Transcript Document

Milagro: A Synoptic VHE
Gamma-Ray Telescope
Gus Sinnis
Los Alamos National Laboratory
Milagro
Gus Sinnis
Milagro NSF Review July 18-19, 2005
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
Gus Sinnis
Milagro NSF Review July 18-19, 2005
Detectors in Gamma-Ray Astrophysics
High Sensitivity
HESS, MAGIC, CANGAROO, VERITAS
Low Energy Threshold
EGRET/GLAST
Large Effective Area
Excellent Background Rejection (>99%)
Low Duty Cycle/Small Aperture
Space-based (small area)
“Background Free”
Large Duty Cycle/Large Aperture
High Resolution Energy Spectra
Sky Survey (<10 GeV)
Studies of known sources
Surveys of limited regions of sky
AGN Physics
Transients (GRBs) <100 GeV
Milagro
Large Aperture/High Duty Cycle
Milagro, Tibet, ARGO, HAWC?
Moderate Area/Large Area (HAWC)
Good Background Rejection
Large Duty Cycle/Large Aperture
Unbiased Sky Survey
Extended sources
Transients (GRB’s)
Solar physics/space weather
Gus Sinnis
Milagro NSF Review July 18-19, 2005
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
Gus Sinnis
Milagro NSF Review July 18-19, 2005
The Central Detector
Milagro
Gus Sinnis
Milagro NSF Review July 18-19, 2005
Protons
Background Rejection in Milagro
Gamma MC
Gammas
Data
Proton MC
Retain 50% g and 9% protons
C
NBottom(  2 Pes)
PEMa x( Bottom)
Not angular resolution – inherent rejection
Milagro
Gus Sinnis
Milagro NSF Review July 18-19, 2005
Effective Areas
Milagro
Gus Sinnis
Milagro NSF Review July 18-19, 2005
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
Milagro
Replace with Crab spectrum plot
Gus Sinnis
Milagro NSF Review July 18-19, 2005
Gamma-Ray Bursts
Milagrito (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
Gus Sinnis
Milagro NSF Review July 18-19, 2005
Gamma-Ray Bursts
• I need some slides from David Noyes and
Pablo here
Milagro
Gus Sinnis
Milagro NSF Review July 18-19, 2005
Sky Survey
Milagro sky map
ApJ 2004, 608, p680
Milagro
Gus Sinnis
Milagro NSF Review July 18-19, 2005
Extended Sources
 Sensitivity to an extended source is relatively better for
an EAS than an ACT because angular resolution is not
as important
 source
Sextended  S point
 detector
 Sensitivity determined by
– inherent proton
rejection
– observation time
– effective area
Not
Detected
(both)
Detected
Milagro only
Milagro
Whipple
Detected
(both)
Courtesy: David Kieda ACT VII, Paris 2005
Milagro
Gus Sinnis
Milagro NSF Review July 18-19, 2005
Extended Sources – Galactic Plane


EGRET data






Milagro
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)
Gus Sinnis
Milagro NSF Review July 18-19, 2005
Milagro data 5x5 degree bins
Crab
Milagro
Significance
Galactic Plane
Gus Sinnis
Milagro NSF Review July 18-19, 2005
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
Gus Sinnis
Milagro NSF Review July 18-19, 2005
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
Gus Sinnis
Milagro NSF Review July 18-19, 2005
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
Gus Sinnis
Milagro NSF Review July 18-19, 2005
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
Gus Sinnis
Milagro NSF Review July 18-19, 2005
Future Instruments: ARGO-YBJ
Milagro
Gus Sinnis
Milagro NSF Review July 18-19, 2005
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
m
g
4 meters
300 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
Gus Sinnis
Milagro NSF Review July 18-19, 2005
Effective Areas: Future Detectors
Milagro
Gus Sinnis
Milagro NSF Review July 18-19, 2005
Survey Sensitivity
Milagro
Gus Sinnis
Milagro NSF Review July 18-19, 2005
HAWC: Simulated Sky Map
 C&G AGN
 Hartmann IR model
 known TeV sources
 Milagro extended
sources
 1-year observation
Milagro
Gus Sinnis
Milagro NSF Review July 18-19, 2005
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
Milagro
Gus Sinnis
Milagro NSF Review July 18-19, 2005