The High-Altitude Water Cherenkov Gamma-Ray Observatory

News and Press from HAWC

Detection of a Lamb wave using cosmic rays
October 16, 2023

The figure shows in red the perturbation in the atmospheric pressure detected near the volcanic explosion. HAWC is located at approximately 9000 kilometers from the islands where the explosion took place. The data from HAWC, in black, shows the change in the detection rate of secondary particles as they propagate in the atmosphere. The dataset from Tonga is shifted in time, so the features of both datasets can be compared.

Air showers are able to sample the instantaneous properties of atmosphere, this makes them very useful tools to study transitory phenomena. An example of the properties that can be studied are sudden changes in the atmospheric pressure. In January, 2022, the eruption of the Hunga-Tonga volcano produced the most violent natural explosion that has happened since the discovery of cosmic rays. Large volcanic explosions, such as the one from Hunga-Tonga or the one from Krakatoa in 1883, take place approximately once every century. Due to his continuous operation, HAWC was able to characterize the effect that this pressure wave, known as a Lamb wave, had in the propagation of the air showers detected by the observatory. In this new paper, we present the observations made with HAWC about this spectacular phenomenon.

You can read this article here: High-altitude characterization of the Hunga pressure wave with cosmic rays by the HAWC observatory, HAWC Collaboration 2023

HAWC Study of the Multi-TeV gamma-ray source HAWC J1844-034
October 10, 2023

Best-fit spectrum of HAWC J1844−034.

We report the latest observation of the source eHWC J1842-035, identified as an energetic gamma-ray source with sufficient extension to be seen by Milagro and HAWC. The origin of gamma-ray was unclear; there are multiple pulsars and a supernova remnant as its possible origin. According to its multi-TeV (~100 TeV) gamma-ray emission, it has been suggested as a PeVatron candidate, a PeV cosmic-ray accelerator. In this paper, we found that the extended source observations at a similar position by other TeV gamma-ray observations (HESS, LHAASO, Tibet AS-gamma) correspond well to the newly observed source, HAWC J1844-034. We also modeled primary particle spectra to estimate the cutoff energy of the primary particle, and we found that the estimated hadron spectrum has a smaller cutoff energy than the expected PeVatron cutoff energy.

You can read this article here: HAWC Study of the Multi-TeV gamma-ray source HAWC J1844-034, HAWC Collaboration 2023

Discovery of gamma rays from the quiescent Sun with HAWC
August 3, 2023

A figure that looks like a heat map shows a bright yellow spot at its center, ringed by “cooler” oranges and purples. This represents the excess of gamma rays observed by the HAWC Collaboration.

We present the first detection of gamma rays from the Sun at the unprecedented energies of more than 1 TeV — roughly a trillion times as energetic as ordinary visible light — challenging our understanding of the most extensively studied star in astrophysics. Physicists did not expect the Sun to glow this brightly at these energies, based on what we know about how charged particles from outer space (cosmic rays) interact with it. The observed signal shows an intriguing dependence on the mysterious 11-year solar cycle, i.e. it is brightest during the solar minimum when the Sun’s magnetic activity is low, and weakest during the solar maximum when the Sun’s magnetic field is at its most violent. This points towards a link between cosmic rays impinging on the solar surface and the observed gamma rays. But exactly how these cosmic rays interact to produce such a bright flux of gamma rays is a puzzle. Resolving this puzzle would fundamentally change what we know about sun-like stars at the highest energies accessible to our experiments. From a purely measurement point-of-view, this is a game-changing observation, because until now we could only look at the Sun at lower energies with detectors on satellites in orbit. HAWC observatory, the first ground-based TeV telescope to detect these gamma rays, has therefore pioneered a new chapter in high energy astrophysics of the Sun.

You can read this article here: Discovery of gamma rays from the quiescent Sun with HAWC, HAWC Collaboration 2023

New TeV Halo Candidate Discovered with HAWC
February 17, 2023

Significance map obtained with 2321 days of HAWC data.

In recent years, very-high energy (> 1 TeV) gamma-ray emission has been observed around pulsars older than 100 kyr. This new type of object has been named “TeV halos”. To date, only seven objects in the TeVCat catalog have this classification, so increasing the number of these types of sources is important to continue studying their formation mechanisms. In this work, we report the detection of gamma-ray emission in the 7-188 TeV energy range using more than 2300 days of observations with HAWC. Based on the spectral and spatial analysis of this emission, and the comparison of the very-high energy flux with observations at other wavelengths, it is concluded that this emission is a TeV halo candidate associated with the pulsar PSR J0359+5414. If confirmed, this TeV halo would share similar properties to other halos, but its pulsar is younger and has no radio emission. These observations imply that TeV halos could commonly exist around pulsars and their formation does not depend on the configuration of the pulsar magnetosphere.

You can read this article here: HAWC Detection of a TeV Halo Candidate Surrounding a Radio-quiet Pulsar, HAWC Collaboration 2023

Search for gamma-ray emission from Classical Nova V392 Per
December 6, 2022

The figure shows the rate of energy emission from the V392 Per nova. The points in red are the observations in the GeV energy range. The points in green are upper limits derived from HAWC archival data during the time of the outburst seen in optical and GeV energies. If the shocks that had produced the GeV emission had also produced TeV emission, one would have expected TeV detections at about the level of the red points. This is disfavored or ruled out by HAWC data for energy above 10 TeV or so.

Galactic novae are among the newest candidates for TeV photon sources. This particular nova (V392 Per) has never been investigated in the TeV energy range. The highest energy level emission detected from galactic novae have been in the GeV energy range, though one example has been seen emitting up to about 1 TeV. Theorists have predicted that the same shocks that produce GeV photon emission might be able to produce photons above TeV as well. This plot compares the Spectral Energy Distribution from Fermi-LAT and from HAWC for the galactic nova V392 Per. If the same shocks produced TeV photons, the emission would be expected to continue at about the same level as the red points in the GeV range. HAWC data strongly disfavors continuation of the GeV spectrum beyond 15 TeV. But HAWC limits are still compatible with a continuation of the Fermi/LAT emission spectrum at lower TeV energies.

You can read this article here: γ-Ray Emission from Classical Nova V392 Per: Measurements from Fermi and HAWC, HAWC Collaboration 2022

Constraints on the very high energy gamma-ray emission from short GRBs with HAWC
September 20, 2022

This Figure shows the allowed density of the surrounding medium as a function of the fraction of energy given to amplify the magnetic field (top panels) and to accelerate electrons (middle panels). The bottom panels exhibit the fraction of energy given to accelerate electrons as a function of the energy to amplify the magnetic field. From left to right panels show GRB 181222A and GRB 170206A. The color scale indicates the percentage of cases remaining from the total cases per bin in the fast cooling regime.

Gamma-ray bursts (GRBs) are among the most luminous sources in the Universe. They are associated to the death of masive stars. Depending on their duration, they are classified into short and long GBRs (short GRBs < 2 seconds and long GRBs > 2 seconds). Detecting gravitational waves (GWs) associated with these events has reaffirmed their importance and interest. In particular, short GRBs have been associated as the electromagnetic counterpart of gravitational waves, for example, GRB 170817A to gravitational event GW170817. Due to its large field of view and duty cycle, HAWC is an ideal observatory to study transient phenomena during and after the gamma emission of GRBs in the TeV energy regime. This paper analyzes a sample of short GRBs detected by different satellites (i. e., Fermi-GBM, Swift-BAT, and Fermi-LAT instruments) in HAWC's field of view. The bulk Lorentz factor and the parameters of the model (density of the interstellar medium, the energy given to accelerate the electrons and to amplify the magnetic field) are restricted considering a redshift of 0.3 for GRB 170206A and GRB 181228A and their respective upper limits. Interstellar density constraints as low as 10−2 cm−3 will be needed for GRBs with the highest keV fluences.

You can read this article here: Constraints on the Very High Energy Gamma-Ray Emission from Short GRBs with HAWC, HAWC Collaboration 2022

HAWC studies the gamma-ray emission from the radio galaxy M87
August 4, 2022

Spectral energy distribution of M87.

M87 is a giant radio galaxy and a powerful gamma ray source located in the Virgo cluster. This object contains an active galactic nucleus characterized by the presence of an accreting supermassive black hole and a relativistic jet. Although M87 was the first radio galaxy detected at very high energies, there is still controversy regarding the physical mechanism that produces the observed emission. In this work we use 1523 days of HAWC observations to try to solve this puzzle. We fit a hybrid model to the spectral energy distribution of this source, which was constructed with observations from radio to gamma rays. The theoretical model includes a synchrotron self Compton component plus an photohadronic component. We conclude that this model is consistent with the observed data and determine that the predicted neutrino flux agrees with the upper limits reported by IceCube.

You can read this article here: Study of the Very High Energy Emission of M87 through its Broadband Spectral Energy Distribution, HAWC Collaboration 2022

Probing the Extragalactic Mid-infrared Background with HAWC
July 18, 2022

95% and 68% containment bands for the EBL intensity different lambda values for the combined results from Mrk 421 and Mrk 501.

The intergalactic space is filled with light emitted by stars and dust throughout the history of the Universe. This diffuse radiation is commonly called Extragalactic Background Light (EBL) and its direct measurement is challenging, particularly in the infrared waveband, due to foreground photon contamination from the Sun. Gamma rays coming from blazars travel cosmic distances through the intergalactic space and interact with diffuse photons from the EBL in the mid to far-infrared region through pair production. By studying this absorption effect in blazar spectra we can infer properties about the IR region of the EBL. In this study, we use HAWC and Fermi-LAT observations from the popular blazars Markarian 421 and Markarian 501, and analyze the feasibility of different EBL absorption scenarios. We then measure the EBL by calculating credible bands in the intensity space. We find these bands to be in agreement with current lower and upper limits, showing a downward trend towards higher wavelength values $\lambda>10 \mu$m also observed in previous measurements. Other features observed in these measurements might be related to intrinsic spectral properties of the sources.

You can read this article here: Probing the Extragalactic Mid-infrared Background with HAWC, HAWC Collaboration 2022

The classification of extensive air showers detected by the HAWC observatory
July 6, 2022

Probability distribution of the NN output for signal and background MC samples.

Separating gamma-ray-induced showers from hadronic showers is one of the main challenges facing high-energy astronomical observatories, such as HAWC, since photons from gamma-ray sources are dominated by hadrons by several orders of magnitude. In HAWC, several techniques are used to discriminate gamma-ray showers from hadronic ones. In this paper, we describe the official HAWC gamma/hadron separation technique, and two new implementations using machine learning techniques. These techniques were applied to three astrophysical sources, the Crab nebula and two blazars, to quantify and compare their relative performance.

You can read this article here: Gamma/hadron separation with the HAWC observatory, HAWC Collaboration 2022

HAWC reports the detection of gamma rays coming from the central zone of two galaxies
April 21, 2022

Significance map of the Markarian 421 blazar.

Using data corresponding to 1038 days of observations, the HAWC Gamma Ray Observatory reported in an article, published by The Astrophysical Journal, that it detects gamma radiation coming from two galaxies called Markarian 421 (Mrk 421) and Markarian 501 ( Mark 501). These two galaxies contain a supermassive black hole at their center, from whose vicinity a jet of ultra-relativistic particles and very energetic photons emanates. The angle that these jets make with our line of sight from Earth is very small, meaning almost all the information we receive from this galaxy comes from the jet. This gives us a great opportunity to study the physical processes involved in the production of very energetic radiation, on the order of trillions of electron volts! In this work, we describe the way in which the photon energy of Mrk 421 and Mrk 501 above 0.5 Tera-electronvolts (TeV) is distributed. It is reported that the maximum energy at which HAWC detects Mrk 421 is at 9 TeV, while this value for Mrk 501 is 12 TeV, which are one of the highest-energy detections reported to date for long-term averaged spectra.

You can read this article here: Long-term Spectra of the Blazars Mrk 421 and Mrk 501 at TeV Energies Seen by HAWC , HAWC Collaboration 2022

HAWC investigates the cosmic ray energy spectrum of protons plus helium at high energies
April 4, 2022

Spectrum for H+He cosmic ray nuclei measured by HAWC (black circles).

The HAWC Collaboration has just published the results of a new study, in which it was measured with good precision and large statistics the energy spectrum of protons and helium in the cosmic ray flux that we receive from the space. Specifically, the study shows data of the energy spectrum at energies between 6 TeV and 158 TeV, in a region that has been poorly explored and that overlaps with direct measurements, which are carried out with particle detectors on board of balloons, satellites and space stations. HAWC results demonstrate the existence of a cut around 24 TeV in the cosmic ray spectrum of protons plus helium nuclei, which seems to point out the presence of a new population of cosmic ray sources in our Galaxy or of a TeV cosmic ray accelerator close to the Earth. HAWC observations are also in agreement with the measurements of the satellite-borne detector DAMPE that also show cuts in the individual spectra of protons and helium at energies around tens of TeV. Besides, the HAWC measurements also confirm previous observations of the ATIC-2, CREAM I-III and NUCLEON direct detectors, which provided first hints about the existence of the cut observed by HAWC. Finally, the HAWC results show that the composition of TeV cosmic rays can also be studied indirectly with high altitude observatories, which detect cosmic rays by means of the extensive air showers that are produced in the atmosphere. In addition, these analyses open the possibility to carry out comparative studies, which could allow to understand the systematic errors that affect both direct and indirect techniques that are used in the research of cosmic rays.

Full Article: Cosmic ray spectrum of protons plus helium nuclei between 6 and 158 TeV from HAWC data , HAWC Collaboration 2022

HAWC Study of the Ultra-High-Energy Spectrum of MGRO J1908+06
March 31, 2022

Significance map of the analysis region in Galactic coordinates. The maximum significance is 38.82 sigma and the contours are the 5, 10, 15, 20, 25, 30 and 35 sigma significance levels.

We report new observations of the source MGRO J1908+06, which is one of the highest-energy gamma-ray sources ever detected, with the emission extending past 200 TeV. This extremely bright source, located in the Galactic plane, is an intriguing source to study. Previously, the origins of the emission were unclear – there is a supernova remnant, molecular clouds, and multiple pulsars in the region. Evidence of hadronic emission could mean that the source is a “PeVatron”, or PeV particle accelerator and therefore a source of Galactic cosmic rays. In this paper, HAWC modeling shows that the emission is predominantly leptonic in nature and originates from the extremely energetic pulsar PSR J1907+0602. However, a hadronic component may be present at the very highest energies (> 50 TeV). The result highlights the importance of multi-wavelength and multi-messenger studies in determining emission mechanisms.

Full Article: HAWC Study of the Ultra-High-Energy Spectrum of MGRO J1908+06 , HAWC Collaboration 2022

Using the largest volcano in Mexico to look for neutrinos
December 22, 2021

Outline of the physical process that can be used to search for neutrino signals using HAWC. A neutrino propagates through the Pico de Orizaba volcano and produces a charged lepton with the same flavor as the initial neutrino. The charged lepton (a muon for example) produces a signal in the detectors that points back to the volcano. The picture of the observatory has a superimposed image of one of the signals that we report in the paper.

Neutrino detection is usually associated with underground observatories, as a way to mitigate the extremely large background produced by cosmic rays. However, more than 20 years ago it was proposed to use mountains as targets to produce charged-current neutrino-nucleon interactions, and to detect the decay products of the produced charged lepton if it is able to escape the mountain. The mountain also works as a shield against the background of horizontal cosmic rays. The HAWC observatory is located next to the Pico de Orizaba volcano, the largest in Mexico. We used this geographical coincidence to implement the neutrino detection method mentioned above. In a new publication from HAWC we characterized the background that is observed when looking for track-like signals that point back to the volcano. We found that the large majority of the signals that point back to the volcano are produced by atmospheric muons that are scattered to almost horizontal trajectories. However, we show that it is possible to control this background by selecting muons with energies larger than 100 GeV. Our work shows that the “Earth-skimming” neutrino detection method is possible using a surface air shower array as HAWC.

Full Article: Characterization of the background for a neutrino search with the HAWC observatory , HAWC Collaboration 2021

A new analysis of the HESS Galactic Plane Survey confirms 4 new HAWC sources
September 1, 2021

Galactic plane maps from HAWC and H.E.S.S. Maps from top to bottom: (1) H.E.S.S. Galactic plane map for E > 1 TeV and a Gaussian kernel of size 0.1°; the ring background method is applied. (2) Same H.E.S.S. data, using a Gaussian kernel of 0.4°. (3) Same H.E.S.S. data, using the field-of-view background method and a Gaussian kernel of 0.4°.(4) HAWC Galactic plane map with 1523 days of data.

In the third HAWC catalog [1], the HAWC collaboration reported 20 sources detected for the first time at TeV energies. Imaging Atmospheric Cherenkov Telescope (IACT) arrays analyzed archival data or collected new data from these sources as part of their observation plan. For example, MAGIC studied archival data from 3 new HAWC sources, without detecting significant emission [2]. VERITAS analyzed 14 new sources using archival data and new data and was able to confirm only one of them [3]. Hence, despite their overlapping energy range, it seems challenging for IACTs to confirm the HAWC observations. One possible explanation is that the HAWC sources may be significantly extended, and have a low surface brightness which make them hard to detect to IACTs for which the angular resolution may be up to an order of magnitude better than HAWC. However, a major effort from the HESS collaboration to reanalyze the HESS Galactic Plane Survey data (HGPS, [4]) using a method adapted for extended sources allowed to confirm 4 new HAWC sources amongst 7 present in the HGPS. The 3 remaining ones have shown to be below HESS sensibility and hence are not expected to be detected. [1] Albert, A., Alfaro, R., Alvarez, C., et al. 2020, ApJ, 905, 76 [2] Ahnen, M. L., Ansoldi, S., Antonelli, L. A., et al. 2019, MNRAS, 485, 356 [3] Abeysekara, A. U., Archer, A., Benbow, W., et al. 2018, ApJ, 866, 24 [4] H.E.S.S. Collaboration, Abdalla, H., Abramowski, A., et al. 2018a, A&A,612, A1

Full Article: TeV emission of Galactic plane sources with HAWC and H.E.S.S. , H.E.S.S. and HAWC Collaborations 2021

HAWC Probes Sea of Cosmic Rays in Giant Molecular Clouds with Gamma Rays
June 21, 2021

95% C.I. upper limits on the gamma-ray flux of the giant molecular clouds studied. The gray band represents the statistical uncertainty in the U.L. (68% and 90% containment). The blue band is the expectation for the gamma-ray spectrum of the clouds based on local cosmic-ray measurements.

The cosmic-ray flux measured in the vicinity of the solar system is assumed to be representative of the whole galaxy. It is referred to as the “sea” of cosmic rays. A way to probe this is, in an indirect way, is to study the gamma-ray emission from distant parts of the galaxy. We looked at regions that can act as targets for cosmic rays: giant molecular clouds. We selected high galactic latitude clouds that are in HAWC's field of view and that are within 1 kpc distance from the Sun. No significant emission was observed from them. However, we were able to constrain the gamma-ray emission from these regions and found that this gamma-ray flux is consistent with a cosmic-ray flux and energy density similar to that measured at Earth.

Full Article: Probing the Sea of Cosmic Rays by Measuring Gamma-Ray Emission from Passive Giant Molecular Clouds with HAWC, A. Albert et al. 2021

HAWC Search for High-Mass Microquasars
April 30, 2021

Residual significance maps of the regions centered around LS 5039 (top left), CYG X-1 (top right), CYG X-3 (bottom left), and SS 433 (bottom right) produced using 1,523 days of HAWC data. We also show in these maps the labelled 3HWC sources fitted and subtracted.

This analysis involved searching for signs of TeV gamma-ray emission at the known locations of the four high-mass microquasars (HMMQs), LS 5039, Cygnus X-1, Cygnus X-3, and SS 433. Also, two hypotheses were assumed to examine different models of particle acceleration and gamma-ray production in these sources as a common mechanism: I) gamma-ray luminosity if proportional to jet power; II) inverse-Compton and synchrotron fluxes are connected by energy densities of magnetic field and radiation field of donor star.

Full Article: HAWC Search for High-Mass Microquasars, A. Albert et al. 2021

HAWC Finds Energetic Young Pulsar Wind Nebula
April 29, 2021

Spectrum of HAWC J2019+368 with spectral modeling overlaid. An age of approximately 7000 years matches well with the gamma-ray spectrum seen by HAWC and VERITAS. The small FoV of the Suzaku instrument may explain a substantial fraction of why the total emission in this model predicted is not observed at X-ray energies. Suzaku data taken from Mizuno, T., Tanaka, N., Takahashi, H., et al. 2017, ApJ, 841, 104. VERITAS data obtained from Abeysekara, A. U., Archer, A., Aune, T., et al. 2018, ApJ, 861, 134, and then scaled based on procedure described in the paper.

This study presents a detailed look at the morphology and spectrum of the MGRO J2019+37 region, discovering two sources of gamma-ray emission: HAWC J2019+368 and HAWC J2016+371. The morphological similarity with VERITAS confirms that this system is asymmetric. The spectrum indicates electrons and positrons accelerated in the pulsar wind produce the gamma-rays HAWC detects via inverse Compton scattering. We associate HAWC J2019+368 with PSR J2021+3651, a young pulsar in the Cygnus Region which lies at the heart of the Dragonfly Nebula. The Dragonfly Nebula is notable for being the only other Pulsar Wind Nebula (PWN) having a double torus structure similar to the Vela PWN. A detailed model of the HAWC spectrum indicates that the pulsar is ~7000 years old, much younger than the its characteristic age of 17,000 years. Further studies like this may reveal other sources for high energy electrons and positrons in our galaxy. This work broadens our understanding how these particles achieve high energies and move throughout the galaxy.

Full Article: Spectrum and Morphology of the Very-high-energy Source HAWC J2019+368, A. Albert et al. 2021

Ultra-high-energy gamma rays originate from pulsar nebulae
April 23, 2021

A new study by HAWC has resulted in the discovery that ultra-high-energy (> 56 TeV) gamma-ray emission is a generic feature near the most powerful pulsars ever observed. This emission is expected to be leptonic and goes against the conventional wisdom that UHE gamma rays would be associated with PeVatrons, or sites where cosmic rays are accelerated to petaelectronvolt energy. With this new study, we have a fuller picture of how the highest energy gamma rays are created in our galaxy.

Full letter: Evidence that Ultra-high-energy Gamma Rays Are a Universal Feature near Powerful Pulsars, A. Albert et al. 2021 ApJL 911 L27.

Press Release.

Statement from the HAWC Leadership
March 24, 2021

The HAWC Collaboration stands in solidarity with the Asian, Desi, and Pacific Islander community and against hate and racism. We are outraged by the recent horrific crime in Atlanta that we see as part of a larger pattern of discrimination and racism targeting Asian Americans. As in our message of June 8, 2020 (below), we reiterate our belief that scientific discoveries are made not by hardware, but by people and that we can only achieve the best scientific results when we include people of all ethnicities, races, gender identities, and backgrounds in the process. We also believe that it is our role to speak out against violence, discrimination and oppression wherever it occurs. We once again pledge to do more to bring change to our collaboration, our institutions, our countries and society as a whole.

Could Star Clusters Be PeVatrons?
March 11, 2021

24 micrometer infrared map from the Cocoon region with Spitzers MIPS overlaid with gamma-ray significance map from HAWC (greenish-yellow to red indicate higher gamma-ray significance). The map is centered at Cocoon with about 4.6 deg in x and y direction (Spitzer image credit: Hora et. al, Spitzer’s Growing Legacy, ASP Conference Series, 2010, P. Ogle, ed.)

HAWC has detected cosmic gamma rays with energies up to at least 200 TeV from the direction of the Cygnus Cocoon. The Cocoon is a superbubble surrounding the birthplace of massive stars. Our analysis shows that these gamma rays could be produced by PeV cosmic rays. These cosmic rays interact with the gas in the region to produce the observed gamma rays. They are accelerated to PeV energies in the enclosed star forming region Cyg OB2. The energy distribution and location of the gamma rays in the Cocoon are different at GeV and TeV energies, which indicates that particles in the Cocoon at different energies were transported differently. These observations give us new clues about the creation and evolution of cosmic rays and the past activity in the superbubble. They also show for the first time, that PeV cosmic rays are born in massive stars environment.

Full letter: HAWC observations of the acceleration of very-high-energy cosmic rays in the Cygnus Cocoon, A.U. Abeysekara et al. 2021

Press Release.

Are photons of extreme energies coming from the Galaxy's largest accelerator?
February 1, 2021

FUGIN CO molecular column density in units of cm^−2 obtained by integrating the line intensity over a range in velocity between 40 and 60 km corresponding to a distance for the molecular gas of 3–4 kpc. The HAWC, and H.E.S.S. source locations, as well as the SNR, PWN, Bubble, star cluster and molecular cloud locations are overlaid.

HAWC discovered a γ-ray source, HAWC J1825-134, whose energy spectrum extends well beyond 200 TeV without a break or cutoff. The source is found to be coincident with a giant molecular cloud. While the nature of this extreme accelerator remains unclear, cosmic rays accelerated to energies of several PeV colliding with the ambient gas likely produce the observed radiation.

Full letter: Evidence of 200 TeV Photons from HAWC J1825-134, A. Albert et al. 2021

Press Release.

HAWC takes a good look at a cosmic accelerator
June 18, 2020

HAWC has detected gamma-ray emission from the supernova remnant G106.3+2.7. Combining the HAWC results with older measurements by the VERITAS collaboration, we find that the gamma-ray emission continues up to more than 100 TeV. The observed gamma-ray emission can be produced either by relativistic protons with energies up to at least 800 TeV, or by relativistic electrons with energies up to at least 270 TeV. These results indicate that G106.3+2.7 may be a PeVatron, a cosmic ray accelerator capable of accelerating protons to PeV energies. More studies are needed to fully constrain the emission mechanism and the source of the high-energy particles.

Full letter: HAWC J2227+610 and Its Association with G106.3+2.7, a New Potential Galactic PeVatron, A. Albert et al. 2020, ApJL 896 L29.

Popular science article featuring the results on AASNova.

Statement from the HAWC Leadership
June 8, 2020

The murders of George Floyd, Ahmaud Arbery, and Breonna Taylor are but recent reminders of the hatred, violence, and racism that are pervasive in the US and around the world. We, the leadership of the HAWC Gamma Ray Observatory, wish to express our solidarity with the Black Lives Matter movement and with the multitude of demonstrators around the world in their protest against systemic racism and police brutality directed at people of color.

Science and society are strengthened when we bring together people with the broadest spectrum of backgrounds and experiences. Despite a stated commitment to diversity, equity, and inclusion in our research activities, we recognize that we must do significantly more to bring change to our collaboration, our institutions, our countries and society as a whole. While we must work to remove barriers that prevent black people as well as other underrepresented groups from gaining access to academic and scientific pursuits and institutions, we recognize that this begins with fully and actively supporting basic human rights for all people.

We, the leadership of the HAWC collaboration, recognize that we are privileged in our positions as scientists and have a responsibility to speak out against discrimination and oppression in all its forms and to demand justice.

HAWC tests Einstein’s theory of relativity with extreme astronomy
January 13, 2020

Sometimes in science, things that don’t occur can be as interesting as those that do. Much of modern physics is founded on Einstein’s special relativity, but physicists have also considered possible models in which it is subtly violated at very high energies. To constrain such theoretical ideas, it is important to test the observational consequences of such exotic theories.

If special relativity were violated at high energy, strange things could happen; gamma rays might travel faster or slower than lower energy light. Or high energy gamma rays could turn into lower energy particles and thus never reach Earth.

In a recent study, published in in the March 30th issue of Physical Review Letters, MSU Department of Physics and Astronomy Professor Jim Linnemann and his now former graduate student Sam Marinelli tested this theory by working on machine learning algorithms to take the patterns observed by the HAWC detectors and translate them into surprisingly accurate estimates–within 30 percent– of the energy of the gamma rays initiating the particle showers.

Overall, the data indicate that Lorentz invariance is preserved up to at least 285 TeV. Such gamma rays carry about 100 trillion times the energy of visible light coming from the Sun. This is the highest-energy test of Einstein's theory of relativity to date.

  • Link to full press release
  • HAWC observations of highest-energy gamma-ray sources reveal birthplaces of high-energy particles
    January 13, 2020

    Galactic plane
    A map of the Galactic plane above 56 TeV. These are the highest-energy gamma-ray sources ever observed.

    There are many unanswered questions about cosmic-ray origins and acceleration mechanisms. Since these particles are charged, they are deflected in magnetic fields on their way to the Earth and do not point back to their sources. High-energy Gamma rays are produced near cosmic-ray acceleration sites and since they are neutral, they can be used to probe cosmic-ray acceleration. High-energy gamma rays can also be produced via other mechanisms, such as lower-energy photons scattering off electrons. This commonly occurs near pulsars.

    HAWC has observed nine gamma-ray sources emitting above 56 TeV, with three of them continuing emission to 100 TeV and beyond. The results have been accepted in Physical Review Letters. These are the highest-energy sources ever observed in our galaxy and have energies of about ten times higher than any particles produced using particle colliders on Earth. All of the sources have extremely energetic pulsars nearby. The number of sources seen may indicate that ultra-high-energy emission is a generic feature of pulsar wind nebula.

    HAWC's funding includes the National Science Foundation, the Department of Energy Office of High-Energy Physics, Consejo Nacional de Ciencia y Tecnologia, and the Laboratory Directed Research and Development (LDRD) program of Los Alamos National Laboratory. For complete funding information see the acknowledgments here.

  • Link to press release
  • 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 at APS
    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.