New observations of FO Aquarii reveal strange behavior within this cannibalistic binary system

Astronomers thought they knew everything they needed to know about this famous binary star system — but they were wrong.
By | Published: January 17, 2017 | Last updated on May 18, 2023
An artist’s rendition of cataclysmic variable binary system, in which a compact star pulls material off its companion and consumes it.

FO Aquarii, a famous binary system comprised of a compact white dwarf stripping mass from a low-mass companion star, has decided to misbehave. When a National Science Foundation Research Experiences for Undergraduates (REU) student at the University of Notre Dame turned the school’s 0.8-meter telescope on these stars, she observed a pattern of behavior that has been, thus far, unexplainable.

FO Aquarii is an intermediate polar system — a cataclysmic variable binary system in which the compact star, typically a white dwarf, pulls matter off its puffed-up companion. This matter forms an accretion disk that funnels material toward the compact star. When it reaches the white dwarf, the star’s intense magnetic fields yank material away from the disk and onto the star in streams, called accretion curtains. This accretion of matter — and angular momentum — keeps the white dwarf spinning at a rate faster than the orbital period of the binary system. The spin of the white dwarf can be determined by measuring the system’s lightcurve, which tracks the excess light created when the incoming material hits the star.

FO Aquarii is located about 500 light-years away in the constellation Aquarius. It’s often called the “king of intermediate polars” because of its significant optical brightness and the large amplitude of brightness changes observable as it spins. These factors combine to make observations of the system easy, even with smaller telescopes. Mark Kennedy, a graduate student at University College, Cork, studied the FO Aquarii system intensely using NASA’s Kepler Telescope. The telescope dedicated three months of time to this single binary system, watching it revolve for several weeks before finally looking away. Once the binary’s orbital period and lightcurve had been well-studied and characterized, Kepler moved on to a different target. Then, last year, University of Notre Dame REU student Erin Aadland turned the school’s Sarah L. Krizmanich Telescope to the same coordinates. Working under Professor Peter Garnavich, Aadland was instructed to determine whether the white dwarf, which spins once every 20 minutes, was speeding up or slowing down.

But Aadland immediately found several things appeared to be amiss. First, the system was seven times fainter than expected. What’s more, it was behaving strangely, exhibiting inexplicable changes in its period as well.

Much of the observed brightness of intermediate polar systems comes from accretion energy, which can be released at wavelengths that range from X-rays to optical light. Thus, these systems are brightest when active mass transfer is occurring between the donor star and its white dwarf companion. FO Aquarii’s dimming, then, means that the transfer had stopped, or at least slowed significantly. While this “low state” behavior has been observed in other such systems, it has never before been seen in FO Aquarii.

Since it was first observed with the Krizmanich Telescope, FO Aquarii has been slowing brightening again, a sign that mass transfer is resuming. Still, “the recovery to normal brightness has been slow, taking over six months to get back to where it was when Kepler observed,” says Colin Littlefield, a member of the Garnavich lab, in a recent press release documenting this discovery. In essence, the team is now following the system as it resumes business as usual, as accretion slowly ramps back up to pre-dimming levels and increases the brightness as a result.

The real question, though, is why the dimming occurred in the first place. One theory suggests that a starspot, a cooler area associated with the white dwarf’s magnetic field, could have disrupted the accretion process, causing the observed dimming. If this were the case, however, it shouldn’t take the system months to resume normal brightness.

What’s more, the system’s lightcurve developed strange behavior during its dimmed state, and the normally steady 20-minute rotational signal changed periods to range between 11 and 21 minutes. “We had never seen anything like this before,” Garnavich says, adding that the behavior would last for hours at a time before changing again. “For two hours, it would flash quickly and then the next two hours it would pulse more slowly.”

This complex behavior points to something else going on within the system and its accretion state. One thing is clear: astronomers definitely don’t yet know all of this system’s secrets.