WoU-MMA: Galactic Gamma-Ray Astrophysics with HAWC (UW-Madison Group Grant)

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The High-Altitude Water Cherenkov Observatory (HAWC) is located at 4100 m above sea level near Puebla, Mexico. The detector has monitored the gamma-ray and cosmic-ray sky in the energy range from 100 gigaelectronvolts (Gev) to 100 teraelectronvolts (Tev) with a nearly 100% duty cycle and a sensitivity unprecedented for a survey instrument, offering an exceptional view into the cosmos. This award provides funding for the UW–Madison HAWC group to research scientific areas in which HAWC can make unique contributions: a search for TeV halos surrounding middle-aged pulsars identified by radio and GeV gamma-ray observations and a search for microquasars. The key questions motivating these studies include: Are TeV halos a universal feature of middle-aged pulsars? What drives the formation of TeV halos? How do microquasars produce high-energy emissions? Can they emit other high-energy messengers such as neutrinos?The HAWC group is in the Wisconsin IceCube Particle Astrophysics Center (WIPAC), a scientific center at UW-Madison with researchers involved in a range of Particle Astrophysics projects, including the IceCube Observatory, the Askaryan Radio Array, the HAWC experiment, and the Cherenkov Telescope Array. The PI will join the vibrant education and outreach activities of the center and contribute to educating and training future leaders in the field of Particle Astrophysics. She will lead an effort to support women and gender minorities at the center. Along with other colleagues, she will initiate monthly or bi-monthly social events to gather up female and gender minority (such as people who are transgender or nonbinary) students, postdocs, and researchers at WIPAC. These diversity meetings will also be open and will facilitate interactions between the local undergraduates, graduate students, and researchers working in Particle Astrophysics.The study of TeV halos and microquasars is closely related to the understanding of high-energy neutrinos. A population of TeV–PeV neutrinos with astrophysical origin has been observed by the IceCube Observatory since 2013. The origin of the bulk of the IceCube neutrinos remains a mystery. Recent multi-messenger follow-up observations provide hints toward the first neutrino sources, including the blazar TXS 0506+056 and the VHE gamma-ray emitting region surrounding the pulsar PSR J1907.9+0602, MGRO J1908+06. With an overlapping energy window, HAWC’s observation of these two types of sources will provide important information for neutrino studies. The gamma-ray analyses will be combined with the study of high-energy neutrinos. Microquasars are also candidate neutrino sources. The PI will work with the IceCube group on the multi-messenger study of the VHE gamma-ray sources, and in particular, to explore the possibility of a joint analysis of HAWC and IceCube observations of the potential sources. As a first attempt, they will use the HAWC spectra to derive the number of expected neutrinos from each source, assuming that protons produce the gamma-ray emission. The results will be compared with the IceCube observation of these sources to understand the role of microquasars as high-energy neutrino emitters.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.