(PhysOrg.com) -- An international team of astronomers has
identified a candidate for the smallest-known black hole using
data from NASA's Rossi X-ray Timing Explorer (RXTE). The
evidence comes from a specific type of X-ray pattern, nicknamed
a "heartbeat" because of its resemblance to an
electrocardiogram. The pattern until now has been recorded in
only one other black hole system. Named IGR J17091-3624 after
the astronomical coordinates of its sky position, the binary
system combines a normal star with a black hole that may weigh
less than three times the sun's mass. That is near the
theoretical mass boundary where black holes become possible. Gas
from the normal star streams toward the black hole and forms a
disk around it. Friction within the disk heats the gas to
millions of degrees, which is hot enough to emit X-rays.
Cyclical variations in the intensity of the X-rays observed
reflect processes taking place within the gas disk. Scientists
think that the most rapid changes occur near the black hole's
even