grad student talk 1-F eb-06 1 Studying Astrophysics and Studying Astrophysics and Particle Physics with Particle Physics with Gamma Rays: Gamma Rays: what we may learn with what we may learn with the upcoming GLAST the upcoming GLAST mission mission -and- -and- The UW Contributions to GLAST The UW Contributions to GLAST Toby Burnett University of Washington
42
Embed
Grad student talk 1-Feb-061 Studying Astrophysics and Particle Physics with Gamma Rays: what we may learn with the upcoming GLAST mission -and- The UW.
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
grad student talk 1-Feb-06 1
Studying Astrophysics and Studying Astrophysics and Particle Physics with Gamma Particle Physics with Gamma
Rays: Rays: what we may learn with the what we may learn with the upcoming GLAST missionupcoming GLAST mission
-and--and-The UW Contributions to GLASTThe UW Contributions to GLAST
Toby BurnettUniversity of Washington
grad student talk 1-Feb-06 2
Context: the photon spectrumContext: the photon spectrum
GAP!
GLAST
(Mike Turner 1989)
grad student talk 1-Feb-06 3
““Seeing” the Universe with gamma Seeing” the Universe with gamma raysrays
the plot and the charactersthe plot and the charactersSource
propagation
“Telescope”
• Massive black holes (AGN, blazars)
• GRB (stellar collapse, magnetars)
• Pulsars (neutron stars)
• CR interactions
• WIMP annihilation?
• Primordial black holes?
• absorption by IR
• Dispersion?
• EGRET / BATSE
• GLAST: LAT/GBM
• MILAGRO (EAS)
• Whipple
• HEGRA
• HESS
• VERITAS
Satellite
Cherenkov
Observer
grad student talk 1-Feb-06 4
Objective: detect gamma rays from Objective: detect gamma rays from astronomical sources with astronomical sources with
Largest possible energy range
High acceptance, A A: effective area, including photon cross section
: field of view
: instrumental efficiency, including dead time
Good energy resolution for spectral measurements
Good angular resolution (buzz-word from telescopes: “point spread function”, or PSF)
Good signal/noise
grad student talk 1-Feb-06 5
ConstraintsConstraints
Good acceptance, PSF: must use pair conversion process
Compton: lose direction information, not high energy
OurOur launch vehicle: Boeing Delta IIH launch vehicle: Boeing Delta IIH
Launch: from Cape Canaveral - September 2007
grad student talk 1-Feb-06 13
Calorimeter
e+ e–
ACD
Tracker
Overview of the LATOverview of the LATPrecision Si-strip Tracker 18 XY tracking planes. Single-sided silicon strip detectors (228 m pitch) Measure the photon direction; gamma ID.
Hodoscopic CsI Calorimeter Array of 1536 CsI(Tl) crystals in 8 layers. (8 X0) Measure the photon energy; image the shower.
Segmented Anticoincidence Detector (ACD) 89 plastic scintillator tiles. Reject background of charged cosmic rays; segmentation removes self-veto effects at high energy.
Electronics System Includes flexible, robust hardware trigger and software filters.
1.7 m
grad student talk 1-Feb-06 14
Performance: 1970’s vs 1990’s technologyPerformance: 1970’s vs 1990’s technology
Data handling and analysisData handling and analysis
Not an imaging device – no pixels as suchDoes that make it not a “telescope”? Webster says: Telescope \Tel"e*scope\, n. [Gr. ? viewing afar, farseeing; ? far, far off + ? a watcher, akin to ? to view: cf. F. t['e]lescope. See Telegraph, and -scope.] An optical instrument used in viewing distant objects, as the heavenly bodies.
Instead of collecting photons with ccd pixels, we record “events”, caused by single incoming photons
trigger logic, including possibility of veto of background (EGRET had both “A-dome” and TOF requirement to keep rate well below 10 Hz.)Many channels to calibratePattern recognition Event reconstructionDiscrimination against backgroundCalibration of response to photons
The Framework: combine simulation, The Framework: combine simulation, reconstruction, event display and some reconstruction, event display and some
analysisanalysis
grad student talk 1-Feb-06 20
The GLAST Data Challenge 2The GLAST Data Challenge 2
We are in the midst of preparing a major end-to-end simulation:
Orbit: start 1-1-08 for 56.3 days (a precession period)Best estimates of particle backgroundsUse scanning/rocking mode (most likely for first year, perhaps entire mission)Now running special Monte Carlo runs to characterize instrument
Background: ~ 1 day (all we can do!)Photons: 10 M at all angles and energiesUse the above to define responses
Defining model of gamma ray sky, including all the known sources, some speculation.
Test with special parametric Monte Carlo based on previous analysis.
The “real” run, for later this year, will use full Monte Carlo with gamma sources, with sampling from the 1-day background
grad student talk 1-Feb-06 21
The orbitThe orbit
Trigger rate (~8 kHz) is dominated by charged particles! Only 1-2 Hz are actual gammas from space.
Orbit and pointing mode: create 56.3 days with rocking, sun-avoidance
Generate the ~50 K incoming particles, with random directions, energies, and spread out over a sphere with cross sectional area 6 m2
Send each into the detector: Discard if no trigger (missed or hits did not satisfy a trigger condition) ~8 kHz remain (20% deadtime)
Apply the onboard filter code that checks for obvious charged, non-interacting particles: ~700 Hz remain
Fully analyze these, corresponding to the downlink rate
Run 8640 such jobs, starting every 10 sec, for 10% of a full day. (using the UW physics condor system for up to 64 jobs)
grad student talk 1-Feb-06 25
What is Condor?What is Condor?
Invented, maintained at UW-Madison.
Basis for managing jobs in much of the “grid”, now called Open Science Grid
Now installed on all physics dept lab and undergraduate machines: ~60 machines, ~25 Gflops of Windows cycles available (except when the machines are used!).
[Note, the UW astronomers are ‘way ahead of us in sharing desktops]
All are welcome: see http://glast-ts.phys.washington.edu/condor/for instructions on how to participate
grad student talk 1-Feb-06 26
The rates, from 864 jobs run at UWThe rates, from 864 jobs run at UW
grad student talk 1-Feb-06 27
Also generate signal eventsAlso generate signal events
All-gamma sample: uniform in log(E) from 16 MeV to 160 GeV, and in the upper hemisphere
Rather different from actual source, but easy to characterize response for given incoming gammas.
Try to estimate reliability of energy and direction measurement
grad student talk 1-Feb-06 28
Background rejection – very difficultBackground rejection – very difficult
Create many variables to measure gamma-like, or charged particle-like quantities
extra hits around a found track
correlation of track direction with hit ACD tile (if any)
correlation of track direction with direction of CAL shower
etc.
Feed them to a set of classification tree trainers (code written for D0 single top analysis)
grad student talk 1-Feb-06 29
A preliminary bottom lineA preliminary bottom line
grad student talk 1-Feb-06 30
Pixels or photons?Pixels or photons?
Astronomers prefer pixels, but physicists like photons!Focusing devices (mirrors, lenses) convert direction to position, CCD’s collect photons, define the pixels
From SDSS web site:
“On a clear, dark night, light that has traveled through space for a billion years touches a mountaintop in southern New Mexico and enters the sophisticated instrumentation of the SDSS's 2.5-meter telescope. The light ceases to exist as photons, but the data within it lives on as digital images recorded on magnetic tape. Each image is composed of myriad pixels (or picture elements); each pixel captures the brightness from each tiny point in the sky.”
For astronomers, pixels are the data
grad student talk 1-Feb-06 31
Our data comes as individual photonsOur data comes as individual photons
Two image processing approachesIndividual photons
Advantage: keep all the information
Disadvantage: processing time: scales with exposure
Fill pixelsAdvantage: all astronomical tools work, easy to deal with:Almost all EGRET analysis was with 0.5 deg pixels
Disadvantage: loose resolution for high-energy photons
grad student talk 1-Feb-06 32
Problems with binning: IProblems with binning: I
Angular resolution varies dramatically with energy:
expect 1/E from multiple scattering
measure E-0.8
Images don’t show localization without removing low energies, increasing resolution
Full information not used in point source searches
Gamma energy (MeV)
Res
olut
ion
scal
e fa
ctor
(
deg)
WMultiple scatter
conversion
Note: 68% containment is ~3
4 decades of energy: 3 decades in resolution!
grad student talk 1-Feb-06 33
Problems with binning: IIProblems with binning: II
Need a spherical projection to 2-d that defines pixels with:
Equal area
No discontinuities (like poles, wrap-around)
Pixels ~uniform in shape (square, triangular)
Simple mapping to/from actual coordinates
Neighbors easy to find
Cartography defines ~150 including equal-area Hammer-Aitoff.
None are appropriate, really want a tesselization based on a regular polygon
The Hammer-Aitoff: popular in astronomy
WMAP microware
grad student talk 1-Feb-06 34
Solution from WMAP: HEALPixSolution from WMAP: HEALPix
~4 M pixels for full sky, > photons, not adequate for 100 GeV.
Intensity is the number of photons in the pixel
grad student talk 1-Feb-06 39
Healpix Healpix densitydensity image imageConstruct 0.1 deg image with density at center of display pixel: sum of counts/solid angle for all contained Healpix pixels in that direction.High energy photons count according to resolution
grad student talk 1-Feb-06 40
Image generation: define a Image generation: define a densitydensity function function
High energy photons are more localized: we express this by defining photons/area
Easily determined from the data base and the Healpix code.
3C273: density vs. all photons above 100 Mev
grad student talk 1-Feb-06 41
Point Source Detection: work in Point Source Detection: work in progressprogress
Motivation was to create a manageable data set for study of point sources, allowing quick projection integrals for candidates
This is actually a “Hough transform”, allowing easy detection of point sources. Comparison with other fixed-scale binning methods is in progress.
Applying wavelet technology developed by Sean Robinson
Allows quick measurement of intensity, position, significance.
Precision expected to be close, within 20% of formal maximum likelihood analysis
grad student talk 1-Feb-06 42
Science GroupsScience Groups
CatalogsDiffuse (Galactic & Extragalactic) and Molecular CloudsBlazars and Other AGNsPulsars, SNRs, and PlerionsUnidentified Sources, Population Studies, and Other Galaxies
Dark Matter and New PhysicsGamma-Ray BurstsSources in the Solar SystemCalibration and Analysis MethodsMultiwavelength Coordinating Group