toby burnett university of washington university of … · university of washington university of...

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Toby Burnett University of Washington

University of Pavia

Physics Colloquium 13 May 2011

Launch of Fermi (née GLAST) June 11, 2008

Understanding the gamma-ray sky: 2+ years of Fermi data

U. Pavia 13 May 2011

The conception: launch and deployment of CGRO: April 5, 1991

Inspired Bill Atwood (SLAC at the time) and Peter Michelson (Stanford) to realize that there was a much better design for a gamma-ray telescope.

Start of HEP/Astrophysics, or DOE/NASA collaboration

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EGRET

Bill Atwood Peter Michelson


2011 Rossi prize

The 2011 Rossi prize is awarded to Bill Atwood, Peter Michelson, and the Fermi Gamma-Ray Space Telescope/LAT team for enabling, through the development of the Large Area Telescope, new insights into neutron stars, supernova remnants, cosmic rays, binary systems, active galactic nuclei, and gamma-ray bursts.

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Bill Atwood Peter Michelson


Final phase

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Launch!


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25.6 Degree Orbit

SAA

http://observatory.tamu.edu:8080/Trakker/

Circular orbit, 565 km altitude (96 min period), 25.6 deg inclination U. Pavia 13 May 2011

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GN

HEASARC

-

-

DELTA

7920H •

White Sands

TDRSS SN

S & Ku

LAT Instrument

Science

Operations Center

(SLAC)

GBM Instrument

Operations Center

GRB

Coordinates Network

• Telemetry 1 kbps •

- •

S

Alerts

Data, Command Loads

Schedules

Schedules

Mission Operations

Center (MOC)

Fermi Science

Support Center

• m sec •

•

•

Fermi Spacecraft

Large Area Telescope

& GBM GPS

Fermi MISSION ELEMENTS


After 1 day First data, 25 June, L+14

½ hour, 417 photons

The very first photons


August 27: First light press release!


Observation modes

Pointing (traditional telescope) • Continuous stare, if not obscured

• Otherwise: – Follow Earth limb (CGRO strategy)

– Slew to a secondary point

– Survey mode

Survey mode – our standard – Look away from Earth, but rock

toward poles to equalize north/south

– Full sky coverage every 3 hours.

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Exposure for one year

Maximum deviation 20%

Note that south pole is less

due to South Atlantic Anomaly

N

S


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Fermi gamma-ray


Primack et al

No significant attenuation below ~10 GeV.

A dominant factor in EBL models is the star

formation rate -- 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)

Lower-energy gamma-rays are special

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.

only e-t of the original source flux reaches us

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Fermi-LAT collaboration

United States • California State University at Sonoma • University of California at Santa Cruz - Santa Cruz Institute of Particle Physics • Goddard Space Flight Center – Laboratory for High Energy Astrophysics • Naval Research Laboratory • Ohio State University • Stanford University (SLAC and HEPL/Physics) • University of Washington

France • IN2P3, CEA/Saclay

Italy • INFN, ASI

Japanese GLAST Collaboration • Hiroshima University • ISAS/JAXA, RIKEN • Tokyo Inst of Technology

Spain • ICREA and Inst de Ciencies de l’Espi

Swedish GLAST Collaboration • Kalmar University • Royal Institute of Technology (KTH) • Stockholm University

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PI: Peter Michelson (Stanford & SLAC)

~270 Members (including ~90 Affiliated

Scientists, plus 37 Postdocs, and 48

Graduate Students)

Cooperation between NASA and DOE, with

key international contributions from

France, Italy, Japan and Sweden.

Managed at Stanford Linear Accelerator

Center (SLAC).


2-years of LAT data

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Back to gamma rays: a journey from very close to very far away

• The closest source:

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e+

e–

Incoming positron

Annihilate here (micrometeorite shield) to two gammas)


Our angular resolution, or PSF

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3 Orders of magnitude

Dashed line is a measurement using the data. Current public representation is wrong by x2 at high energies. This misunderstanding plays a role in at least two external “discoveries”


x30

N

S

W E

Looking down at the brightest source

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At 650 km, The

Earth covers

40% of the sky

Earth-based coordinate system

Horizontal

pointing

Zenith angle

Limb is forward scattering from top of atmosphere: fits well to cosmic ray spectrum

nadir limb


About representing the data

• Pixels and projections: How to represent the entire sky? – Tesselation: the HEALpix scheme

– Fermi issues: • the resolution depends on energy! (next slide on PSF)

The UW-invented sparse representation allows compact storage of data binned in energy and angle

• Large dynamic range: how to show contrast, especially with diffuse background: we adopt a scheme to show photon density, using PSF weighting

• Pseudo-color schemes:

• Convert a number to a color. But what are the colors supposed to convey? – Scale function (linear, sqrt, log)

Offset, bias in mapping important

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B

SLS

heat

grey U. Pavia 13 May 2011

How are they generated?

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Acceleration (shock waves)

Synchroton Inverse compton

Magnetic field

Radiation field

e-

p

ISM: H I, H II, H2, dust

Hadronic interaction

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Dark Matter

Dark matter annihilation

? U. Pavia 13 May 2011

WMAP and Fermi

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Goal: account for every photon

• ~Steady Sources of photons – Point sources

• Pulsars, including binaries

• Galaxies, mostly AGN – AGN probes B, photons

• Unknown

– Galactic diffuse

– Isotropic diffuse • Unresolved point sources

• Proton background

• Unknown

• Transients – GRB

– Nova

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Making the 2FGL catalog

U. Pavia 13 May 2011 22

Understanding the sky: extract a list of sources for the 2FGL catalog

Data: 28 M

Exposure: 52 Ms

Two years (excluding 3 GRBs) “Pass7 processing” [1FGL: 11 months]

2FGL Table 1888 entries [1FGL: 1451]

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Light curves, SED plots, associations

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Bright sources at high latitudes are easy

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Strong source high latitude example: SED plots

Circles are 3, 100 MeV PSF (‘front’ section) (varies by a factor of 30 with energy!)

Use log parabola if better fit

Pulsars fit with exponential cutoff

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Power law

Measuring point source properties: maximize likelihood

Model of the sky must account for all photons PSF Aeff Galactic, isotropic diffuse including CR 1/8 degree grid, pixels centered on plane Earth limb

An important issue: how to measure significance? Test Statistic: TS=-2log(Lfit/Lnull)

We conservatively choose only sources with TS>25.

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TS=14 TS=32 TS=1000

null

max

TS/2

flux

Log(L

)

Detecting sources

Initial list: use 1FGL supplemented with: PGWave, MRfilter: image based

MST (minimal spanning tree): pattern of high energy photons

Likelihood, using current model Test each of 3.1 M 0.1 deg pixels with a trial point source,

(index 2), record the corresponding TS.

Look for clusters with max TS>10, use as seed for new iteration

Efficiency checks: Test with Monte Carlo – generated sources

Recheck by applying PGWave

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The 2FGL process: Stage I

Use the pointlike tool applied between 100 MeV and 300 GeV. Initial list: use 1FGL as seeds supplemented with:

PGWave, MRfilter: image based MST (minimal spanning tree): pattern of high energy photons

Create model with point and extended sources, galactic, isotropic and limb diffuse Require exponential cutoff for all LAT pulsars, use log parabola if

improves fit Fit the entire sky including updating positions (procedure detailed

below) Create 'residual TS' map:

TS for new point source (index 2) at each of 3.1M 0.1 pixels Look for clusters with max TS>10, use as seeds in new model

Provide all with TS>10 to stage II

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CenA Lobes

MSH 15-52

Vela X

IC 443

LMC

SMC W28 W30 HESS J1825-137

W44 W51C

Cygnus Loop

Extended source templates

Details about the sky model

Tessellate sky using HEALPix: 1728 regions

Each ~5 square pixel defines:

Center of circular regions for:

data (5 deg)

sources (10 deg)

sources inside are varied; those outside fixed to results of previous iteration

Note ~x3 overlap of data: not independent

Diffuse component normalizations free

Iteration procedure:

Each region fit (full likelihood maximized) independently

Each fit remeasures point source positions: Apply updates between cycles.

Check changes in log(L): iterate until none changes by more than 10 (8-10 iterations required)

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Colors: HEALPix index

Localization

Basic principle: the likelihood function, as a function of the position of a source, is an estimator of the position, with the curvature defining the resolution.

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Demo with 3C 273.

PSF: 3 @ 100 MeV (front)

PSF: 0.1 for E>10 GeV (front)

Error ellipse defined by 95% contour (2.45). Plot shows contours, and results of fit to quadratic surface

‘TS plot’

galactic 67.3%

isotropic 23.7%

limb 0.5%

sources 8.5%

2FGL skymodel

Stage I Summary

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Example consistency check: all photons in 5 radius circle (Approx. 12 d.o.f.)

Contributions for all energies, full sky

Ch

i squ

ared

Consistency mostly good

Limb distribution

Free parameters

Type Number

Spectra 7603

Diffuse normalization 3456

Location ( 2 per TS>16 source)

5096

It is not all so rosy…

Most sources apparently associated with diffuse structures probably result from inadequate representation of the diffuse itself

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Galactic center is complicated! See talk by T. Porter

Orion molecular cloud: poster by S. Digel & F. Giordano

Sources: TS>10 seeds for 2FGL

Sources: did we miss any?

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TS residual map, threshold=10, ~3 (catalog: 25)


2FGL process: Stage II

Use the standard tool gtlike in binned mode Accept all 3499 TS>10 sources from stage I Similar iterative scheme, but with 933 overlapping

regions chosen to equalize number of sources Refit spectra and diffuse normalization Fit pulsars with exponential cutoff

Everything else with power law: try log parabola Retain all with gtlike TS>25 when generating model

The pointlike fit and TS generally agree well, but some scatter for low values.

Generate: Table spectral plots light curves associations

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TS comparison

pointlike

gtlik

e

Discussion

277 1FGL sources are not represented. Recall that they were only used as seeds at the start of the new process Some reasons:

New requirements for localization

Extended sources were represented by more than one point source

Improved galactic diffuse model

There, but not significant enough (flared during first 11 months)

Dominant effect is new fitting procedure: application of current procedure (especially now using binned vs. unbinned gtlike) to 1FGL accounts for nearly all.

The 5 LAT pulsars that are not (DC) detected are put in ‘by hand’

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2FGL Associations

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Classifications

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Type Number Percentage of total

Active Galactic Nuclei 832 44%

Candidate Active Galactic

Nuclei

268 14%

Unassociated 594 32%

Pulsars (pulsed emission) 86 5%

Pulsars (no pulsations yet) 26 1%

Supernova

Remnants/Pulsar Wind

Nebulae

60 3%

Globular Clusters 11 < 1%

Other Galaxies 7 < 1%

Binary systems 4 < 1%

TOTAL 1888 100%

Very Preliminary - Work Still In Progress

Pulsars

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radio emission cone

-ray

emission

fan beam

Discovered 1967 (Nobel prize 1974) Vela discovered 1968


88 gamma-ray pulsars

U. Pavia 13 May 2011 40

Pulsars seen from above

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Vela: published results with first 2.5 months

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Remarkably

sharp peaks;

features to

~0.3ms.

Turns nearly

completely off

between the

double pulses.

•<2.8% of phase-

averaged pulsed

emission, 95%

confidence

•Stringent limits or

measurement will be

available with more

livetime

Radio pulse

89 ms


Vela: phase-averaged SED

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( / )

0( )b

cE EN E N E e

0.051.51

0.04

2.9 0.1 GeVcE

Consistent with b=1

(simple exponential)

b=2 (super-exponential)

rejected at 16.5

No evidence for magnetic

pair attenuation:

Near-surface emission

ruled out


CTA-1: Discovery of the first gamma-ray-only Pulsar with “blind search” technique

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Quick discovery enabled by new

blind-search technique (Atwood et al ApJ (2009))

P ~ 317 ms Pdot ~ 3.6E-13

• Spin-down luminosity ~1036 erg s-1, sufficient to supply the PWN with magnetic fields and energetic electrons.

• The γ-ray flux from the CTA 1 pulsar corresponds to about 1-10% of Erot (depending on beam geometry)

1420 MHz Radio Map:

Pineault et al., A&A 324, 1152 (1997)

27 total discovered so far U. Pavia 13 May 2011

From our pulsar catalog paper

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Millisecond pulsars and Fermi

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See http://nanograv.org 47 U. Pavia 13 May 2011

Looking for MSP’s: the effect of Fermi

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Supernova Remnants: Origin of Cosmic Rays?

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“How fitting it is that Fermi

seems to be confirming the bold idea advanced over 60 years ago by the scientist after whom it was named,” Roger Blandford,


Nearby galaxies (including our own!)

U. Pavia 13 May 2011 50

Our Nearest Non-blazar AGN

2011 May 10 Fermi Symposium - Cheung

M. Su’s talk yesterday

Our Nearest Non-blazar AGN


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Diffuse Emissions in Other Galaxies

Test Statistic (TS) = -2 log(Lsource - Lno source)

0.68, 0.95, 0.99 confidence level localization contours

Appear as LAT point sources, starburst regions unresolved

Galactic diffuse, isotropic diffuse, and point sources subtracted

Fermi LAT (>200 MeV) Fermi LAT (>200 MeV)

6o x 6o region of the sky 6o x 6o region of the sky


Angle on Classification of Radio Galaxies

Radio Galaxies, Lobe-dominated Sources Blazars


from Urry & Padovani (1995)

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Blazars

(from Sikora, Begelman, and Rees (1994))

3C273 Identified as distant in 1962 by Greenstein and Schmidt (added quasar to the language)


Blazars are variable on monthly scales

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PKS 1502+106

3C273

CTA 1

PKS 2155-304

3C454.3

Vela

BL Lac Crab


3C454.3 flare!

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• Well-known radio source, quasar

at z = 0.859;

also detected by EGRET, AGILE

• Not a simple power law

• Can describe as a broken

power law with a break, 1 ~

2.3 to 2 ~3.5 at Ebr ~ 2 GeV

•Origin of the break?


Multi-wavelength: where we fit in

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radio IR

UV X-ray

Fermi

A previous multiwavelength

radio IR

UV X-ray

Fermi


http://www.iop.org/EJ/article/0004-637X/690/1/1018/apj292051f12.html

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PKS 1502+106 • z=1.84 • Extremely rapid flare just as we started!


PKS 2155-304: 11-day multiwavelength campaign

• BL Lac @ z=0.116

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ATOM RXTE

Swift

Fermi

HESS

EGRET


The galactic diffuse emission

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Used for 1FGL: the “gll_iem_v02” version

Current one includes the “lobes”

Largest component: H I Contribution by ring

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Looking away from the center

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Isotropic diffuse

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(Not really flat: have to take proton contamination into account)

LAT Isotropic Diffuse Flux

|b| > 10º

extragalactic diffuse

CR background

LAT

arXiv: 1002.3603

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Not just unresolved blazars!

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See arXiv:1003.0895


Explosions/Flares: very near and very far

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V407 Cyg: nova on March 11, 2010

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V407 Cyg: in our 2-year catalog

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High Energy Activity from the Crab

No corresponding flare in X-rays with

INTEGRAL (Atel # 2856), Swift (Atel #

2858, 2866), or RXTE (Atel # 2872) or

NIR (Atel #2867). No evidence for active

AGN near Crab (Swift, Atel # 2868). U. Pavia 13 May 2011 70

Crab Flares (>100 MeV)

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Fermi-GBM

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Charles Meegan (PI)

Jochen Greiner (Co-PI) U. Pavia 13 May 2011

GBM Trigger Rate (weekly)

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129 GBM detected GRBs

• 2 SGRs (SGR 0501+4516, SGR 1806-20)

• 1 AXP (AXP 1E1547.0-5408)

• >5 TGFs (Terrestrial Gamma-ray Flashes )


GRB080916C (AKA collaboration meeting burst)

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8 keV – 260 keV

260 keV – 5 MeV

LAT raw

LAT > 100 MeV

LAT > 1 GeV

T0

• The first low-energy peak is not observed at LAT energies

• 14 events above 1 GeV

• The bulk of the emission of the 2nd peak is moving toward later times as the energy increases

• Clear signature of spectral evolution

• new era of GeV GRB lightcurves!

GROND optical follow up [GCN 8257, 8272] Faint (21.7 mag at T

0+32h) and

fading (T0+3.3d) source

RA = 119.8472˚, Dec = −56.6383˚ (±0.5” at 68% C.L.)

Photometric redshift:

z=4.2 +/- 0.3


Some implications of high-energy photons at z=4.35

Bulk Lorentz factor: =608, 887 for time bins b and d

Problem for predictions of EBL absorption?

13.2 GeV observed within 16.5 s

Quantum gravity limit: MQG > 1.3 x 1018 GeV

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Fermi GRBs

GBM: ~250 GRB/yr LAT: ~9 GRB/yr

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THE DATA ARE PUBLIC

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Did Hooper and

Goodenough discover

DM in our data?

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"When I look at this data, it lines up perfectly," he says. "It quacks like a duck."


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And did Ando and Kusenko discover “primordial magnetic fields” by detecting halos around AGN’s that we missed?


Summary • Many discoveries

• But: After two years, we are still trying to understand both the performance of the detector, and subtleties in the sky!

• Dark matter still the holy grail

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The UW group: Key ideas

Basic object model for sources, all-sky

Data representation: variable size HEALPix, stored as sparse list of pixels containing photons. Ideal for all-sky analysis: one compression step good for

entire sky: for 2 years, 3.4 GB (scales with time) 41 MB (~constant), factor of 83.

But needs extraction of subsets for timing

Use for display, easy to weight photons

Likelihood analysis optimized for speed, without loss of precision vs. unbinned technique. Use likelihood technique for source finding: According to

Cramer-Rao thm, the best way!

Allows quick localization

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Multi-wavelength context

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High energy photons often produced in a different physical process to the lower energy emission -> Independent handle on the physical conditions.

Gamma-rays are produced in non-thermal processes

Known classes of gamma-ray sources are multiwavelength objects seen across much of the spectrum

Fermi LAT

High energy gamma-rays explore nature’s accelerators - “Where the energetic things are”

natural connections to UHE cosmic-ray and neutrino astrophysics


toby burnett university of washington university of … · university of washington university of...

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