Following Up Gravitational Wave Event Candidates

1. Following Up Gravitational Wave Event Candidates Roy Williams (Caltech), Peter Shawhan (U Maryland),for the LIGO Scientific Collaborationand Virgo Collaboration LSST All Hands Meeting 2012 August 14 LIGO-G1200782

2. Sources of Gravitational Waves • ►Compact binary coalescence ►Stellar core collapse • e.g. neutron stars or black holes • ►Neutron stars ►Cosmic strings • Periodic from bump • Non-periodic from flare • ►Early Universe • Like CMB • The challenge: Expected strain amplitudes at Earth are 10–21 or less h(t) Bill Saxton, NRAO/AUI/NSF Casey Reed/PSU ))) ►and others…

3. A Full-Size GW Detector LIGO Hanford Observatory (Washington state, USA)

4. Advanced LIGO will be a Vast Improvement Best estimate: will detect dozens of mergers per year* • Factor of ~10 better amplitude sensitivity than initial detectors  Factor of ~1000 greater volume of space Image courtesy Beverly Berger and atlasoftheuniverse.com *”Rates paper” 1003.2480 4

5. GEO-HF Advanced Detector Network, ~2015 and Later KAGRA Advanced LIGO 600 m 4 km4 km 3 km 3 km 4 km LIGO-India (proposed) Advanced LIGO Advanced VIRGO

6. Global GW Observatory Data courtesy LIGO/LSC http://www.ligo.org/science/GW100916/ Sky localization by time differences • Need at least 3 operating detectors to localize signals • Coordinate science runs and downtimes when possible • ROUGH GUIDE to typical error region areas: • 2 detectors: ~1000 square degrees (annulus) • 3 detectors: tens/hundreds square degrees • 4 detectors: ~10 square degrees

7. 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) associations • Classification of transients will be essential • Should have a handle on the normal population of similar transients Multi-messenger astronomy • 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 ] 7

8. 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

9. LSST is Essential • The range of existing SGRB optical afterglows … indicates that observations with LSST are essential. • -- Metzger and Berger observation upper limit On Axis Post merger accretion Van Eerten/MacFadyan1102.4571 Isotropic kilonova Metzger and Berger1108.6056 Figs: Metzger and Berger Off Axis

10. 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 GEO 600 LIGO Hanford KAGRA Virgo Send infoto observers Transfer data LIGO Livingston Swift: NASA E/PO, Sonoma State U., AuroreSimonnet LIGO-India Trigger database Validate Analyze data, identify triggers,infer sky position GW data Select event candidates Estimate background

11. Communication with Follow-up Observers • Assemble event candidate information • Type of signal, significance, time, sky map, estimated physical params (?) • Format as a VOEvent, for instance • Send alert to observers • Plan 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 run • Early on, work with partners through MOUs until a few GWs are detected • Policy: http://dcc.ligo.org/cgi-bin/DocDB/ShowDocument?docid=89391

12. Observing Partners During 2009–2010 • Mostly (but not all) robotic wide-field optical telescopes • Many of them used for following up GRBs, surveying for supernovae and other optical transients XRT UVOT 1.2 m 2 m LSST, 6.7m 1.3 m 1 m 1 m APERTURE

13. Observing Partners During 2009–2010 25 sqdeg • Mostly (but not all) robotic wide-field optical telescopes • Many of them used for following up GRBs, surveying for supernovae and other optical transients 20×20° 3.4 sqdeg XRT UVOT 3.4 sqdeg 7.3 sqdeg 3.4 sqdeg LSST 9.4 sqdeg 5.7 sqdeg 3.4 sqdeg 3.4 sqdeg 3.4 sqdeg FIELD OF VIEW

14. Event of 20100916 the “Big Dog” • Coherent WaveBurst probability sky map: • Top 1000 pixels reported • total area: 129 sqdeg • est. containment: ~19% The “Big Dog” 160 Phase rings

15. 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

16. Follow-up for Big Dog Images taken within 44 min after event, 2 min after LIGO event, and on subsequent nights … turned out to be blind injection  nearby galaxies Zadko SkyMapper Swift TAROT, ROTSE Zadko

17. Gravitational wave detectors are operated as a global network Data combined and analyzed coherently Advanced LIGO and Virgo upgrades are in progress First 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 alerts Supports 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 Summary