New Gamma-ray Observatory Begins Operations
at Sierra Negra Volcano
The High-Altitude Water Cherenkov
(HAWC) Gamma Ray Observatory has
begun formal operations at its site
in Mexico. HAWC is designed to study
the origin of very high-energy
cosmic rays and observe the most
energetic objects in the known
universe. This extraordinary
observatory, using a unique
detection technique that differs
from the classical astronomical
design of mirrors, lenses, and
antennae, is a significant boost to
international scientific and
technical knowledge.
The HAWC Observatory taken in August
2013 from the summit of Sierra
Negra. The image has been digitally
altered to show HAWC as it will
appear when construction is complete
in 2014. The 111 Cherenkov detectors
currently installed (100 Cherenkov
detectors in operation) are colored
white and located in the upper right
quadrant of the array.
The HAWC Observatory taken in August
2013 from the summit of Sierra
Negra. The image has been digitally
altered to show HAWC as it will
appear when construction is complete
in 2014. The 111 Cherenkov detectors
currently installed (100 Cherenkov
detectors in operation) are colored
white and located in the upper right
quadrant of the array.
The HAWC Observatory taken in August
2013 from the summit of Sierra
Negra. The image has been digitally
altered to show HAWC as it will
appear when construction is complete
in 2014. The 111 Cherenkov detectors
currently installed (100 Cherenkov
detectors in operation) are colored
white and located in the upper right
quadrant of the array.
"The HAWC observatory will search
for signals from dark matter and to
study some of the most extreme
objects in the universe, such as
supermassive black holes and
exploding stars," said Brenda
Dingus, principal investigator and a
research fellow at Los Alamos
National Laboratory. Dingus is a
Fellow of the American Physical
Society, and in 2000 was a recipient
of the Presidential Early Career
Award for Scientists and Engineers.
HAWC is located at an altitude of
4100 meters on the slope of the
volcanoes Sierra Negra and Pico de
Orizaba at the border between the
states of Puebla and Veracruz. The
observatory, which is still under
construction, uses an array of
Cherenkov detectors to observe
high-energy cosmic rays and
gamma rays. Currently 100 out of
300 Cherenkov detectors are deployed
and taking data. Each Cherenkov
detector consists of 180,000 liters
of extra-pure water stored inside an
enormous tank (5 meters high and 7.3
meters in diameter) with four highly
sensitive
light sensors fixed to the
bottom of the tank.
"Los Alamos has a long history of
working in this field and built the
predecessor to the HAWC observatory,
called Milagro, located at the Los
Alamos site in New Mexico," Dingus
said.
HAWC 15 Times More Sensitive
Than Predecessor
"HAWC will be more than 15 times
more sensitive than Milagro was, and
it will detect many new sources of
high-energy photons. Los Alamos also
studies these high-energy phenomena
through complex
computer simulations to
understand the physical mechanisms
that accelerate particles to
energies millions of times greater
than man-made accelerators," Dingus
said.
The construction and operation of
HAWC has been made possible by the
financial support of several Mexican
institutions such as the Consejo
Nacional de Ciencia y Tecnología
(CONACYT), the Universidad Nacional
Autónoma de México (UNAM), and the
Instituto Nacional de Astrofísica,
Óptica y Electrónica (INAOE).
Funding has also been provided by
the United States through the
National Science Foundation (NSF),
the Department of Energy (DOE)
Office of Science, the Los Alamos
National Laboratory (LANL), and the
University of Maryland. The
University of Maryland is the
managing institute of the project
overall.
The HAWC array, operating with
100 Cherenkov detectors since August
1 and growing each week, will be
sensitive to high-energy particles
and radiation between 100 GeV and
100 TeV, energy equivalent to a
billion times the energy of visible
light.
In 2009, HAWC was identified as
the Mexican astronomical project
with the highest expected impact on
high-energy astrophysics. Shortly
thereafter a test array with three
Cherenkov detectors was installed at
the volcano Sierra Negra and
successfully observed cosmic rays
and gamma rays. Following these
early tests, a prototype array of
seven Cherenkov detectors was built
in 2009 to test the tank design,
simulate real data-taking, and study
the logistics of deploying a
large-scale observatory in this
remote location. In 2012, the first
30 of 300 HAWC detectors were
deployed, and since that time have
been operated nearly continuously.
The 30-detector stage of HAWC
permitted calibration of the
observatory via the observation of
the shadow of the moon as it blocked
cosmic rays. (1.usa.gov/14jjT8w)
Today, the scientific team of
HAWC will formally begin
observations of the most violent
phenomena in the known universe,
such as supernovae explosions and
the evolution of supermassive black
holes.
Background: The Most Energetic
Particles in the Known Universe
Gamma rays (electromagnetic
radiation of very high frequency)
and cosmic rays (subatomic particles
of very high energy) are products of
the most energetic and cataclysmic
events in the known universe. These
phenomena include the collisions of
two neutron stars, the explosions of
supernovae, binary systems of stars
with stellar accretion, and active
gal actic nuclei which host black
holes millions of times more massive
than the sun.
When high-energy
cosmic rays and gamma rays reach
the Earth, they interact with air
molecules in the upper atmosphere.
Gamma rays, for example, are
converted into pairs of charged
matter and anti-matter particles
(mainly electrons and positrons).
These particles rapidly interact
with other air molecules, producing
additional gamma rays of reduced
energy, which then create further
charged particle pairs. This chain
reaction proceeds until a large
cascade of particles and radiation
reaches ground level, where it can
be recorded in the HAWC detectors.
When the charged particle cascade
from an extra-terrestrial gamma ray
passes through a Cherenkov detector,
its particles are traveling faster
than the speed of light in water.
The resulting effect is similar to
the shock wave produced in the
atmosphere by a supersonic airplane
(a "sonic boom"), but instead of
producing sound the particles
produce a visible cone of light. The
flash of light, called Cherenkov
radiation, is measured by the light
sensor fixed to the bottom of each
detector in HAWC. By combining the
light signal observed in many tanks
with fast electronics and high
precision computing equipment, it is
possible for scientists to determine
the time of arrival, energy, and
direction of the original
extraterrestrial gamma ray or cosmic
ray.