# Presentation: 814408

## Milagro in High  Energy Gamma-Ray    				Astrophysics

- Milagro in High  Energy Gamma-Ray    				Astrophysics

-  Petra Hüntemeyer
- Los Alamos National Laboratory

## High Energy Particle Astrophysics Questions

- What is the origin of cosmic rays?
- Are the accelerators of hadrons different from electrons?
- How high in energy can galactic sources produce particles?
- What extragalactic sources can accelerate particles to even higher energies?
- How do astrophysical sources accelerate particles?
- What is the role of the extreme magnetic and gravitational fields surrounding black holes and neutron stars?
- How are particles accelerated within relativistic jets?
- What interactions cause these particles to lose their energy?
- What constraints can we place on fundamental physics?
- What is the dark matter?
- What are the tightest constraints on Lorentz invariance?
- Are there primordial black holes?

## How do -rays answer these questions?

- -rays are produced by cosmic-ray interactions in their sources
- The highest energy particles produce highest energy -rays

## -rays are produced by cosmic-ray interactions in their sources
The highest energy particles produce highest energy -rays
Several classes of astrophysical objects produce -rays
But -ray fluxes within classes is not predictable

- The highest energy particles produce highest energy -rays
- Several classes of astrophysical objects produce -rays
- But -ray fluxes within classes is not predictable

- How do -rays answer these questions?

## How do -rays answer these questions?

- -rays are produced by cosmic-ray interactions in their sources
- The highest energy particles produce highest energy -rays
- Several classes of astrophysical objects produce -rays
- But -ray fluxes within classes is not predictable
- Hadronic vs leptonic interactions
- Produce different energy spectra and -ray time variability

## How do -rays answer these questions?

- -rays are produced by cosmic-ray interactions in their sources
- The highest energy particles produce highest energy -rays
- Several classes of astrophysical objects produce -rays
- But -ray fluxes within classes is not predictable
- Hadronic vs leptonic interactions
- Produce different energy spectra and -ray time variability
- Goals of  -ray observations
- Detect many sources of many different classes
- Observe highest energies and most rapid variability
- Compare -ray images, spectra, and variability with those from  other wavelengths

## How are γ-Rays detected? Ground-Based Techniques:

-               EAS
- Extensive Air Showers

-                 ACT
- Air Cherenkov Telescope

- EAS Array

## Comparison of Gamma-Ray Detectors 

- _Large Aperture/High Duty Cycle_
- Milagro, Tibet, ARGO, HAWC

**Large Effective Area **
**Good Background Rejection**
**Large Duty Cycle/Large Aperture**

**Sky Survey 1-100 TeV****	****	****Extended sources****	****	****Unknown classes**
**Transients****	****	**
**Solar physics**

**Milagro**

## The Milagro    Detector

- Detect Particles in Extensive Air Showers from Cherenkov light created in 60m x 80 m x 8m pond containing filtered water
- Reconstruct shower direction to ~0.5° from the time different PMTs are hit
- 1700 Hz trigger rate mostly due to Extensive Air Showers created by cosmic rays
- Field of view is ~2 sr and the average duty factor is >90%

- Outrigger array added, finished in 2004

## Inside the Milagro Detector

- 728 PMTs of 8” dia. in a pond of  ~4000 m2 x ~8 m deep and 176 PMTs in ~6 m2 tanks spread over ~105 m2

## Background Rejection in Milagro

- Hadronic showers contain penetrating component: ’s & hadrons
-  Cosmic-ray showers lead to clumpier bottom layer hit distributions
-  Gamma-ray showers give smooth hit distributions

**Notes:**
*When a gamma event core falls on the pond, a lot of energy falls in a small region of the pond, making the event look proton-like, in turn making gamma-hadron separation difficult for these types of events, even though the angular resolution is good
*easier to separate gammas from hadrons for cores falling off the pond, but without ORs, the angular resolution is worse than events falling on the pond
=>ORs have improved off-pond ang res

## Background Rejection (Cont’d)

- A4 > 3 rejects 99% of Hadrons
- 			 retains 18% of Gammas
**	****S/B increases with increasing A****4**

- mxPE:	maximum # PEs in bottom layer PMT
- fTop:	fraction of hit PMTs in Top layer
- fOut:	fraction of hit PMTs in Outriggers
- nFit:	# PMTs used in the angle reconstruction

- Background Rejection Parameter

## 6.5 year data set (July 2000-January 2007)

**Distribution of Excesses in the Galactic Plane**

**cut level**

- Crab
- Nebula

- Mrk 421

- Weighted analysis using A4 parameter
- Crab nebula 15 
- Galactic plane clearly visible

- Cygnus
- Region

## Milagro has discovered 3 new sources & 4 candidate sources in the Galaxy. 

- Milagro has discovered 3 new sources & 4 candidate sources in the Galaxy.
- 5/7 of these TeV sources have GeV counterparts (only 13 GeV counterparts in this region - excluding Crab)
- Probability = 3x10-6

- Abdo, et al. ICRC 2007

## TeV Galactic Diffuse Emission

- Diffuse emission expected from cosmic ray interactions with matter and radiation
- Gamma rays probe cosmic rays outside the near Earth environment

- What is GLAPROP? What does optimized mean?

- -2<b<2

## The Cygnus Region

*Southern *
*Hemisphere*
*Sky*

- Standard Deviations

**Notes:**
Replace with Aitoff plot

## Diffuse Emission from Cygnus Region

- Not only Milagro’s TeV flux  exceeds predictions
- GeV gamma ray fluxes (EGRET) also exceed predictions based on local cosmic ray flux
- GALPROP (Strong & Moskalenko) model
- “optimized”: Fit to EGRET
- Increase 0 and IC component throughout Galaxy
- Milagro flux ~2.5*x* prediction
- Possible explanations:
- Hard spectrum cosmic ray sources?
- Unresolved point sources?

## What’s Next? From Milagro to HAWC

- High Altitude Water Cherenkov (HAWC)
- Increase Altitude to 4300 m from 2650 m
- Increase Area to 22000 m2 from 4000 m2
- Reuse Milagro PMTs and electronics
- HAWC ~15x Sensitivity of Milagro

- e

- 

- 

- HAWC Design:
- Single layer of 900 PMTs (4 m depth vs Milagro’s 2 layers at 1.5 and 6 m) optically separated by curtains or in individual tanks

## Site Exploration (selection July 2007)

- Mexico
- 2 hr drive from Puebla
- 4000 m elevation
- Site of the US/Mexico Large Millimeter Telescope

- Tibet
- 4300 m elevation
- Site of the Chinese/Italian ARGO detector

- Bolivia
- 1/2 hr drive from LaPaz airport
- 4800 m elevation
- Near Mt. Chacaltaya Cosmic Ray Laboratory

## Higher Altitude is Closer to Shower Max.

- 4300m

- 2600m

- Difference between 2600m (Milagro) and 4300m (HAWC):
- 	~ 6x number of particles
- HAWC’s median trigger energy ~ 1 TeV vs Milagro’s ~ 4 TeV

## Gamma/Hadron Separation

- Size of HAWC

- Size of Milagro deep layer

- Energy Distribution at ground level

- Larger Area implies better angular resolution + better cosmic ray background rejection

## Gamma-Ray Sensitivity 

- Gamma-Ray Sensitivity

- GeV

**VERITAS****, ****HESS****, ****MAGIC****, ****Whipple** sensitivity in 50 hours, (~0.2 sr/year)
**GLAST**
-    sensitivity in 1 year (4 sr)
**HAWC, ****Milagro**, sensitivity in 1 year (2  sr)

## Survey Sensitivity

## HAWC Complementary Science

- Complements TeV atmospheric Cherenkov telescopes
- Identifies new and flaring sources for follow up observation of morphology and sub TeV spectra
- Extends TeV spectra to higher energies
- Complements TeV neutrino observations
- Identifies new and flaring TeV sources to improve the sensitivity and interpretation of blind searches
- Complements GeV All Sky Survey
- Determines which of GLAST’s 1000s of sources extend to high energies

## Conclusion

- Milagro has demonstrated the power of the water Cherenkov technique
- Detection of Crab Nebula and Mrk 421, known TeV sources
- Discovery of 7 new TeV sources
- 1st detection of TeV diffuse emission from the Galactic plane
- Future:  HAWC
- Building on expertise with Milagro
- Design improvements in Size, Altitude, Curtains . . .
- 15x Milagro sensitivity
- Milagro /HAWC complementary to other particle astrophysics observatories

**Notes:**
Put 8 in context

## 0.1 GeV

**Milagro 10 TeV gamma-ray**

## TeV -rays: A New Window on the Sky

**0.1 GeV**

**Milagro 10 TeV gamma-ray**

## The Cygnus Region

- Crosses are EGRET sources
- Contours are Molecular (Dame et al, 2001)  and Atomic Hydrogen (Kalberla et al, 2005)
- TeV flux correlated with matter density

- Abdo *et al.*, astro-ph/0611691
- ApJ Letters, March 2007