Searching for Electromagnetic Counterparts - PowerPoint Presentation

Slide1

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: G1100079Slide2

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.

1Slide3

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 RunThe EM-follow-up program in S6-VSR2/3 is a milestone towards advanced detectors era where the chances of GW detections are very enhancedPresentation 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

.

2Slide4

GW Online Analysis

H1

L1

V1

Omega &

cWB

MBTA

for

Unmodeled Bursts for signals from Compact Binary CoalescenceGRACE DBARCHIVE LUMIN GEMfor Optical Telescopes for SwiftEvent 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 3Slide5

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 SwiftGW 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 telescopesLIGO/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)4Slide6

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 cluster5Slide7

O

bserved

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 eventTo discriminate the possible EM counterpart from contaminating transients The expected EM counterpart afterglows guide observation schedule timeMetzger et al.(2010), MNRAS, 406..265Kann et al. 2010, ApJ, 720.1513Kann et arXiv:0804.1959KILONOVASRadioactively 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 curve6R magnitude assuming z=1R magnitude assuming z=1Time (days after burst in the observer frame)Time (days after burst in the observer frame)Time (Days)Luminosity (ergs s-1)

Metzger et al.(2010), MNRAS, 406..265Slide8

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 ROTSE1.85 ° X 1.85° FOV QUEST

9.4 square degree FOV

SkyMapper5.6 square degree FOV

Pi of the Sky20° X 20° FOVPalomar Transient Factory7.8 square degree FOV

Liverpool telescope4.6 X 4.6 arcmin FOVOptical TelescopesSwift Satellite 0.4° X 0.4° FOV X-ray and UV/Optical TelescopeRadio InterferometerLOFAR10 – 250 MHz

Winter

/

Summer

Run

Only

Summer

Run7Slide9

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 ImagesLimited 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

8Slide10

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

transientsSlide11

Catalog-based

Detection Pipeline

for

images

taken by

TAROT and

Zadko telescopesTAROT South/North 0.25 meter telescope FOV 1.86° X 1.86° Single Field Observation of 180 s exposure Red limiting magnitude of 17.5Zadko 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

10Slide12

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 imageMatch 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 Zadkoavoid problemsat image edgesMagnitude consistencyto recover possible transients that overlap with known galaxies/starsRecover from the list of “known objects”: |USNO_mag – TAROT_mag| > 4σ

Octave

Code

11Slide13

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 uncertaintiesreject 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” analysisreject “contaminatingobjects” (galaxy, variable stars, false transients..)Possible

Optical

counterparts

…..

Catalog-based

Detection Pipeline

12Slide14

“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 3Optical 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 effectsRed magnitudeADU countsnot saturatedsaturatedDistance in MpcInitial Red magnitudeSlope Index13Slide15

Image

characterization

-

limiting

magnitude

used a set of

10 test TAROT images

(180 sec exposure) limiting magnitude of 15.5 for all imagesImage 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 magnitudeCounts(0.5 mag bin)/sq degreeCounts(<mag)/sq degree+xx

+

Limiting

magnitude

Limiting

magnitude

USNOA

counts

USNOA

countsTAROT image countsTAROT image counts14Slide16

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

Transients

15

Efficiency

Efficiency

Distance

(

Mpc

)

Distance

(

Mpc

)

Distance

(

Mpc

)

Efficiency

kilonova

SGRB

SGRB0

SGRB8

LGRB

LGRB0

LGRB8Slide17

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