NASA's NuSTAR to Look for Medium Sized Black Holes

NASA has announced that its Nuclear Spectroscopic Telescope Array (NuSTAR) is looking for black holes that come in medium sized category.

There are nearly ten million to a billion black holes in our Milky Way. Black holes are the remnants of massive stars that die in a supernova explosion and exist in two radically different sizes.  The giant ones are called supermassive black holes that are massive and million times bigger than the Sun, and are present at the center of all huge galaxies. There is another category, the smaller black holes, which are just 10 times the size of the Sun. Strangely, the astronomers have never been able to determine if there is another category, the medium-sized black holes.

To solve this mystery, NASA's NuSTAR is observing a class of black holes that may fit into the proposed category of medium-sized black holes.

"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." 

Recently NASA's Chandra, XMM-Newton and Hubble provided strong evidence that mid-sized black holes do exist. NASA using NuSTAR wants to confirm the existence of these mid-size black holes. They claim that the proof may come from the ultraluminuos X-ray sources (ULXs).

The ultraluminous X-ray sources are pairs of objects in which the black hole feed off a normal star, and this process is analogous to the event that takes place around the supermassive black holes.

The intense glow of the X-ray coming from the ULXs is too bright to be appearing from a small black hole. The brightness indicates that the mass of the object is intermediate i.e. 100-10,000 times the mass of the sun. NuSTAR will join the other telescopes to take a look at ULXs. The researchers used the data on NASA's Chandra, Swift and Spitzer space telescopes as well as Japan's Suzaku satellite for further studies.

"We went to town on this object, looking at a range of epochs and wavelengths," said Walton. 

The results showed that the mass of the mid-small sized black holes is nearly 100 times the mass of the Sun, placing it between small and medium sized black holes.

"It's possible that these objects are ultraluminous because they are accreting material at a high rate and not because of their size," said Bachetti. "If intermediate-mass black holes are out there, they are doing a good job of hiding from us." 

The finding was reported in the Astrophysical Journal.