Detector Fermi

8/20/2019 Detector Fermi

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Exploring the Extreme

Universe with FermiGamma-ray Space

TelescopeDave ThompsonNASA GSFC

Deputy Project Scientist

S. Ritz

NASA GSFC and U. Maryland

• Why?• How?

• What?

Rittenhouse Astronomical Society January 14, 2009

Modified by J. Bazo


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Whatsupposedlyfirst turnedDavid Bannerinto the

Hulk?

Gamma Rays!

Becausegamma raysare powerful


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But what if you had gamma-ray vision?


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

diffuse extra-galactic background (flux ~ 1.5x10-5

cm-2

s-1

sr -1

)galactic diffuse (flux ~30 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: V ARI ABILITY in time!

Field of view important for study of transients.

Features of the EGRET gamma-ray sky


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About that Name

Enrico Fermi was an Italian

physicist who immigrated to the

United States before World War II.

He was the first to suggest a viable

way to produce high-energyparticles in cosmic sources. Since

gamma-rays are produced by

interactions of such energetic

particles, his work is the foundationfor many of the studies being done

with the Fermi Gamma-ray Space

Telescope, formerly GLAST.

U. S. Postal Service


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Fermi


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FERMI


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The Observatory

Gamma-ray Burst Monitor (GBM)NaI and BGO Detectors

8 keV - 30 MeV

Large AreaTelescope (LAT)20 MeV - >300 GeV

KEY FEATURES

• Huge field of view –LAT: 20% of the sky at anyinstant; in sky survey mode,expose all parts of sky for~30 minutes every 3 hours.GBM: whole unocculted sky

at any time.• Huge energy range, includinglargely unexplored band 10 GeV -100 GeV. Total of >7 energydecades!

Successors to EGRET and BATSE


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Large AreaTelescope (LAT)


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e+ e –

Overview of LAT: How it works

• Precision Si-strip Tracker (TKR)

Measure the photon direction;gamma ID.

• Hodoscopic CsI Calorimeter

(CAL) Measure the photon

energy; image the shower.

• Segmented AnticoincidenceDetector (ACD) Reject

background of charged cosmic

rays; segmentation removes

self-veto effects at high energy.

• Electronics System Includesflexible, robust hardware trigger

and software filters.

Systems work together to identify and measure the flux of cosmic gamma

rays with energy 20 MeV - >300 GeV.

Calorimeter

Tracker

ACD[surrounds4x4 array ofTKR towers]

Atwood et al, ApJ submitted


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FERMI


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How does a high-energy gamma-ray telescope work?

• The key is “high-energy”

• A gamma ray is a packet ofenergy – lots of energy.

• Who do we call for help?

Prof. Einstein, what do we do withsomething that is just a large amount

of energy?

Energy? That’s E, and E = mc2

Convert the energy to mass.


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e+ e – Calorimeter(energy measurement)

Particle TrackingDetectors

Conversion Foil

AnticoincidenceDetector (background rejection)

Pair-Conversion Telescope

Fermi Large Area Telescope (LAT)

• Gamma rays interact by pair production, the conversion of the

gamma-ray energy into two particles – an electron and a positron

(really an antiparticle); LAT is a particle detector.


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LAT Gamma Candidate Events

The green crosses show the detected posit ions of the charged p art ic les, the blue l ines show the reconstru cted track trajector ies, and the yel low line shows

the candidate gamma-ray est imated direct ion. The red crosses show the detected energy deposit ions in the calor imeter.


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A very broad menu that includes:

• Systems with supermassive black holes (Active Galactic Nuclei)• Gamma-ray bursts (GRBs)

• Pulsars

• Supernova remnants (SNRs), PWNe, Origin of Cosmic Rays

• Diffuse emissions

• Solar physics• Probing the era of galaxy formation, optical-UV background light

• Solving the mystery of the high-energy unidentified sources

• Discovery! New source classes. Particle Dark Matter? Other relicsfrom the Big Bang? Other fundamental physics checks.

Huge increment in capabilities.

Fermi Science

Draws the interest of both the High Energy Particle Physics and

High Energy Astrophysics communities.


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The Accelerator


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Launch!

• Launch from Cape Canaveral

Air Station 11 June 2008 at

12:05PM EDT

• Circular orbit, 565 km altitude

(96 min period), 25.6 deg

inclination.

• Communications:

– Science data link via

TDRSS Ku-band, average

data rate 1.2 Mbps.

– S-band via TDRSS andground stations


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A moment later…


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… and then …


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… on its way!


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GN

HEASARC

-

-

DELTA

7920H•

White Sands

TDRSS SN

S & Ku

LAT InstrumentScience

Operations Center

GBM Instrument

Operations Center

GRB

Coordination

Network (GCN)

• Telemetry 1 kbps •

- •

S

Alerts

Data, Command Loads

Schedules

Schedules

Mission Operations

Center (MOC)

Science

Support Center

• m sec •

•

•

Fermi Spacecraft

Large Area Telescope

& GBMGPS

MISSION ELEMENTS


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What is Fermi seeing?

• A key point - because gamma rays are detected one at atime like particles, the Fermi telescopes do not have

high angular resolution like radio, optical or X-ray

telescopes. No pretty pictures of individual objects.

• Instead, Fermi trades resolution for field of view. The

LAT field of view is 2.4 steradians (about 20% of the

sky), and the GBM field of view is over 8 steradians.

• The Fermi satellite is operated in a scanning mode,

always looking away from the Earth.

• The combination of huge field of view and scanning

means that the LAT and GBM view the entire sky every

three hours!


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Operating modes

• Primary observing mode is SkySurvey

– Full sky every 2 orbits (3 hours)

– Uniform exposure, with eachregion viewed for ~30 minutesevery 2 orbits

– Best serves majority of science,facilitates multiwavelengthobservation planning

– Exposure intervalscommensurate with typicalinstrument integration times forsources

– EGRET sensitivity reached indays

• Pointed observations when appropriate (selected by peer review in lateryears) with automatic earth avoidance selectable. Target of Opportunitypointing.

• Autonomous repoints for onboard GRB detections in any mode.


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• PSF on-orbit as expected (note

intrinsic energy dependence =>

localization is source-dependent)

– verify using on-pulse photons

from Vela, compare with detailed

MC simulation:

LAT Working Very Well On Orbit!

Effective Area

Energy resolution


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Large Area Telescope First Light!

The full gamma-ray sky projected onto a surface - Galactic coordinates

The Fermi Large Area Telescope sees the whole gamma-ray sky every three

hours. This is an important feature, because the high-energy sky isconstantly changing. This image represents just four days of observations.


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Three months of LAT scanning data


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Pulsars - rapidly rotating neutron stars

Vela pulsar -

brightest persistent

source in the

gamma-ray sky.

The actual rotation of the star takes less than 1/10 second.


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Vela Pulsar – Phase-averaged SED

b

c E E e E N E N )/(

0)(

0.051.51

0.04

2.9 0.1 GeVc

E

Consistent with b=1(simple exponential)

b=2 (super-exponential)

rejected at 16.5s

No evidence for magnetic

pair attenuation:

Near-surface emission

ruled out


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The Pulsing Sky

Pulses at

tenth true

rate


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Pulsars

Geminga: P=237 ms

Crab: P =33 ms

Vela: P=89.3 ms


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LAT discovers a radio-quiet pulsar!

P ~ 317 msCharacteristic age ~ 10,000 yrs

Location of EGRET source 3EG J0010+7309,the Fermi-LAT source, and the central X-ray

source RX J0007.0+7303

pulsars found in blind searchesof LAT data.


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Over half the bright sources seen with LAT appear to

be associated with Active Galactic Nuclei (AGN)

• Power comes frommaterial falling

toward a

supermassive

black hole

• Some of thisenergy fuels a jet

of high-energy

particles that

travel at nearly the

speed of light


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Blazars –

supermassiveblack holes with

huge jets of

particles and

radiation pointed

right at Earth.

Gamma rays from blazars

3C454.3 - LAT saw

it flare up 5 times

brighter thanEGRET ever

measured.

PKS 1502+106 - a blazar 10billion light years away,

never detected by EGRET,

flared up overnight to

become one of the

brightest things in the

gamma-ray sky.


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Flaring sources

• Automated search for

flaring sources on 6 hour, 1

day and 1 week timescales.

• Astronomers telegrams

– Discovery of new gamma-

ray blazars PKS 1502+106,

PKS 1454-354

– Flares from known gamma-

ray blazars: 3C454.3, PKS

1510-089,3C273, AO

0235+164, PSK 0208-512,

3C66A, PKS 0537-441,3C279

– Galactic plane transients:

J0910-5041, 3EG J0903-

3531


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Two ATels - Astronomer’s Telegrams www.astronomerstelegram.org

These announcements encourage cooperation from other telescopes, like

Swift, to help understand how these powerful jet sources work.

Gamma-Ray Bursts (GRBs): the most


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Gamma Ray Bursts (GRBs): the most

powerful explosions since the Big Bang

• Originally discovered by

military satellites, GRBs are

flashes of gamma rays

lasting a fraction of a second

to a few minutes.

• Optical afterglows reveal that

many of these are at

cosmological distances

• The GBM and LAT extend the

energy range for studies of

gamma-ray bursts to higher

energies, complementing

Swift and other telescopes.

Multiple detector light curve


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• The bulk of the emissionof the 2nd peak is movingtoward later times as theenergy increases

• Clear signature ofspectral evolution

Multiple detector light curve

Wh N f F i?


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What Next for Fermi?

• We have only scratched the surface of what the Fermi

Gamma-ray Space Telescope can do. – The gamma-ray sky is changing every day, so

there is always something new to learn about theextreme Universe.

• Beyond pulsars, blazars, and gamma-ray bursts,other sources remain mysteries. Nearly 20% of thebrightest sources do not seem to have obviouscounterparts at other wavelengths.

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Detector Fermi