Saturday, January 30, 2010

Magnetic fields in gamma-ray burst jets

Gamma-ray bursts (GRBs) are the most dramatic short-lived violent events observed in the universe. They are often described as releasing a quantity of energy, in less than a minute, that is at least as much as a star like the Sun releases in its entire 10 billion year lifetime. Since the first detection of a gamma-ray burst in 1967, the central question has been to determine the nature of the process or processes that can release so much energy so quickly.

We've discussed gamma-ray burst several times before, such as here, here, and here.

The defining characteristic property of a GRB is a rapid, highly energetic burst of gamma rays, lasting only a few seconds. Most of the GRB energy is released in that event. But beyond that, GRBs exhibit a bewildering diversity of characteristics – reflecting a diversity in the conditions that can produce a GRB.

The most noticeable difference observed in GRBs is that some events are over very quickly – within 1 or 2 seconds – while others include an "afterglow" of radiation less energetic than gamma rays, lasting as long as minutes in some cases. In a few instances there are events that have some properties of both "short" and "long" types of GRB.

Gamma rays cannot penetrate the Earth's atmosphere, so the initial phase of any GRB is detectable only from a satellite-based instrument. This is rather limiting in terms of the types of observations that can be made. For example, satellites lacked instruments that could record a spectrum of a GRB event in lower-energy electromagnetic radiation. Without a spectrum, astronomers cannot measure redshift, and hence the distance of the event.

The development and deployment within the last 10 years of systems that could use satellite detection of GRBs to activate automated ground-based telescopes have dramatically improved this situation. It's now possible to collect much more detailed data, so that astronomers have been able to learn a lot more about GRBs – in many cases.

Even so, many short GRB events are over in just a few seconds, and therefore much less is yet known about short GRBs. The best current guess is that such events are caused by the merger of a pair of binary neutron stars.

The research to be described here, therefore, concerns long GRBs, where it is relatively easy to study the characteristics of the lower-energy electromagnetic radiation, which makes up the afterglow for several minutes or even tens of minutes after the initial burst.

A general consensus has emerged that gamma-ray burst progenitors are certain types of supernovae. Not just any type, either, because the progenitor must be capable of releasing the amount of energy actually observed in a GRB. This rules out Type Ia supernovae, which result from a thermonuclear explosion of matter that has accreted onto the surface of a white dwarf star.

Type Ia supernovae are important in cosmology, because they all have roughly the same intrinsic brightness. This makes it possible to determine the approximate distance of a Type Ia supernova event, just from the observed brightness. By comparing this distance with the redshift of the supernova it is possible to determine how rapidly the universe has expanded in the past. This, in turn, is what made it possible to conclude, in 1997, that the expansion of the universe is accelerating.

However, the energy released by a Type Ia supernova is far too small to account for a GRB. Instead, a different supernova mechanism, known as "core collapse" is needed. Classification of supernovae is a little confusing, since it was originally done on the basis of spectral characteristics. Type II supernovae have a particular line in their spectrum due to hydrogen, while Type I supernovae do not.

Type II supernovae result from the collapse of very massive stars at the end of their lives, when they can no longer support their own weight by the pressure of fusion occuring in their constituent matter. But it turns out that some supernovae lacking the hydrogen spectral line are too energetic to result from the same mechanism as that of Type Ia supernovae. These types are known as Type Ib and Type Ic, and they also result from core collapses of massive stars (that have already burned off all their hydrogen).

So there are significant differences even among core collapse supernovae, resulting from such factors as the total mass of the progenitor star and the original composition of the star, among other things. Such differences can account for some of the differences observed if such supernovae are responsible for GRBs.

But by no means all core collapse supernovae produce a GRB. Theoretical considerations dictate that several other factors must also be present. For one thing, the supernova must result in the formation of a black hole that is massive enough to support a large accretion disk of matter orbiting around it. A neutron star, which is the alternative remnant of a supernova, just isn't massive enough. To get a sufficiently massive black hole, the progenitor star must be at least 40 Solar masses.

High mass alone, however, is not enough. The progenitor star must also be rotating rapidly enough that the angular momentum of the system is large enough to cause most of the matter and energy from the supernova explosion to be focused into a jet of angular width at most about 20 degrees. This concentrates most of the energy of the explosion into a narrow beam, so that the energy emitted in our direction matches what we actually observe. If the beam were not so narrow, the energy would not appear to be the magnitude that we observe.

There are additional factors that affect the varying characteristics of GRBs that we observe. In particular, the distribution of matter in the interstellar medium surrounding the supernova is important. It is the collision between the jets and this matter that determines the intensity and duration of the afterglow we observe for some time after the original burst.

And there's more. The jets of matter and energy from a GRB event may well be powered by the energy of the original explosion. But that's not the only possibility. Suppose there are strong magnetic fields surrounding the progenitor star. Then there will also be a considerable amount of energy in the magnetic flux, and this can also supply power to the jets.

Strong magnetic fields would have another consequence as well. The jets consist partly of electrons moving at relativistic speeds (very close to the speed of light). These electrons will follow a spiral path around lines of magnetic flux. This creates a type of electromagnetic radiation known as synchrotron radiation. If present, this radiation would make up part of the afterglow we can observe.

How would we know if synchrotron radiation, and hence magnetic fields, are present? That's simple – the radiation would be partly polarized, provided that the magnetic fields are orderly and not all tangled up.

And this is precisely what recent research has observed in the case of one particular GRB event (GRB 090102), which was detected January 2, 2009. Specifically, a polarization of 10±1% was observed at optical wavelengths. This degree of polarization is quite rare in astrophysical events, and it strongly suggests the presence of large-scale magnetic fields associated with GRB 090102. These fields should contribute substantially to the observable energy of the GRB.


Ten per cent polarized optical emission from GRB 090102
The nature of the jets and the role of magnetic fields in gamma-ray bursts (GRBs) remains unclear. In a baryon-dominated jet only weak, tangled fields generated in situ through shocks would be present. In an alternative model, jets are threaded with large-scale magnetic fields that originate at the central engine and that accelerate and collimate the material. To distinguish between the models the degree of polarization in early-time emission must be measured; however, previous claims of gamma-ray polarization have been controversial. Here we report that the early optical emission from GRB 090102 was polarized at 10 ± 1 per cent, indicating the presence of large-scale fields originating in the expanding fireball. If the degree of polarization and its position angle were variable on timescales shorter than our 60-second exposure, then the peak polarization may have been larger than ten per cent.
Steele, I., Mundell, C., Smith, R., Kobayashi, S., & Guidorzi, C. (2009). Ten per cent polarized optical emission from GRB 090102 Nature, 462 (7274), 767-769 DOI: 10.1038/nature08590

Further reading:

Magnetic Power Revealed in Gamma-Ray Burst Jet (12/9/09)

Huge Cosmic Explosions Fueled by Magnetism (12/9/09)

Gamma-ray bursts: Magnetism in a cosmic blast (12/10/09)

In the News this month: the role of magnetic fields in GRBs (1/3/10)

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