Posted on behalf of Ron Cowen.
In a new and sharper hunt for the Universe’s most violent events, astronomers may have found two medium-sized galactic monsters. Launched in June, NASA’S NuSTAR (Nuclear Spectroscopic Array) X-ray observatory has discovered what may be two intermediate-mass black holes in a nearby galaxy — a missing link between stellar black holes and supermassive black holes. Principal investigator Fiona Harrison of the California Institute of Technology in Pasadena unveiled the mission’s first science on 7 January at a meeting of the American Astronomical Society in Long Beach, California.
NuSTAR employs a pair of X-ray telescopes that focus photons with energies between 6 and 79 kiloelectronvolts, Harrison says. The two-year mission overlaps in the X-ray spectrum with NASA’s Chandra X-ray Observatory and the European Space Agency’s XMM-Newton satellite, but can also record higher energies.
The new observatory’s pair of eyes has been brought to bear on the spiral galaxy IC 342, some 7 million light years (2.1 million parsecs) from Earth. The NuSTAR picture (above) shows two luminous objects believed to be black holes feeding on their surroundings. The objects are so close together that they couldn’t have been discerned with previous instruments at the same energies.
It’s unclear whether the objects are unusually bright stellar-mass black holes or a long-sought missing link — black holes weighing between 500 and several thousand Suns, which would represent a bridge between the small black holes that pepper the Milky Way and the behemoths, millions to billions of times more massive than the Sun, that lie at the heart of many galaxies. A comparison of the energy emitted by the two bodies with X-rays known to be emitted by small black holes in the Milky Way could determine the true nature of the objects, Harrison says.
Cole Miller of the University of Maryland in College Park notes that it will be difficult to identify the bodies as intermediate-mass black holes without information about the orbits of the objects, which would allow astronomers to weigh them.
In the accompanying image, the two candidate black holes appear to be entangled in the spiral arms of IC 342 and are assigned the false colour magenta, whereas the galaxy itself is portrayed in visible light.