Detecting -ray SourcesBrenda Dingus
[email protected] 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