Gamma Ray Burst Visible At Record Distance

Invisible Pink Unicorn writes "A gamma ray burst detected on March 19 by NASA's Swift satellite has set a new record for the most distant object that could be seen with the naked eye. The burst had a measured redshift of 0.94, meaning the explosion took place 7.5 billion years ago. The optical afterglow from heated gas was 2.5 million times more luminous than the most luminous supernova ever recorded, making it the most intrinsically bright object ever observed by humans in the universe. The previous most distant object visible to the naked eye is the nearby galaxy M33, a relatively short 2.9 million light years from Earth."

Title slightly in error

[Lazarian]

Not to nitpick, but the article title should have been more along the lines of "Most distant naked-eye event recorded", since gamma rays themselves are not visible.

Anyway, it's a good thing that this occurred so far away, instead of nearby. There are a few hypergiant stars known to exist in our galaxy like Eta Carinae and the Pistol Star which are inherently unstable. And in 2004 a GRB was emitted by a magnetar half way across the galaxy that, were it visible, would have been brighter than a full moon. Its been proposed that GRB's may be a factor in past extinction events here on earth.

Re:Title slightly in error

[Chris Burke]

Not to nitpick, but the article title should have been more along the lines of "Most distant naked-eye event recorded", since gamma rays themselves are not visible.

It's not the gamma rays they're talking about, but the afterglow caused by gas heated by the gamma rays, and this afterglow is partly in the visible spectrum, and was in fact visible with the naked eye (magnitude between 5 and 6, which is at the edge of unaided human vision). The title is completely correct.

"No other known object or type of explosion could be seen by the naked eye at such an immense distance," said Swift science team member Stephen Holland of Goddard. "If someone just happened to be looking at the right place at the right time, they saw the most distant object ever seen by human eyes without optical aid."

Re:Phew

[Anonymous Coward]

No. It's already faded from view. The peak brightness in the visual range, however, was estimated to reach about magnitude 5 or 6, which is about the same as the dimmest stars you can see from a really dark location.

Re:Article is wrong

[ajs]

The Universe is, in fact, at least 156 billion light years wide:

http://www.space.com/scienceastronomy/mystery_monday_040524.html [space.com]

Re:Redshift?

[amRadioHed]

You can tell the difference by looking at the spectrum. The light spectrum of any distant source will have absorbtion bands [wikipedia.org] from passing through various elements such as hydrogen. These bands form recognizable patterns and so astronomers can determine the red shift by measuring how far the absorption bands have been shifted from their normal location.

Re:Article is wrong

[JohnFluxx]

(Just for reference, I am doing an MSc in this field.)

Your definition would be what cosmologists call 'comoving distance'. I have never seen a light year defined in this way however. The rate of expansion changes with time, so under your definition you would end up with things like that 2 * 1 light year != 2 light years, etc.

It also means that a light year now, would be a different distance (in km) than a light year was a year ago, etc.

Re:Article is wrong

[JohnFluxx]

Update:

I emailed the author, and they have now corrected the article.

The article now just says:

The explosion was so far away that it took its light 7,500,000,000 (7.5 billion) years to reach Earth! In fact, the explosion took place so long ago that Earth had not yet come into existence.

And the title has also been changed to "A Stellar Explosion You Could See on Earth!" (Instead it was something about that it happened half way across the universe from us)

Re:Article is wrong

[JohnFluxx]

For anyone interested, here's the email that I received from the author:

Hi John,

Thanks for your message. I was the principle author of the press release, so I will try to answer your question. I should note that the press release was reviewed by numerous scientists. But it was edited at NASA headquarters before it was made public.

In my original draft, I purposefully avoided making the statement that the GRB was 7.5 billion light-years from Earth, because as your message implies, it is problematic to express specific distances when one is talking about events that happened in the very distant past, because the universe is rapidly expanding. Such is the case when trying to express a "distance" to GRB 080319B.

The most relevant direct "distance" measurement is the object's redshift, which was measured to be 0.94. As the press release explained, this measurement tells astronomers how much the GRB's light was "stretched" by cosmic expansion. I used this popular website from a renowned UCLA cosmologist to convert the object's redshift to a light-travel time:

http://www.astro.ucla.edu/~wright/CosmoCalc.html [ucla.edu]

When I entered the redshift and the cosmological parameters based on the latest results from the WMAP satellite and large-scale galaxy surveys, the calculator gave me a light-travel time of 7.5 billion years. In other words, the light from this GRB was emitted 7.5 billion years ago.
But at the time the burst occurred, Earth didn't even exist, so how does one express a "distance" between one object and another object that does not exist? In addition, 7.5 billion years ago, the visible universe was a much smaller place than it is now, because cosmic expansion has made the universe much bigger during those intervening 7.5 billion years. The GRB's host galaxy and the Milky Way Galaxy would have been much closer back then than they are today (please note that the Milky Way would have been a lot different back then, but it undoubtedly existed at that time). In fact, back then, the two galaxies would have been much closer than 7.5 billion light-years. And yet because of cosmic expansion, the two galaxies are currently much farther apart than 7.5 billion light-years. So there really is not an ideal way to express such a huge distance.

In my opinion, the best way to express such a huge distance in a rapidly expanding universe at the level of a popular audience is to express distances in terms of light-travel time, which is what I did in the original draft of the press release. And because our best current measurements suggest that the universe is 13.7 billion years old, an event taking place 7.5 billion years ago is roughly halfway across the visible universe. Some of the scientists at NASA probably felt that it was important to specify a distance in a unit of distance rather than in a unit of time, so they translated the light travel time to a distance in light-years. I realize this is imprecise from a strict scientific perspective, but the NASA scientists concluded that there is no better way to express it, and I cannot think of a better way to do it.

The problem, of course, is that the most precise way to express the distance is to state the redshift, which I did in the press release. Unfortunately, the term "redshift" has little meaning to the media and public, and the general public does not have the familiarity with astronomical terminology to be able to translate a redshift of 0.94 into a distance that has any deep meaning.

Best regards,

Robert Naeye, NASA Goddard Space Flight Center

 

Re:Without the red shift...

[K. S. Kyosuke]

I do not have much information, but assuming that the brightest recorded supernova emitted about 10^45 J of energy in about one hundred days (you can fetch a nice paper on this here [arxiv.org]) and this phenomenon is supposed to be two and half million times brighter for about thirty seconds (here [wikipedia.org]), I ended up with 2.5*10^6 * 10^45 * (30 / (86400 * 100)) = ~9*10^45 W of peak apparent power output of an isotropic radiation source (as in the effective radiated power for a transmitter, not the transmitter power output, in the telecommunication systems parlance - my EE education shows up ;-)).

Given that the solar constant is somewhere around 1370 watts per square meter, the distance for the irradiation to match the one we are receiving from the Sun would be about 23 kpc. This is a ridiculous number! Either I am a way off and missing something important (I am no astronomer, I admit that), or it is just that this thing could be *much* further away than those 2700 ly mentioned by the parent and it would still have enough energy to kill us all. I really do not want to imagine what would happen to us at the distance of just ~1 kpc.