( 1) We Would Like a Clarification of Your Science Goals. in Particular, What Do You Expect

(1) We would like a clarification of your science goals. In particular, what do you expect to learn from studying the diffuse emission?

Will you be able to address the question of whether cosmic rays originate in supernova remnants? HAWC will observe 2π sr of the sky with 15 times the sensitivity of Milagro and with 6 times more sensitivity than the HESS observatory at energies above 10 TeV for a 50 hour HESS observation of a single source. The highest energy gamma rays are more likely produced by hadrons than electrons because the Klein-Nishina drop in the inverse Compton scattering cross section reduces the flux of > 10 TeV gamma rays, and because the energy loss rate increases with electron energy limiting the maximum energy attainable for electrons. In contrast, the standard “leaky box” model of cosmic rays requires hadrons to be accelerated to at least the knee (>3000 TeV) with a hard spectrum of differential index of ~-2. When these hadrons interact with matter near their source (before experiencing the energy dependent leakage out of the Galaxy), they will produce gamma rays of energy ~1/10 the hadron energy and consequently this same hard spectrum. Therefore, hard spectrum gamma-ray sources extending beyond 300 TeV are the expected signatures of the sources producing hadronic cosmic rays.

Gamma rays produced by hadrons should also be coincident with regions of higher density matter. In the case of localized sources, the HAWC detections will point to regions requiring deeper observations with atmospheric Cherenkov telescopes to map the TeV emission and correlate it with lower energy maps indicating potential astrophysical sources, such as SNR, and their nearby clouds. However, HAWC will be much more sensitive than atmospheric Cherenkov telescopes for mapping the gamma ray emission from the cosmic rays that have propagated away from their sources. The GALPROP model predicts the gamma-ray emission from this sea of cosmic rays assuming supernova remnants are the source of cosmic rays. HAWC will detect the flux as predicted by the “conventional” GALPROP model (which is tuned to reproduce the observed cosmic ray flux at Earth and matches the GeV emission observed by Fermi at mid Galactic latitudes) over a large range of the Galactic plane with > 5 sigma in 5 x 5 square degree bins.

Milagro observations of the diffuse emission from the Galactic plane are 5 to 8 times larger, depending on the region of the Galaxy, than the expectations of this GALPROP model. However, these Milagro observations are an upper limit on the diffuse flux due to the contribution of unresolved sources. The Milagro flux measurement is obtained by subtracting the currently resolved sources, which contribute ~25% of the total flux. A recently discovered correlation of Fermi GeV sources and weaker Milagro sources (Abdo, et al. arXiv:0904.1018, submitted to ApJ Lett) indicates another 10-20% of the flux is likely due to localized sources. VERITAS will soon survey much of this region to resolve more sources and HAWC will characterize their higher energy emission. As we learn more about these TeV sources we will constrain the luminosity function and make more accurate predictions of the total flux of unresolved sources. HAWC, due to its geographic latitude, will then be able to measure the diffuse emission from Galactic longitude of 10-100 degrees and in the anticenter. The HAWC flux and spectra will measure the sea of cosmic rays in these very different regions of the Galaxy and provide important constraints to our understanding of cosmic rays.

(2) What is the state of the Milagro equipment (phototubes and electronics)? What has to be done to refurbish these components?

Milagro PMT and electronics are in very good shape. We lost only a few PMTs in the entire running period of Milagro. Some of the bases, particularly for those tubes in the outriggers where the temperature cycling was substantial, need to have some of the resistors replaced and this process is on-going. Also many of these tubes will have to have the back cover of the encapsulation replaced as well. Another issue is that Milagro started with underwater connectors on the tubes. A substantial number of these failed (leaked). We developed a better method of connecting to the PMTs that did not involve a connector. (It uses a plastic pipe and water-proof shrink wrap). During the course of Milagro we replaced all the muon (bottom) layer tube connectors and some of the air shower (top) layer connectors. Still about 50% of the PMTs had connectors at the end of the experiment and all of those will be replaced. Again this activity is on-going.

The front-end boards are the one component of the electronics we are planning to reuse. These were custom built by UCSC and have had virtually no failures. We have 1200 channels of these boards. These distribute HV, pick off signals, generate trigger signals, and do time over threshold pulse integration. The output of these boards was fed to multihit Fastbus TDCs in Milagro. In HAWC we will replace the Fastbus with CAEN VME TDCs, but use the same boards. The one modification planned for the boards involves changing a shaping time constant on the signal to match the shorter pulses in HAWC. This is just replacing a capacitor. We are confident these boards will work fine for HAWC. However, in the long run we may want to develop new front-end boards. If we ever want to expand past 1200 channels, we will need new boards. While there is a feeling that waveform digitization might be useful, there is as of yet little evidence this would make much of a difference, but it clearly would require a much greater effort in data acquisition and data handling.

(3) Please expand on the characteristics of the proposed site. What issues do the sub-zero temperatures present for the experiment?

The HAWC site is a plateau near the saddle between Sierra Negra, a 15000 ft peak where the LMT is situated, and the Pico de Orizaba. The site is treeless and with bushes and grasses. The surface soil is a fine volcanic ash with firmer soil below and few large rocks.

Since the site is a volcanic cone surrounded by tropical regions (towards the Gulf) and semitropical (towards Puebla), the median temperature at the site at 4100m altitude is 4.3ºC. At the HAWC site sub-zero temperatures exist only 5% of the time (specifically 10% of the time during winter, which is not enough to cause a freezing problem for the tanks). However, if the water does freeze it will not impact the operation of the detector. We know this because the Milagro detector was constructed and operated in a much colder environment near Los Alamos, NM. The Milagro outrigger tanks were much smaller than the HAWC tanks and only the top surface froze annually. Freezing did not impact the performance of the detector. In HAWC the PMTs will be at the bottom which will never freeze. Shown in the figure is the fraction of time above a given temperature for the LMT site at 4600m altitude, where the median temperature is 1.05 ºC. The 4100m HAWC site is 3.25 ºC degrees warmer, so the time below 0 ºC at HAWC will correspond to the time below -3.25 ºC at Sierra Negra.

These figures show the temperature at the test tank in Los Alamos (where the winter is much colder than the site in Mexico). The second one shows the temperature at the end of the winter. We're just missing the data from January. But the point is that the 'bottom' sensor, closest to the PMT location, never came close to freezing.

(4) What infrastructure will be provided at the site?

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 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 (see below) required an access road wide enough to allow transportation of items up to 6m wide. Electricity and an internet fiber have been installed up to the top of the mountain. The road and infrastructure are currently being extended 1 km from just above the guard house to the HAWC site over mostly flat terrain.

HAWC is located very near the Large Millimeter Telescope (LMT) which 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 and has been continuously improved. There is a fiber optic Internet line running parallel to the electric power lines. 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 for LMT and HAWC will be set up at a lower altitude.

At the HAWC site we will have an electronics trailer (moved from the Milagro site) that will be refurbished and will be oxygenated. We will also have a 20m x 6m building for pumps, storage and a workshop (provided by INOAE).

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 the University of Nagoya and is 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 (Omega) formerly part of the HEGRA array will be installed at the HAWC site. 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 and a greenhouse gas monitor from the Climate Institute likely to start installation work in the next couple of months.

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.

(5) When will the choice of tank design be made? What are the manpower requirement for and schedule implication of building the tanks? What are the challenges?

Using our MRI, we have studied both HDPE and steel tanks with Hypalon bladders. The results of these studies show us that while either would work there are substantial advantages to the steel tanks with bladders and as a result we have modified our baseline design to use this design. Unless our continuing tests show some unforeseen problem, we are planning to use the steel tanks with the HDPE as a fallback. Below we give some details of the study that lead to this choice, but the summary is that HDPE tanks that are big enough are very expensive and difficult to transport. In addition, incremental diameter or height changes add significantly to the cost. Finally, the HDPE tanks of this size are not routinely made. On the other hand, steel tanks of this diameter (and much larger) with liners are made commercially and used to store clean water. They are manufactured in pieces and shipped in containers to the site where they are assembled. There is an economy of scale where making a bigger tank to hold several tubes is less expensive than several smaller ones. These tanks have been engineered to work in zones with high seismic activity.

HDPE tanks:

· We learned how to make them light tight and UV stable for 10 years plus.

· We have 3 companies willing to built them 20miles away to reduce transportation cost.

· But the transportation for larger than 3.6 m diameter is very difficult If not impossible

· Addressing seismic stability will increase the price by adding stronger walls and hold downs, if required. (There are no regulations on single HDPE tanks).