Following Up Gravitational Wave Event Candidates

Roy Williams (Caltech),

Peter Shawhan (U Maryland),for the LIGO Scientific Collaboration

and Virgo Collaboration

LSST All Hands Meeting 2012 August 14

LIGO-G1200782

))) Sources of Gravitational Waves

► Compact binary coalescence ► Stellar core collapsee.g. neutron stars or black holes

► Neutron stars ► Cosmic stringsPeriodic from bumpNon-periodic from flare

► Early UniverseLike CMB

The challenge: Expected strain amplitudes at Earth are 10–21 or less

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► and others…

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))) A Full-Size GW Detector

LIGO Hanford Observatory (Washington state, USA)

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))) Advanced LIGO will be a Vast Improvement

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Factor of ~10 better amplitude sensitivity than initial detectors Factor of ~1000 greater volume of space

Best estimate: will detect dozens of

mergers per year*

*”Rates paper” 1003.2480

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Advanced Detector Network, ~2015 and Later

GEO-HF

Advanced VIRGOAdvanced LIGO

Advanced LIGO

4 km4 km

4 km

600 m

3 km 3 km

LIGO-India (proposed)

KAGRA

))) Global GW Observatory

Sky localization by time differences

Da

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ROUGH GUIDE to typical error region areas:2 detectors: ~1000 square degrees (annulus)

3 detectors: tens/hundreds square degrees

4 detectors: ~10 square degrees

Need at least 3 operating detectors to localize signals

Coordinate science runs and downtimes when possible

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))) Impact of Follow-up Observations

Finding an optical/radio/X-ray/neutrino transient will put the GW event candidate in an astronomical context

Much more science!

May be able to confidently detect a somewhat weaker GW event (spectral)

Localize in a host galaxy (or outside!)

Compare GW and electromagnetic emissions: strength, time, etc.

Allow better parameter estimation from the GW data

Need to manage probability of false (unrelated) associationsClassification of transients will be essential

Should have a handle on the normal population of similar transients

In 2009–2010:Program active for 10 weeks of LIGO-Virgo joint observing

Nine event candidates were followed up by at least one telescope

Including two by Swift (XRT & UVOT)

No stand-out candidates, unfortunately

[ Evans et al., arXiv:1205.1124 ]

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))) Supernova vs Binary Inspiral

Binary Coalescence is an impact

Fainter

But bright along jet axis (= short GRB)

CC Supernova is a detonation

BRIGHT

Can be seen to edge of Universe

Metzger and Berger1108.6056

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))) LSST is Essential

The range of existing SGRB optical afterglows … indicates that observations with LSST are essential.-- Metzger and Berger

On

Axi

sO

ff A

xis

observation

upper limit

Post merger accretionVan Eerten/MacFadyan1102.4571

Isotropic kilonovaMetzger and Berger1108.6056

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))) Rapid Alerts for Follow-up Observations

Goal: Catch a counterpart that would have been missed(or detected only later)

Missed GRB, orphan afterglow from off-axis or “failed” GRB, kilonova, …

Localize accurately, compare GW & EM emissions

LIGO Hanford

LIGO Livingston

GEO 600

Virgo

LIGO-India

KAGRA

GW data

Analyze data, identify triggers,infer sky position

Estimate background

Trigger database

Select event candidates

Validate

Transfer data

Send infoto observers

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ift:

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))) Communication with Follow-up Observers

Assemble event candidate informationType of signal, significance, time, sky map, estimated physical params (?)

Format as a VOEvent, for instance

Send alert to observersPlan to use standard channel(s) like GCN/TAN, VOEventNet

May have revised / refined information to distribute later

LSC and Virgo committed to releasing public alerts in the long runEarly on, work with partners through MOUs until a few GWs are detectedPolicy: http://dcc.ligo.org/cgi-bin/DocDB/ShowDocument?docid=89391

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))) Observing Partners During 2009–2010

XRTUVOT

APERTURE

2 m

1 m

1.2 m

1.3 m

1 m

Mostly (but not all) robotic wide-field optical telescopesMany of them used for following up GRBs, surveying for supernovae and other optical transients

LSST, 6.7m

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))) Observing Partners During 2009–2010

FIELD OF VIEW

XRTUVOT

20×20°

7.3 sq deg

9.4 sq deg

25 sq deg

3.4 sq deg 3.4

sq deg

5.7 sq deg

3.4 sq deg

3.4 sq deg

3.4 sq deg

3.4 sq deg

Mostly (but not all) robotic wide-field optical telescopesMany of them used for following up GRBs, surveying for supernovae and other optical transients

LSST

))) Event of 20100916 the “Big Dog”

Top 1000 pixels reported• total area: 129 sq deg• est. containment: ~19%

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Coherent WaveBurst probability sky map:

The “Big Dog”

Phase rings

160

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))) Galaxy Prior

Probably (maybe not*) GW sources stay near their places of birth …

Use positions of known galaxies within 50 Mpc

White et al., CQG 28, 085016

Star formation proxy = blue light luminosity

Galaxies not so useful at 200 Mpc – too many.

False positives are concentrated on the galaxies

* Fong et al 1012.4009

))) Follow-up for Big Dog

nearby galaxies

TAROT,ROTSE

SwiftSkyMapper

Zadko

Zadko

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Images taken within 44 min after event, 2 min after LIGO event, and on subsequent nights

… turned out to be blind injection

Summary

Gravitational wave detectors are operated as a global networkData combined and analyzed coherently

Advanced LIGO and Virgo upgrades are in progressFirst science runs planned for 2015–16

Might be years to full sensitiviy and good localization

KAGRA and LIGO-India to join too

Have begun a program of producing and sending rapid alertsSupports both prompt and delayed follow-up observations

Many lessons learned from the 2009–10 science run

Now preparing an improved future program with easy MOU

Transition to public alerts planned after detection of 4 GW events