Astrophysical observations of the cosmos allow us to probe extreme physics and answer foundational questions on our universe. Modern astronomy is increasingly operating under a holistic approach, probing the same question with multiple diagnostics including how sources vary over time, how they appear across the electromagnetic spectrum, and through their other signatures, including gravitational waves, neutrinos, cosmic rays, and dust on Earth. Astrophysical observations are now reaching the point where approximate physics models are insufficient. Key sources of interest are explosive transients, whose understanding requires multidisciplinary studies at the intersection of astrophysics, gravity, nuclear science, plasma physics, fluid dynamics and turbulence, computation, particle physics, atomic, molecular, and optical science, condensed matter and materials science, radiation transport, and high energy density physics. This white paper provides an overview of the major scientific advances that lay at the intersection of physics and astronomy and are best probed through time-domain and multimessenger astrophysics, an exploration of how multidisciplinary science can be fostered, and introductory descriptions of the relevant scientific disciplines and key astrophysical sources of interest.

The fundamental nature of Dark Matter is a central theme of the Snowmass 2021 process, extending across all frontiers. In the last decade, advances in detector technology, analysis techniques and theoretical modeling have enabled a new generation of experiments and searches while broadening the types of candidates we can pursue. Over the next decade, there is great potential for discoveries that would transform our understanding of dark matter. In the following, we outline a road map for discovery developed in collaboration among the frontiers. A strong portfolio of experiments that delves deep, searches wide, and harnesses the complementarity between techniques is key to tackling this complicated problem, requiring expertise, results, and planning from all Frontiers of the Snowmass 2021 process.

High-energy cosmic rays that hit the Earth can be used to study large-scale atmospheric perturbations. After a first interaction in the upper parts of the atmosphere, cosmic rays produce a shower of particles that sample the atmosphere down to the detector level. The HAWC (High-Altitude Water Cherenkov) cosmic-ray observatory in Central Mexico at 4,100 m elevation detects air shower particles continuously with 300 water Cherenkov detectors with an active area of 12,500 m$^{2}$. On January 15th, 2022, HAWC detected the passage of the pressure wave created by the explosion of the Hunga volcano in the Tonga islands, 9,000 km away, as an anomaly in the measured rate of shower particles. The HAWC measurements are used to characterize the shape of four pressure wave passages, determine the propagation speed of each one, and correlate the variations of the shower particle rates with the barometric pressure changes, extracting a barometric parameter. The profile of the shower particle rate and atmospheric pressure variations for the first transit of the pressure wave at HAWC is compared to the pressure measurements at Tonga island, near the volcanic explosion. This work opens the possibility of using large particle cosmic-ray air shower detectors to trace large atmospheric transient waves.

The performance of the High Altitude Water Cherenkov Gamma-ray Observatory (HAWC) is mainly shaped by its event reconstruction methodologies, which take as input only the footprint left on the ground by the air shower. We discuss here the revamped reconstruction algorithms, including noise suppression cleaning, a refined core reconstruction method using simulations to fit hit observation, addressing systematic error sources in direction reconstruction, and a new gamma/hadron separation approach based on a more accurate model. As a result, HAWC's angular resolution and gamma/hadron separation reach an improvement of a factor of 2 and 3, respectively, at the highest energies for highly inclined showers and can now observe the Galactic Center at a significance of over 5$\sigma$. Furthermore, we verify the overall performance improvement by observing the Crab Nebula as a reference source which we observe at 250$\sigma$ using 2434 days of data.

Microquasars are one of the classes of powerful Galactic particle accelerators that emit gamma rays beyond multi-TeV energies. Recently, gamma-ray emission above tens of TeV has been detected from some of these objects. The High Altitude Water Cherenkov (HAWC) observatory has previously reported gamma-ray emission with a median energy of 25 TeV from the microquasar SS 433, suggesting that gamma-ray binaries can accelerate particles to above 100 TeV inside the jets. With 2,321 days of data and better reconstruction algorithms, the HAWC observatory detected emissions above 20 TeV from two other binaries, V4641 Sagittarii and LS 5039. The V4641 Sgr is detected with a significance great than 9 sigma. It’s a low-mass X-ray binary(LMXB) that has been observed with a small-scale jet-like structure in the radio wavelength. In this talk, I will present the preliminary results from V4641 Sgr and discuss possible scenarios for gamma-ray production. We will also present future plans to search for additional X-ray binaries with HAWC.

The connection between short Gamma-Ray Bursts (sGRBs) and Gravitational Waves (GWs) has long been a subject of study, motivating the search for counterparts by gamma-ray instruments. Both phenomena are thought to be produced by the same astrophysical event. However, only one event to date has been identified as a simultaneous occurrence of both, a sGRB (GRB 170817A) and a GW (GW170817). GRB 170817A was classified as an unusual burst due to its low-luminosity and prolonged non-thermal emission (afterglow) observed across radio, optical, and X-ray bands, which reached their maximum hundreds of days after the trigger time. Although TeV emissions were not immediately observed for this burst, if they exist, they are most likely generated through synchrotron-self-Compton of the delayed radio emission from each burst. We have identified 8 sGRBs within the Fermi Gamma-ray Burst Monitor catalogue that appear to share some characteristics with GRB 170817A during the time interval spanning from December 5th, 2014 until December 5th, 2022. In this work, we discuss the methodology utilized to identify sGRBs that are alike GRB 170817A and discuss the implications of our results.

The recent discovery of PeV gamma-ray emission especially from the LHAASO observatory, located in the Northern hemisphere, boosted the relevance of observing the Southern sky at such energies. SWGO (SouthernWide-Field Gamma-Ray Observatory) is the largest proposed detector with sensitivity in the 100 TeV-1 PeV energy range. The baseline SWGO idea is a km^2 array of water tanks to be placed above 4,400ma.s.l. in the Andes, South America. In this contribution, we have studied the particle content and the morphology of Extensive Air Showers (EAS) generated by photons and protons in the 0.1 to 10 PeV energy range. We have simulated over 106 gamma-rays and proton induced showers respectively with primary energy in the 0.1-10 PeV energy range. We also show the particle distribution at ground, the lateral profile, the muon content and the average particle properties at ground.

Water-Cherenkov detectors (WCD) have been manufactured in Australia by the company AQUA-MATE as part of the RD activities for SWGO. They consist of a steel tank frame with a bladder on its interior satisfying the SWGO double-layer tank design. Tanks and bladders have been custom designed to optimally accommodate the bladder inside the tank and with minimal material usage. They are delivered in compact boxes that are easy to transport. These boxes are designed to fit 24 tanks in a 20-foot container. The double-layer tank design has introduced new features to improve the discrimination between gamma-rays and cosmic rays. Some of these features created challenges for the manufacturing. Some units have been delivered to one of Peru’s candidate sites at 4800 m and to Mexico (the HAWC Observatory, 4100 m) for prototype tests in real conditions. In this contribution we will describe manufacturing and construction details of the first SWGO prototype WCD. These details were envisaged to facilitate: the transport of the units, the assembly, the deployment and maintenance activities of the detectors. Furthermore, the units need to be resistant to strong winds, rain, snow and earthquakes.The costs are scalable with the detector volume. This information could be of interest to other Observatories that are in RD phase, such as the Global Cosmic Ray Observatory (GCOS) for the study of the highest-energy particles in the Universe and the Tau Air Shower Mountain-Based Observatory (TAMBO) for the search of PeV neutrinos.

The SWGO Collaboration is evaluating the possibility of deploying Water Cherenkov Detectors (WCD) in a high-altitude natural lake. For that, the first challenge is to build a bladder strong enough that could be used as a WCD inside a natural lake. A prototype bladder has been designed for SWGO and two bladders, made of different films, have been deployed for testing at Sibinacocha lake, in Peru, at 5000 masl. In order to monitor the wave intensity in the lake, a low-cost oceanographic buoy was developed using an acceleration sensor MPU6050 and a liquid sensor DS18B20. The development platform used was the Arduino Mega 2560 with some out-of-the shelf modules to achieve a functional and autonomous prototype. A code was developed in Python to process the data and convert the acceleration values into position, allowing estimation of height variations, as a function of time, less than 1 cm. To reduce the environmental impact of the floating structure, the use of metallic materials was minimized and mostly wood, cotton, and PVC pipes were used. This buoy has been installed next to SWGO prototype bladders at the Sibinacocha lake in Peru. In this contribution we will present the details of a low-cost oceanographic buoy built to monitor lake wave intensity.

In 2020, the HAWC Collaboration presented the first catalog of gamma-ray sources emitting above 56 TeV and 100 TeV.With nine sources detected, this was the highest-energy source catalog to date.Here, we present the results of re-analysis of the old data, along with additional data acquired since then.We use a new version of the reconstruction software with better pointing accuracy and improved gamma/hadron separation.We now see more than 25 sources above 56 TeV, with most sources being located in the Galactic plane.The vast majority of these seem to be leptonic pulsar wind nebulae, but some have been shown to have hadronic emission.We will show spectra and discuss possible emission mechanisms of some of the most interesting sources, including the ones the HAWC Collaboration considers PeVatron candidates.

Axion-like particles could be potential dark matter candidates, whose conversion from gamma rays could have an impact on the spectra of extremely powerful astronomical gamma-ray sources. For galactic sources, the overall result of this coupling may be reflected as an attenuation of the gamma-ray spectrum at energies above several tens of TeV. Therefore, multi-TeV observatories like the High Altitude Water Cherenkov (HAWC) Observatory would have a unique opportunity to investigate the parameters of ALPs candidates in the mass range from fractions of a neV up to tens of $\mu$eV. In this study, we present a preliminary study of the spectrum observed of the TeV gamma-ray source eHWC J1908+63, constraining the ALP coupling to better than $10^{-12}$ GeV$^{-1}$

Current cosmic ray anisotropy experiments have shown a significant swing in both the direction and the amplitude of the dipole at energies around tens of TeV. Due to the charged nature of these particles, and the presence of magnetic fields in our galaxy, an underlying composition-dependent dipole swing is expected. For this reason, combining measurements of the composition and the distribution of arrival directions is essential for unveiling the astrophysical origin of this structure. In this work, we study the potential of the upcoming Southern Wide-field Gamma-ray Observatory (SWGO) in contributing to these anisotropy studies. We present a template-based method developed for reconstructing the number of muons and separating primary cosmic rays. Preliminary resolutions of $5-30\%$ in the number of muons and an accuracy of $70-90\%$ in the species separation are found. A clear improvement is seen by considering a dedicated muon-counter layer in a detector, highlighting the future potential of SWGO.

LHAASO J1849-0003 is one of the twelve gamma-ray sources that can emit photons above 100 TeV reported by LHAASO in 2021. It is spatially coincident with HESS J1849-000, which may be powered by PSR J1849-0001. The High Altitude Water Cherenkov (HAWC) observatory has also observed a possible counterpart source named eHWC J1850+001 above 56 TeV. In this work, we use the most up-to-date HAWC dataset with 2398 days of data to perform dedicated multi-source modeling of the region around eHWC J1850+001. We present the spectrum and morphology of eHWC J1850+001 and other nearby sources identified during a systematic source search. We anticipate the HAWC observational results to determine a potential connection between HESS J1849-000 and LHAASO J1849-0003, providing a more complete TeV spectral study of this energetic source.

The High Altitude Water Cherenkov (HAWC) Observatory detected significant TeV gamma-ray emission from the jets of a microquasar, SS 433, in 2018. The gamma-ray emission from the powerful jets can reach up to a few tens of TeV, but the spectral studies at these energies have not been carried out. Compared to the east jet lobe, the west jet lobe is more challenging to analyze due to a higher level of contamination from nearby MGRO J1908+06 and the Galactic plane. With the most up-to-date HAWC data, the west jet lobe has over 8 sigma pre-trial significance. In this work, we use $\sim1,922$ days of HAWC data to model the source confused region around the SS 433 west jet lobe and study its spectrum between 1 and over 200 TeV.

We use data from the Large Area Telescope onboard the Fermi gamma-ray space telescope (Fermi-LAT) to analyze the faint gamma-ray source located at the center of the Sagittarius (Sgr) dwarf spheroidal galaxy. In the 4FGL-DR3 catalog, this source is associated with the globular cluster, M54, which is coincident with the dynamical center of this dwarf galaxy. We investigate the spectral energy distribution and spatial extension of this source, with the goal of testing two hypotheses: (1) the emission is due to millisecond pulsars within M54, or (2) the emission is due to annihilating dark matter from the Sgr halo. For the pulsar interpretation, we consider a two-component model which describes both the lower-energy magnetospheric emission and possible high-energy emission arising from inverse Compton scattering. We find that this source has a point-like morphology at low energies, consistent with magnetospheric emission, and find no evidence for a higher-energy component. For the dark matter interpretation, we find that this signal favors a dark matter mass of $m_{\chi} = 29.6 \pm 5.8$ GeV and an annihilation cross section of $\sigma v = (2.1 \pm 0.59) \times 10^{-26} \,\text{cm}^3/$s for the $b \bar{b}$ channel (or $m_{\chi} = 8.3 \pm 3.8$ GeV and $\sigma v = (0.90 \pm 0.25) \times 10^{-26} \, \text{cm}^3/$s for the $\tau^+ \tau^-$ channel), when adopting a J-factor of $J=10^{19.6} \, \text{GeV}^2 \, \text{cm}^{-5}$. This parameter space is consistent with gamma-ray constraints from other dwarf galaxies and with dark matter interpretations of the Galactic Center Gamma-Ray Excess.

The High Altitude Water Cherenkov (HAWC) Gamma-ray Observatory, located in the mountains of Mexico, has been performing an unbiased survey of the Northern sky at energies above 300 GeV since becoming fully operational in 2015. HAWC’s wide field-of-view enables indirect searches for TeV-scale dark matter from diverse targets including galaxy clusters, dwarf spheroidal galaxies, the Milky Way galactic halo and the Sun. Beyond dark matter, sensitivity to transient bursts of gamma-rays provides a window into the early universe through searches for evaporating primordial black holes and its high energy reach enables searches for violations of the Lorentz symmetry. I will present an overview of beyond-the-Standard-Model searches with HAWC and present some of the world’s strongest constraints on these processes at the TeV scale.

Located in the southern field of view of the High Altitude Water Cherenkov (HAWC) observatory, the eHWC J1825-134 region is one of the most complicated gamma-ray emission sites on the galactic plane. The region contains a few PeVatron candidates that can accelerate particles up to PeV energies. Disentangling the overlapping gamma-ray emission and associating it with accelerators is crucial to understand the mechanism of cosmic-ray acceleration and gamma-ray production near the accelerators. In this talk, I will present each of the gamma-ray sources resolved in this region using 1910 days of HAWC data, including their spectra. Also, we have studied their potential association with astrophysical accelerators, the binary system LS 5039, two pulsar wind nebulae, and a young star cluster.

The number of observed Fast Radio Burst (FRB) events has grown rapidly over the last few years, but their origin remains unknown. Multiwavelength follow-ups have been encouraged by radio astronomers but the transient nature of the events makes targeted follow-ups difficult unless a source is known to repeat. A wide field-of-view instrument, such as HAWC can overcome this limitation using archival data. In this study we search for very high energy (VHE) gamma rays from FRB source locations over the largest population studied at this energy. Using data from the HAWC Observatory we look at over 200 FRB events from 141 unique locations and search for persistent emission and transient activity in 600 s surrounding the burst time. The results are then placed in context of available models and maximum gamma-ray emission levels from FRBs.

We outline the unique opportunities and challenges in the search for “ultraheavy” dark matter candidates with masses between roughly 10 TeV and the Planck scale m_{\rm pl} ≈ 10^{16} TeV. This mass range presents a wide and relatively unexplored dark matter parameter space, with a rich space of possible models and cosmic histories. We emphasize that both current detectors and new, targeted search techniques, via both direct and indirect detection, are poised to contribute to searches for ultraheavy particle dark matter in the coming decade. We highlight the need for new developments in this space, including new analyses of current and imminent direct and indirect experiments targeting ultraheavy dark matter and development of new, ultra-sensitive detector technologies like next-generation liquid noble detectors, neutrino experiments, and specialized quantum sensing techniques.