November 27, 2013
NuSTAR Helps Scientists Hunt For Medium-Sized Black Holes
Lee Rannals for redOrbit.com - Your Universe Online
NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR) is helping scientists determine whether black holes need a medium-sized category.
Black holes are known to either come in masses of only about 10 times that of our sun, or the equivalent in mass of up to 10 billion suns. However, NuSTAR could be finally finding a middle ground.
"Exactly how intermediate-sized black holes would form remains an open issue," Dominic Walton, of the California Institute of Technology, Pasadena said in a statement. "Some theories suggest they could form in rich, dense clusters of stars through repeated mergers, but there are a lot of questions left to be answered."
Massive black holes sit inside the middle of galaxies, while small black holes dot the rest of the galactic landscape. The smaller of the celestial species form under the crush of collapsing, dying stars bigger than our sun.
NASA says that evidence for medium-sized black holes lying somewhere between these two extremes might come from objects called ultraluminous X-ray sources (ULXs). These are pairs of objects in which a black hole ravenously feeds off a normal star. A bright glow coming from the ULXs is too great to be the product of typical small black holes, indicating objects may be intermediate in mass, with 100 to 10,000 times the mass of our sun.
Researchers used NuSTAR, along with plenty of other telescopes, to look at ULXs in more detail. NuSTAR is providing scientists with the first look at these objects in focused, high-energy X-rays.
Scientists writing in the Astrophysical Journal say they have found a ULX that had gone largely unnoticed before. They studied the object with both the NuSTAR and the European Space Agency's XMM-Newton satellite. They also looked through data from NASA’s Chandra, Swift and Spitzer space telescopes, as well as Japan’s Suzaku satellite.
"We went to town on this object, looking at a range of epochs and wavelengths," Walton said.
The study shows the black hole in question is about 100 times the mass of the sun, putting it right at the border between small and medium black holes.
Another group of researchers publishing a separate paper in the same journal studied two ULXs in NGC 1313, which is a spiral galaxy known as the “Topsy Turvy galaxy.” A single viewing with NuSTAR showed how the black holes didn’t fit with models of medium-sized black holes. The team determined both ULXs harbor small, stellar-mass black holes, one of which is about 70 to 100 solar masses.
"It's possible that these objects are ultraluminous because they are accreting material at a high rate and not because of their size," Matteo Bachetti of the Institut de Recherche en Astrophysique et Planétologie said in a statement. "If intermediate-mass black holes are out there, they are doing a good job of hiding from us."