Distant leviathan peeled star before swallowing half.
A closely-watched flare from a gargantuan black hole in a distant galaxy has revealed to astronomers not only the mass of the black hole that ate the snack but the type of star that met its end as the meal.
Astronomers have seen other stars ripped apart by supermassive black holes before now. Two separate events reported last year were found thanks to associated relativistic jets, high-powered beams that shot out as the black hole scarfed down the star. But while previous cases have matched observers’ expectations, not one provided enough information to determine what kind of star had died.
The new result, reported online this week in Nature, is “fantastic” in that respect, says Andrew Levan (University of Warwick, England), who worked on both of last year’s discoveries but was not involved with the current study. “The challenge of finding this sort of event is to find it young,” he says. “The relativistic versions had this because they triggered gamma-ray detectors, but this is the first time we’ve found a non-relativistic event early enough to do all the analysis [the study authors] are able to do.”
The team first found the flare in May 2010 in optical wavelengths using the Pan-STARRS1 survey, a wide-field imaging system attached to a 1.8-meter scope on Mount Haleakala in Hawaii. It was discovered independently a month later in ultraviolet with the Galaxy Evolution Explorer (GALEX) satellite. But figuring out just what the flare was took a long time, explains study coauthor Suvi Gezari (Johns Hopkins University). Because the light from the flare was so bright, observers had to wait a whole year for it to fade before they could see the host galaxy. Once the flare did fade, they could determine things such as the galaxy’s distance (2.7 billion light-years) and the chemical composition of the material.
With detailed data in hand, the international team scrutinized the flare’s behavior. The flare’s rise and decay times matched just what are expected of material falling onto a black hole from a disrupted star. Even though the flare didn’t start until the black hole began feeding, the astronomers could backtrack and determine that the star had been torn apart about 76 days before the flare peaked.
But the really cool part is what the leftovers revealed. A black hole tears a star apart via gravitational influence, a souped-up version of the tidal force that the Moon and Earth exert on each other. This tidal process stretches the star out into a pseudo-banana-shape as it nears the black hole, Levan explains. Once the star swings through its closest approach to the central leviathan, the black hole’s gravity overpowers it, ripping the star apart. Due to the nature of the tidal pull on the star, half the star is thrown out, ejected at high velocity. The other half spreads out into a disk of material that then accretes onto the black hole. It's this accreting material that produces the radiation seen as a flare.
Spectroscopic observations of the ejected debris, made with the 6.5-meter MMT in Arizona, showed ionized helium emission but no sign of hydrogen. Normally, stars are mostly hydrogen. But an evolved star called a red giant has fused all its core hydrogen into helium, leaving the outer layers hydrogen. If a red giant lost its outer hydrogen envelope and was reduced to its helium-rich core, it would exactly match the observations.
Losing the hydrogen envelope is probably part of the process of being gobbled, Gezari says. “It’s a consequence of being in the environment of the black hole,” she says. “The stars that actually have orbits that will bring them close enough to be disrupted are going to get stripped on their way in.” It’s kind of like going through an airport: you probably won’t get to the gate without first having to take off all your outer layers in security.
The way the black hole tears the star apart depends on its mass. Pairing the detailed flare behavior with estimates of the stellar core’s mass and radius, the astronomers were able to “weigh” the beast and came up with about 3 million solar masses. That’s in keeping with indirect mass measurements using the galaxy’s properties, which put the black hole between 2 and 8 million solar masses.
There’s one major question left unanswered in this observational knockout. The accreted material is about one-tenth as hot as expected, and its temperature doesn’t decline with time, both of which should happen. “What we’re seeing is more complicated than what you would expect from just a simple accretion disk,” Gezari says. “We don’t know exactly what’s going on.” It could be the radiation is being absorbed and reemitted by intervening material, but at present that’s just a guess.
If you want to see the black hole in action — at least in a simulation — check out the video below.
The computer simulation shows a star being shredded by the gravity of a massive black hole. The areas in white are regions of highest density, with progressively redder colors corresponding to lower-density regions. The elapsed time corresponds to the amount of time it takes for a Sun-like star to be ripped apart by a black hole a million times more massive than the Sun.
Credit: NASA / S. Gezari (JHU) / J. Guillochon (UCSC)