Milagro

Milagro: A Synoptic VHE Gamma-Ray Telescope

Gus Sinnis

Los Alamos National Laboratory

Milagro

Why A Synoptic Telescope?• Complete unbiased sky survey• Transient phenomena

– Gamma ray bursts– Flares from active galaxies– Solar events (coronal mass ejections)

• Year-round observation of all sources• Extended sources

– Diffuse emission from the Galactic plane• cosmic ray generation and propagation

– Molecular clouds– ??

• Discovery potential• Prototyping of radically new technique

Milagro

Detectors in Gamma-Ray Astrophysics

High SensitivityHESS, MAGIC, CANGAROO, VERITAS

Large Aperture/High Duty CycleMilagro, Tibet, ARGO, HAWC?

Low Energy ThresholdEGRET/GLAST

Large Effective Area

Excellent Background Rejection (>99%)

Low Duty Cycle/Small Aperture

Space-based (small area)

“Background Free”

Large Duty Cycle/Large Aperture

Moderate Area/Large Area (HAWC)

Good Background Rejection

Large Duty Cycle/Large Aperture

High Resolution Energy Spectra

Studies of known sources

Surveys of limited regions of sky

Sky Survey (<10 GeV)

AGN Physics

Transients (GRBs) <100 GeV

Unbiased Sky Survey

Extended sources

Transients (GRB’s)

Solar physics/space weather

Milagro

Milagro

• 2600m asl• Water Cherenkov Detector• 898 detectors

– 450(t)/273(b) in pond– 175 water tanks

• 3.4x104 m2 (phys. area)• 1700 Hz trigger rate• 0.5o resolution• 90% proton rejection

10 m

Milagro

The Central Detector

Milagro

Background Rejection in MilagroP

roto

ns

Ga

mm

as

Gamma MC

Data

Proton MC

)()2(

BottomPEPesNBottom

MaxC Retain 50% and 9% protons

Not angular resolution – inherent rejection

Milagro

Effective Areas

Milagro

Energy Resolution

• Two new techniques– Direct event-by-event

method– Spectral measurement

compactness distribution of event excess

• S/B increases with energy (S/N ~ constant)– With a small significance

we can measure spectra Replace with Crab spectrum plot

Milagro

Gamma-Ray BurstsMilagrito (Milagro prototype)

Operated April 1997-1998

BATSE detected 54 GRBs within Milagrito’s field of view

We scan the region around the BATSE position for an excess

GRB 970417a had a post-trial probability of 1.7x10-3

Milagro

Gamma-Ray Bursts

• I need some slides from David Noyes and Pablo here

Milagro

Sky Survey

Milagro sky map ApJ 2004, 608, p680

Milagro

Extended Sources

Sensitivity to an extended source is relatively better for an EAS than an ACT because angular resolution is not as important

detector

sourcepointextended

SS

Sensitivity determined by – inherent proton

rejection– observation time– effective area

NotDetected(both)

Detected(both)

DetectedMilagro only

MilagroWhipple

Courtesy: David Kieda ACT VII, Paris 2005

Milagro

Extended Sources – Galactic Plane

Cosmic rays interacting with matter in Galaxy produce p’s that decay into g rays

Gamma ray spectrum is sensitive to cosmic ray source models

– inverse Compton component– point sources

EGRET observations up to 20 GeV indicated an excess > 1 GeV

Higher energy observations have proven elusive despite 20 years of effort

Milagro has made the first detection of TeV gamma rays from the Galactic plane

S/B level ~3x10-4

Flux(>1 TeV) = 5.1x10-10 cm-2 sec-1 sr-1

Spectral Index = -2.61± 0.07 (combined EGRET-Milagro fit)

EGRET data

Milagro

Galactic Plane

Milagro data 5x5 degree bins

Sig

nific

ance

Crab

Milagro

Galactic Plane

E-2.51±0.05 Demonstrates the strength of

EAS in finding diffuse and extended sources– Due to good “inherent”

background rejection– Angular resolution unimportant– Large observation time– Large field of view

Milagro flux measurement is ~1/10 of previous upper limits

Milagro

Extended Sources

Search northern sky for large sources

~6 degree source in Cygnus arm of Galaxy

– EGRET observed as brightest region in Northern hemisphere

~3 degree source near the Crab Nebula

– coincident with an EGRET unID

Tibet hotspot

Milagro point source hotspot

Milagro

Solar Physics Coronal mass ejections

are an ideal laboratory to study particle acceleration in the cosmos

By monitoring the singles rates in all PMTs we are sensitive to “low”-energy particles (>10 GeV)

Milagro has detected 4 events from the Sun with >10 GeV particles

Milagro

X7-Class flare Jan. 20, 2005

GOES proton data– >10 MeV– >50 MeV– >100 MeV

Milagro scaler data– > 10 GeV protons– ~1 min rise-time– ~5 min duration

Milagro

Future Instruments: ARGO-YBJ

Milagro

Farther Future:HAWC Build pond at extreme altitude (Tibet 4300m or Chile 5200m) Incorporate new design

– Optical isolation between PMTs– Larger PMT spacing– Deeper PMT depth (in top layer)

e

300 meters

4 meters

~$20M for complete detector~60x sensitivity of Milagro – instantaneous sensitivity of Whipple over 2 sr

Crab Nebula in 30 minutes (now 1 year)GRBs to redshift of >1 (now 0.4)

Milagro

Effective Areas: Future Detectors

Milagro

Survey Sensitivity

Milagro

HAWC: Simulated Sky Map

C&G AGN

Hartmann IR model

known TeV sources

Milagro extended sources

1-year observation

Milagro

Conclusions Milagro has met or exceeded all of its design

goals We have made exciting discoveries

First convincing detection of a TeV gamma ray source with a synoptic instrument

Complete survey of Northern sky Diffuse emission from the Galactic plane Extended sources of TeV gamma rays Possible TeV emission from GRB Clear demonstration of low-energy (5 GeV) capability

for solar physics We have pioneered a radically new technique