Astronomers from the University of Notre Dame have discovered a unique binary dwarf like no other. It appears to have faded with no explanation.
The system, known as FO Aquarii, is located 500 light-years from Earth and it consists of a regular "main sequence" star and a compact dwarf companion. FO Aquarii is known as an intermediate polar: The dwarf is currently stealing material from the main sequence object and that material then falls onto the dwarf, following its strong magnetic field.
FO Aquarii has been studied by planet hunter telescope Kepler, which has collected data every minute for three months. As the material hits the compact companion, the dwarf flashes in X-ray and visible light. By timing those flashes, it is possible to measure how quickly the star is rotating. The analysis suggested a rare but regular system, with the dwarf spinning every 20 minutes.
The star system is visible even with small telescopes, so when Kepler moved on to other targets, the team continued their observations after a break with other instruments. But when observed again, something wasn’t quite right. As reported in the Astrophysical Journal, the star was much dimmer.
“Just after the star came around the sun last year, we started looking at it through the Krizmanich Telescope, and we were shocked to see it was seven times fainter than it had ever been before,” said lead author Colin Littlefield in a statement. “The dimming is a sign that the donating star stopped sending matter to the compact dwarf, and it’s unclear why. Although the star is becoming brighter again, the recovery to normal brightness has been slow, taking over six months to get back to where it was when Kepler observed.”
The team suggested a star spot could resolve the sudden change of flow of material. Star spots have strong magnetic fields, so a complex interaction might explain the missing stream, but it doesn’t clarify why the star is taking so long to recover.
The regular light flashes have faded and the system now shows other types of periodicities that are related to how the two stars orbit each other. The flashes tend to be either 11 or 22 minutes, which correspond to one and two spin-orbit beats.
“We had never seen anything like this before,” senior author Peter Garnavich added. “For two hours, it would flash quickly and then the next two hours it would pulse more slowly.”
The binary system hasn’t returned to its original brightness yet, but hopefully new studies will shed light on this unique phenomenon.