Detecting -ray SourcesBrenda Dingus

dingus@lanl.gov23 January 2006

Outline: I. Detection TechniquesII. Each -ray is an ImageIII. Source Detection

A. Point SourcesB. Extended SourcesC. Variable Sources

-rays Probe Nature’s Particle Accelerators

HST Image of M87 (1994)

Black Hole producing relativistic jet of particles

Binary Neutron Star Coalescing

Artist Conception of Short GRBs

Spinning Neutron Star powering a relativistic wind

Massive Star Collapsing into a Black Hole

SuperComputer Calculation

HESS TeV+ x-ray

TeV image of Vela Jr. Supernova Remnant

Chandra x-ray image

Different Types of Detectors for Gamma-Ray Astrophysics

High SensitivityHESS, MAGIC, CANGAROO, VERITAS

Large Aperture/High Duty CycleMilagro, Tibet, ARGO, HAWC

Low Energy ThresholdEGRET/GLAST

Low Duty Cycle/Small Aperture

Large Effective Area

Excellent Background Rejection

Known Source Spectra

Known Source Lightcurves

Survey of Galactic Plane

Large Duty Cycle/Large Aperture

Space-based (Small Area)

“Background Free”

Sky Survey > 100 MeV

Transient Sources

Extended Sources

Large Duty Cycle/Large Aperture

Moderate Area

Good Background Rejection

Sky Survey >~ 1 TeV

Transient Sources

Extended Sources

High Energy -ray Observatories in Space

Pre Compton Observatory– SAS2– CosB– 10-20 sources

Compton Observatory – 1991-2000– EGRET (spark chamber)– ~300 sources

GLAST– Launch September 2007– >5000 sources

Satellites (30 MeV to 300 GeV -rays) -rays interact via pair production in

dispersed foils Cosmic-ray background (mostly

protons) is rejected by anticoincidence shield AND inverted V-image of electron-positron pair

-ray direction is determined by energy-weighted average of the electron and positron tracks

e+ e– calorimeter (energy measurement)

particle tracking detectors

conversion foil

anticoincidenceshield

Pair-Conversion Telescope

Angular Resolution is dominated by multiple scattering ( 1/Energy) at low energies and by position resolution of tracker at high energies

Energy Resolution is ~10%, but lower energies are always more probable due to source spectra which is typically dN/dE K E-2

Energy Dependent Localization

The number of -rays is small with typically < 100 per source

Use spatially unbinned likelihood analysis (infinitesimally small bins with either 0 or 1 event)

Use Energy Dependent Point Spread Function to calculate the Model in small energy intervals

Require the normalization in each energy interval to fit a power law spectrum with free parameters for the overall normalization and spectral index Diamonds show -rays > 5 GeV.

95 % confidence intervals are Black Circle for Previous Analysis and Blue Area is New Analysis.

Galactic Longitude (deg)

Gal

act

ic L

atitu

de (

deg)

EGRET’s Galactic Center Source

Point Source Survey Source confusion will be

a problem – ~ 1 source/ 4 sq deg

– Point Spread Function at 0.1 (1) GeV is 3.5 (0.4) deg

– Typical (weak) source will have < 100 -rays detected

Most sources vary with time as much as an order of magnitude

Different Spectra also help with harder (higher energy) spectra having better localization

Integral Flux (E>100 MeV) cm-2s-1

Prediction for GLAST Detections Of Active Galactic Nuclei

Time Variable Sources Small # of -rays

limits minimum variability time scale

At least 5 -rays are required to detect a source

Bayesian block statistical technique is needed to distinguish the lightcurve

- GRB940217 (100sec)- PKS 1622-287 flare- 3C279 flare- Vela Pulsar

- Crab Pulsar- 3EG 2020+40 (SNR Cygni?)

- 3EG 1835+59- 3C279 lowest 5 detection- 3EG 1911-2000 (AGN)- Mrk 421- Weakest 5 EGRET source

100 sec

1 orbit

1 day

Galactic Diffuse Emission -rays are produced by interaction of cosmic rays with matter and

photons in the Galaxy Structure (e.g. molecular clouds) is comparable to the size of the -ray

point spread function The uncertainty in the model of diffuse emission is difficult to determine,

but does effect point source detection Use maximum likelihood test with diffuse model + point source vs only

diffuse model to quantify significance of point sources (need Monte Carlo to derive probability from Test Statistic)

-180 -140 -100 -60 -20 20 60 100 140 180

Galactic Diffuse Model& EGRET Data (Hunter et al. 1998)

Water Cherenkov Extensive Air Shower Detectors

8 meters

e

80 meters

50 meters

• Detect Particles in Extensive Air Showers from Cherenkov light created in a covered pond containing filtered water.

• Reconstruct shower direction from the time different photomultiplier tubes are hit.

• 1700 Hz trigger rate (>50 billion events/yr) mostly due to Extensive Air Showers created by cosmic rays

• Field of view is ~2 sr and the average duty factor is nearly 100%Milagro Cross Section Schematic

Angular ReconstructionUse nsec timing from each PMT hit to fit direction of primary particle

Monitor angular reconstruction with the space angle difference between reconstructions of individual events with the Even vs Odd # PMTs (delEO)

delEO is ~ twice the angular resolution due to the error in each subset as well as the improvement when the # of points in the fit is doubled.

Median even-odd = 1.0o implies Gaussian of 0.4o for proton reconstruction Red Monte Carlo Black Data

even-odd in degrees

Event Images in MilagroP

roto

ns

Ga

mm

as

Gamma MC

Data

Proton MC

Cut at C>2.5 to Retain 50% and 9% protons.

71090

50 ..

.Q

QEfficiency

EfficiencyBackground

Signalbackground

gamma

Size of red dots indicate # of photoelectrons detected.

MARS1 (Multivariate Adaptive Regression Splines)

Predicts the values of an outcome variable given a set of independent predictor variables

Calculates probability of vs background for all combinations of parameters MARS Value is ln[P(signal)/P(background)]

– More positive means more -like

1J. Friedman, “Multivariate Adaptive Regression Splines”, Annals of Statistics 19 (1991).

Differential Distribution Integral Distribution

Combining Data with Different Cuts: Weighted Analysis

Hard Cuts: NFIT>=200,C>6.0Std Cuts: NFIT>=20,C>2.5

Excess = 60, Off = 140, S:B = 1:2.3

hadron background =~ 1x10-5

Excess = 5410, Off = 1218288, S:B = 1:225

hadron background =~ 0.1

Weight events byExpected S:B

Milagro’s Crab Signal

Optimal Bin Size for Point Sources:• If Guassian Point Spread

Function, then Radius of Bin is 1.6 x of the Gaussian Point Spread Function

• If Square Bin, then chose dimensions to give same area as square bin

• Milagro Opt Square Bin Size = 2.1o

Point Source Search - Weighted Analysis

Cygnus Region Mrk421 Crab

Vicinity of the Crab

=1.03

Milagro Background Estimation

Variable Source Search

• Search in spatial and time domain• Examine >50 time intervals from < 1 msec to 2 hrs to days, weeks, months• Shortest time intervals (< 1 sec) use starting times of the single events • Longer time intervals are oversampled by factor of two• Monte Carlo is used to access trials penalty of oversampling

For this analysis, searching and oversampling worsens sensitivity by ~ factor of 2, because ~10 result is required to give a 5 chance probability

Extended Source Search

Vary Bin Size from 2.1O to 5.9O (Optimal for ~5O source)

As bin size increases to > 6O background estimation suffers

Cygnus Region Significance: 9.1 Post-trials probability: >7

Cygnus RegionCrab

Milagro FOV

The Northern Sky above 100 MeV (EGRET)CrabCygnus Region

EGRET Data

=1.082

A Closer Look at the Galactic Plane GP diffuse excess

clearly visible from l=25O to l=90O.

Cygnus Region shows extended excess of diameter ~5O-10O.

FCygnus ~= 2x FCrab

Color Map does not show error bars

Map is oversampled which smooths the data

Make Slices in Latitude for different Longitude cutsConsider Region l = 20O-100O

-2<b<2 gives 7.5

Exclude the Cygnus Region: l=20O-75O

-2<b<2 gives 5.8

Galactic longitude 20-75 excludes Cygnus region Galactic longitude 20-100 includes Cygnus region

=1.42 +/- .26

Galactic Latitudinal Distribution

Atomic Hydrogen radio contoursEGRET Diffuse Model

Convolve Cygnus region excess with Milagro PSF(0.75O). Region shows resolvable structure.

Cygnus Region Morphology

HEGRA detected TeV Source: TEV J2032_4130.

PSF

EGRET Unidentified Sources in the Cygnus Region

> 100 MeV/cm2s 1 3EG J2016+3657 (34.7 ± 5.7) x 10-8 2.092 3EG J2020+4017 (123. ± 6.7) x 10-8 2.08 3 3EG J2021+3716 (59.1 ± 6.2) x 10-8 1.864 3EG J2022+4317 (24.7 ± 5.2) x 10-8 2.315 3EG J2027+3429 (25.9 ± 4.7) x 10-8 2.286 3EG J2033+4118 (73.0 ± 6.7) x 10-8 1.967 3EG J2035+4441 (29.2 ± 5.5) x 10-8 2.08 1

2

3

4

5

6

7

3rd EGRET Catalog sources shown with 95% position error circle.

Flux of maximum point: 500mCrab(May be extended)

psf

EGRET Data >1 GeV

1

2

3

4

5

6

7

Weight EGRET >1 GeV -rays by EGRET’s energy dependent psf

Slice of EGRET Data

Cut on the Dec. band around Milagro’s bright spot

2 point sources or 1 extended source?

EGRET catalog sources were fit as point sources ONLY

How close together can GLAST resolve 2 sources of this signal strength?

1st point source

Galactic Diffuse

2nd point source

Max error bar

Summary of the Statistical Issues

Event Reconstruction requires advanced pattern recognition and analysis techniques

Background estimation and uncertainty effects detection significance

Signal events should be weighted by probability of being signal and angular resolution

Effective area of the detector is continuously changing and may vary over the size of the point spread function

Chance probabilities are effected by oversampling and must be simulated by Monte Carlo