Searching for Electromagnetic Counterparts of Gravitational-Wave Transients

Marica Branchesi (Università di Urbino/INFN)on behalf of

LIGO Scientific Collaboration and Virgo Collaboration&

Alain Klotz (TAROT telescope) Myrtille Laas-Bourez (Zadko telescope)

DCC: G1100079

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

Core Collapse of Massive Stars Supernovae LONG GRB

Cosmic String Cusps EM burst

Main motivations for 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

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

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 RunSep 4 to Oct 20 2010 – 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;

Development of image analysis procedures able to identify the EM counterparts.

2

GW Online AnalysisH1 L1 V1

Omega & cWB MBTA for Unmodeled Bursts for signals from Compact Binary Coalescence

GRACE DBARCHIVE

LUMIN GEMfor Optical Telescopes for Swift

Event Validation

• Search algorithms to identify triggers

Send alert to telescope

• Select Statistically Significant Triggers• Determine Pointing Locations10 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 * LikelihoodP = 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, 406..265

Kann et al. 2010, ApJ, 720.1513Kann et arXiv:0804.1959

KILONOVASRadioactively Powered EM-transient

LONG/SOFT GRBMassive 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 m

agni

tude

ass

umin

g z=

1

R m

agni

tude

ass

umin

g z=

1

Time (days after burst in the observer frame)Time (days after burst in the observer frame)Time (Days)

Lum

inos

ity (e

rgs

s-1)

Metzger et al.(2010), MNRAS, 406..265

Ground-based and space EM facilities observing the sky at Optical, X-ray and Radio wavelengths involved in the follow-up program

TAROT SOUTH/NORTH1.86° X 1.86° FOV Zadko 25 X 25 arcmin FOV ROTSE1.85 ° X 1.85° FOV QUEST 9.4 square degree FOV

SkyMapper5.6 square degree FOVPi of the Sky20° X 20° FOVPalomar Transient Factory7.8 square degree FOVLiverpool telescope4.6 X 4.6 arcmin FOV

Optical Telescopes

Swift Satellite 0.4° X 0.4° FOV

X-ray and UV/Optical Telescope

Radio Interferometer

LOFAR10 – 250 MHz

Winter/Summer Run

Only Summer Run

7

Observations Performed with Optical Telescopes:

Winter run 8 candidate GW triggers, 4 observed by telescopes

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, ROTSE and SkyMapper)

• Catalog Cross-Check Methods (for TAROT, Zadko, QUEST 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)

App

aren

t Red

mag

nitu

de

LONG GRB

SHORT GRB

KILONOVA

TAROT limiting magnitudeZadko 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 centralpart of

the imageFOV restricted to region with radius = 0.8 deg for TAROT and 0.19 deg for Zadko

avoid problemsat 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 Zadkochosen 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 backgroundevents

“Light curve” analysis

reject “contaminatingobjects” (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 log10 (Luminosity)]

expect Luminosity α [time- β] magnitude α [2.5 β log10(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 TransientsBlack squares = contaminating objects

Contaminating objects that could pass the cut are only variable AGN or Cepheid stars

Saturation effects

Red magnitudeA

DU

cou

nts

not saturatedsaturatedDis

tanc

e in

Mpc

Initial Red magnitude

Slop

e In

dex

13

Image characterization - limiting magnitude

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 Counts Integral Source Counts

R magnitudeR magnitude

Cou

nts(

0.5

mag

bin

)/sq

degr

ee

Cou

nts(

<mag

)/sq

degr

ee

+x x

+Limiting magnitudeLimiting magnitudeUSNOA countsUSNOA counts

TAROT image 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 imageSHORT/HARD GRB Kilonova Objects LONG/SOFT GRB

SGRB, LGRB intrinsic luminosity range parametrized by a magnitude offset 0 ÷ 8SGRB, LGRB: random offset 0 ÷ 8SGRB0, 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 15 100 ------ 400 2500 80 10

Simulations using different sets of images and different time schedule wrt the GW trigger give similar results

Preliminary tests on sensitivity : obtained by running the Detection Pipeline over images with injections of Fake Transients

15

Effic

ienc

y

Effic

ienc

y

Distance (Mpc)Distance (Mpc)Distance (Mpc)

Effic

ienc

y

kilonovaSGRBSGRB0SGRB8

LGRBLGRB0LGRB8

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