APOD and General Astronomy Discussion Forum
https://asterisk.apod.com/
In one famous case (GRB080319B), simultaneous wide-field cameras recorded an optical counterpart that reached visual magnitude 5.8, so a keen-eyed observer looking in the right direction would have seen it for a period of some seconds - at a redshift z=0.93, distance about 7 billion light-years. (In that particular case, almost certainly no one actually did because the Moon was full, but the point remains - another decade will probably see one just as bright in the visual range). GRB 130427A would have shown up in binoculars for a minute or two had someone been looking in the right direction and noticed a new faint "star". Its optical counterpart has been fading very rapidly, something like one magnitude per 24 hours for a few days (meaning that by now it's vanished from detectability if that rate continued).Strangerbarry wrote:I wonder if any GRBs have been or could be seen with the naked eye ? You'd have to be looking at the right place at the right time - pure chance - and most people likely wouldn't know what they're seeing. Still, the thought that one might see light that is billions of years old, without a telescope, seems fascinating
Of course, that's why it's called an experiment.rstevenson wrote:Presumably you're thinking of using high-energy collisions, such as in the large Hadron Collider, to make a black hole? The Wikipedia page about Black Holes says...
RobEven if micro black holes should be formed in these collisions, it is expected that they would evaporate in about 10−25 seconds, posing no threat to the Earth.
http://en.wikipedia.org/wiki/Gamma-ray_burst_progenitors wrote:<<Gamma-ray burst progenitors are the types of celestial objects that can emit gamma-ray bursts (GRBs). GRBs show an extraordinary degree of diversity. They can last anywhere from a fraction of a second to many minutes. Bursts could have a single profile or oscillate wildly up and down in intensity, and their spectra are highly variable unlike other objects in space. The near complete lack of observational constraint led to a profusion of theories, including evaporating black holes, magnetic flares on white dwarfs, accretion of matter onto neutron stars, antimatter accretion, supernovae, hypernovae, and rapid extraction of rotational energy from supermassive black holes, among others.Click to play embedded YouTube video.
As of 2007, there is almost universal agreement in the astrophysics community that the long-duration bursts are associated with the deaths of massive stars in a specific kind of supernova-like event commonly referred to as a collapsar or hypernova. Very massive stars are able to fuse material in their centers all the way to iron, at which point a star cannot continue to generate energy by fusion and collapses, in this case, immediately forming a black hole. Matter from the star around the core rains down towards the center and (for rapidly rotating stars) swirls into a high-density accretion disk. The infall of this material into the black hole drives a pair of jets out along the rotational axis, where the matter density is much lower than in the accretion disk, towards the poles of the star at velocities approaching the speed of light, creating a relativistic shock wave at the front. If the star is not surrounded by a thick, diffuse hydrogen envelope, the jets' material can pummel all the way to the stellar surface. The leading shock actually accelerates as the density of the stellar matter it travels through decreases, and by the time it reaches the surface of the star it may be traveling with a Lorentz factor of 100 or higher (that is, a velocity of 0.9999 times the speed of light). Once it reaches the surface, the shock wave breaks out into space, with much of its energy released in the form of gamma-rays.
Three very special conditions are required for a star to evolve all the way to a gamma-ray burst under this theory: the star must be very massive (probably at least 40 Solar masses on the main sequence) to form a central black hole in the first place, the star must be rapidly rotating to develop an accretion torus capable of launching jets, and the star must have low metallicity in order to strip off its hydrogen envelope so the jets can reach the surface. As a result, gamma-ray bursts are far rarer than ordinary core-collapse supernovae, which only require that the star be massive enough to fuse all the way to iron.
This consensus is based largely on two lines of evidence. First, long gamma-ray bursts are found without exception in systems with abundant recent star formation, such as in irregular galaxies and in the arms of spiral galaxies. This is strong evidence of a link to massive stars, which evolve and die within a few hundred million years and are never found in regions where star formation has long ceased. Second, there are now several observed cases where a supernova has immediately followed a gamma-ray burst. While most GRBs occur too far away for current instruments to have any chance of detecting the relatively faint emission from a supernova at that distance, for lower-redshift systems there are several well-documented cases where a GRB was followed within a few days by the appearance of a supernova. These supernovae that have been successfully classified are type Ib/c, a rare class of supernova caused by core collapse. Type Ib and Ic supernovae lack hydrogen absorption lines, consistent with the theoretical prediction of stars that have lost their hydrogen envelope. The GRBs with the most obvious supernova signatures include GRB 060218 (SN 2006aj), GRB 030329 (SN 2003dh), and GRB 980425 (SN 1998bw), and a handful of more distant GRBs show supernova "bumps" in their afterglow light curves at late times.
Possible challenges to this theory emerged recently, with the discovery of two nearby long gamma-ray bursts that lacked the signature of any type of supernova: both GRB060614 and GRB 060505 defied predictions that a supernova would emerge despite intense scrutiny from ground-based telescopes. Both events were, however, associated with actively star-forming stellar populations. One possible explanation is that during the core collapse of a very massive star a black hole can form, which then 'swallows' the entire star before the supernova blast can reach the surface.>>
Not entirely... It is apparently all part of the same jet acting upon the star material.DTT wrote:
The background image pre-explosion appears to be different from the background shown with the explosion. Can someone explain that to me?
http://science.nasa.gov/science-news/science-at-nasa/2008/21mar_nakedeye/ wrote:<<Most gamma ray bursts occur when massive stars run out of nuclear fuel. Their cores collapse to form black holes or neutron stars, releasing an intense burst of high-energy gamma rays and ejecting particle jets that rip through space at nearly the speed of light. When the jets plow into surrounding interstellar clouds, they heat the gas to incandescent visibility. It is this gaseous "afterglow" which was visible to the human eye on March 19, 2008.>>
Well, did it? Reported was only, that the visible light arrived earlier. I realize that interstellar space is a very thin medium, but then 3e9 ly is a lot of medium.neufer wrote:
Why visible light would escape from the star so long before the gamma rays do is a mystery.
But gamma ray bursts take place throughout the known universej_m_h wrote:
I'm a bit confused at the statement that 5 billion light years is considered "in the neighborhood". If the universe is on the order of 14 billion years old then something is 5 billion light years away over a 1/3 (1/6?) of the universe away isn't it?
The corrections were made in the APOD: http://apod.nasa.gov/apod/ap130508.htmlBoomer12k wrote:
From my re-reading the article....no corrections have been made....
This is why it's generally best to stick with redshift, and not distances. It is probably more correct to say that this object is now 4.3 billion light years away, meaning it occurred 3.7 billion years ago. Once we get up into fractional redshifts, the comoving disance and light travel time start diverging significantly due to the expansion of space.Daburb1953 wrote:It was observed on the 27th of April. I discovered (today) that GRB 130427 means 'Gamma Ray Burst 2013-04-27' or April 27th 2013
If the distance is now reported at 3.6 billion light years. So it occurred 3.6 billion years ago - or thereabouts - give or take a month or two :)
We all have a innate desire to feel both important and special.Perk Cartel wrote:
I saw the APOD May 8 page today May 11 ( been a bit busy) and the 0.34 redshift was given as 5 billion years ago for the GRB so again humans witness an event well before the sun, solar system and the planet upon which we evolved ever existed - with this kind of miraculous understanding, why do we seem to need religion and imaginary friends in the sky?
Presumably because they aren't gamma ray sources at all, but rather represent the noise floor of the instrument.Good Dog Lucy! wrote:Why do the blue "background" gamma-ray sources change position so radically between the two frames of the animation?
Neufer wrote:
If we ever make contact with extraterrestrials we will know that we also aren't special.
CaptainFanSpaztic wrote:
Can someone explain the other two "hot spots" (On the horizon at 1:00 and near the horizon at 4:00) in these images?
