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, near the theoretical boundary where black-hole status is first becomes possible. 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 IGR J17091 and one other system, named GRS 1915+105, exhibit so-called rho-class oscillations that astronomers describe as a ‘‘heartbeat’‘ reflecting the accretion and ejection of matter.
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 stops 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.