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in-betweeners sprang into existence ready-
made, either from the explosive deaths of
gargantuan stars or from the direct collapse of
clouds of gas, there would have been enough
time for some to merge into supermassive
monsters. But if that’s the case, there should
be plenty of IMBHs left over that didn’t merge.
“We should be able to observe them now in the
local universe,” says Mezcua. Where are they
all hiding?
Until recently, we thought we had it
cracked. The answer seemed to lie in a series of
unusually bright X-ray flashes that telescopes
around the world had been detecting since the
1980s. As matter spiralling into black holes
travels faster the closer it gets to destruction,
friction with neighbouring material heats it
until it glows in X-rays. The greater the mass
of the central object, the faster the spiralling
matter flies, the greater the friction and the
brighter the X-rays. Less powerful X-ray blasts
had already been used to pinpoint small black
holes, but these new ones, more than a million
times brighter than the sun, appeared too
bright to be coming from black holes of only
stellar mass size. The evidence seemed to
point to IMBHs instead.
That picture continued to hold as data
accumulated, but there was a nagging doubt.
“There had been hints for some years that
the spectra of these objects didn’t quite fit
with IMBHs,” says Matteo Bachetti from the
Cagliari Astronomical Observatory in Italy.
Then in 2014 came a bombshell: one of the
X-ray blasts was found to be pulsating. >

20 January 2018 | NewScientist | 33