My Dark Sky

 …continue from Part 1 – its history and Part 2 – its location…

Now we know that gamma ray bursts (GRBs) are far far away and releasing extremely HUGE amount of energy. They are by far the brightest and most energetic phenomena in the known universe, second only to the Big Bang itself.

What could possibly cause them?

In fact, we are not really sure…

But we have good guesses…

Before we go into what could possibly cause a gamma rays burst, let us first look at the types of GRBs because different types of bursts are created fundamentally by different processes. In general, GRBs are separated into two classes – short-duration bursts and long-duration bursts.

The short-duration bursts last less than 2 seconds and range anywhere from a few milliseconds to 2 seconds with an average duration time of about 0.3 seconds. The long-duration bursts are those last more than 2 seconds and up to few minutes. The average time for long-duration burst is about 30 seconds. 

A gamma-ray burst heralds the birth of a black hole – an artist’s concept. Taken from Science@NASA.

Astronomers now are quite sure what causes the long-duration GRBs. The explanation was first put forth by Dr. Stan Woosley of University of California in his “collapsar” model. At the end of the life of a very very massive star, about 50 to 100 times more massive than our Sun, its core collapsed and formed a black hole. At the same time, the star’s outer layers exploded energetically and it funnel the energy out through the two poles and form a GRB. The energy outputs of these so-called “hypernovae” are much greater (by about 100 times) than a normal supernova.

Although we are now fairly confident of what drives the long-duration GRBs, the short-duration ones still remains a mystery. The durations of the bursts are too short to be supernovae, and the energies involved do not add up to an exploding star. It was found that the short-duration ones release more energy relative to the longer ones.

Our best guess is that short duration GRBs are caused by the extremely violent collisions of two compact objects, such as colliding of two neutron stars or merging of a neutron star and a black hole. When these very compact objects merged, there are going to release a huge amount of energy and ended up as a new black hole (in the case of two colliding neutron stars) or a bigger black hole (in the case of a neutron star and a black hole merging).

An artist’s concept of a neutron star-neutron star collision. Taken from Science@NASA.

We never “see” these kinds of collisions in action, so how do we know that this explanation is true?

Before the two objects collide, they would orbit each other as a binary system. At the same time, they lose their rotational energy to gravitational energy, i.e. their orbit decays, and they spiral in toward each other. As they spiral in, these dense objects intense gravitation field will cause ripples in the fabric of spacetime and can be detected as gravitational waves. The frequency of these waves would ramp up in a characteristic pattern as the objects get closer and closer to each other until… bang! they collided in a fiery explosion, and release burst of gamma rays.

<= Gravitational wave – “ripples” in the curvature of space-time – generated by the motion of massive objects. Credit: K. Thorne (Caltech) , T. Carnahan (NASA GSFC)

Astronomers are trying to detect these waves now. The Laser Interferometer Gravitational-wave Observatory (LIGO) and future Laser Interferometer Space Antenna (LISA) hopefully will be able to detect these gravitational waves and help verifying the model.

These are currently some of our best guesses. It may also be that GRBs originated from something that we have not thought of yet.

Next, we are going to look at the space telescopes with gamma rays eyes.  …to be continued…

Posted in General Astro Talk
Tags: gamma ray burst

continue from last week: What is a Gamma Ray Burst? Its History…

We now know that gamma ray bursts (GRBs) came from space. But exactly where? From our Solar System? within our Milky Way Galaxy? or galaxies far far away?

In 1990s, the Compton Gamma-Ray Observatory (CGRO) was launched. Within a few years, this satellite had detected thousands of bursts. If we plot these bursts on a “sky map”, it turned out that their distribution was uniform across the sky. This means that the bursts should come from outside our galaxy. If they were related to our Milky Way, then they should be more or less confined to the plane of our galaxy, though we weren’t 100% certain because it could be something that was associated with the halo.

BATSE map of its 2704 detected GRBs. Note that the GRBs are distributed all over the sky.

While many astronomers were convinced GRBs came from far far away, some still think that they should be local because of the extreme energy release. These bursts seemed impossibly powerful: to appear so bright from so far away, they must vastly outshine entire galaxies containing hundreds of billions of stars.

To know the distance of a GRB, we must be able to pinpoint its location and transmit the coordinates fast enough back to Earth so that we can zoom in with our ground-based telescopes to find out what it was, how far it was located and what kind of galaxies it was in (if the bursts occurred in galaxies).

The answer came in 1997. BeppoSAX, an Italian-Dutch satellite, manage to identify the location of a GRB (GRB 970228) fast enough that astronomers on the ground were in time to photograph its optical afterglow. After the burst had faded away, they found a faint galaxy there, took its spectra and figure out that it’s a galaxy far far away.

BeppoSAX observations of GRB970228. On the left is the original observation of the burst on 28 February 1997. The right image shows the burst object has dimmed 3 days later.

Afterglow is the radiation emitted after a GRB, and can be seen in other parts of the electromagnetic spectrum – X-rays, optical and radio wavelengths. Afterglows can last for days to several years, and they fade away over time in a well-understood manner. Most of what we know about GRBs comes from afterglow observations. That’s why it is important to pinpoint the exact location of GRBs fast enough to observe their afterglows.

Afterglows helped astronomers determine the distance to GRBs. The discovery of afterglow made redshift measurements possible, which can be translated into distance. And we found out that all of the GRBs are really very very far away, at a distance of few billion light-years. The current record-holder is located 12.8 billion light-years away, almost to the edge of the observable Universe.

So, gamma ray bursts are far far away, releasing extremely HUGE amount of energy (GRB only needs a few seconds to release the energy our Sun produce in its entire 10 billion years lifetime). They are by far the brightest and most energetic phenomena in the known universe, second only to the Big Bang itself.

What could possibly cause them?

…to be continued…

Posted in General Astro Talk
Tags: gamma ray burst

If you ask any astronomer what is the most powerful explosions in the Universe, the answer will definitely be gamma ray burst.

How powerful is it?

Imagine the energy that our Sun put out in its entire 10 billion years lifetime, gamma ray burst only needs a few seconds to release them all!

Basically, gamma rays are just “light” (electromagnetic waves), same as the familiar visible light that we can see but with much shorter wavelengths and thus contains higher energy. In fact, it is the highest energy photons in the electromagnetic spectrum, billions of times more energetic than visible light.

Gamma ray bursts (GRB) are very intense and short bursts of gamma-ray radiation in the sky. They were actually first discovered by accident. In the 1960s during the Cold War, US launched the Vela satellites to make sure that the Soviet Union (specifically and other nations generally) adhered to the nuclear test ban treaty. The satellites will look for nuclear testing by detecting gamma rays that are given off during nuclear explosion.

And Vela did detected bursts of gamma ray. However, the distributions of these gamma rays don’t look the same as the ones you get from nuclear test; the bursts are not coming from our planet, but from space. They seemed to be coming from all over the universe at random and at unexpected times – they are totally unpredictable.

So, now we know the bursts came from space. But exactly where? From our Solar System? within our Milky Way Galaxy? or galaxies far far away?

…to be continued…

Posted in General Astro Talk
Tags: gamma ray burst