Searching for Electromagnetic Counterparts of Gravitational-Wave Transients Marica Branchesi (Università di Urbino/INFN) on behalf of LIGO Scientific Collaboration and Virgo Collaboration LIGO Scientific Collaboration and Virgo Collaboration& Alain Klotz (TAROT telescope) Myrtille Laas-Bourez (Zadko telescope) Myrtille Laas-Bourez (Zadko telescope) DCC: G

A goal of LIGO and Virgo interferometers is the first direct detection of gravitational waves from ENERGETIC ASTROPHYSICAL events:  Mergers of NeutronStars and/or BlackHoles SHORT GRB Kilonovas Kilonovas  Core Collapse of Massive Stars Supernovae LONG GRB LONG GRB  Cosmic String Cusps EM burst Main motivations for joint GW/EM observations: Increase the GW detection confidence;  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

Low-latency GW data analysis pipelines allow the use of GW triggers in real time to obtain prompt EM observations and to search for EM counterparts 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 to Jan – Winter Run Sep 4 to Oct – Summer Run The EM-follow-up program in S6-VSR2/3 is a milestone towards advanced detectors era where the chances of GW detections are very enhanced Presentation Highlights:  Methods followed to obtain low-latency Target of Opportunity EM follow-up observations; observations;  Development of image analysis procedures able to identify the EM counterparts. EM counterparts. 2

GW Online Analysis H1L1V1 Omega & cWB MBTA for Unmodeled Bursts for signals from Compact Binary Coalescence GRACE DB ARCHIVE LUMIN GEM for Optical Telescopes for Swift Event Validation Search algorithms to identify triggers Send alert to telescope Select Statistically Significant Triggers Determine Pointing Locations 10 min. 30 min. LIGO (H1 and L1) and Virgo (V1) interferometers 3

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 several disconnected patches Necessity of wide field of view telescopes LIGO/Virgo horizon : a stellarmass BH/ NS binary inspiral detected out to 50 Mpc distance that includes thousands of galaxies GW observable sources are likely to be extragalactic Limit regions to observe to Globular Clusters and Galaxies within 50 Mpc (GWGC catalog White et al. 2011) 4

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

Observed on-axis LONG and SHORT GRB afterglows peak few minutes after the EM/GW prompt emission Kilonova model afterglow peaks about a day after the GW event To discriminate the possible EM counterpart from contaminating transients The expected EM counterpart afterglows guide observation schedule time Metzger et al.(2010), MNRAS, Kann et al. 2010, ApJ, Kann et arXiv: KILONOVAS Radioactively Powered EM-transient LONG/SOFT GRB Massive star Progenitors SHORT/HARD GRB Compact Object mergers  EM observations as soon as 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 R magnitude assuming z=1 Time (days after burst in the observer frame ) Time (Days ) Luminosity (ergs s -1 ) Metzger et al.(2010), MNRAS,

Ground-based and space EM facilities observing the sky at Optical, X-ray and Radio wavelengths involved in the follow-up program 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 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 Optical Telescopes Swift Satellite 0.4° X 0.4° FOV X-ray and UV/Optical Telescope Radio Interferometer LOFAR 10 – 250 MHz Winter/Summer Run Only Summer Run 7

Observations Performed with Optical Telescopes: Winter run 8 candidate GW triggers, 4 Winter run 8 candidate GW triggers, 4 observed by telescopes Summer run 6 candidate GW triggers, 4 Summer run 6 candidate GW triggers, 4 observed by telescopes Analysis Procedure for Wide Field Optical Images Limited Sky localization of GW interferometers Wide field of view optical images Requires to develop specific methods to detect the Optical Transient Counterpart of the GW trigger 8

LIGO/Virgo collaborations are actually testing and developing several Image Analysis Techniques based on: Image Subtraction Methods (for Palomar Transient Factory, Image Subtraction Methods (for Palomar Transient Factory, ROTSE and SkyMapper) ROTSE and SkyMapper) Catalog Cross-Check Methods (for TAROT, Zadko, QUEST Catalog Cross-Check Methods (for TAROT, Zadko, QUEST and Pi of the Sky) and Pi of the Sky) 9 The rest of the talk focuses on a Catalog-based Detection Pipeline under development for TAROT and Zadko: methodology and preliminary results obtained using images with simulated transients

Catalog-based Detection Pipeline for images taken by TAROT and Zadko telescopes TAROT South/North 0.25 meter telescope FOV 1.86° X 1.86° Single Field Observation of 180 s exposure Red limiting magnitude of 17.5 Zadko 1 meter telescope FOV 25 X 25 arcmin Five Fields Observation of 120 s exposure Red limiting magnitude of 20.5 Afterglow Light Curves (source distance d=50 Mpc) Time (days) Apparent Red magnitude LONG GRB SHORT GRB KILONOVA TAROT limiting magnitude Zadko limiting magnitude “Afterglow light curves” for LONG/SHORT and Kilonova transients at a distance of 50 Mpc 10

detect sources in each image select “unknown objects” (not in USNO2A) Catalog-based Detection Pipeline to identify the Optical Counterparts in TAROT and Zadko images SExtractor to build catalog of all the objects visible in each image Match Algorithm (Valdes et al 1995; Droege et al 2006) to identify “known stars” in USNO2A (catalog of 5 billion stars down to R ≈ 19 mag) select central part of the image FOV restricted to region with radius = 0.8 deg for TAROT and 0.19 deg for Zadko avoid problems at image edges Magnitude consistency to recover possible transients that overlap with known galaxies/stars Recover from the list of “known objects”: |USNO_mag – TAROT_mag| > 4σ Octave Code 11

search for objects in common to several images Spatial cross-positional check with match-radius of 10 arcsec for TAROT and 2 arcsec for Zadko chosen on the basis of position uncertainties reject cosmic rays, noise, asteroids... select objects in “on-source” “On-source region” = regions occupied by Globular Clusters and Galaxies up to 50 Mpc ( GWGC catalog, White et al 2011 ) reject background events “Light curve” analysis reject “contaminating objects” (galaxy, variable stars, false transients..) Possible Optical counterparts …..Catalog-based Detection Pipeline 12

“Light curve” analysis - cut based on the expected luminosity dimming of the EM counterparts recall magnitude α [ -2.5 log 10 (Luminosity)] expect Luminosity α [ time - β ] magnitude α [ 2.5 β log 10 (time)] slope index = measurement of ( 2.5 β) to discriminate expected light curve from “contaminating events” The expected slope index for SHORT/LONG GRB is around 2.7 and kilonova is around 3 Optical counterparts the ones with slope index > 0.5 Coloured points = Optical LGRB Transients Black squares = contaminating objects Contaminating objects that could pass the cut are only variable AGN or Cepheid stars Saturation effects Red magnitude ADU counts not saturatedsaturated Distance in Mpc Initial Red magnitude Slope Index 13

Image characterization - limiting magnitude used a set of 10 test TAROT images (180 sec used a set of 10 test TAROT images (180 sec exposure) limiting magnitude of 15.5 for all images Image Limiting Magnitude: point where Differential/Integral Source Counts distribution (vs magnitude) bends and moves away from the power law of the reference USNOA Differential Source CountsIntegral Source Counts R magnitude Counts(0.5 mag bin)/sq degree Counts(<mag)/sq degree + x x + Limiting magnitude USNOA counts TAROT image counts 14

Monte Carlo simulations at the Computing Center in Lyon: Injections of fake transients modelled using on-axis GRB and Kilonova afterglow light curves Time origin (time of GW trigger) set 1 day before the first image SHORT/HARD GRBKilonova ObjectsLONG/SOFT GRB SGRB, LGRB intrinsic luminosity range parametrized by a magnitude offset 0 ÷ 8 SGRB, LGRB: random offset 0 ÷ 8 SGRB0, LGRB0: offset set to 0 SGRB8,LGRB8: offset set to 8 Preliminary results on Distance_50% horizon (Mpc): SGRB SGRB0 SGRB8 LGRB LGRB0 LGRB8 Kilonova Simulations using different sets of images and different time schedule wrt the GW trigger give similar results Preliminary tests on sensitivity : Preliminary tests on sensitivity : obtained by running the Detection Pipeline over images with injections of Fake Transients 15 Efficiency Distance (Mpc) Efficiency kilonova SGRB SGRB0 SGRB8 LGRB LGRB0 LGRB8

Conclusions: The first EM follow-ups to GW candidates have been performed by the LIGO/Virgo community in association with several Partner EM Observatories “EM counterpart Detection pipelines” based on different analysis techniques are under test and development Preliminary results suggest that “Catalog-based Detection Pipeline” is able to detect on-axis GRB and Kilonova afterglows for TAROT and Zadko images Efforts are in progress to improve the efficiency and to extend the use to other telescope images The analysis of the images observed during the Winter/Summer LIGO/Virgo run is on-going Winter/Summer LIGO/Virgo run is on-going