The easiest way to spot a supermassive black hole (SMBH) is when it expels a huge jet of matter in one of the most energetic displays in the Universe. While astronomers have spotted these huge black holes at the centers of most galaxies, not all are active—meaning the jet isn't there, and the SMBH is hiding. However, even inactive black holes may give themselves away if we can spot them eating stars: the disruption of a star by gravitational forces can produce a burst of light.

As reported in Nature, the Pan-STARRS1 (PS1) galaxy survey spotted a burst of intense ultraviolet and visible light from the center of a galaxy with no known SMBH. S. Gezari et al. performed a spectral analysis on the flare, and determined it to be consistent with the destruction of a red giant star with a helium-rich core. The likely culprit for the star's disruption is a black hole with a mass between 2.7 and 2.9 million times that of our Sun.

Gravity will tear us apart

Astronomers have observed SMBHs at the cores of a substantial fraction of large galaxies (including the Milky Way), leading to a widespread consensus that all galaxies have them. In many known SMBHs, a disc of hot gas surrounds the black hole; the acceleration of the gas creates huge jets of particles and produces light in radio and X-rays. These SMBHs are known as active galactic nuclei. Not every SMBH is active. Some, including our Milky Way, are relatively quiet, requiring other methods to observe and characterize them.

Although black holes can devour objects directly, a more typical scenario involves disruption via the tidal force of gravity. (This is simply a stronger version of the force that raises tides on Earth and makes the same side of the Moon face Earth.) Supermassive black holes exhibit stronger tidal forces, strong enough to tear stars apart—but this phenomenon is rare and transient, so only three such events have ever been spotted, including the present study. From a statistical analysis, astronomers expect one tidal disruption event per galaxy every ten thousand years; watching for them requires either a lot of patience, or a lot of galaxies.

Tidal disruption by SMBH is a function of the black hole's mass as well as the size and mass of the star being torn apart. Stars with large diameters are more likely to be shredded than smaller stars, and lower mass increases the probability as well. (If the black hole is too massive compared to the star, it will swallow the star whole.) During disruption, roughly half the star's mass is swallowed by the black hole, while the remainder is shot out into space at high speed. Unlike gas falling into a black hole, tidal disruption of a star produces visible and ultraviolet light, but not X-rays.

Watch what you eat

While the theory of tidal disruption by SMBHs is well understood, it's another thing to spot it in action. Gezari et al. spotted a bright visible light flare in data from the Panoramic Survey Telescope & Rapid Response System (Pan-STARRS1) survey of galaxies. The flash also appeared in data from the Galaxy Evolution Explorer (GALEX), which is an ultraviolet instrument. Tracking its visibility over time, the astronomers determined the flare was located right at the center of a galaxy, to a high degree of confidence.

The researchers followed up observations using the Chandra X-ray Observatory, and failed to find anything at the location of the flare. This rules out gas accretion onto a black hole, meaning the SMBH candidate is not active. Similarly, the strong ultraviolet signal rules out the possibility that the flare was due to a supernova explosion. On the other hand, the flare showed a high fraction of ionized helium—also something unlikely to be present either in ordinary gas accretion or a supernova.

Matching the spectrum and duration of the flare to the model for tidal disruption, Gezari et al. determined the most likely candidate for the event was a red giant torn apart by a SMBH. Red giants are stars near the end of their lives: they have converted the hydrogen in their cores into helium.

When the astronomers fit both the parameters for the star and the black hole to the data, they determined the star was more massive than our Sun, but not enormously so. They also found a best-fit value for the SMBH mass to be 2.8±0.1 million times the mass of the Sun. (For comparison, the Milky Way's SMBH is about 4 million times the mass of the Sun.)

The lack of gas accretion may rule out the possibility that the SMBH is an active galactic nucleus, but it's also puzzling for tidal disruption. The envelope of the disrupted star should have formed a disc of gas around the black hole, producing accretion behavior that we should be able to observe. The researchers suggest the star may already have lost its outer layers through tidal stripping before the final disruption, or perhaps the envelope may have been blown away through another process, leaving only the helium-rich core.

With large surveys of galaxies to work with, astronomers should be able to spot many more events like this. Observations will settle how often these disruptions occur, and provide a better comparison with the theory, teaching us much about black hole behavior in galaxies.

Nature, 2012. DOI: 10.1038/nature10990 (About DOIs).