and Fermi LAT GCN Notices"!
Here's a link with more info: http://www.aavso.org/observing/programs/hen/
April 29th, 2009 at 5:04 am
@Nereid, Thanks for pointing out the fantastic work of the AAVSO and its' High Energy Network, which I neglected to mention above. This is a great example of pro-am cooperation.
April 29th, 2009 at 8:49 am
@ Scibuff
Thanks very much, I will check that out.
April 29th, 2009 at 12:01 pm
Is a ten-second burst considered "long duration?" If so, it may involve two neutron stars colliding, right?
April 29th, 2009 at 3:13 pm
scibuff:
While the object that emitted the gamma ray burst is now 46.5 billion light years away, the light itself only had to travel 13 billion light years to get here, since space has been expanding "behind" it on its journey; space that it did not have to cover to get here.
April 30th, 2009 at 6:50 am
I wonder if the high-energy gamma ray component was observed before the low energy GR component, as this issue was just recently mentioned in another article here at UT?