The HAWC Site
The HAWC site is inside the Parque Nacional Pico de Orizaba, a Mexican national park comprising Citlaltepetl or Pico de Orizaba, the highest peak in Mexico at 5610m, and Sierra Negra, a 4600m volcano 7km SW Citlaltepetl. The Large Millimeter Telescope (LMT) is located on top of Sierra Negra, and HAWC will be located on a 200m x 450m plateau near the saddle between the two peaks. The exact geographical coordinates of the site are latitude 18º59’41”N, longitude 97º18’28”, altitude 4100 meters above sea level. The latitude of the Sierra Negra site provides an excellent visibility of celestial objects: HAWC will see 15% more of the celestial sphere within a 45º field of view compared to Milagro. When considering a cone of 45º the survey solid angle reaches 8.4sr, or 2/3 of the entire sky. The Crab culminates at 3º from the zenith and will be visible for slightly more than 6 hours each day; Cygnus reaches 20º zenith angle and the Galactic plane will be covered such as to include over a dozen of the VHE sources observed by HESS. Even the Galactic Center, at 48º from zenith, will be observable at the highest energies. The coverage of HAWC will have a 90% overlap with that of IceCube. The longitude of the site is also favorable, as its visibility has good overlap with observatories in the US, Mexico and Chile, which can be promptly alerted of any interesting activity and can aim to simultaneous observations of objects in the field of view of HAWC.
Climate
High sites with tolerable conditions are scarce. The site is located close enough to the equator to have weather conditions as benign as could be wished for its altitude. Weather conditions have been monitored for over six years at the summit of Sierra Negra, 500 meters above the HAWC site with a horizontal distance of 1km. The median temperature (adding the 6.5º thermal gradient to the 4600m measurements) becomes 4.3ºC for the site, with sub-zero temperatures only 5% of the time (specifically 10% of the time during winter). Water freezing inside the detector will not be an issue. Wind velocities are generally mild, with a median of 4 m/s for the recorded data. Occurrence of wind above 10 m/s is rare. Still, the recent passage of a hurricane Dean some 100 km North of the site provided winds up to 150 km/h, the largest measured in the 6 years of meteorological monitoring. The Large Millimeter Telescope has been designed to withstand winds up to 250 km/h, equivalent to a hurricane of category 5, which is not considered possible for a site over 4km high and 100 km inland.
Site accessibility
The coordinates of the HAWC observatory are defined tentatively, with over 200m of freedom in approximately the EW direction, as the northern base of the Sierra Negra can fit a square of (200m)2 in more than one location. An even a larger rectangle of 450m ´ 200m can be placed between two 20m topographic contour levels.
The HAWC site is located just over 2 hours from Puebla, a city of 2 million inhabitants with a relatively small international airport currently in expansion. Puebla itself is 2 hours by road from either Mexico City (to its East) or the HAWC site (to its West). Most of the distance between Puebla and the LMT HAWC site is through the Puebla-Veracruz motorway, with the last 40 minutes on minor roads. Veracruz is a major international port within 2.5 hours drive to the site.
The LMT project required an access road wide enough to allow transportation of items up to 6m wide. Electricity and internet have been installed up to the top of the mountain. The road and infrastructure would need to be extended 1 km to reach the HAWC site over mostly flat terrain.
The Large Millimeter Telescope
The Large Millimeter Telescope (LMT) is the largest scientific project ever undertaken in Mexico(120M$) and was constructed by a joint US/Mexico Collaboration. LMT is a single dish 50 meter telescope for millimeter-wave astronomy located at 4600 meter and due to operate in the frequency range of 80 to 350 GHz. Sierra Negra was the highest of close to twenty candidate sites monitored for water vapor content in the atmosphere and was selected as the LMT site in February 1997. Construction of the telescope began in 2000, with the antenna inaugurated by President Fox in November 2006. The surface of the telescope is presently being completed, set and tested while the LMT scientific instruments prove their performance in other telescopes.
The construction of the LMT required the development of the site infrastructure. The electric grid was extended 13 km to reach the LMT site to supply up to 1 MegaWatt of power during operation, with a potential peak supply of 5 MW. The road was constructed mostly during 1998-1999 to be able to allow pieces up to 6m wide and has been continuously improved. A fiber optic Internet line running parallel to the electric power has been set and is currently kept to just 2 Mb/s, but will be expanded once LMT enters the operation phase. The LMT installations are already able to lodge scientists in oxygen enriched areas. However, staying at the site is not encouraged and a base camp will be set at a lower altitude.
Sierra Negra Consortium
The construction of the LMT and the development of the Sierra Negra site brought the opportunity for other instruments to benefit from the high altitude site. Nine such facilities are in different stages:
- the Telescopio de Neutrones Solares in a solar neutron telescope installed by the Instituto de Geofísica of UNAM and in operation since 2005. It detected a major solar event in September 2006.
- RT5 is a 5m radio telescope in construction, due to perform daily monitoring of the Sun at 43 GHz during daytime and astronomical observations during nighttime. It can also function as test-bed for LMT instrumentation. RT5 is a joint project of INAOE and the Institutos de Astronomía and Geofísica of UNAM.
- two Cerenkov telescopes formerly part of the HEGRA array will be installed at the top of Sierra Negra, at about 1 km horizontal distance from HAWC. These will monitor blazars during the GLAST era and can also complement HAWC observations.
- the Instituto de Física of UNAM is to build an antineutron detector.
- the University of Puebla (BUAP) has set an array of cosmic ray detectors on the top of the mountain. These are small water tanks with individual PMTs. BUAP is also setting an array of larger tanks in the slope of Citlaltepetl to be complemented with a fluorescence detector at Sierra Negra.
- non astrophysical facilities include a seismological station from BUAP already operational, a greenhouse gas monitor from the Climate Institute and a geo-reference point from INEGI, the latter two in planning stage.
Together with HAWC and LMT, these facilities are members of the Sierra Negra Consortium (CSN, for its abbreviation in Spanish), a non-profit organization charged with organizing the joint operation of the site. The CSN will act as a provider of common services like site access, electricity, Internet, communications (with special consideration to preventing RFI to the LMT), water supply, security (there is a gate house with a 24 hour a day guard), etc. and the respective maintenance; in exchange the consortium members will cover their share of the operations cost, according to the location and characteristics of each experiment.
Site permission
The HAWC site is inside a National Park, with the land formerly owned by the Mexican Federal government. Permission for using the site can be granted by the Secretaría del Medio Ambiente y Recursos Naturales (SEMARNAT), the federal body in charge of the environment. An environmental impact declaration for the installation of HAWC in the Parque Nacional Pico de Orizaba was submitted to SEMARNAT on June 2007 and a conditioned permission was granted in September 2007. The permit allows a construction phase of three years and up to ten years operation, allowing the installation of the experiment, its peripheral infrastructure and water acquisition systems. A separate permit is required for the construction of the 1km access road and power line, which we plan to apply for by November. The conditions imposed to the project through the SEMARNAT permit are directed to minimize the environmental impact and to compensate it through a reforestation program related to the HAWC project. The permit is online at the HAWC proposal web site.
Water availability
This proposal includes funding to pump water for HAWC from a nearby valley. However, given the location and the reasonably high precipitation in the region (100 cm/year with a marked seasonal dependence), we will likely be able to acquire the water locally, through a deep well nearer to the HAWC site or a water capture system. A 3D topographic model of the region was constructed to model water flows in the vicinity of the site and was complemented with geo-electrical studies to determine the most suitable location for an extraction well. The demands on the capacity of the well are reasonable: considering the volume of water required for the 900 tanks the demand is only 2.4 liter/second for acquiring the water within one year using this method alone. Extending the time for water acquisitions relieves the demands on the well with the same proportion.
We already have defined the position for a test well and plan to perform its drilling in the next month. Depending on the water extractable, the test well can be expanded to a full extraction well. This will require widening its diameter, installing a high power pump and about 2km of pipes to transport the liquid to the HAWC observatory.
A second water acquisition option is a capture system placed below a convergence point, a natural nozzle, of water running down Citlaltepetl Sierra Negra, which was identified some 7 km WNW from the the HAWC site. This capture system will be a large concrete parallelepiped, which would be particularly efficient during the rain season (May-October), while the well would provide water in a more continuous