Searching for Electromagnetic Counterparts of Gravitational-Wave Transients

1. Searching for Electromagnetic Counterparts of Gravitational-Wave Transients MaricaBranchesi(UniversitàdiUrbino/INFN) on behalf of LIGO Scientific Collaboration andVirgoCollaboration & Alain Klotz(TAROT telescope) MyrtilleLaas-Bourez(Zadkotelescope) DCC: G1100079

2. A goal of LIGO and Virgo interferometers is the first direct detection of gravitational waves from ENERGETIC ASTROPHYSICALevents: • Mergersof NeutronStarsand/or BlackHolesSHORT GRB • Kilonovas • Core Collapse of Massive Stars Supernovae • LONG GRB • Cosmic String CuspsEM burst • Mainmotivationsfor joint GW/EM observations: • Increase the GW detection confidence; • Get a precise (arcsecond) localization, identify host galaxy; • Provide insight into the progenitor physics; • In the long term start a joint GW/EM cosmology. 1

3. Low-latency GW data analysis pipelines allow the useof GW triggers in real time to obtain prompt EM observations and to search for EM counterparts • The first program of EM follow-up to GW candidates has been performed during two LIGO/Virgo observing periods: • Dec 17 2009 to Jan 8 2010 – Winter Run • Sep 4 to Oct 20 2010 – Summer Run • The EM-follow-up program in S6-VSR2/3 is a milestonetowards • advanced detectors era where the chances of GW detections are very enhanced • PresentationHighlights: • Methods followed to obtain low-latency Target of Opportunity EM follow-up • observations; • Development of image analysis procedures able to identify the • EM counterparts. 2

4. GW Online Analysis • LIGO (H1 and L1) and • Virgo (V1) interferometers H1 L1 V1 Omega & cWB MBTA forUnmodeledBurstsforsignalsfrom Compact BinaryCoalescence • Searchalgorithmsto • identifytriggers GRACE DB ARCHIVE • SelectStatisticallySignificant • Triggers • DeterminePointingLocations LUMIN GEM forOpticalTelescopesforSwift 10 min. EventValidation Sendalerttotelescope 30 min. 3

5. Requirements to select a trigger as a candidate for the EM follow-up: • Triple coincidence among the three detectors; • Power above a threshold estimated from the distribution of background events: • Target False Alarm Rate • Winter Run < 1.00 event per Day • Summer Run < 0.25 event per Day for most of optical facilities • < 0.10 event per Day for PTF and Swift GW Source Sky Localization:signals near threshold localized to regions of tens of square degrees possibly in severaldisconnectedpatches Necessity of wide field of view telescopes LIGO/Virgohorizon: a stellarmass BH/ NS binary inspiral detected out to 50 Mpc distance that includes thousands of galaxies GW observable sources are likelytobeextragalactic Limit regions to observe to Globular Clusters and Galaxies within 50 Mpc (GWGC catalog White et al. 2011) 4

6. Nearby galaxies and globular clusters (< 50 Mpc) are weighted to select the most probablehost of a GW trigger: Mass * Likelihood P = Distance • Likelihood based on GW data • Mass and Distance of the galaxy • or the globular cluster Probability Skymap for a simulated GW event • Black crosses nearby galaxies locations • Rectangles pointing telescope fields • chosen to maximize • chance to detect the • EM counterpart 5

7. The expected EM counterpart afterglows guide observation schedule time SHORT/HARD GRB Compact Objectmergers LONG/SOFT GRB Massive star Progenitors KILONOVAS RadioactivelyPoweredEM-transient R magnitudeassuming z=1 R magnitudeassuming z=1 Luminosity (ergs s-1) Metzger et al.(2010), MNRAS, 406..265 Time(Days) Time(daysafterburst in the observer frame) Time(daysafterburst in the observer frame) • Observedon-axisLONG and SHORT GRBafterglows peakfew minutesafter the EM/GW prompt emission • Kilonova model afterglow peaks about a day afterthe GW event • To discriminate the possible EM counterpart from contaminating transients Metzger et al.(2010), MNRAS, 406..265 Kannet al. 2010, ApJ, 720.1513 KannetarXiv:0804.1959  EM observations as soonas possible after the GW trigger validation EM observations a day after the GW trigger validation  repeated observations over several nights to study the light curve 6

8. Ground-based and space EM facilities observing the sky at Optical, X-ray and Radio wavelengths involved in the follow-up program Winter/SummerRun OnlySummerRun Optical Telescopes X-ray and UV/Optical Telescope Swift Satellite 0.4° X 0.4° FOV SkyMapper 5.6 square degree FOV Pi of the Sky 20° X 20° FOV Palomar Transient Factory 7.8 square degree FOV Liverpool telescope 4.6 X 4.6 arcmin FOV TAROT SOUTH/NORTH 1.86° X 1.86° FOV Zadko 25 X 25 arcmin FOV ROTSE 1.85 ° X 1.85° FOV QUEST 9.4 square degree FOV Radio Interferometer LOFAR 10 – 250 MHz 7

9. Observations Performed with Optical Telescopes: Winter run8 candidate GW triggers,4observed by telescopes Summer run 6 candidate GW triggers, 4observed by telescopes Analysis Procedure for Wide Field Optical Images LimitedSky localizationof GW interferometers Wide fieldofviewopticalimages Requirestodevelopspecificmethodstodetect the OpticalTransientCounterpartof the GW trigger 8

10. LIGO/Virgocollaborationsare actuallytesting and developingseveralImageAnalysisTechniquesbased on: • ImageSubtractionMethods(forPalomarTransientFactory, • ROTSE and SkyMapper) • CatalogCross-CheckMethods(for TAROT, Zadko, QUEST • and Pi of the Sky) The restof the talk focuses on a Catalog-based Detection Pipeline underdevelopmentfor TAROT and Zadko: methodology and preliminaryresultsobtainedusingimageswithsimulatedtransients 9

11. Catalog-based Detection Pipeline forimagestakenbyTAROTandZadkotelescopes • TAROT South/North • 0.25metertelescope • FOV 1.86° X 1.86° • Single FieldObservation • of180 sexposure • Red limitingmagnitude • of17.5 • Zadko • 1 metertelescope • FOV 25 X 25 arcmin • FiveFieldsObservation • of120 sexposure • Red limitingmagnitude • of20.5 Afterglow Light Curves (source distance d=50 Mpc) “Afterglowlight curves” forLONG/SHORT and Kilonovatransientsat a distanceof 50 Mpc LONG GRB • SHORT GRB • KILONOVA Apparent Red magnitude TAROT limitingmagnitude Zadkolimitingmagnitude Time (days) 10

12. Catalog-based Detection Pipeline toidentify the OpticalCounterparts in TAROT and Zadkoimages Octave Code detectsources in eachimage SExtractortobuildcatalogofall the objectsvisible in eachimage selectcentral part of the image avoidproblems at imageedges FOV restrictedtoregionwithradius = 0.8 degfor TAROTand 0.19 degforZadko Match Algorithm(Valdeset al 1995;Droege et al 2006) to identify “known stars” in USNO2A (catalog of 5 billion stars down to R ≈ 19 mag) select “unknownobjects” (not in USNO2A) Magnitudeconsistency torecoverpossibletransients thatoverlapwithknowngalaxies/stars Recoverfrom the listof “knownobjects”: |USNO_mag – TAROT_mag| > 4σ 11

13. …..Catalog-based Detection Pipeline Spatialcross-positionalcheck withmatch-radiusof10 arcsecfor TAROT and 2 arcsecforZadko chosen on the basisof position uncertainties searchforobjects in common toseveralimages rejectcosmicrays, noise, asteroids... reject background events “On-source region” = regions occupied by Globular Clusters and Galaxies up to 50 Mpc (GWGCcatalog,White et al 2011) selectobjects in “on-source” reject “contaminating objects” (galaxy, variablestars, false transients..) “Light curve” analysis PossibleOpticalcounterparts 12

14. “Light curve” analysis- cut based on the expectedluminositydimming of the EM counterparts recall magnitudeα[-2.5 log10 (Luminosity)] expect Luminosityα[time-β]magnitudeα[2.5 βlog10(time)] slope index = measurement of (2.5 β) to discriminate expected light curve from “contaminatingevents” The expected slope index for SHORT/LONG GRB is around 2.7 and kilonova is around 3 Saturationeffects Distance in Mpc SlopeIndex ADU counts saturated notsaturated Initial Red magnitude Red magnitude Colouredpoints= OpticalLGRBTransients Blacksquares = contaminatingobjects Opticalcounterparts the oneswithslopeindex > 0.5 Contaminatingobjectsthatcould pass the cut are onlyvariableAGN or Cepheidstars 13

15. Imagecharacterization - limitingmagnitude used a set of 10 test TAROT images (180 sec exposure) limiting magnitude of 15.5 for all images Differential Source Counts Integral Source Counts x TAROT imagecounts x TAROT imagecounts + USNOA counts + USNOA counts Limitingmagnitude Limitingmagnitude Counts(<mag)/sqdegree Counts(0.5 magbin)/sqdegree R magnitude R magnitude Image Limiting Magnitude: point where Differential/Integral Source Counts distribution (vs magnitude) bends and moves away from the power law of the reference USNOA 14

16. Preliminarytests on sensitivity :obtainedbyrunning the Detection Pipeline overimageswithinjectionsofFakeTransients Monte Carlo simulations at the Computing Center in Lyon: Injectionsoffaketransientsmodelledusingon-axis GRB and Kilonovaafterglow light curves Timeorigin (timeof GW trigger) set 1 daybefore the first image SHORT/HARD GRB LONG/SOFT GRB KilonovaObjects kilonova SGRB SGRB0 SGRB8 LGRB LGRB0 LGRB8 Efficiency Efficiency Efficiency Distance (Mpc) Distance (Mpc) Distance (Mpc) SGRB, LGRBintrinsicluminosityrangeparametrizedby a magnitude offset 0 ÷ 8 SGRB, LGRB: random offset 0 ÷ 8SGRB0, LGRB0: offset set to 0 SGRB8,LGRB8: offset set to 8 Preliminaryresults on Distance_50% horizon (Mpc): SGRB SGRB0 SGRB8 LGRB LGRB0 LGRB8 Kilonova 15 100------ 400 250080 10 Simulationsusingdifferentsetsofimages and differenttimeschedulewrt the GW trigger givesimilarresults 15

17. Conclusions: The first EM follow-upsto GW candidateshavebeenperformedby the LIGO/Virgo community in associationwith several Partner EM Observatories “EM counterpart Detection pipelines” based on differentanalysistechniquesare under test and development Preliminaryresultssuggestthat “Catalog-basedDetection Pipeline” isabletodetecton-axis GRB and Kilonovaafterglows for TAROT and Zadkoimages • Efforts are in progress toimprove the efficiency and toextend the usetoothertelescopeimages • The analysisof the imagesobservedduring the • Winter/Summer LIGO/Virgorunison-going