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Using a new high-speed photometer, a team of scientists including a researcher from the Universitat Politècnica de Catalunya (UPC) and the Institute for Space Studies of Catalonia, may have discovered a new sub-category of astronomical objects. It could be a magnetar emitting flares in the visible part of the spectrum, instead of the bursts of X-rays and gamma rays that are characteristic of these objects. The research will be published in an article in the journal Nature on 25 September.

On the night of 10 June 2007, Alexander Stefanescu and his team were monitoring their OPTIMA instrument, a photometer, attached to a 1.3-meter telescope at the Skinakas observatory, 1750 m above sea level on the island of Crete. The development of the OPTIMA had taken many years of work at the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching (Munich, Germany) and this valuable instrument was then transported to Greece and connected to the telescope. This year the second round of observations began on Skinakas. As in the previous year the team would spend weeks on the mountain waiting by the telescope every night to receive notice of a new gamma-ray burst (GRB) and to make use of the exceptional capabilities of the OPTIMA, the only instrument in the world combining a high time resolution with the capacity to immediately observe unexpected astronomical events.

That night notice of the discovery of the new object was received directly from the NASA Swift satellite. It indicated a brief high-energy blast in the sky, most probably a new GRB. Alexander Stefanescu and his team acted immediately, interrupting the observations programmed for that night and reorienting the telescope to begin observing the area of the sky where the blast of gamma rays had been located, just 421 seconds after their detection by the Swift satellite. At first the team did not find anything, but shortly afterwards they saw the first flash of light. They quickly realized that what they were observing was not a normal gamma-ray burst, which is initially bright but then becomes progressively dimmer over a matter of hours or days. In this case, the astronomers observed sudden bright flashes. The results became more and more surprising when the activity of the new stellar source had not diminished the following night but had actually increased, and only ceased completely after several nights. The new object was given the name SWIFT J195509+261406.

Stefanescu’s team included Glòria Sala, who joined the Astronomy and Astrophysics Group of the UPC (of the Department of Physics and Nuclear Engineering and the Department of Applied Physics) this September. The team examined the radiation emitted during the X-ray burst and found that part of the X-rays had been absorbed by hydrogen gas that the photons passed through on their journey towards the Earth. From maps of the gas present in the direction of the source it was clear that the new object was most likely situated inside our own galaxy. This implied that it could not possibly be a gamma-ray burst (GRB), since these are known to usually only occur in remote galaxies.

The most important new discovery was due to the special capabilities of the OPTIMA instrument. Instead of obtaining images during a certain exposure time, like the majority of cameras, the detectors of the device record the arrival time of each individual photon with a time resolution of just four millionths of a second. This enables the scientists to make a detailed reconstruction of the variations in brightness of objects in the sky. The detection of individual photons is a common practice in high-energy astronomy but OPTIMA is one of the few instruments capable of detecting them in visible light. The rapid and large-scale variability of the object, observable only with a high time resolution, was crucial in ruling out the possibility that it was a classic GRB.

The variability of the emission detected enabled the scientists to determine that the new star must be less than one tenth the size of the Sun but also a hundred times brighter. Supposing that the emission was of thermal origin, as is the case with the Sun, extraordinarily high temperatures would be needed to explain the great luminosity of the object. “So high, in fact, that it's hard to see how an object of this size can heat up and then immediately cool down so quickly”, explains Alexander Stefanescu, lead author of the Nature article. “So the only possible conclusion was that we had observed a nonthermal process: light that is not produced by heat as in a light bulb or in a candle but, for example, by particles in a magnetic field.”

The scientists thought that the short bright flashes observed over several days resembled the nonthermal emission of high-energy bursts from what are known as Soft Gamma Repeaters (SGRs). Not only the shape, but also the statistical distribution of the brightness of individual flashes, as well as a slight indication of periodic emission, were quite similar to what is observed in SGRs. Therefore, scientists think that the newly discovered object could be of the same nature as SGRs: a magnetar, which is a special type of young neutron star with extraordinarily potent magnetic fields. This hypothesis is supported by a second article on multi-wavelength observations of the same object, also published in Nature by the team led by Alberto Castro-Tirado, of the Instituto de Astrofísica de Andalucíca (IAA-CSIC, Granada).

Neutron stars are the last remnants of a massive star that has expelled its outer layers during a supernova explosion. The nucleus of the star contracts until a very dense structure forms, with a solid crust and a liquid interior that is thought to be made up mainly of neutrons. If a recently formed neutron star is spinning very rapidly about its own axis, its strong magnetic field can be amplified by a factor of 1,000, resulting in a magnetic field of some 100 gigatesla, more than a billion times stronger than the strongest magnetic fields generated in Earth laboratories. The magnetic field is so strong that the electron clouds of the surrounding atoms are distorted into thin needle shapes. And a credit card placed at a distance similar to that from the Earth to the Moon would be wiped clean automatically.

Changes in the configuration of the magnetic field during the first 10,000 years of the life of a neutron star produce such great forces that the crust of the neutron star can be heated until it cracks. The result is a quake on the surface of the star producing the high-energy radiation bursts that are characteristic of magnetars and that are very similar to the visible flashes from the new object.

But what caused this peculiar object to emit radiation in the visible part of the electromagnetic spectrum, instead of gamma rays like all other known magnetars? One possible theory is that the highly charged ions are ripped off the surface of the magnetar and spin along the lines of the magnetic field. Since the ions are much heavier than the electrons (which are responsible for high energy emissions from magnetars), they spin much more slowly, emitting much lower energy electromagnetic radiation in the visible range.

The majority of observations of magnetars until now have taken place in the high-energy regime (X-rays and gamma rays). “We know 15 other magnetars, but up to now, no optical flashes of these have ever been seen,“ say Glòria Sala and Alexander Stefanescu. “Accordingly the main efforts of theoreticians were made in the high-energy regime. That's why we don't have an adequate theory against which to compare the observations with OPTIMA.” The next step for the scientists will therefore be to extend the established theories on magnetars and to look at how they can explain the emission of visible light observed.