Sky and Telescope - July 2018 - 10

NEWS NOTES

Galaxy's Center Is Home
to Many Black Holes

Radius = 3 light-years

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
center. Now, a study published in the
April 5th Nature 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 analyzed
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 center are white
dwarfs in binary systems, which siphon
gas off of their ordinary stellar companions and radiate more of their energy at

Radius = 0.7 light-year

p A Chandra X-ray image of the Milky Way's
center is overlaid with circles around unresolved
X-ray sources. Red circles indicate white dwarf
binaries, while blue circles denote likely black
hole binaries.

lower X-ray energies. But 12 of the 26
sources the team found are relatively
brighter at 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
center should have found all neutron
star binaries by now via their outbursts.

STARS

Astronomers Catch Cache of "Fast Supernovae"
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, but their
origin remained unclear. With DES data
-8 d

-4 d

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 10,000K
and 30,000K.
These ﬁndings agree with another
recent result from NASA's Kepler mission. In 2015 Kepler spotted a FELT
known as KSN 2015K, measuring its
+8 d

+14 d

+18 d

p One of the 72 fast-evolving luminous transients (FELTs) observed by the Dark Energy Survey

J U LY 2 0 1 8 * S K Y & T E L E S C O P E

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 center. 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 10,000 black
holes in the galaxy's core.
But whether all 12 sources are
black holes and how they got there
remain uncertain. Hailey and his team
acknowledge that as many as half of
their X-ray sources could be rotationpowered millisecond pulsars: fast-spinning neutrons stars that exhibit fewer
outbursts. Then there might be several
hundred instead of thousands of black
holes in the galactic center. 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 astronomers
pin down the number and origin of
massive objects in our galaxy's core.
■ MONICA YOUNG

brightness every 30 minutes over a
three-week period. In the March 26th
Nature Astronomy, 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

X-R AY: C. H AILE Y E T A L. / NATUR E; SUPER NOVA E: M. PURSIAINEN /
UNIV ERSIT Y OF SOU TH A MPTON A ND DES COLL A BOR ATION

MILKY WAY

Sky and Telescope - July 2018

Table of Contents for the Digital Edition of Sky and Telescope - July 2018