Transcript Document

The Milagro Gamma-Ray Observatory
James Linnemann and Aous Abdo
Department of Physics and Astronomy, Michigan State University
Milagro, the Spanish word for miracle, is a new type of astronomical telescope. Like conventional telescopes, Milagro is sensitive to light but the similarities end there. Whereas
"normal" astronomical telescopes view the Universe in visible light, Milagro "sees" the Universe at very high energies. The "light" that Milagro sees is in the TeV Range.
What is Milagro?
Milagro is a water Cherenkov extensive air shower (EAS) detector located near Los Alamos, NM at 2630m above sea level, consisting of a
~5,000 m2 central (pond) detector surrounded by an array of 175 instrumented water tanks, (outriggers) that span an area of roughly 40,000
m2. The Milagro detector has 723 photomultiplier tubes (PMTs) submerged in a 24 million liter water reservoir. The PMTs are arranged in
two layers, each on a 2.8 x 2.8 m grid. The top layer of 450 PMTs (under 1.4 m of water) is used primarily to reconstruct the direction of the
air shower. By measuring the relative arrival time of the air shower across the array, the direction of the primary cosmic ray can be
reconstructed with an accuracy of roughly 0.75o.The bottom layer of 273 PMTs(under 6 m of water) is used primarily to discriminate between
gamma-ray Initiated air shower and hadronic air showers. The sides of the reservoir are sloped (2:1) so that the area of the bottom of the
reservoir is smaller than the top, leading to the smaller number of PMTs in the bottom layer.
Why Milagro?
When one views the heavens in the TeV range the picture is quite different
from what we see when we look up at the night sky. The number of objects we
Bottom-layer tubes
see are much fewer and much more "extreme". We see super massive black
holes and neutron stars. Some of these sources are known to be highly
Top-layer tubes
variable, flaring on timescale of minutes to days. In addition we hope to
discover new sources of TeV photons, possibly observe TeV emission from
Gamma-Ray Bursts, discover primordial black holes, or discover completely
new phenomena. Until the advent of Milagro there was no instrument capable
of continuously monitoring the entire overhead sky in the TeV energy regime.
The existing instruments had to be pointed at small regions of the sky (usually
The Milagro pond with the cover inflated for servicing. known sources) and could only look at a source during the time of year it was
overhead at night. Even then they could only look at the source if the weather
was good and the moon was set. Milagro is ideally suited to monitor the
Cosmic Rays and Extensive Air Showers variable TeV Universe and discover new sources of TeV gamma rays.
Arial view of the Milagro detector.
The Earth is immersed in a "sea" of high-energy nuclei known as cosmic rays. Cosmic rays are composed of all nuclei, from the simple
hydrogen nucleus (a proton) to the iron nucleus and beyond (transuranic elements have been observed in cosmic rays). The energy
spectrum of cosmic rays has been measured up to 109 TeV. When a high-energy cosmic ray enters the atmosphere it loses its energy via
interactions with the nuclei that make up the air. At high energies these interactions create particles. These new particles go on to create
more particles, etc. This multiplication process is known as a particle cascade. This process continues until the average energy per particle
drops below about 80 MeV. At this point the interactions lead to the absorption of particles and the cascade begins to die. This altitude is
known as shower maximum. The particle cascade looks like a pancake of relativistic particles traveling through the atmosphere at the speed
of light. Though the number of particles in the pancake may be decreasing, the size of the pancake always grows as the interactions cause the particles to diffuse away from each
other. When the pancake reaches the ground it is roughly 100 meters across and 1-2 meters thick. If the primary cosmic ray was a photon the pancake will contain electrons,
positrons, and gamma rays. If the primary cosmic ray was a nucleus the pancake will also contain muons, neutrinos, and hadrons (protons, neutrons, and pions). The number of
particles left in the pancake depends upon the energy of the primary cosmic ray, the observation altitude, and fluctuations in the development of the shower.
AGN
Shadow of the Moon
The shadow of the Moon in cosmic
rays can be used to determine the
performance
characteristics of
Milagro. At TeV energies the Moon’s
shadow is offset from the actual
position of the Moon because the
cosmic rays are bent in the earth’s
magnetic field. From the position
and shape of the observed shadow
one can determine the angular
resolution of the detector and the The Shadow of the Moon as
observed by Milagro.
absolute energy response of the
detector.
Active galaxies emit
radiation over the entire electromagnetic spectrum

from radio waves to TeV gamma rays. Thermal emission emanates
from the accretion disk (infrared to X-rays) and the torus (infrared).
Non-thermal emission (radio and gamma rays) comes from the jets.
One of the more exciting discoveries of the 1990s has been the
observation of TeV emission from several AGNs. TeV emission has
been observed from Mrk 421, Mrk 501, and 1ES2344+514,
1ES1959+65 . Mrk 501 and 1ES2344+514 are the first gammaray sources discovered by ground-based instruments. Milagro
data was used to study Mrk 421 while it was flaring during the period
of January to April of 2001 and again in November of 2002. During
Mrk 421 during the 2001 flare.
the 2001 period we observed a 4.7s excess and during the 2002 flare a
3s excess.
All-Sky Survey
In a manner identical to that used to analyze data from the region of the Crab nebula, the entire sky is
searched for excesses over the background cosmic rays using data from Dec. 15, 2000 to Dec. 15, 2001.
The sky is binned into 0.1x0.1 degree bins and the expected background and actual number of events
detected in each bin is determined. These small bins are then summed into larger bins, commensurate
with the angular resolution of Milagro. The resulting sky map is shown in the Figure. The circles are
drawn around 26 active galaxies identified in Costamante and Ghisellini 2002 as likely sources of TeV
gamma rays, including the five, which have all been observed at TeV wavelengths by other
observatories: the Crab nebula, Mrk 421, Mrk 501, 1ES1426+428, and 1ES2344+514 . The brightest
point in the TeV sky over this time period was Mrk 421. Most of the observed signal in this data set came
from an outburst that began in December of 2000 and lasted for several months. The next brightest point
in the sky is not associated with any of the drawn circles and is to the northwest of the Crab.
Map of the Northern sky in TeV gamma rays. The scale is the
significance of each point in the sky. The circles mark the locations
of AGN and known TeV sources. Mrk 421 is the brightest object in
the sky over this data set.
The Crab Nebula
The Crab nebula was the first source convincingly
detected in TeV gamma rays . Since the original
detection in 1989 the Crab has become the standard
candle of TeV astronomy. The luminosity of the
Crab is constant (within the accuracy of the
measurements made to date) at 2.68(±0.42stat
±1.4sys)x10-7 (E/1TeV)-2.59 m-2 s-1 TeV-1. As a
standard candle it is useful for cross calibrating the
sensitivity of different instruments. From the shadow
of the Moon and Monte Carlo simulation of the
detector the angular resolution of Milagro is 0.8
degrees. The square angular bin that maximizes the
significance of a signal has a width 2.8 times the
angular resolution of the detector . Therefore an
angular bin of width 2.1 degrees is used in this
analysis.
Data taken in the Crab Nebula
region with 6 s in the position of the
Crab.
The Galactic Plane
Diffuse emission from the galactic plane is the dominant source in the MeV
gamma ray sky. Milagro detected, for the first time, the galactic plane in the
TeV range. The emission seems to be concentrated in the Cygnus region
Other topics that we are currently studying include: the study of Gamma Ray Bursts; Solar physics; and Dark Matter. The Milagro collaboration consists
of more than ten institutions. To date more than ten Ph.D. theses have been completed.