Posted: September 25, 2008
A mysterious celestial object that emitted 40 visible light flashes before disappearing again is the first object to exhibit a powerful magnetic field and show strong visible light activity, and could be a missing link in the family of neutron stars.
The peculiar object was originally identified as a gamma-ray burst (GRB 070610), a phenomenon that represents the last cry of a dying star. Shortly after the initial gamma-ray pulse, however, things began to get strange: A three day period of activity during which 40 visible light flares were observed was followed by a brief near-infrared flaring episode 11 days later, then the source became dormant again. Astronomers using the Swift satellite found that part of the X-ray radiation emitted in the outburst had been absorbed by hydrogen gas on the way from the object to Earth, and after mapping the gas masses along the line of sight it became clear that the object was most probably situated within our own Galaxy. This meant it could not have been a normal GRB, because those usually do not occur in our near galactic neighbourhood.
Artist impression of a possible magnetar that emitted 40 visible pulses before disappearing again. The optical pulses are likely due to ions gyrating around the field lines. Image: ESO/L.Calcada.
"We are dealing with an object that has been hibernating for decades before entering a brief period of activity", explains Alberto J. Castro-Tirado, lead author of one of two papers in this week's issue of Nature that describe the object referred to as SWIFT J195509+261406.
Alexander Stetanescu of the Max Plank Institute for extraterrestrial physics and lead author of the complementary Nature paper, along with Castro-Tirado and colleagues, think the most likely culprit is a magnetar, a young neutron star with an ultra-strong magnetic field a billion billion times stronger than that of the Earth. A magnetar would be capable of wiping the information from all credit cards on Earth from a distance halfway to the Moon. This particular magnetar was pinpointed to a spot in our own Milky Way Galaxy, about 15,000 light-years towards the constellation of Vulpecula.
"Magnetars remain quiescent for decades,” says Castro-Tirado’s co-author Antonio de Ugarte Postigo. “It is likely that there is a considerable population in the Milky Way, although only about a dozen have been identified."
J195509+261406 was determined to be about a tenth the size of the Sun, but at the same time almost a hundred times as bright. Under ‘normal’ conditions, extraordinarily high temperatures would be necessary to explain this kind of luminosity. "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", says Stefanescu. "So the only possible conclusion was that we had observed a non-thermal process: light that is not produced by heat as in a light-bulb or in a candle, but e.g. by particles in a magnetic field."
The newly discovered object, known as SWIFT J195509+261406, showed intense flaring activity over a three day period. One of such flare is demonstrated here (click for animation). The images, obtained with the 0.8-m IAC80 telescope at Teide Observatory (Spain), show the source over a time span of about 30 minutes, during which the source brightens quickly then fades away. Image: A.J. Castro-Tirado/IAC80/ESO.
The observations reminded the scientists of non-thermal high-energy outbursts of so-called Soft Gamma Repeaters (SGRs), leading them to the conclusion that the same type of object is involved as in SGRs: a magnetar. Changes in the configuration of the magnetic field during the first 10,000 years of a neutron star’s existence exert forces of such strength on the crust of the magnetar itself that its surface can crack, resulting in powerful star-quakes that produce outbursts of high-energy radiation, similar to the optical outbursts observed in this case. The fact that optical emissions were preferred over gamma rays suggests that highly charged ions were ripped out of the surface of the magnetar and gyrated along the field lines. Since ions are much heavier than electrons, they gyrate a lot slower, emitting electromagnetic radiation of much lower energy, in stark contrast to anything ever before seen in this typically high-energy form of star.
"We know 15 other magnetars, but up to now, no optical flashes of these have ever been seen", says 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 our observations."
The next piece of the puzzle therefore must be to study the consequences that established magnetar theories predict for optical emission. Some scientists have noted that magnetars should be evolving towards a pleasant retirement as their magnetic fields decay, but no suitable source had been identified up to now as evidence for this evolutionary scheme.