Optical Image Analysis to detect EM-Counterparts of GW-Transients Marica Branchesi (Università di Urbino/INFN) & Eric Chassande-Mottin (APC/CNRS) Virgo Ego Scientific Forum School (Cascina 2-6 May 2011)

Ground Based and Space Telescopes ASTRONOMICAL OBSERVATIONS Electromagnetic Spectrum Ground Based and Space Telescopes

Panoramic of ASTRONOMICAL IMAGES in different wavelength bands Optical Images X-ray and Radio Images Silla Observatory Keck Observatory VLA Chandra-Observatory HST

Electromagnetic Radiation from Astrophysical Objects LUMINOSITY is the amount of energy an object radiates in unit time: Units: erg/s (cgs) or watts (SI) Intrinsic quantity for a given objects Not dependent on the observer’s distance or viewing angle SPECTRUM: distribution of luminosity as a function of wavelength or frequency The shape of the spectrum depends on the radiation emission process(es) Monochromatic Ll, Ln: relative to a given wavelength or frequency (erg s-1 Å-1 or erg s-1 Hz-1) Bolometric L: integrated over all wavelengths or frequencies (ers s-1 )

What is observed and mesured of EM Radiation from Astrophysical Objects? Types of Astronomical Spectra FLUX is the radiative energy per unit time passing through a unit area Monochromatic fl, fn intensity in (ergs or photons) per unit area, time, l, or n (erg cm-2 s-1 Å-1 or erg s-1 Hz-1) Total Flux (units erg cm-2 s-1 )

In the “optical band” there are many bandpasses (photometric systems) Real Detectors are sensitive over a finite range of l (or n) In the “optical band” there are many bandpasses (photometric systems) Some example of optical bandpass curves (Fukugita et al. 1995)

FLUX - Inverse Square Law If d is the distance from the center of the source to the observer: The 1/d2 fall-off of flux with distance from the source

Optical Wavelengths: Magnitude For historical reason fluxes in the optical band are measured in magnitudes The scale was defined by Pogson in 1856 using a logarithmic scale as the “logarithmic response” of the human eye: Apparent Magnitude an object 2.5 magnitudes brighter than another  has a 10 times larger flux smaller magnitudes correspond to brighter objects Absolute Magnitude: magnitude that an object would have at a distance of 10 parsec d = source distance in parsec measure of intrinsic luminosity Distance Modulus d = source distance in parsec

Astronomical Image Processing 1 Step) IMAGE CALIBRATION: different steps to follow and software to use on the basis of observed wavelength band 2 Step) IMAGE ANALYSIS: different analysis to perform and software to use on the basis of the project goal IMAGE CALIBRATION: process by which astronomers convert electronic signal from the telescope into meaningful astronomical data Raw Data Calibrated Image calibration

Optical Image Calibration Process 1) Dark Frame Subtraction to compensate for  Thermal effects that add (taken preventing ligth entering the camera) unwanted intensity  Readout Noise, electronics produce noise when reading and transmitting data 2) Flat Field Correction to overcome  image variations due to a (taken observing a source with uniform illumination) not uniform response/ sensitivity of the detector 3) Bad Pixel Masks to exclude  “hot and cold pixels”, pixels that saturate prematurely or do not produce signal 4) Fringe correction  to remove the fringe pattern due to atmospheric OH emission (in i- and z-band)

Are the images ready to be analyzed? In order to make astrometry (evaluation of object position) and photometry (measure of the flux) two other steps are necessary: Astrometric Calibration Photometric Calibration Astrometric Calibration to find mathematical transformation relating the positions of pixels in the image CCD to celestial coordinates on the sky by using Reference Stars with well known celestial coordinate in the FOV Star Catalogs suitable for astrometric use: GSC, HST Guide Star Catalog USNO, US Naval Observatory Astrometric Catalog

Celestial Coordinates - Right Ascension and Declination Celestial Sphere Ecliptic Celestial Equator Celestial Equatorial Coordinate System Dec  measured from the celestial equator from 0° to +90° towards North Pole and from 0° to -90° towards South Pole. RA  measured east from the Vernal Equinox Point in hours, 0h to 24h or in degree 0° to 360°

Photometric Calibration – Standard Stars General Formula for a object’s magnitude in a filter i: where DN = net source counts, Exp = exposure time ZPi (1 sec) = Zero-Point to determine or given as ZPi (exp) = ZPi (1 sec) + 2.5log10(exp)  mi = -2.5log10(DN) + ZPi (exp) For a standard star mi is known and tabulated for different filters. ZPi is determined by evaluating DN for standard stars visible in the image FOV or for standard stars observed during the same night Standard Star Catalogs suitable for photometric use: BSC, Bright Star Catalog Landolt (1992, AJ, 104, 336) Tycho Catalog

Zero-point is dependent on airmass Due to Earth’s Atmospheric Extinction (absorption and scattering of light ) a source appears bigther observed at the zenith and fainter close to the horizon Airmass = 1/cos(z) , z=zenith distance (ratio between the thickness of the Earth’s atmosphere at the observing altitude and at the zenith) Select STANDARD STARS whose airmass is the same as the target airmass

“not calibrated magnitude” xxxxxxxxxxxxxxxxxxxxxx ) Photometric Calibration – “Clear Filter” Wide Field Telescope There are telescopes , likeTAROT, that observe with a “Clear filter” The conversion of “Clear filter observed flux (DN)” into “standard reference magnitude system” can be done estimating the Zero-Point (ZP) by using a linear least squares fit between: “not calibrated magnitude” xxxxxxxxxxxxxxxxxxxxxx ) and “reference-catalog magnitude” for common stars in the FOV

musno = mimage+ ZP Example for TAROT images Reference Star Catalog USNO-A2.0 that lists magnitudes in a standard red filter: POSSI-R1 POSS-I 103aE 1.0 0.8 0.6 0.4 0.2 The POSSI red magnitude is chosen as reference Trasmission(%) 6000 6400 6800 Wavelength (Angstroms) -2.5log10 (DN(ADU counts))+ZP Linear Least Squares Fit musno = mimage+ ZP using USNO stars brigther than mag=16 R1 magnitude USNOA-2

IMAGE ANALYSIS: DS9-Saoimage File Name Pixel Count in ADU DS9 - Astronomical imaging and data visualization application Celestial Coordinates (RA,Dec) Image Pixel Coordinates (X,Y) Astronomical Image and Table Format FITS - Flexible Image Transport System

Simple Aperture Photometry m = - 2.5 log (Source_counts) + ZP Source_counts =Total_counts – Bkg_counts Sum counts in all pixels aperture Sky background counts in annulus or separate region Star Brightness Profile Define the correct size of aperture Size  comprimise between including all the light from the star and excluding excessive amount of noisy background FWHM good choice  1.5 or 2.0 X FWHM

Larger aperture telescope Image Resolution In the absence of aberrations and atmospheric turbulence, the Point-Spread Function (the response to a point-source) is the Airy pattern Diffraction-Limited PSF The IMAGE RESOLUTION: minimum angular separation at which two equally bright stars would just be distinguished Larger aperture telescope Higher Resolution D =aperture diameter l = wavelength of light First Diffraction Ring Airy Disk

The resolution of ground-based telescope is limited by the atmosphere Lens or mirror aberrations and atmospheric turbulence cause the width of the PSF to broaden and its shape to become distorted Central maxima of PSF expanded by atmospheric turbulence is called “seeing” The “seeing” is estimated by measuring the FWHM (full width at half maximum) of the star brightness radial profile The FWHM is estimated by fitting with a Gaussian model the brightness profile of a sample of not saturated stars (e.g. IRAF) The observation “seeing” gives the measure of IMAGE RESOLUTION

Signal To Noise Ratio For a counting process (e.g photons) the error is the “Poisson noise”: Since the source is seen over a background: where Bsky are the sky background counts and Bother are readout and dark noise in the same area occupied by the source.

SExtractor for large field photometry SExtractor is an astronomical software to extract and build catalogs of objects from optical images Steps: 1) Estimate Background and its RMS noise 2) Detect objects (thresholding) 3) Deblend merged objects 4) Measure shapes and position 5) Perform Photometry 6) Classify objects: star-like/galaxy 7) Output catalog star galaxy Detection  SExtractor consider a “minimum number” of adjacent pixel above a “certain threshold” an object detection Deblending  use a multiple pass thresholding to separate neighbour objects detected as single source Photometry  isophotal, isophotal-corrected, automatic, best-estimate and fixed circular aperture approaches

ISOPHOTAL  the user defines the threshold above which SExtractor does photometry: pixels above this threshold constitute an isophotal area ISOPHOT-CORRECTED  objects rarely have all their flux within neat boundaries, some of the flux is in the “wings” of the profile. Sextractor do a correction for that, assuming a symmetric Gaussian profile for the object AUTOMATIC  SExtractor uses an adaptive elliptical aperture around every detected object by analyzing the objet’s light ditribution and using the Kron (1980) approach: the ellipical sizes are defined in order to capture most (> 90%) of the objetc flux BEST  is usually equal to AUTO photometry, but if the contribution of other nearby sources exceeds 10%, it is ISOPHOT-CORRECTED FIXED CIRCULAR APERTURES  the user specified fixed circular aperture where the flux is estimated

Image Sensitivity – Limiting Magnitude Example for TAROT images For large FOV images the survey sensitivity can be estimated by comparing “image Source Counts” with a “Reference-Catalog Source Counts” in the same region of the sky Example for TAROT images Differential Source Counts Integral Source Counts x TAROT image counts x TAROT image counts + USNOA counts + USNOA counts Limiting magnitude Limiting magnitude Counts(0.5 mag bin)/sq degree Counts(<mag)/sq degree R magnitude R magnitude Limiting Magnitude: point where Differential/Integral Source Counts distribution (vs magnitude) bends and moves away from the power law of the reference USNOA

Analysis Procedure for Optical Images taken with Wide Field Telescopes Searching for Electromagnetic Counterparts of Gravitational-Wave Transients Analysis Procedure for Optical Images taken with Wide Field Telescopes

(GWGC catalog White et al. 2011) 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 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)

Compact Object mergers Massive star Progenitor The expected EM counterpart are transient objects whose brightness changes with time: Optical Afterglows SHORT/HARD GRB Compact Object mergers LONG/SOFT GRB Massive star Progenitor KILONOVAS Radioactively Powered Object R magnitude assuming z=1 R magnitude assuming z=1 Luminosity (ergs s-1) Metzger et al.(2010), MNRAS, 406..265 Kann et arXiv:0804.1959 Time (Days) Time (days after burst in the observer frame) Time (days after burst in the observer frame) Metzger et al.(2010), MNRAS, 406..265 Kann et al. 2010, ApJ, 720.1513 The study of transient objects requires the analysis of images taken over several nights to sample flux variation as a function of time - light curve study

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 Main steps for a EM-counterpart Detection Pipeline: Find all “Transient Objects” visible in the images Select the EM-counterpart from the “Contaminating Transients”

Catalog-based Detection Pipeline Octave Code detect sources in each image SExtractor to build catalog of all the objects visible in each image select central part of the image FOV restricted to region with radius = 0.8 deg for TAROT avoid problems at image edges select “unknown objects” (not in USNO2A) 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) Magnitude consistency to recover possible transients that overlap with known galaxies/stars Recover from the list of “known objects”: |USNO_mag – TAROT_mag| > 4σ 11

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

to discriminate expected light curve from “contaminating events” “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 Slope Index Distance in Mpc Contaminating objects that could pass the cut are only variable AGN or Cepheid stars Initial Red magnitude Coloured points = Optical LGRB Transients Black squares = contaminating objects

Thank you for your attention!! Ready to start the practical section.... You are provided with a set of 10 images taken by TAROT telescope observing the same region of the sky during three consecutive nights after a fictitious GW trigger Some optical transients have been injected in the images by using LONG and SHORT GRB and kilonova models The transients were injected in nearby galaxies (within the LIGO/Virgo horizon of 50 Mpc) with an offset from the galaxy center in the range of the observed ones for GRBs (within 100 Kpc)