Smallest known black hole revealed by its heartbeat
NASA said today (December 15, 2011) that one of its satellites has identified a candidate for the smallest-known black hole. The evidence comes from a specific type of X-ray pattern – nicknamed a “heartbeat” because of its resemblance to an electrocardiogram – that until now has been recorded in only one other black hole system.
The smallest black hole has a long name – IGR J17091-3624. It’s thought to be a binary system where a normal star orbits with a black hole that might weigh less than three times the sun’s mass. In other words, this object is near the theoretical boundary where black-hole status is first becomes possible.
In the animation above, the X-ray “heartbeats” of IGR J17091 and another black hole – designated GRS 1915+105 by astronomers – are compared. Both ingest gas from companion stars.
GRS 1915 has nearly five times the mass of IGR J17091.
Many black hole binaries show distinct and highly structured patterns of X-ray changes, which scientists distinguish by Greek-letter names. But to date only these two black holes – IGR J17091 and GRS 1915 – exhibit so-called rho-class oscillations that astronomers describe as a “heartbeat.”
The heartbeat pattern comes from the accretion (adding on) and ejection of matter from the vicinity of the black hole.
Flare-ups occur when 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 radiate X-rays.
It’s thought that strong magnetic fields near the black hole’s event horizon eject some of the gas into dual, oppositely directed jets that blast outward at nearly the speed of light. The peak of its heartbeat emission corresponds to the emergence of the jet. Changes in the X-ray spectrum observed by RXTE during each beat in GRS 1915 reveal that the innermost region of the disk emits enough radiation to push back the gas, creating a strong outward wind that staunches the inward flow, briefly starving the black hole and shutting down the jet. This corresponds to the faintest emission. Eventually the inner disk gets so bright and so hot that it essentially disintegrates and plunges toward the black hole, re-establishing the jet and beginning the cycle anew.
In GRS 1915+105, which at 14 solar masses is by for the more massive of the two, this cycle occurs in as little as 40 seconds. It occurs eight times faster in IGR J17091.
The data about IGR J17091 come from NASA’s Rossi X-ray Timing Explorer (RXTE).
Bottom line: On December 15, 2011, NASA said that its Rossi X-ray Timing Explorer (RXTE) satellite has identified a candidate for the smallest-known black hole. The evidence comes from a specific type of X-ray pattern – nicknamed a “heartbeat” because of its resemblance to an electrocardiogram – that until now has been recorded in only one other black hole system. The other system is designated GRS 1915+105 by astronomers.