Astronomy: New direction for γ-rays
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understanding of its signalling repertoire is finally beginning to catch up. ■ Richard H. Masland is at the Howard Hughes Medical Institute, Massachusetts General Hospital, Harvard Medical School, Boston, Massachusetts 02114, USA. e-mail: [email protected] 1. Ölveczky, B. P., Baccus, S. A. & Meister, M. Nature 423, 401–408 (2003). 2. Masland, R. H. Nature Neurosci. 4, 877–886 (2001). 3. Vaney, D. I., Peichl, L. & Boycott, B. B. Proc. R. Soc. Lond. B 235, 203–219 (1988). 4. Famiglietti, E. V. J. Comp. Neurol. 316, 422–446 (1992). 5. Euler, T., Detwiler, P. B. & Denk, W. Nature 418, 845–852 (2002). 6. Fried, S. I., Münch, T. A. & Werblin, F. S. Nature 420, 411–414 (2002). 7. Roska, B. & Werblin, F. Nature Neurosci. doi:10.1038/nn1061 (2003). 8. Chiao, C.-C. & Masland, R. H. Invest. Ophthalmol. Vis. Sci. Suppl. 3255 (2003). 9. Masland, R. H. Curr. Opin. Neurobiol. 11, 431–436 (2001). G amma-ray bursts (GRBs) are short flashes of g-rays, typically tens of sec- onds long 1 . First detected in the 1960s, GRBs are observed at a rate of roughly one per day. Although their sources are known to reside in distant galaxies, several billion light years away, what these sources are remains a mystery. But a new clue is provided by Wayne Coburn and Steven Boggs 2 , who, on page 415 of this issue, report the detection of polari- zation — a particular orientation of the electric-field vector — in g-rays from a burst. This discovery may shed light on the identity of the sources of GRBs, as well as on the mech- anism by which the g-rays are produced. The huge energy release associated with a GRB is thought to be created by the gravita- tional collapse of a star to form a black hole or neutron star 3 . The contraction causes gravi- tational energy to be released. The typical energy output of a GRB corresponds to the conversion of about 1% of the Sun’s mass into energy: in comparison, the energy out- put of an atom bomb is equivalent to the con- version to energy of about 1 gram of matter. The energy released in the collapse seems to be carried away from the source in the form of a highly relativistic jet (Fig. 1), in which particles move at nearly the speed of light. As the jet reaches a radius of about 100 million kilometres from its source, part of this energy is converted to g-rays, which become the GRB. At a later stage still, as the jet expands to a scale of 10 billion kilometres, an ‘afterglow’ of lower-energy radiation, at X-ray, optical and radio wavelengths, is produced. The detection over the past few years of GRB afterglows 4 provides strong support for this picture. But as afterglow radiation is produced at a large distance from the collapsing object, key questions remain unanswered 3 : what, for instance, is the nature of the collapsing object? The most popular candidate is a massive star, about ten times the mass of the Sun, whose life ends with the collapse of its core. How the gravita- tional energy released becomes a relativistic jet and how jet energy is converted to g-rays are also not well understood. From earlier observations of the g-ray spectrum of GRBs 1 , it was concluded that the most likely mechanism for g-ray production is ‘synchrotron emission’ — the emission of radiation by highly energetic electrons gyrating in a strong magnetic field. But other mechanisms, such as thermal emission or energy loss by relativistic electrons in intense radiation fields, are also possibilities 3 . The radiation released through synchrotron emission is highly polarized, but the other suggested mechanisms do not naturally produce large polarization. That Coburn and Boggs detect a clear polarization in the g-rays from a burst provides direct evidence in support of synchrotron emission as the mechanism of g-ray production. Their observations also reveal more about the nature of the magnetic field in which synchrotron emission occurs. In a GRB, the g-rays are produced in different regions inside the jet (Fig. 2). These regions are unresolved by detectors close to the Earth, so the source appears to be point-like. The detected polarization signal is, then, an average over the polarization of radiation produced at different points within the source. If the direction of polarization varies randomly from place to place in the jet, then the observed polarization signal is likely to average out to zero. But this ‘washing out’ of the polarization signal will not happen if the polarization direction is the same every- where. For polarization produced by the synchrotron mechanism, this means that the g-ray-producing region is suffused by an ordered magnetic field, oriented in the same direction everywhere (Fig. 2a). The direction of polarization reported by Coburn and Boggs 2 remained constant throughout the duration of the GRB, but the g-ray flux varied significantly. So it seems unlikely that such a strong, constant, ordered field could be generated in the region where the g-rays are produced. Rather, this suggests that the strong field originates near the col- lapsing object, and is then carried by (or perhaps even drives) the jet outwards from the source: the mechanism by which gravita- tional energy is extracted and powers the jet is, possibly, electromagnetic. But there could be another explanation. news and views 388 NATURE | VOL 423 | 22 MAY 2003 | www.nature.com/nature Figure 2 Information-processing machinery in the retina. As well as the input (rods and cones) and output (ganglion) cells, the retina contains a hugely diverse population of neurons. Each cell type is thought to have a specific role in vision 9 . The most extreme diversity is exhibited by amacrine cells, a collection of which is depicted here: their differing shapes and sizes are reflections of their wide variety of synaptic patterns. The widely spreading ‘polyaxonal’ amacrine cells, which come in several subtypes and are highlighted at lower right, are the type that Ölveczky et al. 1 suggest provide the mechanistic basis of local motion detection. (Data for the rabbit, adapted from ref. 2.) Astronomy New direction for g-rays Eli Waxman The origin of energetic g-ray bursts is still unknown. But the detection of polarization of the g-rays provides fresh insight into the mechanism driving these powerful explosions. © 2003 Nature Publishing Group
Transcript of Astronomy: New direction for γ-rays
understanding of its signalling repertoire isfinally beginning to catch up. ■
Richard H. Masland is at the Howard HughesMedical Institute, Massachusetts General Hospital,Harvard Medical School, Boston, Massachusetts02114, USA.e-mail: [email protected]
1. Ölveczky, B. P., Baccus, S. A. & Meister, M. Nature 423,
401–408 (2003).
2. Masland, R. H. Nature Neurosci. 4, 877–886 (2001).
3. Vaney, D. I., Peichl, L. & Boycott, B. B. Proc. R. Soc. Lond. B 235,
203–219 (1988).
4. Famiglietti, E. V. J. Comp. Neurol. 316, 422–446 (1992).
5. Euler, T., Detwiler, P. B. & Denk, W. Nature 418, 845–852
(2002).
6. Fried, S. I., Münch, T. A. & Werblin, F. S. Nature 420, 411–414
(2002).
7. Roska, B. & Werblin, F. Nature Neurosci. doi:10.1038/nn1061
(2003).
8. Chiao, C.-C. & Masland, R. H. Invest. Ophthalmol. Vis. Sci.
Suppl. 3255 (2003).
9. Masland, R. H. Curr. Opin. Neurobiol. 11, 431–436 (2001).
Gamma-ray bursts (GRBs) are shortflashes of g-rays, typically tens of sec-onds long1. First detected in the 1960s,
GRBs are observed at a rate of roughly one perday. Although their sources are known toreside in distant galaxies, several billion lightyears away, what these sources are remains amystery. But a new clue is provided by WayneCoburn and Steven Boggs2, who, on page 415of this issue, report the detection of polari-zation — a particular orientation of the electric-field vector — in g-rays from a burst.This discovery may shed light on the identityof the sources of GRBs, as well as on the mech-anism by which the g-rays are produced.
The huge energy release associated with aGRB is thought to be created by the gravita-
tional collapse of a star to form a black hole orneutron star3. The contraction causes gravi-tational energy to be released. The typicalenergy output of a GRB corresponds to theconversion of about 1% of the Sun’s massinto energy: in comparison, the energy out-put of an atom bomb is equivalent to the con-version to energy of about 1 gram of matter.
The energy released in the collapse seemsto be carried away from the source in the formof a highly relativistic jet (Fig. 1), in whichparticles move at nearly the speed of light. Asthe jet reaches a radius of about 100 millionkilometres from its source, part of this energyis converted to g-rays, which become theGRB. At a later stage still, as the jet expands toa scale of 10 billion kilometres, an ‘afterglow’
of lower-energy radiation, at X-ray, opticaland radio wavelengths, is produced.
The detection over the past few years ofGRB afterglows4 provides strong support for this picture. But as afterglow radiation is produced at a large distance from the collapsing object, key questions remainunanswered3: what, for instance, is thenature of the collapsing object? The mostpopular candidate is a massive star, about tentimes the mass of the Sun, whose life endswith the collapse of its core. How the gravita-tional energy released becomes a relativisticjet and how jet energy is converted to g-raysare also not well understood.
From earlier observations of the g-rayspectrum of GRBs1, it was concluded that themost likely mechanism for g-ray productionis ‘synchrotron emission’ — the emission of radiation by highly energetic electronsgyrating in a strong magnetic field. But othermechanisms, such as thermal emission orenergy loss by relativistic electrons in intenseradiation fields, are also possibilities3. Theradiation released through synchrotronemission is highly polarized, but the othersuggested mechanisms do not naturally produce large polarization. That Coburnand Boggs detect a clear polarization in theg-rays from a burst provides direct evidencein support of synchrotron emission as themechanism of g-ray production.
Their observations also reveal moreabout the nature of the magnetic field inwhich synchrotron emission occurs. In aGRB, the g-rays are produced in differentregions inside the jet (Fig. 2). These regionsare unresolved by detectors close to theEarth, so the source appears to be point-like.The detected polarization signal is, then, anaverage over the polarization of radiationproduced at different points within thesource. If the direction of polarization variesrandomly from place to place in the jet, thenthe observed polarization signal is likely toaverage out to zero. But this ‘washing out’ ofthe polarization signal will not happen if the polarization direction is the same every-where. For polarization produced by thesynchrotron mechanism, this means that the g-ray-producing region is suffused by anordered magnetic field, oriented in the samedirection everywhere (Fig. 2a).
The direction of polarization reported by Coburn and Boggs2 remained constantthroughout the duration of the GRB, but theg-ray flux varied significantly. So it seemsunlikely that such a strong, constant, orderedfield could be generated in the region wherethe g-rays are produced. Rather, this suggeststhat the strong field originates near the col-lapsing object, and is then carried by (or perhaps even drives) the jet outwards fromthe source: the mechanism by which gravita-tional energy is extracted and powers the jetis, possibly, electromagnetic.
But there could be another explanation.
news and views
388 NATURE | VOL 423 | 22 MAY 2003 | www.nature.com/nature
Figure 2 Information-processing machinery in the retina. As well as the input (rods and cones) andoutput (ganglion) cells, the retina contains a hugely diverse population of neurons. Each cell type isthought to have a specific role in vision9. The most extreme diversity is exhibited by amacrine cells, a collection of which is depicted here: their differing shapes and sizes are reflections of their widevariety of synaptic patterns. The widely spreading ‘polyaxonal’ amacrine cells, which come in severalsubtypes and are highlighted at lower right, are the type that Ölveczky et al.1 suggest provide themechanistic basis of local motion detection. (Data for the rabbit, adapted from ref. 2.)
Astronomy
New direction for g-raysEli Waxman
The origin of energetic g-ray bursts is still unknown. But the detection ofpolarization of the g-rays provides fresh insight into the mechanism drivingthese powerful explosions.
© 2003 Nature Publishing Group
Strong polarization might also arise in a ran-domly oriented magnetic field — a structurethat would be expected if the field is generatedin the g-ray production region — providedthat the line-of-sight to the GRB lies close tothe jet edge5,6 (Fig. 2b). In this case, the polar-ization signal is not averaged out: radiationreaches the observer only from points lying to one side of the line-of-sight, closer to the jet centre; radiation from points on the otherside, outside the jet cone, is ‘missing’. For ahighly relativistic jet, with a velocity that is99.99% that of light, such an orientation islikely to occur by chance only if its openingangle is close to 0.01 radians (0.6 degrees); thecloser the jet velocity is to the speed of light,the smaller the opening angle required. After-glow observations suggest that jet openingangles are typically about 0.05 radians, withmore powerful GRBs produced by narrower
jets7. The burst reported by Coburn andBoggs2 is exceptionally bright, and its jet maytherefore have been very narrow. So the existence of a randomly oriented magneticfield cannot be discounted.
Both of these interpretations of the datapose challenges to models, which need toexplain how a constant ordered field or ahighly collimated jet might be produced.Future observations will determine whichone is valid. ■
Eli Waxman is in the Faculty of Physics, WeizmannInstitute of Science, Rehovot 76100, Israel. e-mail: [email protected]. Fishman, G. J. & Meegan, C. A. Annu. Rev. Astron. Astrophys.
33, 415–458 (1995).2. Coburn, W. & Boggs, S. E. Nature 423, 415–417 (2003).3. Meszaros, P. Annu. Rev. Astron. Astrophys. 40, 137–169 (2002).4. Costa, E. et al. Nature 387, 783–785 (1997).5. Medvedev, M. V. & Loeb, A. Astrophys. J. 526, 697–706 (1999).6. Gruzinov, A. Astrophys. J. 525, L29–L31 (1999).7. Frail, D. A. et al. Astrophys. J. 562, L55–L58 (2001).
news and views
NATURE | VOL 423 | 22 MAY 2003 | www.nature.com/nature 389
Figure 2 Magnetic fields and polarization. Energetic electrons gyrating in a strong magnetic fieldinside the jet of material ejected by a collapsing star would emit polarized g-rays. The strong degreeof polarization seen by Coburn and Boggs2 suggests that the magnetic field is ordered, provided thatthe observer’s line-of-sight to the g-ray burst (GRB) is close to the axis of the jet cone (a). But if theline-of-sight to the GRB runs along the edge of the jet cone (b), the same degree of polarization couldbe seen even if the magnetic field is oriented randomly.
Figure 1 The creation of a g-ray burst (GRB). If a dying star a few times heavier than our Sun collapses to a diameter of just one kilometre, a large amount of gravitational energy is released as a black holeforms. The energy is carried away by a highly relativistic jet of material, propagating at nearly the speedof light and generating, from inside the jet, a short flash of g-rays. The electric field associated with thepropagating g-rays lies in a perpendicular plane and may point in any direction within this ‘plane ofpolarization’. Coburn and Boggs2 have detected a specific orientation, or polarization, of the electricfield that could be the consequence of a strong, constant and well-ordered magnetic field in the regionsurrounding the source of the GRB. Further from the source, the jet interacts with the surroundingmedium, generating an afterglow of X-rays, optical and radio waves that lasts a few days or months.
100 YEARS AGOWe learn from the Athenaeum that aNorwegian expedition, commanded byCaptain Roald Amundsen, left Christiania afew days ago with the object of fixing theexact situation of the magnetic North Pole.The party are expected to be absent for fouryears, the route taken being by LancasterSound, Boothia Felix, where a magneticobservatory will be established for a period oftwo years under control of two members of thescientific staff, and back by the North-WestPassage, Victoria Land, and the Behring Straits.
ALSO...A Paris correspondent states that on May 8, aballoon built for MM. Lebaudy made a notableperformance. The balloon left the MoissonAërodrome in the morning and returned to itafter having navigated round Mantes at adistance of 10 kilometres… The length of the air-ship is 56 metres, and the volume2300 cubic metres. The engine is a 40 horse-power. There were two persons on board, M. Juchmès, a well-known professionalaëronaut, and a mechanician.From Nature 21 May 1903.
50 YEARS AGOSometimes, after heavy and prolongedonshore storms, great masses of foam aredrifted in from the sea on to local rocks andbeaches. A curious feature of these lather-like masses is the way in which they persistfor relatively long periods even when blownabout by the wind. Ordinary lathers soonrevert to their former unlathered state: butsea foams may persist for a day or morewhen the weather continues moist andstormy and no sun shines. It is possible thatthe foam is ‘held’ by the presence of someprotein material, and Miss E. M. Mooresuggests that this protein material might befound in the alginate products of sea-weeds.This, however, does not explain how thesurface tension of sea-water could be solowered that the whipping action of stormwaves could create a frothy mass… A likelyexplanation of the phenomenon is that,during storms, large numbers of planktonicorganisms are destroyed and broken upbecause of the battering they receive inchoppy seas. The surface tension-loweringchemicals which are thus released into thesea-water would allow the waves to whip upa froth and, if there is also present enoughprotein matter released from the alginate, a foam with lasting qualities might result.From Nature 23 May 1953.
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