Smallest black hole was found

The binary star system IGR J17091-3624 may contain the smallest black hole known. Scientists discovered it by studying the heartbeat-like pattern of X-rays thought to be emitted from a disk of accreting matter around the black hole. CREDIT: NASA/Goddard Space Flight Center/CI Lab

Using the Rossi X-ray Timing Explorer (RXTE) satellite of NASA observing the stars in the field of X-rays , a group of astronomers has found what appears to be the smallest known black hole . It’s light curve is similar to electrocardiogram .
The stellar black holes are formed during the collapse of a gravitational star mass. Stars like the Sun just aren’t massive enough to become black holes. Instead, in several billion years, the Sun will cast off its outer layers, and its core will form a white dwarf . So the star must exceed several times the mass of the Sun in order to become a black hole.

Typically, when this happens, the star explodes in a supernova . But recent studies regarding the black hole Cygnus X1 , suggest that this is not always the case. Still, a stellar black hole should not have a mass less than the so-called Chandrasekhar, in other words about 1.44 solar mass. If one were to discover a black hole of smaller mass, for example through the observations of Kepler , the energy from the gravitational collapse is not sufficient to produce the neutrons of a neutron star, so the collapse is halted by electron degeneracy to form white dwarfs.

Researchers think the system, officially called IGR J17091-3624, includes one normal star with a companion black hole, the black hole, if it truly exists, would weigh  three times less than the mass of the sun, putting it near the theoretical minimum mass required for a black hole to be stable. Mass would stream off this normal star and fall toward the black hole, forming a flattened disk around it. As friction in the disk heats the gas to millions of degrees, the disk would emit high-energy X-rays that can be seen across the galaxy.

As changes occur inside the disk, cyclical variations can be seen in the X-rays streaming from it, which pulse in varied intensity like a heartbeat.

“We think that most of these patterns represent cycles of accumulation and ejection in an unstable disk, and we now see seven of them in IGR J17091,” researcher Tomaso Belloni of the Brera Observatory in Merate, Italy, said in a statement. “Identifying these signatures in a second black hole system is very exciting.”

The astronomers recognized the signal from this system because of its similarity to another black hole system called GRS 1915+105 that pulses in much the same way. This other system contains a black hole that weighs about 14 times the sun’s mass, which sends out X-rays in highly structured patterns that last between seconds and hours.

In comparison, the newly observed system has an X-ray heartbeat that pulses 20 times fainter than GRS 1915 and cycles back to the beginning of the pattern about eight times faster, in as little as 5 seconds.

“Just as the heart rate of a mouse is faster than an elephant’s, the heartbeat signals from these black holes scale according to their masses,” said Diego Altamirano, an astrophysicist at the University of Amsterdam in the Netherlands and lead author of a paper reporting the findings in the Nov. 4 issue of the Astrophysical Journal Letters.



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