Wednesday, February 27, 2013

NuSTAR on the spin of supermassive BHs

Some month ago, in this post, we reported about the launch of the Nuclear Spectroscopic Timing Array (NuSTAR).
Today NASA scheduled a media teleconference to announce one of the first scientific achievements of this mission: the first precise measurement of the spin of a supermassive black hole. 

The discovery will appear in this week Nature's issue.

It turns out that NGC 1365 hosts a rapidly spinning supermassive black hole at its center, and this black hole rotates very close to its theoretical limit imposed by Einstein's General Relativity.

one usually refers to these black holes as "extremal" because their spin is close to its maximum value. In "God-given" natural units, the angular momentum of an extremal black hole is J=M^2. 
Indeed, what is usually called the "Kerr limit" is J/M^2 = 1.  This is by no means an "extremal" value!! [For example, Earth also spins around its axis and it has J/M^2~ 10^9 in natural units!]

This doesn't mean that these black monsters in the sky don't spin fast, quite the opposite: because their mass is HUGE, the angular momentum of an extremal black hole of about 10^6 solar masses [roughly the mass of the black hole at the center of the NGC 1365 galaxy]  is as large as 10^13 times that of Earth... a pretty huge and fast spinning top!

The first author of the paper, Guido Risaliti is an Italian astrophysicists who works here at the Harvard-Smithsonian CfA and at Italian Institute for Astrophysics in Arcetri.
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