1. Proposal Submitted to NSF/DoE HEP Given the large increase in sensitivity for a moderate cost, we have decided to propose to build HAWC. A proposal was submitted to NSF and DoE HEP. A strong collaboration has been formed and detailed hardware design and cost estimates have been obtained. Due to the remoteness of the site, the design must be robust and remotely operable using computer based interfaces. Our experience with Milagro has proven that we can build such a detector and it will be successful. HAWC (High Altitude Water Cherenkov) Observatory for Surveying the TeV Sky Brenda Dingus (dingus@lanl.gov) for the HAWC Collaboration: Los Alamos National Lab (Brenda Dingus, Gus Sinnis), Univ. of Maryland (Jordan Goodman, Andrew Smith, Buckley Hopper, David Berley, Greg Sullivan), Univ. of Utah (David Kieda, Stephan LeBohec, Miguel Mostafá), Univ. of New Mexico (John Matthews, W. Miller), UC Irvine (Gaurang Yodh), UC Santa Cruz (Michael Schneider), Michigan State Univ. (Jim Linnemann), Penn. State Univ. (Ty DeYoung), Univ. of New Hampshire (Jim Ryan), NASA/GSFC (Julie McEnery), UNAM (Magda Gonzaléz), INAOE (Alberto Carramiñana), BUAP (Humberto Salazar) Abstract The HAWC observatory is a proposed, large field of view (~2 sr), high duty cycle (>95%) TeV gamma-ray detector which uses a large pond of water (150 m x 150 m) located at 4300 m elevation. The pond contains 900 photomultiplier tubes (PMTs) to observe the relativistic particles and secondary gamma rays in extensive air showers. This technique has been used successfully by the Milagro observatory to detect known, as well as new, TeV sources. The PMTs and much of the data acquisition system of Milagro will be reused for HAWC, resulting a cost effective detector (~5.5M$) that can be built quickly in 2-3 years. The improvements of HAWC will result in ~15 times the sensitivity of Milagro. HAWC will survey 2  sr of the sky every day with a sensitivity of the Crab flux at a median energy of 1 TeV. After one year of operation half of the sky will be surveyed to 50 mCrab. This sensitivity will likely result in the discovery of new sources as well as allow the identification of which GLAST sources extend to higher energies. Building on the Expertise of Milagro Milagro is located at LANL and is the most sensitive wide field of view, very high energy, gamma-ray detector in the world. Milagro was built and is operated by a collaboration of six universities and LANL. The analysis of Milagro data has lead to the first observation of TeV emission from the plane of the Milky Way [Atkins, R. et al. 2005, Physical Review Letters, 95(25), 251103]. Milagro Observation Technique Showers of particles initiated by very high energy gamma rays are detected in a 3500 m 2 pond of water with photodetectors observing Cherenkov light. Timing the shower front with sub-nsec accuracty measures the direction of the gamma- ray source to ~0.5 degrees. Detection of particles penetrating the 7 m deep pond allows rejection of the hadronic cosmic ray background. High Altitude Site When a gamma-ray interacts in the atmosphere, it produces a shower of particles. The number of particles in the shower increases as it penetrates into the atmosphere until a point, called shower maximum, after which the particles begin to range out. Shower maximum is typically over 10 km above the Earth’s surface for very high energy gamma rays. Because the ability to detect a shower depends on the number of particles, the sensitivity of a gamma-ray detector improves when it is placed at as high an altitude as possible. Several high altitude sites have been investigated at elevation above 4000 m (13100 ft). No suitable site exists in the United States due to winter weather conditions and environmental considerations. Mexico: Sierra Negra 4030 m elevation Tibet: Yang Ba Jing 4300 m elevation Figure 4 Comparison of  -ray sensitivity between HESS and HAWC 2 year sky surveys as a function of source angular diameter. The HESS detected sources are shown. Figure 5 Flux limits for HAWC versus the HESS sky survey and the proposed VERITAS sky survey over the next two years. HAWC is assumed to be located in Mexico (19 O N). HESS & VERITAS sensitivity is shown for point (red) sources and for sources extended by 0.25 O (green). A Crab-like spectrum is assumed. The HAWC limits will be lower for harder sources. Figure 6 Comparison of the flux necessary for a GLAST detection of 5  -rays above 10 GeV with the HAWC 5  detection threshold for a source differential photon flux of spectral index -2 that is cut off due to extragalactic background absorption. The absorption is calculated assuming the model of Kneiske, et al. 2005, and the energy at which the flux is attenuated by 1/e is 700, 260, and 170 GeV for z=0.1, 0.3, and 0.5, respectively. The gap between the lines on the left and right is due to the Earth blocking the view of the source. © A. Simmonet Site Preparation (Electricity, Internet, Water)$0.5M Pond & Liner$1.1M Cover or Building$1.9M PMT Refurbishment & Calibration System$0.4M Cabling,Electronics, Computers$0.7M Contingency 20%$0.9M Total$5.5M Gamma Area:  5.0,  <1.0 O 200PMT Trigger 80 PMT Trigger 20 PMT Trigger Pond Area Figure 1 Effective Area (after passing  /hadron and angular resolution cuts) for three different triggers thresholds. Effective Area Design of HAWC HAWC would use the 900 Milagro photomultiplier tubes and much of Milagro’s data acquisition system in a pond that is 6 times larger than Milagro and placed at a altitude of ~4300m above sea level. The elevation of Milagro is 2650 m. The design and sensitivity of HAWC is based on the same Monte Carlo technique that has confirmed the performance of Milagro. Overhead View of Milagro and HAWC detectors Side View of HAWC detector e   150 meters 4 meters Milagro HAWC MilagroHAWC Detector Area4000 m 2 (surface) 2500 m 2 (muon) 22,500 m 2 Time to 5  on the Crab 120 days1 day Median Energy4 TeV1 TeV Angular Resolution0.40 0 – 0.75 o 0.25 o – 0.50 o Hadron Rejection eff.90%95%-99%* Gamma-Ray Efficiency50% Q for gamma/hadron rejection1.62.5-5* Time to detect 5 Crab flare at  5 days10 minutes Eff. Area at 100 GeV5 m 2 100 m 2 Eff. Area at 1 TeV10 3 m 2 20x10 3 m 2 Eff Area at 10 TeV20x10 3 m 2 50x10 3 m 2 Volume of Universe where 10 -6 erg/cm 2 GRB detectable 2 Gpc 3 47 Gpc 3 Flux Sensitivity to a Crab-like source (1 year) (5  detection) 625 mCrab50 mCrab Cost$3.3M$5.5M Science Goals HAWC will monitor (for >4 hours every day), every point in ~ 2  sr of the sky. Over a 5 year observation period HAWC will perform an unbiased sky survey with a detection threshold of ~20 mCrab, enabling the monitoring of known sources, the discovery of new sources of known types, and the discovery of new classes of TeV gamma ray sources. The sensitivity of HAWC to extended sources surpasses that of IACTs for sources larger than 0.25 o. HESS observes many diffuse galactic sources clustered around their sensitivity limit, they may only be seeing “the tip of the iceberg”. With HAWC’s sensitivity at high energies, we will probe the knee of the cosmic ray spectrum answering questions about the origin and propagation of cosmic rays. With the sensitivity to detect a flux of 5 times that of the Crab in just 10 minutes over the entire overhead sky, HAWC will observe many flares from AGN. While an important measurement in its own right, this will also enable many multi- wavelength observations of these flares. IceCube sensitivity can be significantly enhanced by knowing the location and time of flares. HAWC’s sensitivity to the prompt emission from gamma-ray bursts is unique. With HAWC’s low energy threshold, GRBs with a TeV fluence comparable to their keV fluence will be detectable to a redshift of ~1, while for closer GRBs much lower fluences can be detected. HAWC’s sensitivity to transient phenomena will extend the field of time-domain astrophysics to TeV energies. Figure 3 Sensitivity of HAWC (1 year – solid blue) and HESS (50 hrs - red), and Milagro (dashed blue) as a function of spectral index. While the sensitivity of HAWC to the Crab, a relatively soft source, is ~3 times worse than HESS, the sensitivity of both instruments to point sources with hard spectra is comparable. Point Source Sensitivity Extended Source Sensitivity Survey Sensitivity Transient Sensitivity Gamma Ray Burst Sensitivity Figure 7 Fluence sensitivity as emitted at the source for a 5  detection of a 10 second GRB vs. redshift for HAWC (left) and Milagro (right). The different color lines indicate the sensitivity for GRBs at different zenith angles. The superimposed triangles indicate the keV-MeV fluence and redshift of satellite detected GRBs. Figure 2 Angular resolution for HAWC. Three trigger thresholds are shown. Angular Resolution Comparison of HAWC and Milagro HAWC Budget The total cost of HAWC is ~< $5.5M. The reuse of the 900 8” PMTs and electronics from Milagro reduces the cost of HAWC. The other costs are derived from current prices and are consistent with inflation adjusted costs paid by Milagro. Milagro received $2.7M from DoE and NSF over a 6 year period beginning in 1994 and an additional $0.6 M from NSF in 2002-2004 for the construction of the outriggers. Milagro used an existing pond but did have to purchase a new liner and cover in addition to the PMTs and electronics. HAWC Milagro Comparison of HAWC with Other  -ray Observatories Figure 8 – Integral Flux sensitivity vs minimum energy for various  -ray observatories to a point source. For the pointed telescopes— Whipple, HESS, and VERITAS—a 50 hr observation was assumed. For the wide field of view observatories—GLAST, Milagro, and HAWC—a one year observation of was assumed. GeV