Friday, September 17, 2010

Birth of a Quark Star

Quarks are elementary particles and they are a fundamental constituent of matter. For instance, a neutron is made up of one up-quark and two down-quarks, and a proton is made up of two up-quarks and one down-quark. Neutrons and protons then make up the nuclei of atoms. Quarks have never been studied individually because when two quarks move apart, the force between them increases until it becomes more energetically favorable at some point for a new quark-antiquark pair to appear out of the vacuum than for the two quarks to continue separating. The phenomenon whereby quarks cannot be individually isolated is called color confinement and the phenomenon whereby quark-antiquark particles can appear out of the vacuum is called hadronization.

A quark star is a type of exotic star that is made up of ultra-dense quark matter and they are even denser than neutron stars. Given sufficient pressure from a neutron star’s immense gravity, individual neutrons can break down into their constituent quarks and a neutron star can turn into an even more compact quark star. A typical quark star has roughly the mass of the Sun packed into a diameter of only around 10 kilometers and just a cubic centimeter of its ultra-dense material can have a mass of a few billion metric tons!

Gamma-ray bursts are the most energetic electromagnetic events known to occur in the universe and they emit titanic bursts of gamma-rays which last anywhere from milliseconds to several minutes. Gamma-ray bursts are believed to be narrow bipolar beams of incredibly intense radiation created during powerful supernova explosions and a typical gamma-ray burst produces as much energy in a few seconds as the Sun does over its entire 10 billion years lifespan!

There are two types of gamma-ray bursts – the long duration gamma-ray bursts and the less common short duration gamma-ray bursts. Long duration gamma-ray bursts last longer than 2 seconds and they are generally linked to the deaths of very massive stars. Additionally, long duration gamma-ray bursts are followed by bright and lingering afterglows. On the other hand, short duration gamma-ray bursts last less than 2 seconds and they produce very little afterglows as compared to long duration gamma-ray bursts. The true nature of short duration gamma-ray bursts still remains an enigma and the leading hypothesis is that these events originate from the coalescence of binary neutron stars.

A gamma-ray burst is generally characterized by an initial powerful blast of gamma-rays followed by an afterglow with a rapidly decaying intensity. In this article, I will only focus on the afterglows of long duration gamma-ray bursts and the observed plateau in the light curves of a number of these gamma-ray burst afterglows. Such a plateauing of the afterglow light curve of a gamma-ray burst can be attributed to the cooling behavior of a newly formed quark star.

Immediately after a gamma-ray burst, the newly formed quark star cools by emitting vast amounts of neutrinos and photons. This initial afterglow phase is characterized by a light curve with a gradually decaying intensity. The light curve of the afterglow then stops decaying and plateaus out with a constant intensity. This observed phenomenon can be explained by the solidification of the quark star as it undergoes a phase transition from liquid to solid. The latent heat released during the phase transition can provide a steady and constant supply of energy to power the afterglow of the gamma-ray burst. This is because the temperature of the central quark star will remain constant as it undergoes its phase transition.

After the phase transition, the light curve of the afterglow abruptly decays due to the extremely low heat capacity of the solid quark star. The entire phase transition of the quark star from liquid to solid occurs over a timescale of roughly 1000 seconds and the amount of energy generated from the phase transition alone is roughly equal to the total amount of energy the Sun gives off over a period of 10 billion years!

Therefore, the amount of energy produced during the phase transition of a quark star is sufficient and steady enough to produce the plateau in the light curve observed in the afterglow of a gamma-ray burst. Gamma-ray bursts are the most powerful explosions in the universe and when they do occur, they blaze with the glory of a billion billion Suns. Nevertheless, as magnificent as they are, their fleeting nature makes them elusive and challenging to study.

Friday, September 3, 2010

Around Sagittarius A*

Our Sun is one of the hundreds of billions of stars in the Milky Way Galaxy and it is located approximately 26000 light years from the center of the galaxy. One light year is the distance light travels in a year and its value is 9.46 trillion kilometers. A supermassive black hole named Sagittarius A* sits right in the center of the Milky Way Galaxy and this colossal black hole is estimated to have a mass of four million Suns.

Located within the close vicinity of Sagittarius A* is a very intriguing group of stars called the “S stars”. These stars are the closest known stars to the center of the Milky Way Galaxy and they orbit around Sagittarius A* at very high orbital velocities. Each of the “S stars” are several times more massive than our Sun and being much more massive than our Sun, these stars undergo a much more rapid rate of nuclear fusion which means that they have a lifespan of just several million years. In comparison, our Sun has a lifespan of over 10 billion years.

The “S stars” are intriguing because the environment around such a supermassive black hole is hostile to the formation of stars and the “S stars” would have to form somewhere much further out before migrating to their current extraordinarily close proximity to Sagittarius A*. However, the timescales involved in such a migration is longer than the short ages of the “S stars” and hence, these “S stars” constitute a “paradox of youth”.

S2 is the designation given to one of the “S stars” and what distinguishes S2 from the other stars is that S2 is by far the closest star in orbit around the supermassive black hole - Sagittarius A*. S2 orbits Sagittarius A* in a highly elliptical orbit and in such an extreme gravitational environment near a supermassive black hole, S2 takes just 15.5 years to complete one orbit around Sagittarius A* even though S2 is located at an average distance of about 140 billion kilometers from Sagittarius A*. In comparison, Pluto orbits the Sun once every 248 years at an average orbital distance of 6 billion kilometers from the Sun.

At its closest approach, S2 comes within just 17 light-hours from Sagittarius A* and this is roughly three times the distance of Pluto from the Sun. The highly elliptical orbit of S2 also brings it out as far as 10 light-days from Sagittarius A*. During closest approach, S2 zips around Sagittarius A* at a incredible velocity of over 5000 kilometers per second (about 2 percent the speed of light). The remarkable orbit of S2 around Sagittarius A* makes it uniquely valuable for testing various general relativistic and even extra-dimensional effects.