The High-Altitude Water Cherenkov Gamma-Ray Observatory

News and Press from HAWC

HAWC and IceCube Join Efforts to Analyze the Anisotropy of Cosmic Rays
December 17, 2018

Cosmic rays swirling through space constantly bombard Earth from every direction. Out of every 1,000 cosmic rays there is roughly one cosmic ray with a preferred (non-random) arrival direction. This 0.1% effect is called the cosmic-ray anisotropy. The anisotropy is associated with the distribution of cosmic sources in the Milky Way and the properties of the Galactic magnetic field through which the cosmic rays propagate.

The all-sky distribution in the relative intensity of 10 TeV cosmic rays observed by the HAWC and IceCube Observatories. The red color indicates an excess of cosmic rays with respect to the mean flux, and blue indicates a deficit. On the left, the white arrow indicates the direction of motion of the solar system through the local interstellar medium. The black lines indicate the local interstellar magnetic field outside the heliosphere. The right plot shows the opposite side of the sky. (Image credit: Savannah Guthrie, WIPAC.)

Measurements of the anisotropy are performed by ground-based observatories such as HAWC and the IceCube Neutrino Observatory. Unfortunately, the field of view of any single ground-based experiment is limited, since only a fraction of the sky can be observed. Therefore, HAWC (located in the Northern Hemisphere) and IceCube (at the South Pole) have combined their cosmic ray datasets to construct the first all-sky map of cosmic rays at 10 TeV. The results have been accepted for publication in The Astrophysical Journal and include measurements showing how the anisotropy varies as a function of angular scale.

The analysis, performed by Juan Carlos Díaz-Vélez (UdG, UW-Madison), Paolo Desiati (UW-Madison), Dan Fiorino (UMD), and Markus Ahlers (NBI) has resulted in a measurement of the large-scale structure of the anisotropy — its so-called dipole component — that is free of bias due to partial sky coverage. The measurements indicate that the dipole excess and deficit are aligned with the direction of the local interstellar magnetic field. This fact allowed the team to infer the North-South component of the dipole on the celestial sphere, a measurement not possible with only one instrument. The anisotropy is likely caused by the structure of the magnetic field and the location of the cosmic ray accelerators nearest to Earth. A future study extending the combined analysis beyond 10 TeV will be used to disentangle the two effects.

HAWC Observes Particle Acceleration in Astrophysical Jets
October 3, 2018

Active galactic nuclei (AGN) produce powerful jets of ionized matter which are believed to accelerate the highest energy hadronic cosmic rays. Observing gamma rays from jets would be strong evidence of such particle acceleration. Unfortunately, most AGN are too far away to spatially resolve their jets with the current generation of gamma-ray telescopes. However, it is possible to observe jets in microquasars, extreme star systems in the Milky Way that behave like AGN in miniature.

SS 433
Left: gamma rays recorded by HAWC from the extended pulsar wind nebula MGRO J1908+06 (center) and the lobes of SS 433/W50 (lower right). Right: the gamma-ray "lobes" of SS 433/W50 after subtraction of gamma rays from MGRO J1908+06. The light semicircular region indicates the part of the sky used to fit and subtract the emission.

Using 1,017 days of data from HAWC, researchers have made the first observation of jets from the Galactic microquasar SS 433. SS 433 is a binary system located inside the supernova remmant W50. It produces two strong jets which interact with the remnant and create intense "lobes" of radio and X-ray emission. The measurements from HAWC, published today in Nature, indicate gamma rays in spatial coincidence with the radio and X-ray "lobes." The data are consistent with gamma rays of at least 25 TeV in energy, suggesting the presence of a population of charged particles with energies up to 1 PeV.

SS 433
The spectral energy distribution of radio, X-ray, and gamma-ray emission from the eastern X-ray lobe of SS 433/W50. The solid and dashed lines indicate the best fit emission models assuming the radiation is produced by electrons and positrons; the dash-dotted line indicates the very high-energy radiation produced by protons.

Modeling of the gamma-ray flux from the lobes suggests that most if not all of the radiation may originate from leptonic rather than hadronic cosmic rays. However, the measurements from HAWC do not rule out the possibility that hadronic cosmic rays are accelerated in this system. Future measurements with HAWC and imaging air Cherenkov telescopes will better constrain the energy spectrum and help identify the origin of the radiation.

Stefan Westerhoff: 1967-2018
August 10, 2018

It is with great sorrow that we mourn the loss of our colleague Stefan Westerhoff, who passed away on August 5, 2018 after battling a long illness. Stefan played a huge role in making HAWC a success. He was an excellent scientist and an even better friend. We will miss him dearly.

HAWC Collaboration
Stefan with HAWC colleagues in Houghton, Michigan, September 2013.

A memorial service will be held in his honor on August 25, 2018 in Madison, WI. Information about the service and memorial donations is available here.

HAWC Gamma-Ray Data Now Public
January 31, 2018

Data from the 2HWC survey of very-high energy gamma rays and other HAWC papers are now available for public download from the HAWC data repository:


Users can download point source data from the 2HWC catalog with full flux and significance estimates. The repository also provides a coordinate view of the entire HAWC skymap, allowing users to query the flux and statistical significance of excess gamma rays from any location in the HAWC field of view. To use this feature, navigate to:

HAWC Data Repository: Coordinate View

We ask that you please acknowledge any use of the data. We would also appreciate it if you please email our spokespersons (and possibly the relevant science convener, listed here), if you intend to submit a publication using the HAWC public data.

HAWC Sheds Light on Origin of Excess Antimatter
November 16, 2017

In 2008, astronomers observed an unexpectedly high number of positrons (the antimatter cousins of electrons) above Earth's atmosphere. Ever since, scientists have been split over the origin of the anomalous positrons: the particles might come from nearby pulsars, or from decay and annihilation of dark matter.

TeV sky map; source orientation
Left: spatial morphology of TeV emission from Geminga and its neighbor PSR B0656+14. Right: schematic illustration of the observed region and the line of sight to Earth. Positrons from Geminga and PSR B0656+14 must diffuse 250 pc to affect the local flux.

Using new data from HAWC, researchers have made the first detailed gamma-ray maps of two pulsars thought to be the best candidate sources of the positron excess. The measurements, published today in Science, show that these pulsars are not the source of anomalous positrons observed at Earth. While the measurements do not rule out the pulsar hypothesis, they do eliminate two of the most probable local accelerators.

TeV sky map; source orientation
Estimated positron flux at Earth from Geminga (blue solid line), computed using the surface brightness of the pulsar wind nebula measured with HAWC. The green points show the positron flux measured with AMS-02.

HAWC Featured on Science Podcast
April 4, 2017

The Relatively Certain podcast (produced by the Joint Quantum Institute at University of Maryland) has featured HAWC for its recently relaunched show. Check it out!

HAWC Reveals New Look at the Very High-Energy Sky
April 18, 2016

Today members of the HAWC Collaboration presented a new survey of the gamma-ray sky to the public and the scientific community at the annual April Meeting of the American Physics Society in Salt Lake City, Utah.

TeV sky map
A view of two-thirds of the entire sky with very high-energy gamma rays observed by HAWC during 340 days of data taking between November 2014 and November 2015. Clearly visible are many sources in the Milky Way and the extragalactic objects Markarian 421 and 501. Several well-known constellations are shown as a reference.

The new gamma-ray sky map shows many objects along the plane of the Milky Way Galaxy, several of which have not been previously observed.

TeV sky map
A zoomed-in view of the Galactic Plane showing sources of TeV gamma rays observed in the HAWC survey.
HAWC Collaborators at the APS meeting. Back row: P. Harding (LANL), T. Yapici (MSU), A. Albert (LANL), Z. Hampel-Arias (UW), M. Mostafá (PSU), R. Lauer (UNM), T. Weisgarber (UW), C. Brisbois (MTU), I. Wisher (UW). Front row: H. Zhou (MTU), K. Malone (PSU), B. Dingus (LANL), C.M. Hui (NASA-Marshall), A.W. Smith (UMD), D. Kieda (UU), A. Barber (UU), H. Ayala-Solares (MTU).

For more information, see:

HAWC in the press:

HAWC Announces its First Transient Trigger
April 7, 2016

HAWC has made its first externally-announced transient trigger, a gamma-ray flare from the active galaxy Markarian 501. This object is known to exhibit periods of strongly increased gamma-ray emission. HAWC monitors Mrk 501 every day and observed the source emitting more than 5 times its average flux on April 6, 2016.

For more information, see:

The HAWC online monitoring display showing the signficance of emission from Mrk 501 (top left) on April 6, 2016, and the light curve for the previous week (bottom left). The April 6 flare is circled in red.

HAWC Awarded Funding to Construct an Array of Outrigger Tanks
September 25, 2015

HAWC Collaborator Brenda Dingus (Los Alamos National Laboratory, USA) has been awarded a Laboratory Directed Research and Development grant to construct a sparse array of "outrigger" tanks around the central HAWC array. The purpose of the outriggers is to enhance the effective area and sensitivity of the detector to air showers above 10 TeV by a factor of 2 to 4.

OR array
Left: location of HAWC between Sierra Negra and Pico de Orizaba. Right: a zoomed-in view of the HAWC site showing the completed central array and a possible layout of the outrigger array.

The outriggers will be used to identify the core positions of high-energy air showers which fall outside the central array. Because their shower cores are not contained inside the main array, these events are difficult to reconstruct with the current configuration of the HAWC detector. (See arXiv:1509.04269 for more information.)

The HAWC Collaboration is grateful to Los Alamos National Laboratory for its support of this crucial detector upgrade. Dingus and collaborators will begin carrying out surveys and design studies for the outriggers in October 2015, followed by an environmental impact review. Completion of the outrigger array is expected in early 2017.

HAWC Observatory Formally Inaugurated in Ceremony at Sierra Negra
March 20, 2015

Construction of the HAWC Observatory began in 2010. Today, the detector was formally inaugurated by members of the collaboration, representatives from collaborating institutions and the particle astrophysics community, and representatives from CONACyT, the US National Science Foundation (NSF), and the US Department of Energy (DOE) in a ceremony at the detector site at Sierra Negra.

Enrique Cabrero, General Director of CONACyT, and France Córdova, Director of the NSF, complete the HAWC Inauguration Ceremony by starting a new detector run. (Image credit: La Razón de San Luis Potosí.)

The ceremony capped a two-day event celebrating the completion of HAWC. On Thursday March 19, over 250 people participated in an inaugural symposium surveying the state of high energy particle astrophysics at the Complejo Cultural Universitario of BUAP in Puebla. The speakers included guests from most of the major gamma-ray, cosmic-ray, and neutrino observatories in operation around the world, as well as leading theorists in the field.

Today HAWC collaborators and invited guests traveled up the mountain to tour the HAWC site and view work by school children and artists from the town of Texmalaquilla. At noon, Enrique Cabrero, director of CONACyT, and France Córdova, director of NSF, officially inaugurated the detector by starting a new data acquisition run.

Water Filtration and Delivery Completed
January 15, 2015

Today the last empty tanks at HAWC were filled with purified water, completing the process of construction, bladder deployment, water delivery, and purification.

Water plant
The water filtration plant in the HAWC Utility Building at Sierra Negra.

The total weight of water moved, filtered, and delivered for the tanks is 55 kilotons, corresponding to a volume of 55 million liters. This volume is equivalent to one can of soda for every man, woman, and child in Mexico. The weight is equivalent to 110 fully loaded 747 jumbo jets.

This achievement was made possible by many members of the collaboration, as well as the guards at the site and the drivers who brought much of the water from the filtration plant in Esperanza to Sierra Negra. In particular, we wish to recognize the efforts of these individuals:

Eleazar Sandoval
Miguel Angel (Plumber)
Luis M Bautista
Alberto Carramiñana
Alberto Carrera
Jorge Cotzomi
Scott Delay
Fernando Garfias
Juan Gonzalez
Jordan Goodman
Eibar Hernandez
Arturo Iriarte
Alejandro Lara
Oscar Martinez
Epifanio Ponce
Noe Sanchez
Andres Sandoval
Manuel Sandoval
Michael Schneider
Ibrahim Torres
Andres Velazquez
Manuel Zamora (Softener)

Final HAWC Tank Deployed at Sierra Negra
December 15, 2014

Today at noon, the assembly of the HAWC tanks ended with the completion of the steel frame of tank X01, the 300th tank to be deployed on the observatory platform at Sierra Negra. During the next few weeks, X01 and approximately 40 other tanks at the site will have bladders installed, be filled with water, and be incorporated into the data acquisition system, thus completing the full array after four years of construction.

Tank X01
HAWC site construction crew in front of completed WCD X01, tank number 300.

The HAWC Collaboration wishes to recognize the tremendous dedication of the construction crew, many of whom hail from the nearby towns of Atzitzintla and Texmalaquia. Without their efforts, the timely and safe completion of the HAWC detector would not have been possible.

Karen Caballero Receives L'Oreal-UNESCO Award from Mexican Academy of Science
November 6, 2014

Congratulations to HAWC Collaborator Karen S. Caballero-Mora, who has received the L'Oreal-UNESCO-AMC (Academia Mexicana de Ciencias) award for women in science. Dr. Cabellero-Mora is a researcher in the Physics Department of the Centro de Investigación de Estudios Avanzados (Center for Advanced Studies, or CINVESTAV) of the Instituto Politécnico Nacional in Mexico City.

Karen S. Caballero-Mora
Dr. Karen S. Caballero-Mora, HAWC member and researcher at CINVESTAV.

Dr. Caballero-Mora has been recognized for her work on the mass composition of cosmic rays measured by the Pierre Auger Observatory and her efforts to improve the reconstruction and background suppression algorithms used in HAWC.

Reference: "Avances en la Investigación de los Rayos Cósmicos de Alta Energía," Boletín AMC/380/14 (in Spanish).

First Science Results from HAWC: Observation of the Anisotropy of Cosmic Rays
October 16, 2014

Between the start of regular operations in June 2013 and February 2014, the HAWC detector recorded close to 50 billion cosmic rays and gamma rays. With these high statistics, the collaboration has been able to measure a significant small-scale anisotropy in the arrival direction distributions of cosmic rays above 1 TeV.

Celestial coordinates of cosmic-ray hotspots.
Celestial coordinates of small-scale cosmic-ray hot spots observed with the first 9 months of data from HAWC. From arXiv:1408.4805 [astro-ph].

The anisotropy, also observed by the IceCube Neutrino Observatory in the Southern Hemisphere, includes large-scale structures of amplitude 10-3 and small-scale 10° "hot spots" of amplitude 10-4. Although the hotspots have been observed in the Northern Hemisphere by the Milagro, Tibet, and ARGO detectors (among others) their origin is not known. They may be created by turbulence in the Galactic magnetic field, or more exotic processes such as the decay of quark matter in pulsars or the self-annihilation of dark matter in the Galaxy.

Reference: "Observation of Small-Scale Anisotropy in the Arrival Direction Distribution of TeV Cosmic Rays with HAWC," A.U. Abeysekara et al. [HAWC Collaboration]. Accepted by the Astrophysical Journal, arXiv:1408.4805 [astro-ph].

Sensitivity of HAWC to Gamma Rays Produced in the Self-Annihilation of Dark Matter
October 14, 2014

HAWC was designed to detect cosmic rays and gamma rays from astrophysical sources. However, by observing massive objects that are expected to produce relatively few astrophysical gamma rays, the detector can also be used to search for gamma rays created in the self-annihilation of multi-TeV dark matter.

HAWC sensitivity to dark matter XX->WW.
Estimated limits on the dark matter self-annihilation cross section after five years of observations of the extended objects M31 and the Virgo Cluster with HAWC. The dashed lines show the limits after including Sommerfeld enhancements of the cross section. From arXiv:1405.1730 [astro-ph].

The detector is particularly well-suited to observe extended objects, because the gamma-ray signal from dark matter can be boosted by substructure (or "clumpiness") in these objects. Further gains in sensitivity are possible if the dark matter undergoes low-velocity resonant scattering, known as Sommerfeld enhancement.

The sensitivity of HAWC to dark matter annihilation under various scenarios has been calculated in a paper to appear in Physical Review D. With several years of data from the full array, HAWC has a good chance of either observing high-mass dark matter or placing a strong limit on its mass and cross section above 1 TeV.

Reference: "The Sensitivity of HAWC to High-Mass Dark Matter Annihilations," A.U. Abeysekara et al. [HAWC Collaboration]. Accepted by Physical Review D, arXiv:1405.1730 [astro-ph].

New Limits Placed on Evaporation of Primordial Black Holes using Data from Milagro and HAWC
October 6, 2014

Primordial black holes are hypothetical objects formed by the gravitational collapse of extreme density fluctuations in the early universe. These objects are expected to lose mass via Hawking radiation, and when they evaporate they can produce a burst of GeV and TeV gamma rays.

Milagro limit/HAWC sensitivity to PBH evaporation.
Milagro limits and HAWC sensitivity to the evaporation of primordial black holes as a function of the duration of the gamma-ray burst. From arXiv:1407.1686 [astro-ph].

The HAWC Observatory and its predecessor, Milagro, are sensitive to these gamma rays and can be used to observe the evaporation of black holes with an initial mass of 5×1011 kg (about 100 times the mass of the Great Pyramid of Giza). Using gamma rays recorded by Milagro between 2000 and 2008, the HAWC and Milagro collaborations have estimated the most sensitive current limits on the evaporation of black holes in this mass range.

Reference: "Milagro Limits and HAWC Sensitivity for the Rate-Density of Evaporating Primordial Black Holes," A. Abdo et al. [Milagro Collaboration] and A.U. Abeysekara et al. [HAWC Collaboration]. Accepted by Astroparticle Physics, arXiv:1407.1686 [astro-ph].

Construction of 250th water Cherenkov Tank Completed
May 15, 2014

On May 15, 2014, the 250th water tank was completed at the HAWC site. Thanks to the hard work and dedication of the site crew, construction of HAWC has proceeded on schedule. The newest tanks will begin operating when the data acquisition system is expanded in June.

HAWC Begins Operations at the Sierra Negra Site
August 1, 2013

On August 1, 2013, the High-Altitude Water Cherenkov (HAWC) Gamma Ray Observatory formally began operations. HAWC is designed to study the origin very high-energy cosmic rays and observe the most energetic objects in the known universe.

HAWC Observes its First Source: the Moon
April 15, 2013

The HAWC Observatory has unvieled its first observation: the shadow of the moon in cosmic rays. Although HAWC currently comprises only 30 out of its eventual 300 water Cherenkov detectors, the measurement of the moon shadow is above 13 sigma in significance. The measurement was presented at the April meeting of the Americal Physics Society by postdoctoral research Tom Weisgarber (University of Wisconsin-Madison). The observation has also been featured in a science article by BBC News.