Sky and Telescope - September 2017 - 10

NEWS NOTES
PHYSICS

LIGO Detects Third Black Hole Merger

SCIENTISTS WITH the Laser Interferometer Gravitational-Wave Observatory
(LIGO) have announced their discovery
of another black hole merger, revealed
when the spacetime ripples it created
squeezed and stretched the two LIGO
sites' arms by approximately 1/1000 the
width of a proton.
The newly announced event, designated GW170104, was detected on January 4th during LIGO's second observing run. The larger of the two merging
black holes had a mass between 25 and
40 times that of the Sun; the smaller,
between 13 and 25 Suns. The resulting black hole has a mass of roughly
50 solar masses, with a couple of solar
masses carried away as gravitational
radiation, the collaboration reports in
the June 2nd Physical Review Letters.
This is the third firm detection
of gravitational waves. LIGO scientists detected the previous two events
in September and December 2015,
announcing each of them last year (see
page 24). The final mass of the black
hole produced by the latest merger lies
betwixt those of the previous two LIGO
discoveries. Based on how "loud" the
signal was, it happened roughly 3 billion light-years away, approximately

10

twice as far from us as previous events.
However, the team cannot pinpoint its
exact location; it could lie anywhere in
a long, skinny region on the sky spanning roughly 1,200 square degrees.
Scientists derive things like the
black holes' masses, spin, and distance
based on a careful breakdown of the
gravitational-wave pulse. Its frequency,
for example, is inversely proportional to
the total mass of the black holes: Higher
frequency means lower mass.
One aspect of GW170104's signal
has astrophysicists excited: the black
holes' spins. Although the team can't
determine the exact direction and speed
of the two black holes' spins before the
merger, it does appear that at least one of
the objects was spinning in a direction
opposite that of its orbital motion.
The reasoning goes something like
this: If the two black holes were spinning exactly aligned with the axis of
their orbit, they would have needed to
shed some of the system's total rotational energy before they could merge.
Such a merger would take a few more
orbits than if the spins weren't aligned,
explains LIGO deputy spokesperson
Laura Cadonati (Georgia Tech). But the
team didn't see this "hang-up" effect,

S E P T E M B E R 2 0 1 7 * SK Y & TELESCOPE

■ CAMILLE M. CARLISLE

STELLAR

Potential "Failed Supernova" Discovered
ASTRONOMERS MIGHT have watched

a star collapse directly into a black hole
without going supernova.
Scott Adams (Caltech) and colleagues caught the rare event using the
Large Binocular Telescope in Arizona.
They were using the paired 8.4-meter
telescopes to monitor a million aging
stars in 27 nearby galaxies, waiting for
the stars to go pop.
Typically, a star more than eight
times as massive as the Sun ends its
life with a bang. Yet simulations suggest that some massive stars will never
explode. "The 'explodability' of a star
seems to be dependent on the density

LIGO / CA LTECH / MIT / SONO M A STATE (AURORE SIM ONNE T )

p Artist's concept of two black holes about to merge, spinning on axes that are tilted with respect
to their orbital plane.

so they think at least one black hole
wasn't perfectly lined up.
This hint of a misalignment is tantalizing. If the two black holes formed
from two stars that began life together
as a binary system, then they'd likely
be spinning at roughly the same angle
as their orbital motion. But if the black
holes joined up after they formed - say,
by sinking to the center of a dense stellar cluster - then their two spins could
easily be totally different. "This is an
important clue in understanding how
black holes form," Cadonati says.
The spin values themselves might
provide another clue. All three of the
final black holes whose birth LIGO has
detected spin at rates that are about 70%
of the maximum allowed. This result
matches recent calculations by Maya
Fishbach (University of Chicago) and
others, suggesting that when black holes
of similar masses merge, the resulting
object will have a rotation period around
this 70% value. If this correlation holds
up, then whenever we find a black hole
with this spin rate, we might be able to
say that it's the product of a merger.



Sky and Telescope - September 2017

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