Archive for April, 2008

Some of the most powerful explosions in the Universe: gamma ray bursts

Posted in Science on April 23, 2008 by Grad Student

I just attended an awesome talk given by Dr. Niccolo Bucciantini about one of the most puzzling astronomical phenomena out there: gamma ray bursts. These bursts are just pulses of photons that last from 1 second to ~1000 seconds, and the photons are at extremely high energies. All in all these bursts are some of the most luminous objects in the universe. Yet the puzzling thing is that the bursts are very dissimilar, some are just one simple pulse, others have a variety of flares before they die out. The classic joke is, “if you’ve seen one gamma ray burst, … then you’ve seen one gamma ray burst.” We can at least create two categories: long bursts (100-1000 sec.) and short bursts (1-2 sec). The talk I attended concerns long bursts.

The colossal power and compactness of these objects actually puts some serious constraints on what could be powering them. To date, compact objects (either neutron stars or black holes) are the most efficient way to produce prodigious amounts of energy. As it turns out, many models of gamma ray bursts start with a star in its death throes whose core collapses into a compact object. Then, the rest of the star falls onto the compact object, which then produces copious amounts of the energy. In fact, around 10% of the rest mass energy of the in-falling matter is converted into light (less than 1% of the rest mass energy in converted to energy in nuclear fusion (what powers the stars).

Another interesting property of gamma ray bursts can be deduced from the short variability (in time) in the amounts of energy we detect. Even though compact objects, are, well…compact, it turns out they are just too big to be the causes of such short variability. As Bucciantini put it, “Shaq can’t move as fast as the smaller point guards.” Thus, the light we see is being emitted from a small region, which I’ll call a blob. Unfortunately this causes some serious problems. If you have a bunch of highly energetic photons in a small blob, they will not escape! They will slam into each other, producing an electron and a positron. Thus we won’t see any photons because all of them will produce electron-positron pairs. The solution to this problem is actually quite simple: we can say this blob is moving towards Earth at a speed very close to the speed of light. Then all these photons that we see as being so energetic are only energetic because they were produced in a blob that had a lot energy (due to its motion) in the first place! So if you’re riding along with the blob then the photons aren’t as energetic, meaning they wouldn’t be able to produce electron-positron pairs. This then allows for the highly energetic photons to be produced in a small blob and for them to be able to escape without slamming into each other and metamorphosing into electron-positron pairs. (I’m leaving out a few details. Look here for the whole story of what’s called “relativistic beaming.”) In conclusion, gamma ray bursts are supernova that send out a highly relativistic jet (meaning ~99% of the speed of light) that is pointed right at Earth.

Bucciantini and collaborators are working on a model that has a unique type of neutron star, a magnetar as the “engine” which powers these bursts. A magnetar is just a neutron star with a huge magnetic field (14 orders of magnitude stronger than Earth’s magnetic field). Its field is like Earth’s in that the true “north” pole (or spin axis) does not line up with the magnetic pole. This misalignment coupled with a neutron’s stars extremly fast spin, causes the star to emit lots of light. This light, which is electromagnetic energy, comes directly from the neutron star’s spinning energy. Therefore, by conservation of energy the magnetar actually spins down due to all the electromagnetic energy (light) produced (this is known as magnetic breaking).

What’s so great about all this magnetar stuff? As it turns out we can measure the spinning rate of normal neutron stars that don’t spin down like this, and calculate the amount of “spinning energy.” It turns out this energy is just the amount needed to power a typical long gamma ray burst! Also, when you calculate how long it takes for a typical magnetar to spin down, it takes 100-1000 seconds, precisely the duration of a long gamma ray burst!

The next really exciting part of Bucciantini et al’s model is that the magnetic field in the presence of the collapsing star seems to be able to produce the jet that the short variability arguments support!

All in all, it was an excellent talk about a very interesting and puzzling subject in astrophysics. Kudos to Bucciantini.