LIGO

The
Journey of a
Gravitational
Wave Signal
Along time ago in a part of space
far, far away, two black holes collide
- creating another, more massive black
hole whilst emitting enormous amounts of
gravitational waves. These waves travel at the
speed of light, gradually getting weaker. They
arrive at Earth where the LIGO detectors are
operating nominally, about to start their first
observing run. The gravitational waves cause
the space-time in each of the LIGO-Livingston
arms to stretch and squeeze, and 7 ms later
the same thing happens at LIGO-Hanford.
This stretching and squeezing causes a phase
change in the laser light resonating in the
arms, which registers as an electronic signal
at the output of the interferometer.
In the weeks leading up to an observing run,
many measurements are made at each LIGO
site which allow the calibration team to accurately
convert this electrical signal in to
the dimensionless gravitational wave unit
- strain. This is defined as the change in the
length of the detector arms caused by a gravitational
wave divided by the length of the
arms themselves.
Strain data from each interferometer are
transferred in close to real time to a central
location, where several “low latency” data
analysis algorithms are ready, waiting. Only
when both LIGO detectors are operational
at the same time do these algorithms begin
searching through the data to find a gravita-
Normalized spectrograms of GW150914 in LIGO-
Hanford (top) and LIGO-Livingston (bottom).
tional wave signature. These analyses search
for modeled and unmodeled gravitational
wave signals, and have their own methods for
identifying a signal. However should any analysis
identify a potentially interesting signal an
alert is sent out to collaboration members.
The collaboration has a team of scientists on
standby, 24 hours a day, 7 days a week, waiting
for any alert sent through this system.
Mobile phone alerts and emails are sent to
the rapid response team within minutes of a
gravitational wave signal being recorded by
each interferometer. This team immediately
meets to decide if there are any reasons to
suspect the validity of the signal. For example,
a list of instrumental monitors are
checked and discussions are had with experts
on site to ensure the interferometers
were operating nominally. If no problems are
found, a further team then starts the process
to notify astronomers of the possible identification
of a gravitational wave signal so they
can point their telescopes and capture any
potential electromagnetic counterpart.
The “low latency” algorithms give the collaboration
a first glimpse into the parameters of
a signal, such as the masses of the original
compact objects or where the signal came
from on the sky. Parameter estimation algorithms
are then launched on the data around
the signal to help pinpoint the parameters to
a greater certainty. In addition, offline analyses
use large periods of data to confirm and
search for further gravitational wave signals.
Typically the offline searches use at least 5
days of coincident data which ensures any
signal can be found to a statistically significant
level to claim a detection.
These offline analyses are conducted in a
blind fashion, meaning that any gravitational
wave signals an analysis might identify are
not presented in the initial output of the
analysis pipelines. Instead, these pipelines
split the data between “foreground” and
“background”. Foreground data may include a
gravitational wave signal, and their results are
placed in a so-called “closed box”. Background
data are data which cannot possibly include
a real gravitational wave signal, and is used
to estimate the probability of anything in the
foreground being of astrophysical origin. It is
these data that scientists evaluate to check an
analysis was conducted in the manner intended.
Once these checks have been completed,
the “box” can be opened. In practice this is
simply changing permissions on a webpage,
but this process is very exciting. This is usually
done during a teleconference with the rest of
the collaboration, where hundreds of scientists
are constantly refreshing a webpage to
see if any of the offline pipelines identified a
signal to detection significance. In the case
of this event, the box opening occurred on a
Monday, 3 weeks after the signal initially arrived
at the detectors. From this moment the
signal was identified as a possible detection
and the previously agreed procedure for a detailed
analysis was started.
- Laura Nuttall
2016
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