Prompt Alerts for
Gravitational-Wave
Event Candidates
Peter Shawhan
(U. of Maryland / JSI)
for the LIGO Scientific Collaboration and Virgo Collaboration
Hot-Wiring the Transient Universe IV
Santa Barbara — May 14, 2015
LIGO-G1500161-v1
GOES-8 image produced by M. Jentoft-Nilsen, F. Hasler, D. Chesters
(NASA/Goddard) and T. Nielsen (Univ. of Hawaii)
)))
Gravitational Waves and GW Detection
The Einstein field equations have wave solutions for
weak perturbations of spacetime
Changes the spacetime metric (i.e, the effective distance
between inertial points) transverse to direction of travel
Mirror
Emitted, for instance, by a
compact-object binary
system in a tight orbit
Beam splitter
Laser
Mirror
Photodiode
Produces a
dimensionless strain,
ℎ(𝑡) = Δ𝐿(𝑡)/𝐿 , with
amplitude ∝ 1/distance
Interferometer measures
difference in arm lengths
2
)))
The Experimental Challenge
Compact binary systems are out there,
inexorably approaching merger
A handful of relativistic NS-NS systems
known in our Galaxy via pulsars
But compact binary mergers and
other (plausible) events producing
detectable GWs are rare
 Have to be able to search a
large volume of space
Strain amplitude is inversely
proportional to distance from source
Weisberg, Nice & Taylor,
ApJ 722, 1030 (2010)
Spacetime is intrinsically very stiff
Typical strain amplitude at Earth:
ℎ ~ 10–21 or smaller !
3
)))
Advanced GW Detector Network –
Under Construction  Operating
2015
2018?
GEO-HF
2011
LIGO Hanford
600 m
4 km
3 km
3 km
4 km
4 km
LIGO Livingston
2015
2022?
(pending)
Virgo
2016-17
3 separate collaborations
working together
4
)))
Advanced LIGO Optical Layout
Comprehensive upgrade of
Initial LIGO instrumentation
in same vacuum system
Improvements
Advanced Virgo
and KAGRA have
similar designs
Higher-power laser
Larger mirrors
Higher finesse arm cavities
Stable recycling cavities
Signal recycling mirror
Output mode cleaner
Goal: 10× lower noise
 1000× more volume
of space searched
and more …
5
)))
Advanced LIGO Status: Looking Good!
Installation complete at both LIGO observatories
Now focusing on tuning alignment and other
control systems, hunting down noise sources
and improving stability
6
)))
Advanced LIGO Commissioning Progress
Full interferometer lock achieved at LIGO Livingston in May 2014
Now routinely reaches
~65 Mpc NS-NS range
(prelim. calibration)
With improvements to
low-frequency noise and
medium laser power
LIGO Hanford was
staged later and is now
focus of commissioning
efforts – has reached
~35 Mpc
G1401390-v8
7
)))
LIGO / Virgo Observing Run Schedule
Projection made in 2013 (arXiv:1304.0670) still seems on target
Was based on guesses at how fast commissioning would progress
LIGO detectors on track to take data from Sep-Dec of this year –
to be called “O1”
Hoping for Virgo to join late next year with decent sensitivity, then KAGRA
Still very uncertain when we’ll detect the first GW signal(s)
8
Motivation for Multi-Messenger Approach
Gravitational wave transient sources are highly
energetic astrophysical events
(and must be relatively close to be detectable by
LIGO / Virgo / KAGRA)
W. Benger, LSU
)))
GW emissions are only weakly beamed, and
GW detectors are only weakly directional
GWs come directly from the central engine
Not obscured or scattered by material
 Can match up with EM or neutrino emission even
if that is under-luminous and/or delayed
 Complements photon diagnostics of surface, outflows,
circumburst medium, and astronomical context
Bill Saxton, NRAO/AUI/NSF
 Monitor the whole sky for sources with all inclinations
 Not dependent on being within the cone of a jet
9
)))
Generating and Distributing Prompt Alerts
LIGO Hanford
GEO 600
Virgo
Transfer data
LIGO Livingston
KAGRA
LIGO-India
Send info
to observers
GW
data
Analyze data,
identify triggers,
infer sky position
Estimate background
Validate
(data quality, etc.)
Trigger
database
Select event
candidates
Challenge: GW reconstructed sky regions are large !
Typically several tens of square degrees or more, until KAGRA & LIGO-India join
Swift: NASA E/PO, Sonoma State U., Aurore Simonnet 10
)))
Searches for GW Transient Sources
Two or more GW data streams analyzed jointly
Initially LIGO Hanford+Livingston and Virgo; later others too
Two main types of transient searches:
Compact Binary Coalescence (CBC)
Known waveform  Matched filtering
Templates for a range of component masses
(spin affects waveforms too, but not so important
for initial detection)
Unmodelled GW Burst
(< ~1 sec duration)
e.g. from stellar core collapse
Arbitrary waveform  Excess power
Require coherent signals in detectors,
using direction-dependent antenna response
11
)))
Coherent CBC Position Reconstruction
Bayesian parameter estimation methods tested and compared
with simulated events from first 2 years of advanced GW detectors
Singer et al., arXiv:1404.5623; Berry et al., arXiv:1411.6934;
http://www.ligo.org/scientists/first2years/
Sample
LIGO-only
Fast reconstruction (BAYESTAR)
and
LIGO-Virgo
events:
Fast reconstruction
MCMC-based
reconstruction
(BAYESTAR)
12
)))
Partnerships
There’s a lot to be gained from finding counterparts
But confident detection of first few GW signals will require time
and care—need to avoid misinformation / rumors / media circus
 Established a standard MOU framework to share information
promptly while maintaining confidentiality for event candidates
LIGO/Virgo will need to carefully validate the first few detections, at least
Once GW detections become routine, there will be prompt public alerts of
high-confidence detections
LIGO & Virgo have signed MOUs with ~70 groups so far!
Broad spectrum of transient astronomy researchers and instruments
Optical, Radio, X-ray, gamma-ray, VHE
Transient surveys and various instruments that can be pointed to follow up
Telescope owners, instrument science teams and ToO proposers
Encourage free communication among all “inside the bubble”
13
)))
Communication Tools
GW event candidates will be distributed to partners as GCN Notices
With a controlled list of recipients
Primary alert format is VOEvent
Significance indicated as estimated false alarm rate (FAR) for detector
noise fluctuations to produce such a transient signal in the data
Sky probability maps are FITS files containing HEALpix pixel data
Utilize citation feature to support update and retraction messages
Other GCN delivery methods available too (distilled contents)
Direct access to “GraceDB” GW candidate event database
A “bulletin board” will allow observers to report observations
We ask observers to report where and when they have observed
We encourage observers to coordinate as they see fit
14
)))
Types of GW Alerts
Elapsed time goal:
Preliminary: A transient in the GW data has been
detected, but not yet checked for good data quality
3-5 min
May be useful to wake up a human or a scheduling computer
For some instruments, could initiate buffering of data
It should not be considered a validated GW candidate
Initial: Event candidate has passed basic data quality
checks, and a sky-position probability map is available
5-10 min
Automated data quality checks
Optionally require an OK from on-site humans
Update: Updated sky map and/or estimate of significance
hours
Retraction: In case we later that the data quality or
significance estimate was bad
May be issued at any stage
15
)))
Example “Initial” VOEvent – part 1
<voe:VOEvent xsi:schemaLocation="http://www.ivoa.net/xml/VOEvent/v2.0 http://www.ivoa.net/xml/VOEvent/VOEvent-v2.0.xsd"
version="2.0" role="test" ivorn="ivo://gwnet/gcn_sender#G96195-2-Initial">
<Who>
<Date>2014-12-03T16:03:34</Date>
<Author>
<contactName>LIGO Scientific Collaboration and Virgo Collaboration</contactName>
</Author>
</Who>
<What>
<Param name="Pkt_Ser_Num" dataType="string" value="2"/>
<Param name="GraceID" dataType="string" value="G96195" ucd="meta.id" unit="">
<Description>Identifier in the GraceDb database</Description> </Param>
<Param name="EventPage" dataType="string" value="https://gracedb.ligo.org/events/G96195" ucd="meta.ref.url">
<Description>Web page for evolving status of this candidate event</Description> </Param>
<Param name="AlertType" dataType="string" value="Initial" ucd="meta.version" unit="">
<Description>VOEvent alert type</Description> </Param>
<Param name="FAR" dataType="float" value="7.8034122e-8" ucd="arith.rate;stat.falsealarm" unit="Hz">
<Description>False alarm rate for GW candidates with this strength or greater</Description> </Param>
<Param name="Pipeline" dataType="string" value="gstlal" ucd="meta.code" unit="">
<Description>Low-latency data analysis pipeline</Description> </Param>
<Param name="Search" dataType="string" value="LowMass" ucd="meta.code" unit="">
<Description>Low-latency search type</Description> </Param>
<Param name="ChirpMass" dataType="float" value="0.912880957127" ucd="phys.mass" unit="solar mass">
<Description>Estimated CBC chirp mass</Description> </Param>
<Param name="Eta" dataType="float" value="0.2484173" ucd="phys.mass;arith.factor" unit="">
<Description>Estimated ratio of reduced mass to total mass</Description> </Param>
<Param name="MaxDistance" dataType="float" value="57.5933" ucd="pos.distance" unit="Mpc">
<Description>Estimated maximum distance for CBC event</Description> </Param>
This example is for a low-mass CBC event
16
)))
Example “Initial” VOEvent – part 2
<Group type="GW_SKYMAP" name="BAYESTAR">
<Param name="skymap_fits_shib" dataType="string" value="https://gracedb.ligo.org/events/G96195/files/skymap.fits.gz"
ucd="meta.ref.url" unit="">
<Description>Sky Map FITS Shibboleth protected</Description>
</Param>
<Param name="skymap_fits_x509" dataType="string" value="https://gracedb.ligo.org/api/events/G96195/files/skymap.fits.gz"
ucd="meta.ref.url" unit="">
<Description>Sky Map FITS X509 protected</Description>
</Param>
<Param name="skymap_fits_basic" dataType="string" value="https://gracedb.ligo.org/apibasic/events/G96195/files/skymap.fits.gz"
ucd="meta.ref.url" unit="">
<Description>Sky Map FITS basic auth protected</Description>
</Param>
<Param name="skymap_png_shib" dataType="string" value="https://gracedb.ligo.org/events/G96195/files/skymap.png"
ucd="meta.ref.url" unit="">
<Description>Sky Map image Shibboleth protected</Description>
</Param>
<Param name="skymap_png_x509" dataType="string" value="https://gracedb.ligo.org/api/events/G96195/files/skymap.png"
ucd="meta.ref.url" unit="">
<Description>Sky Map image X509 protected</Description>
</Param>
<Param name="skymap_png_basic" dataType="string" value="https://gracedb.ligo.org/apibasic/events/G96195/files/skymap.png"
ucd="meta.ref.url" unit="">
<Description>Sky Map image basic auth protected</Description>
</Param>
</Group>
</What>
<WhereWhen>
<ObsDataLocation>
<ObservatoryLocation id="LIGO Virgo"/>
<ObservationLocation>
<AstroCoordSystem id="UTC-FK5-GEO"/>
<AstroCoords coord_system_id="UTC-FK5-GEO">
<Time> <TimeInstant> <ISOTime>2015-03-01T03:57:59.210675</ISOTime>
</TimeInstant> </Time>
<Position2D> <Value2><C1>0.000000</C1><C2>0.000000</C2></Value2> <Error2Radius>180.000000</Error2Radius> </Position2D>
</AstroCoords>
</ObservationLocation>
</ObsDataLocation>
</WhereWhen>
<How>
<Description>L1: LIGO Livingston 4 km gravitational wave detector</Description>
<Description>V1: Virgo 3 km gravitational wave detector</Description>
<Description>Candidate gravitational wave event identified by low-latency analysis</Description>
</How>
<Citations>
<EventIVORN cite="supersedes">ivo://gwnet/gcn_sender#G96195-1-Preliminary</EventIVORN>
<Description>Initial localization is now available</Description>
</Citations>
<Description>Report of a candidate gravitational wave event</Description>
</voe:VOEvent>
17
)))
FAR Threshold / Alert Rate for O1
We will generate alerts from CBC and Burst low-latency searches
We plan to set the FAR threshold so that we expect ~1 CBC alert
and ~1 Burst alert per calendar month during O1
 Total of about 6 alerts expected during O1
Note: the event type and FAR is included in the alert, so follow-up
observers can be more selective if they want
We’re saying we will generate alerts for ~6 false event candidates,
plus however many real event candidates are loud enough to pass
the FAR threshold too
That might be zero for O1, or it might not!
18
Summary
Advanced LIGO is on track for a first observing run this year
Advanced Virgo and KAGRA to join over next few years; LIGO-India later
Multi-messenger astronomy is an important component of the
GW science program
Some fraction of GW events should also be detectable by EM observers
GW signature complements photon diagnostics of surface, outflows,
circumburst medium, and astronomical context
Now preparing to provide rapid alerts to observers
Support correlation with surveys, as well as prompt & delayed
follow-up observations
Now is a good time for observers to plan how to
receive and act on alerts, and what science
can be obtained from joint observations