Astronomy

S. OSSOKINE, A. BUONANNO (MAX PLANCK INSTITUTE FOR GRAVITATIONAL PHYSICS), D. STEINHAUSER (AIRBORNE HYDRO MAPPING GMBH)
ASTRONEWS
MOON VILLAGE. In late February, the head of the European Space Agency announced plans for a permanent
Moon base to follow the International Space Station. So far, few other details exist.
GRAVITATIONAL WAVES —
SEEN AND HEARD?
12h
Starting with the announcement on
February 11 that scientists had detected
gravitational waves for the first time, a
flurry of related studies have appeared as
researchers around the world joined in the
gravitational wave craze.
The Laser Interferometer Gravitationalwave
Observatory (LIGO) detected the signal
of two colliding black holes September 14,
2015. And while the LIGO team was hard at
work verifying the signal and writing up the
paper, it also reached out to other observatories
in search of corroborating evidence.
Some of LIGO’s potential gravitational
wave sources include collisions between
neutron stars (the dense cores of stars after
their ordinary lives end as supernovae)
or between neutron stars and black holes.
These events should give off gamma-ray
bursts followed by afterglow across the
spectrum that fades on the order of hours to
days. In order to track these events, followup
observations are built into the LIGO
process; in fact, LIGO tested such follow-up
searches during the initial science run in
2009 for a handful of detections that proved
insignificant.
In all, 74 partners signed up with LIGO,
encompassing both ground- and spacebased
observatories. Whenever LIGO
sees a detection — even before it has been
verified — these observatories coordinate
to search the skies for potential electromagnetic
matches to a gravitational wave
event. But they have to search a lot of sky.
With only two stations, in Louisiana and
COSMIC COLLISION. This simulation of two
colliding black holes shows the black holes near
center and the gravitational waves in vibrant colors,
though no visible light should be emitted.
0°
–30°
16h
Sun
Moon
–60°
20h
Washington state, LIGO can identify the
general direction of a source by which station
first detects it, but it can’t point to a
precise position.
Theoretically, two colliding black holes,
the source of LIGO’s only known detection
so far, should not emit any electromagnetic
radiation. And 24 of 25 teams that
responded to LIGO’s call reported no suspicious
detections. Yet the Fermi Gamma-ray
Space Telescope may have spotted something,
flying in the face of theory. It caught
a weak gamma-ray burst less than half a
second after LIGO intercepted the gravitational
waves, and in the right direction. A
fellow gamma-ray observatory, INTEGRAL,
could not verify Fermi’s detection, but given
the burst’s weakness and the differences
between the two telescopes, this does not
mean Fermi’s sighting is unreal.
But if colliding black holes don’t give
off electromagnetic radiation, then what
did Fermi see? Avi Loeb, an astronomer
at Harvard University, suggested in a
paper published in the March 10 issue of
The Astrophysical Journal Letters that the
answer might go beyond just two black holes
8h
24h
GW
Radio
Optical/infrared
X-ray
SEARCHING THE SKY. Observatories searched the spectrum from gamma rays to radio waves for electromagnetic
signals to match LIGO’s gravitational wave event. LIGO narrowed the source’s origins to within the
black contours, and partner observatories (radio searches in red, optical in green, X-ray in blue, and all-sky
gamma ray searches not shown here) searched the area in a patchwork collaboration. ABBOTT ET AL., 2016
crashing together. He posits a single massive
star — more than 100 times the Sun’s mass
— that collapsed in a supernova event into
a dumbbell shape, forming two black holes
separated by only 1,200 miles (2,000 kilometers),
which then quickly smashed together.
Furthermore, astronomers think that a common
way to form such enormous stars is to
merge two smaller suns, meaning the joint
Fermi-LIGO detection — if real — might
indicate a chain of cosmic collisions.
Until LIGO detects more events, and
until it can better pinpoint the locations
for its observing partners, some details
of gravitational wave origins will likely
remain a mystery. But help is on the way
from similar gravitational wave observatories
across the world: Virgo in Italy will
begin a testing phase later this year, and
KAGRA in Japan should be completed in
2018. LIGO India was approved February
17 and could be online as soon as 2023.
Every additional station will better enable
the LIGO collaboration to dig smaller
signals out of the noise, and to zero in on
their origins. The era of gravitational wave
astronomy is just beginning. — K. H.
16 ASTRONOMY • JUNE 2016