Clues about Black Hole’s Diet
Andrea Merloni and members of his
team, from the Max-Planck Institute for Extraterrestrial Physics (MPE) in
Garching, near Munich, were exploring the huge archive of the Sloan Digital Sky
Survey (SDSS) in preparation for a future X-ray satellite mission. The SDSS has
been observing a large fraction of the night sky with its optical telescope. In
addition, spectra, where light is dispersed across wavelengths, allowing
astronomers to deduce properties like composition and temperature, have been
taken of distant galaxies and black holes.
For a variety of reasons, some
objects got the spectra taken more than once. And when the team was looking at
one of the objects with multiple spectra, they were struck by an extraordinary
change in one of the objects under study, with the catalogue number SDSS
J0159+0033, a galaxy in the constellation of Cetus. The huge distance to the
galaxy means that we see it as it was 3.5 billion years ago.
"Usually distant galaxies do
not change significantly over an astronomer's lifetime, i.e. on a timescale of
years or decades," explains Andrea Merloni, "but this one showed a
dramatic variation of its spectrum, as if the central black hole had switched
on and off."
This happened between 1998 and
2005, but nobody had noticed the odd behaviour of this galaxy until late last
year, when two groups of scientists preparing the next (fourth) generation of
SDSS surveys independently stumbled across these data.
Luckily enough, the two flagship
X-ray observatories, the ESA-led XMM-Newton and the NASA-led Chandra took
snapshots of the same area of the sky close in time to the peak of the flare,
and again about ten years later. This gave the astronomers unique information
about the high-energy emission that reveals how material is processed in the
immediate vicinity of the central black hole.
Gigantic black holes are at home
in the nuclei of large galaxies all around us. Most astronomers believe that
they grew to the enormous sizes that we can observe today by feeding mostly on
interstellar gas from its surroundings, which is unable to escape its
gravitational pull. Such a process takes place over a very long time (tens to
hundreds of millions of years), and is capable to turn a small black hole
created in the explosion of a heavy star into the super-heavyweight monsters
that lurk at the center of galaxies.
However, galaxies also contain a
huge number of stars. Some unlucky ones may happen to pass too close to the
central black hole, where they are destroyed and eventually swallowed by the
black hole. If this is compact enough, the strong, tidal gravitational forces
tear the star apart in a spectacular way. Subsequently bits and pieces swirl
into the black hole and thus produce huge flares of radiation that can be as
luminous as all the rest of the stars in the host galaxy for a period of a few
months to a year. These rare events are called Tidal Disruption Flares (TDF).
Merloni and his collaborators
quite quickly realized that "their" flare matched almost perfectly
all the expectations of this model. Moreover, because of the serendipitous
nature of the discovery, they realized that this was an even more peculiar
system than those which had been found through active searches until now. With
an estimated mass of 100 million solar masses, this is the biggest black hole
caught in the act of star-tearing so far.
However, the sheer size of the
system is not the only intriguing aspect of this particular flare; it is also
the first one for which scientists can assume with some degree of certainty
that the black hole was on a more standard "gas diet" very recently
(a few tens of thousands of years). This is an important clue on which sort of
food black holes mostly live on.
"Louis Pasteur said: 'Chance
favour the prepared mind' -- but in our case, nobody was really prepared,"
marvels Merloni. "We could have discovered this unique object already ten
years ago, but people did not know where to look. It is quite common in
astronomy that progress in our understanding of the cosmos is helped by
serendipitous discoveries. And now we have a better idea of how to find more
such events, and future instruments will greatly expand our reach."
In less than two years' time a
new powerful X-ray telescope eROSITA, which is currently being built at MPE,
will be put into orbit on the Russian-German SRG satellite. It will scan the
entire sky with the right cadence and sensitivity needed to discover hundreds
of new tidal disruption flares. Also, big optical telescopes are being designed
and built with the goal of monitoring the variable sky, and will greatly
contribute to solving the mystery of the black hole eating habits. Astronomers
will have to be prepared to catch these dramatic last acts of a star's life.
But however prepared they'll be, the sky will be full of new surprises.