Using NASA’s Rossi X-ray Timing Explorer (RXTE), astronomers have detected the pulsating heartbeat of what may be the smallest known black hole.

Black holes reside at the centres of galaxies and swallow everything that falls into their gravitational clutches such that nothing, not even light, can escape. They are usually known for their imposing size – indeed, the largest supermassive black holes, capable of swallowing our Solar System whole several times over, were reported just last week – but now RXTE has found a black hole that could represent the lower boundary for a black hole’s mass at just three solar masses.

Known as IGR J17091-3624 after the astronomical coordinates of its sky position, the black hole is one half of a binary system that feeds on its normal star neighbour and was identified by its distinctive X-ray emission. Gas syphoned off the star circles around the black hole and is heated to millions of degrees, hot enough to shine in X-rays. But the distinct pattern of X-ray emission, which resembles the pattern printed on an electrocardiogram in response to a heartbeat, has only ever been observed in one other black hole system until now.

“We used to have one of them, now we have two – this is important to study their peculiarities as such, which can help us understand better the process of accretion of matter onto a black hole,” says Tomaso Belloni at Brera Observatory, Italy. “We think that most of these patterns represent cycles of accumulation and ejection in an unstable disc, and we now see seven of them in IGR J17091. Identifying these signatures in a second black hole system is very exciting.”

The binary system first made itself known in 2003 with an outburst event and archival data showed that this was a regular occurrence. The latest outburst began in February 2011 and is ongoing.

“After a normal start, indistinguishable from dozens of others, its luminosity suddenly started to change wildly,” explains Tomaso, who adds that its behaviour resembled closely that of one other source, GRS 1915+105. “The similarity in shape of the time variations between the two sources is almost incredible, but there are two main differences: the first is that the “heartbeats” can come much faster, as fast as every two seconds; the second that the detected flux from this system is much lower than that from GRS 1915+105.”

Tomaso tells Astronomy Now that there are only two possibilities to explain the differences: either the new source is farther away or its mass is lower. “However, there is a limit as to how distant a source like this can be, since it would be very unlikely to have it lying outside our Galaxy. Combining distance and mass, one can estimate that to keep it at a reasonable distance (GRS 1915+105 is itself quite distant from us, 11 kiloparsecs), the mass has to be low. The fact that its 'heart' beats faster is compatible with a lower mass, since the fastest variations from a source of this type also depend on the black hole mass: lighter black holes can allow for faster variability.”

While GRS 1915 is estimated to weigh in at about 14 times the Sun’s mass, IGR J17091 is just three solar masses.

The astronomers will be keen to see what the black hole is doing once it moves out of the glare of the Sun in mid-January.