Twinkle, twinkle, quasi-star: cosmic lenses could tell us what you are.
Billions of years ago, the universe was a different place, where gorging supermassive black holes cast a glow so brilliant, they could outshine entire galaxies.The era of the so-called quasars is over now — the nearest bona fide example, 3C 273, lies 2 billion light-years away. And the starlike points of light remain mysterious because they’re too small (usually barely the size of our solar system) and too far away to resolve directly with current technology.
Now Andy Lawrence (University of Edinburgh, UK) and his colleagues might have a way to do just that, according to results presented at the recent National Astronomy Meeting of the Royal Astronomical Society. Their observations have uncovered dozens of temporary magnifying glasses that could reveal quasars’ innermost structures — if the results pan out.
The team is one of many that use the Pan-STARRS1 (PS1) telescope on Mount Haleakala, Hawaii, which surveys three-quarters of the sky roughly every two months in a project called the 3π survey. The 1.8-meter telescope is the first of four planned for the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS), a robotic system designed to detect transient objects in the night sky. Pan-STARRS isn’t fully online yet — eventually the system will survey the sky with a weekly rather than monthly cadence — but among its transient detections is the recent Comet PanSTARRS.Lawrence and his colleagues have been trawling through the immense amount of incoming data to search for flares near the centers of inactive galaxies. The comparison galaxies came from the Sloan Digital Sky Survey database, which holds images taken roughly a decade ago. The flares had to be at least 1.5 magnitudes brighter than the galaxy to make it into the team’s collection. Lawrence followed up PS1 detections with weekly observations on the Liverpool Telescope, as well as spectra from the William Herschel Telescope.
Lawrence’s team was on the lookout for star-shredding black holes, and out of 41 flares they found some good candidates; they also found some faraway supernovae. But 33 of the transients they selected were something else entirely: quasars. The mystery is, these quasars don’t seem to reside in the SDSS-imaged galaxies, even though the positions on the sky are nearly identical. The galaxies lie between 1 billion and 5 billion light-years away, but the quasars lie even further away, up to 10 billion light-years.
The simplest explanation, perhaps, is the flare of a faraway quasar coincidentally lying behind a much closer galaxy. But although it’s not impossible for a quasar to flare by at least 1.5 magnitudes over the course of a decade, it’s extremely rare, Lawrence says. An independent study published last year shows that such extreme variability (brightening or fading by more than 2 magnitudes) would happen to only one quasar in 10,000.So Lawrence has a different proposal. What if, he asks, a single star in the foreground galaxy lined up with the quasar just right so that the star’s gravity bent the light of the background quasar, magnifying it temporarily? The magnification could potentially brighten the quasar for two years or so before it faded back into obscurity.
The chance of one of these microlensing events isn’t very high either, but it’s not impossible. About 0.03% of foreground galaxies line up with a distant quasar, and with 100 million galaxies to choose from, the PS1 telescope might see a few dozen microlensing events at any one time. “Microlensing events ought to be there,” Lawrence adds. “If this isn't them, where are they?”
These kind of events have been detected before, but the advent of robotic transient surveys could allow many more to be detected and studied.
Michael Irwin (University of Cambridge, UK), who discovered the first microlensed quasar in 1989, agrees that microlensing is “certainly a potential explanation for [these quasars’] properties.” But both he and Lawrence agree that much work remains to be done before they can settle on that explanation.
Lawrence and his colleagues continue to monitor the quasars, watching to see whether they’ll fade as expected for microlensing events. If the hypothesis pans out, measuring how quickly the quasars fade at different wavelengths will help astronomers gauge the size of structures surrounding the black hole, such as the gas accretion disk.
“We haven’t proved the microlensing hypothesis yet,” Lawrence cautions, “but my hunch is that it’s probably the right explanation. Right now I think I give it 90:10 odds on microlensing to extreme variability.”