Gamma-ray bursts are the most powerful explosions known in the universe. They can be detected even if they happen in the farthest reaches of space. Since light travels at a finite speed, we can observe gamma-ray bursts that happened in the very distant past, when the universe was much younger than today. They can not only help us discover the nature of galaxies and stars in the early and late universe, but their light also carries a signature of all the matter (interstellar gas and dust as well as intergalactic gas) it has passed through on its way to us. In astrophysics, gamma-ray bursts also help us understand the final stages of massive stars, while they are of interest to general physics as laboratories for ultra-relativistic explosions originating in areas with very strong gravitational and magnetic fields – that is, they make it possible to study the laws of nature in conditions so extreme they would be impossible to create on Earth.
Figure 1: Gamma-ray burst
Gamma-ray bursts appear as unforeseeable bursts of gamma light approximately once a day at random points in the sky, outshine all other sources of gamma radiation in the sky (Did you know the Moon ‘shines’ in gamma rays?) and usually disappear after a few seconds or minutes. As gamma radiation cannot penetrate the Earth’s atmosphere, they were first discovered in the age of satellites, in the late 60s. Nowadays the most important means for detecting them is NASA’s Swift satellite, which less than a minute after detecting the burst pinpoints its location in the sky and communicates it to the telescopes on Earth. These hasten to observe the location of the burst and in roughly half of the cases, astronomers can also observe in that location a source of visible light known as an afterglow. The afterglow usually disappears after a few hours or days. For the first 30 years after the discovery, practically nothing was known about gamma-ray bursts, not even whether they originated within or outside our galaxy. After 1997, though, when the first afterglows were spotted, it was possible to conclude that the bursts originate in other galaxies and that there are two types: one is the result of the dense remains of two stars (two neutron stars or black holes) merging, while the other has its origin in a rapidly-rotating massive star’s core collapsing and forming either a black hole or a magnetar (a neutron star with an extremely powerful magnetic field). This causes the most powerful explosions since the Big Bang, which can in a few seconds release as much energy as the Sun will produce in millions, if not billions of years.
Figure 2: Gamma-ray burst. A merger of the dense remains of two stars, neutron stars or black holes (compact merger) produces short gamma-ray bursts (shorter than 2 secs), while a collapse of a massive star’s core (collapsar) produces long gamma-ray bursts (longer than 2 secs). In both cases a black hole or a magnetar is formed in the centre as well as a pair of oppositely directed plasma jets, in which shock waves and light with different wavelengths emerge: from gamma-ray radiation to radio light. Source: Gomboc 2012.
The theoretical model describing the source of gamma-ray bursts states that the energy released in the collapse of a star’s core drives outwards two oppositely directed jets of high-energy plasma, which shoot through the star’s envelope and travel into space at 99.99% of the speed of light. Two such jets are likewise hypothesised to occur when two neutron stars and/or black holes merge (Figure 2). The jets’ sheath contains layers with slightly different speeds, which catch up and collide with one another, producing internal shock waves that through Fermi acceleration cause electrons to reach a velocity close to the speed of light. Because plasma also contains magnetic fields (either local, unordered magnetic fields or ordered global magnetic fields connected to the central black hole or magnetar), electrons emit synchrotron radiation, high-energy light, which can be observed as gamma radiation. Slightly later, the collision of plasma jets with the surrounding interstellar matter generates external shock waves. Electrons accelerated by these have lower energy and emit synchrotron light of longer wavelengths – these can be observed as x-ray, optical and radio afterglows.
Since the launch of the Swift satellite less than a decade ago, our understanding of gamma-ray bursts has greatly improved. But as frequently happens, more data can lead to new open questions, as some simple explanations turn out to be insufficient and the desire for an even deeper understanding of the observed phenomenon increases.
In the case of gamma-ray bursts, one of the major open questions concerns the role of the magnetic field: Is it generated locally in the plasma, in magneto-hydrodynamic instabilities in the jet, or does it originate in the central object and is carried outwards by the flow of matter? The role of the magnetic field cannot be determined with photometric observations alone, since various theoretical models predict similar behaviours of spectrograms. They differ, however, in their predictions of the degree of linear polarisation of light (what fraction of light’s electric field oscillates in a particular direction) the optical afterglow is thought to have very soon after the burst. A key factor in answering this question is measurement of the linear polarisation of afterglows during the few minutes after the detection of a burst, of which, since they are technically highly demanding, only a handful have been taken. The first was published in 2007, while recently we could report on the GRB 120308A burst, on which occasion a team that included Slovene astronomers used the Liverpool Telescope to measure the highest recorded degree of optical linear polarisation and its change over time. The results showed that in gamma-ray bursts there is a strong, long-lived and globally ordered magnetic field, thereby importantly contributing to the understanding of the physics of these objects.
Figure 3: Very Large Telescope – VLT at the Paranal Observatory in Chile. Source: Wikipedia.
The other key open question is which mechanisms govern the acceleration of electrons and the generation of light. Our international team of astrophysicists recently made some astonishing discoveries. In the case of GRB 121024A, which is almost 11 billion light years away, we could for the first time measure the circular polarisation of an afterglow. In our article in Nature magazine, we report the measurements made using the FORS2 instrument on the 8-metre Very Large Telescope (Figure 3) at the European Southern Observatory in Chile. While existing theoretical models predict a low degree of linear polarisation a few hours after the burst, they also predict the degree of circular polarisation (what fraction of light’s electric field spins in a helical pattern) to be immeasurably low or zero. Thus it was commonly believed that even attempting to carry out the technically-demanding measurements was futile. In spite of that, on 24 October 2012, just under three hours after detecting a minute-long gamma-ray burst, our team began taking measurements with the Swift satellite. The afterglow at the time had 20th magnitude, meaning it was 400,000 times darker than stars that can still be observed with the naked eye. The afterglow’s degree of linear polarisation was measured at about 4% and the degree of circular polarisation at 0.6%. Although this may sound low, it is a few orders of magnitude higher than theoretical predictions. We checked whether circular polarisation could be produced by light scattering on interstellar dust or by plasma propagation effects, eliminating both possibilities. It was concluded that the circular polarisation is intrinsic and that existing theoretical models require thorough reassessment. It is possible that the jets’ energy is not transmitted into the interstellar matter as effectively as we had thought and that for description of electron acceleration more sophisticated models are required. Furthermore, solving this puzzle will also require repeating these measurements on larger bursts. But they are, as Forrest Gump’s mother would say: “like a box of chocolates. You never know what you’re gonna get.”
In recent years, the universe has rewarded us with some exceptional bursts.
The GRB 080319B burst was so powerful we could have observed its afterglow with the naked eye (if we had known when and where to look in the sky), even though it happened as much as 7.5 billion light years away. In 2013 the GRB 130427A burst raised a lot of eyebrows as it was discovered that, despite having occurred in relatively nearby space and only recently (“only” 3.7 billion years ago), it was very similar to bursts that occurred farther away, in the early universe. This article in Nature, however, shows that we can also learn a lot from bursts that do not stand out in any way, yet we still successfully reveal their secrets using special and difficult measurements – with knowledge, hard work and a bit of earthling luck.
Figure 4: Part of the sky where GRB 121024A appeared. The arrow on the left-hand photo taken 2.6 hours after the burst using the Very Large Telescope shows an afterglow. The right-hand photo taken later after the afterglow disappeared shows the galaxy where the burst happened. Source: Nature
Figure 5: Comparison of the degree of optical circular and linear polarisation in quasars and in the GRB 121024A afterglow (yellow diamond symbol). GRB 121024A has the highest detected degree of circular polarisation and is the most distant object whose circular polarisation was measured. Source: Nature
Author: Andreja Gomboc, astrophysicist. Researches the most powerful explosions in the universe and lectures on astronomy and astrophysics at the Faculty of Mathematics and Physics in Ljubljana. Since she wants to propagate curiosity and knowledge about outer space, in her spare time she runs a Slovene website called the Portal to the Universe.
Title photo: Gamma-ray burst. Via Wikimedia.
Translated by: Peter Mesarič.