## Gamma Ray Bursts by Danielle ThompsonJuly 24, 2012

Posted by missthompsondhs in Astronomy, Gamma Ray Bursts, General.
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In an extremely distance galaxy far far away, billions of light years away from Earth, something remarkable happens nearly every day. The brightest and most energetic events known to the universe perform an electromagnetic lightshow. This extravagant phenomenon releases as much energy in a few seconds as the Sun does in its entire lifetime. This amazing occurrence is thought to be connected to the explosive death of a massive star or the collision of neutron stars. These spectacular incidents, known as gamma ray bursts, that only occur on average for 20-40 seconds produce sudden intense flashes of gamma radiation that outshines everything else in the sky.

Gamma Ray Burst

The discovery of the first gamma ray burst was a fortunate derivative of nuclear war defense using U.S. Vela satellites in the late 60’s. The US military satellites were carrying gamma ray detectors because nuclear reactions from bomb tests would give off gamma radiation. The satellites detected a flash of gamma radiation uncharacteristic of any nuclear weaponry. Surprisingly, this discovery was not of urgent concern to the US and over the next ten years with improved technology more information was collected and finally published in a scientific journal.

A later version of an Italian-Dutch satellite, BeppoSAX, launched in 1996 was equipped with not only a gamma ray but an x-ray detector allowing for the observation of the first “afterglow” of a gamma ray burst. An afterglow is caused from the burst colliding with the interstellar gases emitting longer wavelengths. Today NASA satellites are used to create the Gamma-ray Burst Coordinates Network (GCN) which coordinates space and ground-based observations to allow for better viewing of gamma ray bursts’ afterglows.

Further investigation into gamma ray bursts due to the improvements of satellites has allowed for the classification of long and short duration bursts. Long bursts have to last for more than 2 seconds and astronomers are fairly certain the cause of long duration gamma ray bursts is a rapidly rotating massive star, greater than 100 solar masses, and known as a supernova that is collapsing to form a black hole. Short duration bursts make up 30% of all bursts and are thought to be caused by neutron stars colliding. While studying long and short duration bursts, it has been discovered that no two bursts have the same light curve, this is a mystery that still plaques astronomers today.

Light Curves

A new possible explanation for gamma ray burst is a hypernova. Scientists refer to a hypernova as a “failed supernova”, which is still a massive star whose core has collapsed but didn’t go boom. The hypernova’s shock wave doesn’t blow off the outer layers like a supernova does. The outer layers fall into the central neutron star or black hole and produces enormous amount of heat and radiation with an outcome of higher luminosity than a supernova. A hypernova has become the favored possible explanation because gamma ray bursts are more luminous than a supernova. The actually existence of hypernovae is still a hot debate.

Some astronomers suffer from ergophobia, the fear of energy, and the fear that our galaxy the Milky Way could experience a bad day. The scenario of a gamma ray burst firing its extremely energetic radiation at planet Earth is dishearting. The intense gamma rays would be stopped by the Earth’s stratosphere but the ozone layer would be destroyed. Would the depletion of the ozone layer inevitable cause a mass extinction? Gamma ray bursts fuel the speculation that there is a conceivable end to life as we know it on Earth.

The Milky Way

LIVE UPDATE OF GAMMA RAY BURSTS AND THEIR AFTERGLOWS

## Astronomy Club Tonight: Gamma Ray BurstsNovember 3, 2010

Posted by jcconwell in Astronomy, Gamma Ray Bursts.
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Astronomy Club tonight in Room 2153, at 8:00 PM in the Physical Science Building. Dr J Conwell will be talking about Gamma Ray Bursts; the most violent explosions since the big bang.

## Earth’s Deadliest Nemesis: The Gamma Ray Burst?July 24, 2009

Posted by heisman50 in Astronomy, Gamma Ray Bursts.
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Maybe not right now, nor maybe not even tomorrow, but it could happen. Life on Earth could be in serious jeopardy. And the enemy is not anyone or anything on this planet, but rather the workings of an enemy 5000 to 8000 light years away. The sad thing is, there may be no way to see it coming, and most certainly nothing we can do about it. There are over 300,000 gamma ray bursts a year so there is a chance that one could be directed straight towards Earth.

When an enormous star detonates into a supernova, radioactive nuclei are shot out into interstellar space releasing countless photons. This radiation tends to be very low wavelength, high frequency in the gamma ray portion of the electromagnetic spectrum.

To quantify the amount of energy we can use Plank’s Law. According to Plank’s law the energy of a photon can be expressed by the following equation:

E = energy of a photon
h = Plank’s constant = 6.625 x 10-34 J*s or 4.135 x 10-15 eV*s
λ = wavelength of light

Using this equation, the photons of gamma rays will have the highest energy of all as their wavelength is exceptionally small. Thus, a gamma ray photon would produce the largest amount of energy of any type of electromagnetic radiation in the electromagnetic spectrum. While the release of one photon may seem small, the radiation from a supernova directed towards Earth may carry the energy of seemingly countless photons. The result is a tremendously high amount of energy bound for the Earth. To put the amount of energy into perspective, Dr. Edo Berger, an astrophysicist at Harvard University, the energy released by a gamma ray burst is equivalent to powering the world for 1027 years.[1]

The effects could be substantial. One scenario popularized by the History Channel’s series Mega Disasters depicts the gamma radiation attacking and destroying the ozone layer of the Earth while releasing a smog of nitrogen oxide, nitrogen dioxide, and dinitrogen trioxide compounds that effectively block the Sun, break down ozone and cause acid rain. Without the Sun’s rays, a global cooling would exist and plant life could not continue, abruptly disrupting the food chain for all on Earth. According to Dr. Derek Fox, an astrophysicist at Penn State University, a gamma ray burst could irradiate the Earth to the equivalence of 100,000 atom bombs exploding just outside of the Earth’s atmosphere.[2]

So how likely is the threat? The biggest possibility comes from WR104 that sits about 8000 light-years away. If directed perfectly at Earth, the effect on life could be devastating and the fate of the planet may be in jeopardy. But don’t get all worked up just yet. According to David Thompson, a NASA astrophysicist and deputy project director on the Fermi Gamma-ray Space Telescope, the likelihood of a gamma ray burst from WR104 is equivalent to, “the danger I might face if I found a polar bear in my closet in Bowie, Maryland. It could happen, but it is so unlikely that it is not worth worrying about.”[3]

1.http://www.astro.caltech.edu/~ejb/faq.html
2. History Channel. MEGA DISASTERS: GAMMA RAY BURST ©2007

## Extreme Universe: The most distant object known!April 28, 2009

Posted by jcconwell in Extreme Universe, Gamma Ray Bursts.
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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 z = 8.2, the highest ever measured.

Combined X-ray, UV image from Swift

The cataclysmic explosion of a giant star early in the history of the Universe is the most distant single object ever detected by telescopes.

The colossal blast was picked up first by Nasa’s Swift space observatory which is tuned to see the high-energy gamma-rays emitted from extreme events. Other telescopes then followed up the signal, confirming the source to be more 13 billion light-years away. Scientists say the star’s destruction probably resulted in a black hole.

“This gets us into a realm where we’ve never been before,” said Nial Tanvir, of the University of Leicester, UK.  This is the most remote gamma-ray burst (GRB) ever detected, and also the most distant object ever discovered.”

“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.”

GRB 090423 Infrared afterglow as seen by Gemini North

The burst occurred some 13.1 billion years ago, or perhaps a bit more accurate, when the Universe was only 630 million years old, a mere one-twentieth of its current age. Astronomers like to use age rather than distance because when you get this close to the big bang, there are three ways (at least) of referring to distance.

There is a Luminosity distance which ASSUMES  a 1/ (distance squared) law, which works when the space in between in FLAT.

There is the way that you’ll see it referred to in the press, most of the time, since the light has been traveling for 13.1 billion years, the distance is 13.1 billion light-years. Not wrong, but it assumes no expansion.

Then there is….sound of can opener, opening up can of worms….

the proper distance… the distance you would measure if you could take into account all the extra real-estate the universe has added for 13.1 billion years, the expansion of the universe.

To give a little background in redshift and cosmology, a redshift is an increase in the wavelength of the light. There are three types of redshift. The first is Doppler caused by the motion of the source away from the observer. The second is a gravitational redshift caused by light climbing out from a strong gravitational field, like a black hole or neutron star. The third is what we see here the cosmological redshift, caused by the expansion of the universe.

All three are measured by a number called z. This number is the fractional change in the wavelength of the light, or

z = (λ-λ0)/λ0

Where λ0 is the wavelength emitted from far away and λ is what we see in our telescope. The new gamma ray burst had a z = 8.2, meaning an ultraviolet line of Hydrogen emitted at 121 nm. would be shifted all the way down to infrared at 996 nm, (visible is between 750 nm and 380 nm)

Now in cosmology, General Relativity gives a relation between the AGE  of the object, since the big bang, with t=o as the moment of the big bang and its redshift z. Using a model of the expansion of the universe, redshift can be related to the age of an observed object, the so-called cosmic time–redshift relation. This depends on the shape and density of the universe, if we denote a density ratio as Ω0:

$\Omega_0 = \frac {\rho}{ \rho_{crit}} \ ,$

with ρcrit the critical density dividing a universe that eventually crunches from one that simply expands. This density is about three hydrogen atoms per cubic meter of space. At large redshifts one finds, with H0 as the Hubble constant at the present time:

$t(z) = \frac {2}{3 H_0 {\Omega_0}^{1/2} (1+ z )^{3/2}} \ ,$

But finding the distance is a little more complicated.

Picture walking along a sidewalk to your friends house one block away. Now if you had a insane  construction crew adding sidewalk as you were walking, by the time you got to your friend’s house and looked back you might see a lot more than one block of sidewalk.

Well the mad construction crew of the universe can add a lot in 13.1 billion years, so that if you look back now to the gamma ray burst you might find it around 40 billion light years away.

## Astronomy Club: Gamma Ray BurstsMarch 23, 2009

Posted by jcconwell in Astronomy, Gamma Ray Bursts.
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Gamma Ray Bursts will be the topic of Robert Gaki’s talk this Wednesday at 8:00PM in Room 2153, Physical Science Building. Robert, who’s a club member, will be discussing, what many people think, are the  most mysterious and violent explosions short of the big bang.

## Extreme Universe: Magnetic Fields and MagnetarsMarch 12, 2009

Posted by jcconwell in Astronomy, Extreme Universe, Gamma Ray Bursts, Neutron Stars.
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Neutron Stars are extreme to begin with, but magnetars add a whole new level of extreme to these exotic objects. Magnetars,  as the name implies, are neutron stars with ultra high magnetic fields. As a matter of fact, the most extreme magnetic fields ever found in the universe!

An artist's rendering of a magnetar, a type of neutron star. (Image Credit: NASA, CXC, M. Weiss)

There are about 15 magnetars known, they are all examples of a class of objects called “soft gamma repeaters” . The most magnetic one, and the most magnetized object in the known universe is SGR 1806-20. The magnetic field of this magnetar is estimated to be about 2 x 1011 Teslas or 2 x 1015 gauss, one Tesla being equal to 10,000 gauss.

Now, to give you some sense of how big this is, the Earth’s magnetic field is about 1/2 gauss or .00005 Tesla.  The magnet in a hospital’s  MRI is about 3.2 Tesla or 32,000 gauss, and the largest sustained magnetic field created in a lab is about 40 Tesla.

So we’re talking about magnetic fields 1000 trillion times bigger than the Earth’s field. Very weird things can happen with fields this large. One thing that’s interesting  is how much energy is stored in such a field. So let’s break out an equation from physics and use an example I did in my electricity  & magnetism  class last week. If you look it up,  you’ll find  the energy per cubic meter, or energy density, of a magnetic field is given by:

## u = B2/2 μ0

u is the energy density given in Joules per cubic meter. A Joule is the energy you use to lift a kilogram about 10 centimeters off the ground.

B is the strength of the magnetic field  given in Teslas, and  μo is a constant that has a value of 4π x 10-7 (it has units , but we’ll ignore them).  Using a field of B = 2 x 1011 Teslas, the most powerful magnetar, we will get a huge number…

# 1.6 x 1028 Joules/(cubic meter)

or every cubic meter contains this amount of energy. To put this in context, the largest hydrogen bombs have a yield of 20 Megatons of TNT, which is about 1017 Joules of energy. So in each cubic meter of magnetic field has the stored energy of 160,000,000,000 (160 billion), 20 Megaton bombs.

Since we’re having so much fun, lets think about it this way. Einstein showed mass and energy are equivalent, so how much mass would one cubic meter of this HUGE magnetic field have? Well…

E=mc2

or  m = E/ c2 = 1.6 x 1028 Joules/(3 x 108m/s)2 = 1.78 x 1011kilograms

Each cubic centimeter of magnetic field would have a mass of 178 metric tons!!! If you multiply this by the number of cubic meters in the Magnetar, about 40 trillion, assuming the whole neutron star is magnetized, you get a lot of magnetic energy stored in Magnetar.

To give you an idea of what a small amount of this energy would do, consider the events of December 27, 2004. On that day the magnetar we’ve been using as a example, SGR 1806-20, under went a “superflare”. The “superflare,” from a magnetar named SGR 1806–20, irradiated Earth with more total energy than a powerful solar flare. Yet this object is an estimated 50,000 light-years away in Sagittarius.  During that flicker of time it outshone the full Moon by a factor of two. The gamma rays struck the and created more ionization which briefly expanded the ionosphere. Assuming that the distance estimate is accurate, the magnetar must have let loose as much energy as the Sun generates in 250,000 years.

## Extreme Universe:Most Extreme Gamma-Ray Blast Ever!February 24, 2009

Posted by jcconwell in Astronomy, Extreme Universe, Gamma Ray Bursts.
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