glast lat projectscipp, may 8, 2002 1 glast science at scipp overivew of glast science areas of...

GLAST LAT Project SCIPP, May 8, 2002

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GLAST Science at SCIPP

• Overivew of GLAST Science

• Areas of focus at SCIPP/UCSC

• Contributions to GLAST Analysis Soft


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GLAST will do fundamental science, with a very broad menu that includes:

• Systems with supermassive black holes• Gamma-ray bursts (GRBs)• Dark Matter• Galactic Sources: Pulsars & our Sun• Origin of Cosmic Rays• Probing the era of galaxy formation• Discovery! Hawking Radiation? Other relics from the Big Bang?

– Huge increment in capabilities.GLAST draws the interest of both the the High Energy GLAST draws the interest of both the the High Energy

Particle Physics and High Energy Astrophysics Particle Physics and High Energy Astrophysics communities.communities.

Science Overview


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Measurement techniques

~103 g

cm

-2

~30

km

Atmosphere:

For E < ~ O(100) GeV, must detect above atmosphere (balloons, satellites, rockets)

For E > ~ O(100) GeV, information from showers penetrates to the ground (Cerenkov)

Energy loss mechanisms:

E=mc2. If 2x the rest energy of an electron (~0.5 MeV) is available (i.e., if the photon energy is large enough), in the presence of matter the photon can convert to an electron-positron pair.


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Unified gamma-ray experiment spectrum

The next-generation ground-based and space-based experiments are well matched.

Complementary capabilitiesground-based* space-based

angular resolution good goodduty cycle low excellentarea HUGE ! relatively smallfield of view small excellent (~20%

of sky at any instant)

energy resolution good good, w/ small systematic uncertainties

*air shower experiments have excellent duty cycle and FOV, and poorer energy resolution.


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Unified Gamma-ray Experiment Spectrum

“sensitivity”


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Active Galactic Nuclei (AGN)Active galaxies produce vast amounts of energy from a very compact central volume.

Prevailing idea: powered by accretion onto super-massive black holes (106 - 1010 solar masses). Different phenomenology primarily due to the orientation with respect to us.

Models include energetic (multi-TeV), highly-collimated, relativistic particle jets. High energy -rays emitted within a few degrees of jet axis. Mechanisms are speculative; -rays offer a direct probe.

HST Image of M87 (1994)


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EGRET and 3C279

EGRET discovery image of gamma-ray blazar 3C279 (z=0.54) E>100 MeV (June 1991)

Prior to EGRET, the only known extra-galactic point source was 3C273; however, when EGRET launched, 3C279 was flaring and was the brightest object in the gamma-ray sky! VARIABILITY:

EGRET has seen only the tip of the iceberg.


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How do AGNs Produce Gamma Rays?

Wald Model: (1978)

B

Frame dragging at theEquator gives rise to an Electric Field around the equator.

Since E is of just one signthe BH attaches charge.This is the “Wald Charge”

This charge produce an additional Electric fieldof opposite sign around the spin axis at the poles

These fields can be very intense and accelerate beams of particles

Spinning Black Hole(Kerr BH)

Immersed in a magnetic field


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Blanford - Zynif Blazar Model

ACREATION DISKACREATION DISK

Power = V I = V2/R

Take R = 377 (the vacuum)

PAGN = 1043 j/s

V = 6 x 1022 Volts

Current Flow


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How Close Can You Go?


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EGRET All Sky Map (>100 MeV)

CygnusRegion

3C279

Geminga

Vela

Cosmic RayInteractionsWith ISM

LMC

PKS 0528+134

PKS 0208-512

Crab

PSR B1706-44


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Is it really isotropic (e.g., produced at an early epoch in intergalactic space) or an integrated flux from a large number of yet unresolved sources? GLAST has highersensitivity to weak sources, with better angular resolution.

Diffuse Extra-galactic Background Radiation

GLAST will bring alive the gamma-ray sky! The origin of the

diffuse extragalactic gamma-ray flux is a mystery. Either sources are there for GLAST to resolve (and study!), OR there is a truly diffuse flux from the very early universe.


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AGN: what GLAST will do

EGRET has detected ~ 70 AGN. Extrapolating, GLAST should expect to see dramatically more – many thousands:

• Allows a statistically accurate calculation of AGN

contribution to the high energy diffuse extra-galactic

background.• Constrain acceleration and emission models. How

do AGN work?

Joining the unique capabilities of GLAST with other detectors will provide a powerful tool.

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

• Large acceptance and field of view allow relatively fast monitoring for variability over time --

correlate with other detectors at other wavelengths.

• Probe energy roll-offs with distance (light-light attenuation): info on era of galaxy formation.

• Long mission life to see weak sources and transients.


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Mrk 501

EGRET 3rd Catalog: 271 sources GLAST 1st Catalog: >9000 sources?

+ new source classes also anticipated

Some AGN shine brightly in the TeV range, but are barely detectable in the EGRET range. GLAST will allow quantitative investigations of the double-hump luminosity distributions, and may detect low-state emission:


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AGN, the EBL, and CosmologyIFIF AGN spectra can be understood well enough, they may provide a means to probe the era of galaxy formation:(Stecker, De Jager & Salamon; Madau & Phinney; Macminn & Primack)

If c.m. energy > 2me, pair creation will attenuate flux. For a flux of -rays with energy, E, this cross-section is maximized when the partner, , is

For 10 GeV- TeV - rays, this corresponds to a partner photon energy

in the optical - UV range. Density is sensitive to time of galaxy formation.

eVE

TeV

1

3

1

ussource E

lowerussource

E higher


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Opacity (Salamon & Stecker, 1998)

No significant attenuation below ~10 GeV.

A dominant factor in EBL models is the time of galaxy formation -- attenuation measurements can help distinguish models.

Photons with E>10 GeV are attenuated by the diffuse field of UV-Optical-IR extragalactic background light (EBL)

An important energy band for Cosmology

EBL over cosmological distances is probed by gammas in the 10-100 GeV range.

In contrast, the TeV-IR attenuation results in a flux that may be limited to more local (or much brighter) sources.


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(1) thousands of blazars - instead of peculiarities of individual sources, look for systematic effects vs redshift. Favorable aspect ratio important here.

(2) key energy range for cosmological distances (TeV-IR attenuation more local due to opacity).

• Effect is model-dependent (this is good):

Primack & Bullock

Salamon & Stecker

• How many blazars have intrinsic roll-offs in this energy range (10-100 GeV)? (An important question by itself for GLAST!) Again, power of statistics is the key.

• What if there is conspiratorial evolution in the intrinsic roll-of vs redshift? More difficult, however there may also be independent constraints (e.g., direct observation of integrated EBL).

• Most difficult: must measure the redshifts for a large sample of these blazars!

• Intrinsic roll-offs also for pulsar studies.

No EBL

GLAST Probes the Optical-UV EBL

Caveats


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Aside: some definitions

0 1 2 3 4 50

1000

2000Histogram of Data

lower upper

5 0 55

0

5

y

x

68% 95%

Effective area (total geometric acceptance) • (conversion probability) • (all detector and reconstruction efficiencies). Real rate of detecting a signal is (flux) • Aeff

Point Spread Function (PSF) Angular resolution of instrument, after all detector and reconstruction algorithm effects. The 2-dimensional 68% containment is the equivalent of ~1.5 (1-dimensional error) if purely Gaussian response. The non-Gaussian tail is characterized by the 95% containment, which would be 1.6

times the 68% containment for a perfect Gaussian response.


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Features of the gamma-ray sky

EGRET all-sky map (galactic coordinates) E>100 MeV

diffuse extra-galactic background (flux ~ 1.5x10-5 cm-2s-1sr-1)

galactic diffuse (flux ~O(100) times larger)

high latitude (extra-galactic) point sources (typical flux from EGRET sources O(10-7 - 10-6) cm-2s-1

galactic sources (pulsars, un-ID’d)

An essential characteristic: VARIABILITY in time!Combined, the improvements in GLAST provide a ~ two order of magnitude

increase in sensitivity over EGRET.

The wide field of view, large effective area, highly efficient duty cycle, and ability to localize sources in this energy range will make GLAST an important fast trigger for other detectors to study transient phenomena.


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AGN shine brightly in GLAST energy range

Power output of AGN is remarkable. Multi-GeV component can be dominant!

Estimated luminosity of 3C 279: ~ 1045 erg/scorresponds to 1011 times total solar luminosityjust in -rays!! Large variability within days.

1 GeV

Sum all the power over the whole electromagnetic spectrum from all the stars of a typical galaxy: an AGN emits this amount of power in JUST rays from a very small volume!


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172 of the 271 sources in the EGRET 3rd catalog are “unidentified”

Cygnus region (15x15 deg)

Unidentified Sources

EGRET source position error circles are ~0.5°, resulting in counterpart confusion.

GLAST will provide much more accurate positions, with ~30 arcsec - ~5 arcmin localizations, depending on brightness.


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Supernova remnants as acceleratorsWhat is the origin of cosmic rays? What are the acceleration mechanisms?

Seminal work: Fermi (1949)Current ideas: shock acceleration from supernovae (< 30% of released energy sufficient to produce all cosmics up to ~ 1014 eV)

expect: interaction of CR’s with gas swept up byblast should produce . Flux O(10-7 ph/cm2/s) at 1kpc.

Many shell remnants resolvable in other bands. Subtended angle typ. O(1°).

GLAST can resolve SNRs spatially and spectrally:

(S. Digel et al, simulation of Cygni)


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Transients Sensitivity100 sec

1 orbit**

1 day^̂

work done by Seth Digelwork done by Seth Digel(updated March 2001)

*zenith-pointed, ^“rocking” all-sky scan

EGRET Fluxes

- 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

During the all-sky survey, GLAST will have sufficient sensitivity after one day to detect (5) the weakest EGRET sources.


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GRBs and Deadtime

Time between consecutive arriving photons

Distribution for the 20th brightest burst in a year

GLAST opens a wide window on the study of the high energy behavior of bursts!


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...)1( QGE

EcV Amelino-Camelia et al,

Ellis, Mavromatos, Nanopoulos

Effects could be O(100) ms or larger, using GLAST data alone. But ?? effects intrinsic to

bursts?? Representative of window opened by such old photons.


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Pulsars

• Can distinguish acceleration models by observing high-energy roll-offs

Credit: Alice Hardingpreliminary

PREDICTED PULSAR POPULATIONS

POLAR CAP OUTER GAP

Sturner & Gonthier et al. Yadigaroglu & Zhang, ZhangSOURCE Dermer(1996) (2000) Romani (1995) & Cheng (2000)

EGRET Radio-loud 4 17 5 10Radio-quiet 1 5 17 22

GLAST Radio-loud 132 80

Radio-quiet : point source 212 1100 : pulsed 21

(NPSR = 1/100 yr) (NPSR = 1/100 yr)

(No geometry or spectral effects)

• Models also predict very different statistics. • Independent of models, GLAST sensitivity probes further through the Galaxy. preliminary


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The Dark Matter Problem

Observe rotation curves for galaxies:

For large r, expect:

GM

r

v r

rv r

r2

2 1

( )( ) ~

see: flat or rising rotation curves

Other signatures: e.g., direct detection, high energy neutrinos from annihilations in the core of the sun or earth [Ritz and Seckel, Nucl. Phys. B304 (1988); Ellis, Flores, and Ritz, Phys. Lett. 198B(1987)

Kamionkowski, Phys. Rev. D44 (1991) ].

r

Begeman/Navarro


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New Source Classes?• Unidentified EGRET sources are fertile ground. example: mid-latitude sources separate population (Gehrels et al., Nature, 23 March 2000)

• Radio (non-blazar) galaxies. EGRET detection of Cen A (Sreekumar et al., 1999)

• “Gamma-ray clusters”: emission from dynamically forming galaxy clusters (Totani and Kitayama, 2000)

• Various hypotheses for origin of the extragalactic diffuse, if not from unresolved blazars.

• SURPRISES ! SURPRISES ! ((most importantmost important))


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Result of hard work by many people.

GLAST Large Area Telescope (LAT)

Burst Monitor (GBM)


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Experimental Technique Instrument must measure the direction, energy, and arrival time of high energy photons (from approximately 20 MeV to greater than 300 GeV):

- photon interactions with matter in GLAST energy range dominated by pair conversion: determine photon direction

clear signature for background rejection

e+ e– calorimeter (energy measurement)

particle tracking detectors

conversion foil

anticoincidenceshield

Pair-Conversion Telescope

• instrument must detect -rays with high efficiency and reject the much higher flux (x ~104) of background cosmic-rays, etc.;

• energy resolution requires calorimeter of sufficient depth to measure buildup of the EM shower. Segmentation useful.

Energy loss mechanisms:

- limitations on angular resolution (PSF) low E: multiple scattering => many thin layerslow E: multiple scattering => many thin layers high E: hit precision & lever armhigh E: hit precision & lever arm


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e+ e–

Primary Design Impacts of Science Requirements

Energy range and energy resolution requirements set thickness of calorimeter

Effective area and PSF requirements drive the converter thicknesses and layout. PSF requirements also drive the design of the mechanical support.

Field of view sets the aspect ratio (height/width)

Time accuracy provided by electronics and intrinsic resolution of the sensors.

Electronics

Background rejection requirements drive the ACD design (and influence the calorimeter and tracker layouts).

On-board transient detection requirements, and on-board background rejection to meet telemetry requirements, drive the electronics, processing, flight software, and trigger design.

Instrument life has an impact on detector technology choices.Derived requirements (source location determination and point source sensitivity) drive the overall system performance.


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LAT Instrument Basics• 4x4 array of identical towers4x4 array of identical towers Advantages

of modular design.• Precision Si-strip Tracker (TKR) Precision Si-strip Tracker (TKR)

Detectors and converters arranged in 18 XY tracking planes. Measure the photon direction.

• Hodoscopic CsI Calorimeter(CAL)Hodoscopic CsI Calorimeter(CAL) Segmented array of CsI(Tl) crystals. Measure the photon energy.

• Segmented Anticoincidence Detector Segmented Anticoincidence Detector (ACD(ACD First step in reducing the large background of charged cosmic rays. Segmentation removes self-veto effects at high energy.

• Central Electronics System Central Electronics System Includes flexible, highly-efficient, multi-level trigger.

Systems work together to identify and measure the flux of cosmic gamma Systems work together to identify and measure the flux of cosmic gamma rays with energy 20 MeV - >300 GeV.rays with energy 20 MeV - >300 GeV.


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Performance Plots

Derived performance parameter: high-latitude point source sensitivity Derived performance parameter: high-latitude point source sensitivity (E>100 MeV), 2 year all-sky survey: (E>100 MeV), 2 year all-sky survey: 1.6x101.6x10-9-9 cm cm-2-2 s s-1-1, a factor > 50 better , a factor > 50 better

than EGRET’s (~1x10than EGRET’s (~1x10-7-7 cm cm-2-2ss-1-1).).

(after all background rejection cuts, being updated)

FOV w/ energy measurement due to favorable aspect ratio

Effects of longitudinal shower profiling


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Benefits of Modularity• Construction and Test more manageable, reduce costs and schedule risk.

• Early prototyping and performance tests done on detectors that are full-scale relevant to flight.

• Aids pattern recognition and background rejection.

• Good match for triggering large-area detector with relatively localized event signatures.

Must demonstrate that internal dead areas associated with support material and gaps between towers are not a problem.


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Design and SimulationsThe GLAST baseline instrument design is

based on detailed Monte Carlo simulations.

Two years of work was put into this before any significant investment was made in hardware development.

– Cosmic-ray rejection of >105:1 with 80% gamma ray efficiency.

– Solid predictions for effective area and resolutions (computer models now verified by beam tests). Current reconstruction algorithms are existence proofs -- many further improvements are possible.

– Practical scheme for triggering.– Design optimization.

Simulations and analyses are all OO (C++), based on GISMO toolkit.

Zoom in on a corner of the instrument

First TKR module plane

module walls

scintillators

front scintillators

gaps, dead areas included

The instrument naturally distinguishes most cosmics from gammas, but the details are essential. A full analysis is important.

proton

gamma ray


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X Projected Angle3-cm spacing, 4% foils, 100-200 MeV

Data

Monte Carlo

Simulations validated in detailed beam tests

Experimental setup in ESA for tagged photons:

101 102 103 104

Energy (MeV)

0.1

1

10

Con

tain

men

t S

pace

Ang

le (

deg)

68% Containment95% Containment

(errors are 2)

GLAST Data

Monte Carlo


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Instrument Triggering and Onboard Data Flow

Hardware trigger based on special signals from each tower; initiates readout

Function: • “did anything happen?”

• keep as simple as possible

• TKR 3 x•y pair planes in a row**

workhorse workhorse triggertriggerx

xx

Instrument Total L1T Rate: <5 kHz>

Function: • reject background efficiently & quickly with

loose cuts,

• reduce computing load

• remove any noise triggers

xx

x

• tracker hits ~line up

• track does not point to hit ACD tile

L3T: full instrumentFunction: reduce data to fit within downlink

Total L3T Rate: <30 Hz>

• complete event reconstruction

• signal/bkgd tunable, depending on analysis cuts: cosmic-rays~ 1:~few

Spacecraft

OR

(average event size: ~7 kbits)

rates are orbit averaged; peak L1T rate isapproximately 10 kHz. L1T rate estimate being revised. **ACD may be used to throttle this rate, if req.

Upon a L1T, all towers are read out within 20s

Level 1 Trigger

• CAL: LO – independent check on TKR trigger. HI – indicates high energy event ground

Level 2 Processing Level 3 Processing

L2 was motivated by earlier DAQ design that had one processor per tower. On-board filtering hierarchy being redesigned.

On-board science analysis:On-board science analysis: transient detection (AGN

flares, bursts)


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Summary• GLAST will address many important questions:

– What is going on around black holes? How do Nature’s most powerful accelerators work? (are these engines really black holes?)

– What are the unidentified sources found by EGRET?– What is the origin of the diffuse background?– What is the high energy behavior of gamma ray bursts?– What else out there is shining gamma rays? Are there further surprises in the

poorly measured energy region?– When did galaxies form?

• Large menu of “bread and butter” science• Large discovery potentialLarge discovery potential

Expect the community of investigators interested in Expect the community of investigators interested in

gamma-ray data to ggamma-ray data to grrooww enormously during enormously during the GLAST era!the GLAST era!

glast lat projectscipp, may 8, 2002 1 glast science at scipp overivew of glast science areas of...

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