8 AUSTRALIAN SKY & TELESCOPE July 2018
A NEW X-RAY STUDY has uncovered a
dozen potential stellar-mass black holes
within 3 light-years of the supermassive
black hole lurking in our galaxy’s core
— and these might be just the tip of the
proverbial iceberg.
We’ve long known that a
supermassive black hole lurks in the
Milky Way’s centre. Now, a new study
makes the case that up to 10,000 or
so stellar-mass black holes could be
keeping it company.
Beginning with 16 days’ worth of
observations that the Chandra X-ray
Observatory had collected over the past
12 years, Charles Hailey (Columbia
University) and colleagues analysed
26 sources that remain unresolved at
X-ray wavelengths and lie within 3 light-
years of the supermassive black hole.
Typically, most X-ray emitters found near
our galaxy’s centre are white dwarfs in
binary systems, which siphon gas off of
their ordinary stellar companions and
radiate more of their energy at lower
X-ray energies. But 12 of the 26 sources
the team found are relatively brighter atMilky Way’s core is home to lots of black holes
higher X-ray energies. They also appear
to be binaries, but with more massive
neutron stars or black holes instead of
white dwarfs. Hailey and colleagues
argue that the sources are more likely to
be black holes, as long-term monitoring
of the galactic centre should have found
all neutron star binaries by now via their
outbursts.
If there are a dozen stellar-mass
black hole binaries that we can see,
then many more isolated (and therefore
invisible) black holes might exist in
the galactic centre. Exactly how many
depends on the binaries’ origin. If they
formed when the stellar remnants
captured old, low-mass stars — instead
of beginning life as a pair of stars —
then there could be as many as 10,
black holes in the galaxy’s core.
But whether all 12 sources are
black holes and how they got there
remains uncertain. Hailey and his
team acknowledge that as many as
half of their X-ray sources could be
rotation-powered millisecond pulsars:
fast-spinning neutrons stars that exhibitS A Chandra X-ray image of the Milky
Way’s centre is overlaid with circles around
unresolved X-ray sources. Red circles indicate
white dwarf binaries, while blue circles denote
likely black hole binaries.THE DARK ENERGY SURVEY’S
SUPERNOVA (DES-SN) Program has
detected 72 fast and furious explosions,
Miika Pursiainen (University of
Southampton) announced at the
European Week of Astronomy and Space
Science 2018 in Liverpool, UK. These
fast-evolving luminous transients (FELTs)
have the energy of a regular supernova
explosion, but they brighten and fade
within days or weeks rather than
months or years.
A few dozen of these ‘fast
supernovae’ were already known, butAstronomers catch cache of ‘fast supernovae’
S One of the 72 fast-evolving luminous transients (FELTs) observed by the Dark Energy Surveyfewer outbursts. Then there might be
several hundred instead of thousands of
black holes in the galactic centre. Daryl
Haggard (McGill University, Canada),
who wasn’t involved in this research,
notes that future radio studies could
distinguish between black holes and
neutron stars, which would help pin
down the number and origin of massive
objects in our galaxy’s core.
■ MONICA YOUNGtheir origin remained unclear. With
DES data in hand, Pursiainen and
colleagues conclude that the emission
from a typical FELT comes from a hot,
expanding shell of material — possibly a
dense cocoon around an exploding star.
The shell may span anywhere between
a few and 100 astronomical units (the
average distance between Earth and
the Sun), with a temperature between
10,000°C and 30,000°C.
These findings agree with another
recent result from NASA’s Kepler
mission. In 2015 Kepler spotted a FELTknown as KSN 2015K, measuring its
brightness every 30 minutes over a
three-week period. Armin Rest (Space
Telescope Science Institute) and
colleagues report that KSN 2015K’s
rise to peak brightness occurred in just
2.2 days, before it faded by a factor of
two over 5 days. The richly sampled light
curve enabled the team to exclude all
but one viable scenario: A giant star had
shed a huge amount of gas and dust less
than a year before it went supernova.
When it ultimately exploded, the star’s
outer layers slammed into the thick
cocoon, turning most of the kinetic
energy into heat and light.
Pursiainen and Rest agree that it’s
unclear whether this cocoon model
would apply to all 72 DES-SN supernovae.
Future DES observations will doubtless
reveal more FELTs, and studying larger
numbers will help elucidate their true
nature. ■ GOVERT SCHILLING–8 d – 4 d 0 d +8 d +14 d +18 dX-RAY: C. HAILEY ET AL. /NATURE; SUPERNOVAE: M. PURSIAINEN / UNIVERSITY OF SOUTHAMPTON AND DES COLLABORATIONRadius = 3 light-yearsRadius = 0.7 light-yearNEWS NOTES