At 2 p.m. Pacific time on April 24, 2004, NASA’s Galaxy Evolution Explorer (GALEX), an ultraviolet (UV) space telescope, was studying galaxies in the constellation Virgo when a faint star in its field of view abruptly brightened by more than 10,000 times.
The flaring star is a red dwarf known as GJ 3685A, located about 45 light-years away. The star is 30 percent the size of the Sun and about half as hot, but stars like this comprise 75 percent of those in our galaxy. “At peak flux, the flare was 10 times brighter in the UV than the star itself,” Welsh notes.
GALEX picks up large flares from stars every 2 months or so, but they usually result in brightness increases of 100 times. The 2004 superflare was more than 100 times brighter than these — and about a million times more energetic than flares from our own Sun.
“I was surprised by how often we have observed stellar flares and by the amazing size of some of them,” says Chris Martin, GALEX’s principal investigator. “Nature rarely disappoints us.”
GALEX has an unusually large field of view for a space telescope — 1.5º — and a detector that actually counts individual photons, measuring both their positions and arrival times to within about a microsecond. “This is very different from a CCD, which just stares and accumulates light,” Welsh explains. As a result, the GALEX team recorded the flare in unprecedented detail. “I could show you a movie of the flare with just 0.05 second between frames.”
Although designed to study galaxies, GALEX has recorded flare stars, dwarf novae, and pulsating stars, along with asteroids, satellites, and space debris.
“We had no idea the ultraviolet sky would be filled with so many things that go bump in the night,” Welsh says, noting that a catalog of flare events will soon be published in The Astronomical Journal. “All of these observations are a bonus to astronomers, since they come free when the telescope is aimed at distant galaxies.”
Because GALEX’s imager records the arrival times of individual photons, these serendipitous events can be removed cleanly from the spacecraft’s all-sky survey. GALEX’s mission is to scan the entire sky at ultraviolet wavelengths for clues to how the earliest galaxies evolved into mature galaxies like our own Milky Way.