At 20:52 UT on June 10, 2007 the Burst Alert Telescope aboard NASA’s Swift satellite detected a possible gamma ray burst 5 seconds long. Initial studies of this radiation by a group of scientists at the Max Planck Institute for Extraterrestrial Physics (MPE) suggested this was not a standard gamma ray burst, but something different. Seven minutes after the first detection optical observation of the object began and it remained observed for the next 5 nights.
The high resolution tracking of the object’s brightness was done with OPTIMA, a high speed photometer built by MPE and mounted at the 1.3m Telescope of the Skinakas Observatory in Crete. The system detectors enable the recording of each photon’s arrival time at an accuracy of four millionths of a second. This enabled the scientists to reconstruct in detail the object’s changes in brightness.
The data collected showed a number of bright flares that rose rapidly followed by slower decay. The source in fact changed its brightness by a factor of over 200 in a mere four seconds. Study of the light’s behavior led to the estimate that the object has a radius of no more than a tenth the radius of the sun. In addition, it was discovered that this object lies within the Milky Way galaxy, providing further contradictory evidence to the source being a gamma ray burst as these rarely lie within our own galaxy.
The speedy changes in the object’s luminosity in high probability rule out a thermal process as the cause of these emissions, as the object wouldn’t be able to heat up and cool down so rapidly. This suggests that the object observed was a magnetar, a neutron star with an extremely powerful magnetic field. As Alexander Stefanescu, a member of the OPTIMA team explained, “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 for example, by particles in a magnetic field.”
If the object observed was indeed a magnetar, this detection constitutes the first observation of optical flares in a magnetar, as they usually radiate x-rays and gamma-rays. Not only did the object glow in the optical spectrum, but no x-ray radiation was detected at the corresponding times.
Since to date most magnetar observations were in the high energy band and not in the visible energy region, the theory behind such emissions has been somewhat neglected. “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 with which to compare the observations from OPTIMA.”
One proposed theory suggests that emission in the microwave and radio wavelengths was emitted near the neutron star and then absorbed higher in the magnetosphere by ions. These ions in turn re-emit in optical wavelengths. This theory, along with others, must be studied in depth before a complete understanding of the observed object and its behavior can be reached.
TFOT reported on NASA’s Spitzer Space Telescope’s recent discovery of a magnetar, which may help scientists figure out whether a star’s mass determines if it becomes a magnetar when it dies. Another related article covers NASA’s Rossi X-ray Timing Explorer observation of a pulsing neutron star acting like a magnetar. This discovery can shed light on the mysterious evolutionary relationship between pulsars and magnetars. NASA’s Swift satellite, which made the initial discovery of the object discussed above, was also the satellite that enabled the first real time observation of a supernova.
Further information on the recent discovery, which was published in the September 25th issue of Nature magazine, can be found in the Max Planck Society press release and in the Arxiv website.
Image: Illustration of a magnetar with optical flares (Credit: Max Planck Institute for Extraterrestrial Physics).