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Press Release February 18, 2005

( Press Release of the Max Planck Society (in German) SP 8 / 2005 (26) )

Gamma Burst hits the Earth

(G. Lichti, A. von Kienlin)

Astrophysicists from the Max-Planck-Institut für extraterrestrische Physik in Garching near Munich measure the strongest burst from a magnetar. On December 27, 2004 at 21:30:26 UT the earth was hit by a huge wave front of gamma and X-rays. It was the strongest flux of high-energetic gamma radiation measured so far. It was measured by the team of Dr. Roland Diehl and Dr. Giselher Lichti from the Max-Planck-Institut für extraterrestrische Physik (MPE) in Garching near Munich. The wave front was in addition more intense than the strongest radiation burst measured so far from our sun. The remarkable aspect of this discovery is the origin of this radiation: it originates from a tiny celestial body with an extreme density, a neutron star, a so-called magnetar, and with an extremely strong magnetic field which is located on the other side of our Milky Way at a distance of about 50 000 light years. The astrophysicists from Garching are confident that this event will cast new light on the physics of magnetars and that it will contribute to solve an old puzzle concerning the gamma-ray bursts.
Magnetar

Fig. 1: Magnetar with extreme strong magnetic field.
Such an object is thought to be the source of the intense gamma-ray burst observed.

Image: R. Mallozzi/NASA

"We owe this spectacular event to the magnetar with the prosaic name SGR 1806-20", says Giselher Lichti. "This neutron star has a diameter of an average capital city and a mass which is comparable to the sun. It suffered a huge magnetic instability, whereby its strong magnetic field reoriented itself into a lower energy state", argues the astrophysicist. Within the first 0.2 s the same amount of energy was emitted as from our sun in about a quarter of a million years. This burst was about 100 times stronger than the hitherto strongest measured burst from a magnetar."

Magnetars are neutron stars with magnetic fields which are 1000 times stronger than ordinary neutron stars have. One estimates that about ten percent of all neutron stars belong to this class of stars. Neutron stars originate during the collapse of stars of a certain mass range in a supernova explosion. They have a typical diameter of about 20 km and a magnetic field of the order of 1012 Gauß (for comparison: the magnetic field of the Earth has a strength of about one Gauß), which is the result of the laws of electrodynamics according to which the product of the cross-section of the star and the magnetic field remains constant. The magnetic field of a newly-born magnetar, which is 1000 times stronger, is generated within few seconds via a complex dynamo effect deep in its interior caused by convection and fast rotation.

The spectacular wave front was measured by a detector of the MPE aboard the INTEGRAL satellite, the anticoincidence shield of the INTEGRAL-spectrometer SPI, one of the most sensitive gamma-ray burst detectors which orbits the Earth. "Already the measurement of this burst has justified the building of this detector, which was funded by the Deutsches Zentrum für Luft- und Raumfahrt (DLR)", says Giselher Lichti, under whose leadership this detector was developed and manufactured by the companies Jena-Optronik and Astrium. But not only the INTEGRAL satellite recorded this event. It was also seen by 13 other X-ray and gamma-ray detectors which perform measurements in the interplanetary space between the Earth and Saturn. "Even the Russian Coronas-F stellite saw this burst, although it was at the time of the burst hidden by the Earth so that it could not measure the direct radiation from this source", explains Giselher Lichti. "The analysis of the arrival times revealed that the Russian instrument measured gamma rays which were reflected from the surface of the moon."

light curve
Fig. 2: Light curve of the burst as measured with the anti-coincidence shield of the INTEGRAL spectrometer SPI.
At 0 seconds, the main peak is visible, but to make the finer structure later visible, the peak has been cut. It was actuallu 22 times higher than in the plot. Clearely visible is the pulsating afterglow which lasts at least 200 seconds.

Image: A. von Kienlin, MPE

Because of the strength of the burst and because of its penetrating radiation this burst was also measured by instruments which were not directly pointed in the direction of the sky from where the radiation came. The gamma radiation was able to penetrate the shieldings from metal or crystals which surround the detectors and brought them for a short time into saturation.
Whether the instrument on the INTEGRAL satellite was driven into saturation still has to be investigated. "The behaviour of our detectors under such a high irradiation must still be explored in order to be able to permit an estimate of the energy which was emitted by this event", says Andreas von Kienlin, also astrophysicist at MPE who has calibrated the instrument and set it in action. " The eruption of the neutron star began with the emission of energetic gamma radiation which contained the bulk of the emitted energy but lasted only a fraction of a second. This eruption was followed by a weaker gamma emission which lasted for about 6 minutes and whose intensity oscillated with a period of 7.56 seconds. This oscillation is associated with the known rotation period of the neutron star. Our measurements showed that the energy distribution of the emitted gamma quanta is characteristic for an ultra-hot thermal plasma", explains Andreas von Kienlin. "That is exactly what is expected from a magnetar which expels light high-energy particles. Most of these particles annihilate in pure gamma rays which then escape into interstellar space."
The oscillating gamma emission comes obviously from remaining electrons and positrons which are enclosed by the magnetic field of the magnetar, the astrophysicists think. The theory predicts that such a hot enclosed fireball should shrink and evaporate within few minutes. Its brightness seems to oscillate because the fireball is bounded via the magnetic field to the surface of the rotating neutronstar.

The huge amount of energy of the burst from December 27, 2004 suggests a new solution for an old problem of the gamma-ray burst astronomy: this involves the question after the sources of the so-called "Short-Duration Gamma-Ray Bursts". During the last 35 years hundreds of short (<2 seconds) mysterious thunderbolts of high-energetic radiation from the deep space have been measured without knowing where they originate. One hypothesis says that this radiation is generated when two compact objects (e. g. two neutron stars or a neutron star and a black hole) merge. The new observations allow now another interpretation: some of these observations could partially be explained by outbursts like that observed on December 27, 2004. This idea is proposed by Kevin Hurley from the University Berkeley (California) and his team. According to him such short and intense outbursts could be observed from far distant galaxies. An event with the strength as measured recently could be seen up to a distance of some hundred million light years. "Since there are many galaxies in this distance range one ought to see such events frequently. One could thus explain the observations at least partially, if not completely", says Giselher Lichti.

light curve

Fig. 3: Overview and details of the burst of SGR 1806-20.

Image: A. von Kienlin, MPE

How can one explain the enormous energy output from such a magnetar? The inventor of the magnetar model, the theoreticians Robert Duncan (University of Texas, Austin) and Christopher Thompson (Canadian Institute of Theoretical Astrophysics, Toronto), propose the following scenario in order to explain the gigantic energy output of such a flare. In order to understand their idea one first has to imagine the extremely strong magnetic field of a magnetar which is 1000 times stronger than that of an ordinary neutron star (1012 Gauß). In such strong magnetic fields a hydrogen atom for example is distorted so incredibly that it becomes like a needle (i. e. it is ~200 times narrower than long). Such a star has deep in its interior a strongly twisted magnetic field whose field lines wind themselves around the spin axis like the mainspring of a clock. Its exterior magnetic field, however, is the one of a normal dipole field similar to that of a bar magnet (like the Earth´s magnetic field).
It is believed that the twisted interior field is the remainder of the fast rotation which the neutron star gained during its birth. It contains the largest fraction of the magnetic energy of the star. This magnetic field exerts a force onto the one kilometre thick crust of the star which has a radius of 10 km and displaces it. This has on the one hand the consequence that the exterior magnetic field is twisted and on the other hand that strong currents flow around the star. If the magnetic field lines twist more and more then these currents cause the star to shine brightly in the low-energy gamma-ray range. The twisting of the exterior magnetic field has also an influence on the rotation of the star and leads to a stronger deceleration.

This seems also have to happened with the magnetar SGR 1806-20. From March 2004 until the outburst in December, SGR 1806-20 has shown many single weak bursts which pointed to a displacement of the crust. SGR 1806-20 therefore became brighter and brighter in gamma rays with emission of harder and harder gamma photons and a stronger deceleration. All these measurements indicated that the exterior magnetic filed twisted more and more. According to the model of Duncan and Thompson the twisting became so large that the star with its crust became unstable. The tension of the exterior magnetic field itself erupted in an enormous outburst and readjusted in a lower and untwisted energy state. At the time of the outburst the magnetar was only 5º away from the sun. It is located in the constellation Sagittarius near the centre of our galaxy. Kevin Hurley succeeded with the help of the Interplanetary Network via triangulation to identify the position of the outburst with the magnetar SGR 1806-20. This position was confirmed from radio astronomers of the Very-Large Array telescope in Socorro (New Mexico) by measuring a fading afterglow at radio wavelengths. The observation of this afterglow yields further important information about the explosion mechanism. This will contribute to a better understanding of the observed phenomenon.

"Life on Earth, however, was never threatened by the outburst of this magnetar, since the atmosphere is opaque for this kind of radiation. Gamma rays ionise the atoms of the upper atmosphere and are absorbed by this process", notes Giselher Lichti. (TN) Original publication:

Hurley, K., Boggs, S. E., Smith, D. M., Duncan, R. C., Lin, R., Zoglauer, A., Krucker, S., Hurford, G., Hudson, H., Wigger, C., Hajdas, W., Thompson, C., Mitrofanov, I., Sanin, A., Boynton, W., Fellows, C., von Kienlin, A., Lichti, G., Rau, A., Cline, T., A Tremendous Flare from SGR1806-20 with Implications for Short-Duration Gamma-Ray Bursts, Nature (submitted), 2005

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