Discovery of Gamma Rays from the Quiescent Sun with HAWC.

We report the first detection of a TeV γ-ray flux from the solar disk (6.3σ), based on 6.1 years of data from the High Altitude Water Cherenkov (HAWC) observatory. The 0.5-2.6 TeV spectrum is well fit by a power law, dN/dE=A(E/1 TeV)^{-γ}, with A=(1.6±0.3)×10^{-12} TeV^{-1} cm^{-2} s^{-1} and γ=3.62±0.14. The flux shows a strong indication of anticorrelation with solar activity. These results extend the bright, hard GeV emission from the disk observed with Fermi-LAT, seemingly due to hadronic Galactic cosmic rays showering on nuclei in the solar atmosphere. However, current theoretical models are unable to explain the details of how solar magnetic fields shape these interactions. HAWC's TeV detection thus deepens the mysteries of the solar-disk emission.

Constraints on Lorentz Invariance Violation from HAWC Observations of Gamma Rays above 100 TeV.

Because of the high energies and long distances to the sources, astrophysical observations provide a unique opportunity to test possible signatures of Lorentz invariance violation (LIV). Superluminal LIV enables the decay of photons at high energy. The high altitude water Cherenkov (HAWC) observatory is among the most sensitive gamma-ray instruments currently operating above 10 TeV. HAWC finds evidence of 100 TeV photon emission from at least four astrophysical sources. These observations exclude, for the strongest of the limits set, the LIV energy scale to 2.2×10^{31} eV, over 1800 times the Planck energy and an improvement of 1 to 2 orders of magnitude over previous limits.

Multiple Galactic Sources with Emission Above 56 TeV Detected by HAWC.

We present the first catalog of gamma-ray sources emitting above 56 and 100 TeV with data from the High Altitude Water Cherenkov Observatory, a wide field-of-view observatory capable of detecting gamma rays up to a few hundred TeV. Nine sources are observed above 56 TeV, all of which are likely galactic in origin. Three sources continue emitting past 100 TeV, making this the highest-energy gamma-ray source catalog to date. We report the integral flux of each of these objects. We also report spectra for three highest-energy sources and discuss the possibility that they are PeVatrons.

Publisher Correction: Very-high-energy particle acceleration powered by the jets of the microquasar SS 433.

In this Letter, owing to a production error, the penultimate version of the PDF was published. The HTML version was always correct. The PDF has been corrected online.

Very-high-energy particle acceleration powered by the jets of the microquasar SS 433.

SS 433 is a binary system containing a supergiant star that is overflowing its Roche lobe with matter accreting onto a compact object (either a black hole or neutron star)1-3. Two jets of ionized matter with a bulk velocity of approximately 0.26c (where c is the speed of light in vacuum) extend from the binary, perpendicular to the line of sight, and terminate inside W50, a supernova remnant that is being distorted by the jets2,4-8. SS 433 differs from other microquasars (small-scale versions of quasars that are present within our own Galaxy) in that the accretion is believed to be super-Eddington9-11, and the luminosity of the system is about 1040 ergs per second2,9,12,13. The lobes of W50 in which the jets terminate, about 40 parsecs from the central source, are expected to accelerate charged particles, and indeed radio and X-ray emission consistent with electron synchrotron emission in a magnetic field have been observed14-16. At higher energies (greater than 100 gigaelectronvolts), the particle fluxes of γ-rays from X-ray hotspots around SS 433 have been reported as flux upper limits6,17-20. In this energy regime, it has been unclear whether the emission is dominated by electrons that are interacting with photons from the cosmic microwave background through inverse-Compton scattering or by protons that are interacting with the ambient gas. Here we report teraelectronvolt γ-ray observations of the SS 433/W50 system that spatially resolve the lobes. The teraelectronvolt emission is localized to structures in the lobes, far from the centre of the system where the jets are formed. We have measured photon energies of at least 25 teraelectronvolts, and these are certainly not Doppler-boosted, because of the viewing geometry. We conclude that the emission-from radio to teraelectronvolt energies-is consistent with a single population of electrons with energies extending to at least hundreds of teraelectronvolts in a magnetic field of about 16 microgauss.

Extended gamma-ray sources around pulsars constrain the origin of the positron flux at Earth.

The unexpectedly high flux of cosmic-ray positrons detected at Earth may originate from nearby astrophysical sources, dark matter, or unknown processes of cosmic-ray secondary production. We report the detection, using the High-Altitude Water Cherenkov Observatory (HAWC), of extended tera-electron volt gamma-ray emission coincident with the locations of two nearby middle-aged pulsars (Geminga and PSR B0656+14). The HAWC observations demonstrate that these pulsars are indeed local sources of accelerated leptons, but the measured tera-electron volt emission profile constrains the diffusion of particles away from these sources to be much slower than previously assumed. We demonstrate that the leptons emitted by these objects are therefore unlikely to be the origin of the excess positrons, which may have a more exotic origin.

A limit on the variation of the speed of light arising from quantum gravity effects.

A cornerstone of Einstein's special relativity is Lorentz invariance-the postulate that all observers measure exactly the same speed of light in vacuum, independent of photon-energy. While special relativity assumes that there is no fundamental length-scale associated with such invariance, there is a fundamental scale (the Planck scale, l(Planck) approximately 1.62 x 10(-33) cm or E(Planck) = M(Planck)c(2) approximately 1.22 x 10(19) GeV), at which quantum effects are expected to strongly affect the nature of space-time. There is great interest in the (not yet validated) idea that Lorentz invariance might break near the Planck scale. A key test of such violation of Lorentz invariance is a possible variation of photon speed with energy. Even a tiny variation in photon speed, when accumulated over cosmological light-travel times, may be revealed by observing sharp features in gamma-ray burst (GRB) light-curves. Here we report the detection of emission up to approximately 31 GeV from the distant and short GRB 090510. We find no evidence for the violation of Lorentz invariance, and place a lower limit of 1.2E(Planck) on the scale of a linear energy dependence (or an inverse wavelength dependence), subject to reasonable assumptions about the emission (equivalently we have an upper limit of l(Planck)/1.2 on the length scale of the effect). Our results disfavour quantum-gravity theories in which the quantum nature of space-time on a very small scale linearly alters the speed of light.

Discovery of localized regions of excess 10-TeV cosmic rays.

The 7 year data set of the Milagro TeV observatory contains 2.2 x 10(11) events of which most are due to hadronic cosmic rays. These data are searched for evidence of intermediate scale structure. Excess emission on angular scales of approximately 10 degrees has been found in two localized regions of unknown origin with greater than 12sigma significance. Both regions are inconsistent with pure gamma-ray emission with high confidence. One of the regions has a different energy spectrum than the isotropic cosmic-ray flux at a level of 4.6sigma, and it is consistent with hard spectrum protons with an exponential cutoff, with the most significant excess at approximately 10 TeV. Potential causes of these excesses are explored, but no compelling explanations are found.

Evidence for TeV gamma-ray emission from a region of the galactic plane.

Gamma-ray emission from a narrow band at the galactic equator has previously been detected up to 30 GeV. We report evidence for a TeV gamma-ray signal from a region of the galactic plane by Milagro, a large-field-of-view water Cherenkov detector for extensive air showers. An excess with a significance of 4.5 standard deviations has been observed from the region of galactic longitude l E (40 degrees, 100 degrees) and latitude /b/ < 5 degrees. Under the assumption of a simple power law spectrum, with no cutoff in the EGRET-Milagro energy range, the measured integral flux is phi gamma(>3.5 TeV) = (6.4 +/- 1.4 +/- 2.1) x 10(-11) cm(-2) s(-1) sr(-1). This flux is consistent with an extrapolation of the EGRET spectrum between 1 and 30 GeV in this galactic region.

A gamma-ray burst with a high-energy spectral component inconsistent with the synchrotron shock model.

Gamma-ray bursts are among the most powerful events in nature. These events release most of their energy as photons with energies in the range from 30 keV to a few MeV, with a smaller fraction of the energy radiated in radio, optical, and soft X-ray afterglows. The data are in general agreement with a relativistic shock model, where the prompt and afterglow emissions correspond to synchrotron radiation from shock-accelerated electrons. Here we report an observation of a high-energy (multi-MeV) spectral component in the burst of 17 October 1994 that is distinct from the previously observed lower-energy gamma-ray component. The flux of the high-energy component decays more slowly and its fluence is greater than the lower-energy component; it is described by a power law of differential photon number index approximately -1 up to about 200 MeV. This observation is difficult to explain with the standard synchrotron shock model, suggesting the presence of new phenomena such as a different non-thermal electron process, or the interaction of relativistic protons with photons at the source.