First Light at the HAWC High-Altitude TeV Gamma-Ray Detector in Mexico

Daniel W. Fiorino University of Wisconsin-Madison, Department of Physics, 1 150 University Ave.,

Madison, WI, USA, 53705

The High Altitude Water Cherenkov (HAWC) Observatory - currently under construction at 4100m altitude at Pico de Orizaba in Mexico - is a 1003 duty cycle, large field of view detector for gamma rays at TeV energies. These gamma rays will reveal the underlying mechanics of cosmic ray accelerators. Data taking at our smaller test array (VAMOS) is currently under way. I will present an overview of the observatory and our science goals as well as the first sky map from our data.

1 Exploring A New Energy Frontier

The High-Altitude Water Cherenkov (HAWC) Gamma-Ray Observatory is a next-generation extended air shower array designed to observe TeV gamma rays and cosmic rays with an instan­taneous field of view that covers more than 15% of the sky. With this large field of view, the detector will be exposed to half of the sky during a 24-hour period. Located at an altitude of 4100 meters above sea level, HAWC will be used to perform an unbiased search of the gamma­ray sky within an energy range of 100 GeV to 100 TeV. HAWC is currently under construction near Puebla, Mexico in the Sierra Negra mountain range on the flank of the dormant volcano named Pico de Orizaba.

The HAWC detector comprises an array of 300 water-Cherenkov tanks. Each tank contains a light-tight bladder filled with 200,000 gallons of filtered water. The tanks are all four meters tall and have a seven-meter diameter. The inter-tank spacing will be about one meter on average. The array will cover 22,000 square meters. Four upward-facing photomultiplier tubes (PMTs) are installed at the bottom of every tank. The PMTs are used to collect the timing and charge distribution of Cherenkov light from air shower particles. The water-Cherenkov detection method allows for continuous observation resulting in a near 100% duty cycle.

HAWC will be used to detect diffuse and point sources of TeV gamma rays in hopes of uncovering vital clues to high-energy cosmic-ray production. The HAWC Observatory builds

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VAMOS Sky: October 2 0 1 1 - March 2012

Figure 1 : First Light: Equatorial-coordinate map of statistical significance from VAMOS reconstructed event data (I-degree bins).

upon the success of the Milagro Observatory. In the energy range of 10 to 50 TeV, Milagro cataloged many sources that had a previously-known lower-energy component 1 in addition to several new diffuse and point sources 2. HAWC will have a 15-fold increase in sensitivity with respect to Milagro due to an improved pointing resolution, energy resolution, and background rejection 3 .

HAWC will also serve as a monitor for time-dependent signatures like those from flaring active galactic nuclei or gamma-ray bursts(GRBs). If HAWC discovers that gamma-ray emission of GRBs extends up to several hundred Ge Vs, then many astrophysical properties can be deduced from the spectral cutoff such as the bulk Lorentz boost factor of GRB jets and the intermediate extragalactic background light spectra 4 .

2 Current Progress

In June 2011, a test array of seven tanks called VAMOS (Verification and Monitoring of Systems) began performing measurements of cosmic-ray rates. Between June 2011 and March 2012, 3.2 billion air shower candidates were observed. The sky map in Figure 1 is the result of a source detection analysis using these events. The site of the HAWC water tanks is fully leveled and tank construction is underway. Installation of the electronics will begin in June of 2012 and a fully operational 30-tank subarray is set to be finished by late fall of 2012.

Acknowledgments

This work has been made possible by the National Science Foundation and the United States Department of Energy. I would like to personally acknowledge the University of Wisconsin Alumni Research Foundation for their support.

References

1 . A. A. Abdo et al., Astrophysical Journal Letters 700 (2009) L127-L131 2. A. A. Abdo et al, Astrophysical Journal Letters 664 (2007) L91-L94. 3. http://hawc.umd.edu/ 4. A. U. Abeysekara et al, Astroparticle Physics 35 (2012) , pp. 641-650.

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