Ultra-high Energy Photons Research Articles

Constraints on metastable superheavy dark matter coupled to sterile neutrinos with the Pierre Auger Observatory

Dark matter particles could be superheavy, provided their lifetime is much longer than the age of the Universe. Using the sensitivity of the Pierre Auger Observatory to ultrahigh energy neutrinos and photons, we constrain a specific extension of the Standard Model of particle physics that meets the lifetime requirement for a superheavy particle by coupling it to a sector of ultralight sterile neutrinos. Our results show that, for a typical dark coupling constant of 0.1, the mixing angle θm between active and sterile neutrinos must satisfy, roughly, θm≲1.5×10−6(MX/109 GeV)−2 for a mass MX of the dark-matter particle between 108 GeV and 1011 GeV. Published by the American Physical Society 2024

Constraints on the proton fraction of cosmic rays at the highest energies and the consequences for cosmogenic neutrinos and photons

Over the last decade, observations have shown that the mean mass of ultra-high-energy cosmic rays (UHECRs) increases progressively toward the highest energies. However, the precise composition is still unknown and several theoretical studies hint at the existence of a subdominant proton component up to the highest energies. Motivated by the exciting prospect of performing charged-particle astronomy with ultra-high-energy (UHE) protons we quantify the level of UHE-proton flux that is compatible with present multimessenger observations and the associated fluxes of neutral messengers produced in the interactions of the protons. We study this scenario with numerical simulations of two independent populations of extragalactic sources and perform a fit to the combined UHECR energy spectrum and composition observables, constrained by diffuse gamma-ray and neutrino observations. We find that up to of order 10% of the cosmic rays at the highest energies can be UHE protons, although the result depends critically on the selected hadronic interaction model for the air showers. Depending on the maximum proton energy (E max p) and the redshift evolution of sources, the associated flux of cosmogenic neutrinos and UHE gamma rays can significantly exceed the multimessenger signal of the mixed-mass cosmic rays. Moreover, if E max p is above the GZK limit, we predict a large flux of UHE neutrinos above EeV energies that is absent in alternate scenarios for the origin of UHECRs. We present the implications and opportunities afforded by these UHE proton, neutrino and photon fluxes for future multimessenger observations.

Search for Ultra-high-energy Photons from Gravitational Wave Sources with the Pierre Auger Observatory

A search for time-directional coincidences of ultra-high-energy (UHE) photons above 10 EeV with gravitational wave (GW) events from the LIGO/Virgo runs O1 to O3 is conducted with the Pierre Auger Observatory. Due to the distinctive properties of photon interactions and to the background expected from hadronic showers, a subset of the most interesting GW events is selected based on their localization quality and distance. Time periods of 1000 s around and 1 day after the GW events are analyzed. No coincidences are observed. Upper limits on the UHE photon fluence from a GW event are derived that are typically at ∼7 MeV cm−2 (time period 1000 s) and ∼35 MeV cm−2 (time period 1 day). Due to the proximity of the binary neutron star merger GW170817, the energy of the source transferred into UHE photons above 40 EeV is constrained to be less than 20% of its total GW energy. These are the first limits on UHE photons from GW sources.

Pulsars as candidates of LHAASO sources J2226+6057, J1908+0621, and J1825-1326: The leptonic origin* *Supported by the National Nature Science Foundation of China (12075249, 11690022) and the Key Research Program of the Chinese Academy of Sciences (XDPB15)

Recently, from 12 γ-ray Galactic sources, the LHAASO has detected ultrahigh-energy photons up to 1.4 PeV. The γ-ray spectra of the sources J2226+6057, J1908+0621, and J1825-1326 and the suggested origin pulsars near the sources have been published. In our previous work, we studied the hadronic γ-ray spectra of the sources J2226+6057, J1908+0621, and J1825-1326 in terms of the Hertzian dipole model of pulsars. In this paper, we investigate the possibility of the leptonic origin of the γ-ray. We use the Hertzian dipole model to describe the pulsars around the sources. The electrons around the pulsars can be accelerated to PeV by the electromagnetic fields of pulsars. Under the assumption that the initial electrons are uniformly distributed in a spherical shell between and m around the pulsar, we obtain the energy distribution of electrons. The leptonic γ-ray spectra can be calculated through inverse Compton scattering processes. The leptonic γ-ray can roughly conform to the observation of LHAASO.

Multi-messenger studies with the Pierre Auger Observatory

The combination of data from observatories measuring ultra-high energy cosmic rays, photons, neutrinos and gravitational waves has provided new insights into the most extreme phenomena in the Universe. Sharing information within a broad community is the foundation of the multi-messenger approach. The Pierre Auger Observatory, the world’s largest cosmic ray detector, provides sensitivity to photons and neutrinos above 1017eV, thus contributing significantly to this joint effort. The latest results from diffuse and targeted searches will be reviewed here, along with results from follow-up analyses and future perspectives. In particular, preliminary limits on photon fluence from a selection of gravitational wave sources detected by LIGO/Virgo and results of the search for ultra-high energy neutrinos from binary black hole mergers will be presented.

Searching for multi-messenger signals with the Pierre Auger Observatory

The Pierre Auger Observatory [1], primarily designed for the detection of ultra-high energy (UHE) cosmic rays, has been proven to be also an excellent tool in multi-messenger searches. With its unprecedented exposure to UHE particles, it is exploited to set stringent upper limits on the diffuse flux of UHE photons and neutrinos and to look for neutral particles associated with steady sources and transient events, such as gravitational waves. All these searches can provide key information to investigate the most energetic phenomena in the Universe and answer some of the most important still-open questions in astrophysics.

Diffuse flux of ultra-high energy photons from cosmic-ray interactions in the disk of the Galaxy and implications for the search for decaying super-heavy dark matter

When propagating to Earth, UHECRs can interact with the gas in our Milky Way and produce secondary particles including photons. This can impact the search for UHE photons as we face a diffuse flux of UHE photons resulting from their propagation. This flux, together with the photon flux expected from the GZK interactions, participates in the diffuse photon flux which is limiting the detection of UHE-gamma sources. We present an estimate of the diffuse flux resulting from the interactions of UHECRs in our galaxy above 1017 eV, using results from the Pierre Auger Collaboration concerning the flux and its composition, and two different interstellar gas density models. We also discuss the impact of the evaluated diffuse flux of UHE photons on SHDM searches, as the former can be considered as a floor below which other signals would be overwhelmed. Similarly, the neutrino flux produced during the same process can be evaluated, by accounting for the neutrino mixing.

Ultrahigh energy neutrinos from high-redshift electromagnetic cascades

We study the impact of the muon pair production and double pair production processes induced by ultrahigh energy photons on the cosmic microwave background. Although the muon pair production cross section is smaller than the electron pair production one, the associated energy loss length is comparable or shorter than the latter (followed by inverse Compton in the deep Klein-Nishina regime) at high redshift, where the effect of the astrophysical radio background is expected to be negligible. By performing a simulation taking into account the details of $e/\ensuremath{\gamma}$ interactions at high energies, we show that a significant fraction of the electromagnetic energy injected at $E\ensuremath{\gtrsim}{10}^{19}\text{ }\text{ }\mathrm{eV}$ at redshift $z\ensuremath{\gtrsim}5$ is channeled into neutrinos. The double pair production plays a crucial role in enhancing the multiplicity of muon production in these electromagnetic cascades. The ultrahigh energy neutrino spectrum, yet to be detected, can in principle harbor information on ultrahigh energy sources in the young universe, either conventional or exotic ones, with weaker constraints from the diffuse gamma ray flux compared to their low redshift counterparts.

Blazar Jets as Possible Sources of Ultra-High Energy Photons: A Short Review

In this paper, I present a qualitative discussion on the prospect of production of ultra-high photons in blazars. The sources are a subclass of active galactic nuclei which host supermassive black holes and fire relativistic jets into the intergalactic medium. The kpc-scale jets are believed to be dominated by Poynting flux and constitute one of the most efficient cosmic particle accelerators, that potentially are capable of accelerating the particles up to EeV energies. Recent IceCube detection of astrophysical neutrino emissions, in coincidence with the enhanced gamma-ray from Tev blazar TXS 0506 + 056, further supports hadronic models of blazar emissions in which particle acceleration processes, such as relativistic shocks, magnetic re-connection, and relativistic turbulence, could energize hadrons, e.g., protons, up to energies equivalent to billions of Lorentz factors. The ensuing photo-pionic processes may then result in gamma-rays accompanied by neutrino flux. Furthermore, the fact that blazars are the dominant source of observed TeV emission encourages search for signatures of acceleration scenarios that would lead to the creation of ultra-high-energy photons.

Simulation of the Isotropic Ultra-High Energy Photon Flux in the Solar Magnetic Field

Both the lack of observation of ultra-high energy (UHE) photons and the limitations of the state-of-the-art methodology being applied for their identification motivate studies on alternative approaches to the relevant simulations and the related observational strategies. One such new approach is proposed in this report and it concerns new observables allowing indirect identification of UHE photons through cosmic ray phenomena composed of many spatially correlated extensive air showers or primary cosmic rays observed at one time. The study is based on simulations of interactions of UHE photons with the magnetic field of the Sun using the PRESHOWER program with some essential modifications. One of the expected results of such interactions is a generation of cosmic ray ensembles (CREs) in the form of very thin and very elongated cascades of secondary photons of energies spanning the whole cosmic ray energy spectrum. Upon entering the Earth’s atmosphere, these cascades or their parts may generate uniquely characteristic walls of spatially correlated extensive air showers, and the effect is expected also in cases when primary UHE photons are not directed towards the Earth. Particle distributions in these multi-primary UHE photon footprints are expected to have thicknesses of the order of meters and elongations reaching even hundreds of millions of kilometers, making them potentially observable with a global, multi-experiment approach, including re-exploring of the historical data, with the expected event rate exceeding the capabilities of even very large cosmic ray observatories. In this report, we introduce for the first time the methods allowing for simulating the isotropic flux of UHE photons in the Sun’s vicinity. Presented methods were verified and optimised in such a way that they would successfully model the cumulative spatial distribution of secondary photons at the top of the atmosphere. The preliminary results of simulations for the UHE photon flux of energy 100 EeV demonstrate the possibility of simulating potentially observable quantities related to CRE induced by UHE photons: densities, energy spectra and geographical orientations of secondary particles at the top of the Earth’s atmosphere. A measurement of at least one of these quantities would be equivalent to a confirmation of the existence of UHE photons, which would give an insight into fundamental physics processes at unprecedentedly high energies, far beyond the reach of man-made accelerators. On the other hand, a lack of such an observation would allow for further constraining of these fundamental processes with the physically new upper limits on UHE photon fluxes after careful analysis of the technical observation ability. The novel advantage of such an approach would lay in the purely electrodynamical character of the underlying simulations which are fully independent on extrapolations of hadronic interaction models by many orders of magnitude. Such extrapolations are necessary in the UHE photon identification methods based on the analyses of properties of individual extensive air showers presently used to determine the UHE photon upper limits.

Pulsars as candidates of LHAASO sources J2226+6057, J1908+0621, and J1825−1326

ABSTRACT The Large High Altitude Air Shower Observatory (LHAASO) Collaboration has observed ultrahigh-energy photons up to 1.4 PeV from 12 $\gamma$-ray Galactic sources. In particular, the γ-ray spectra of the sources J2226+6057, J1908+0621, and J1825−1326 have been published. We investigate the possibility of suggested origin pulsars near the sources as the PeVatrons. The pulsar is described by a rotating magnetic dipole. Assuming protons are uniformly distributed out of the light cylinders, we obtain the Lorentz distribution of proton energy spectrum. It is found that the protons around pulsar could be accelerated to PeV at short times. The hadronic γ-ray spectra of the suggested origin pulsars are in good agreement with the LHAASO observed γ-ray spectra of the sources J2226+6057, J1908+0621, and J1825−1326.

On the Issue of Magnetic Monopoles in the Prospect of UHE Photon Searches

Ultra-high energy (UHE) photons with energies exceeding 1018eV can potentially be observed. They are produced in various processes involving electrically charged particles. However, more exotic scenarios are also possible. UHE photons could be emitted in encounters of massive magnetically charged monopole-antimonopole pairs or in processes associated with monopoles accelerated to high energies, typically 1021eV or beyond. Observing UHE photons can pose constraints on the properties of magnetic monopoles. There are compelling theoretical reasons in favor of the presence of magnetic monopoles in nature. The predicted observational signatures of these particles are therefore searched for in dedicated experiments currently in operation. Despite these attempts, magnetic monopoles have yet to be empirically proved. There are also theoretical reasons why magnetic monopoles allowed by Dirac’s theory might not be realized in nature in the form of isolated particles. Detection or non-detection of UHE photon signatures of magnetic monopoles would bring us closer to solving this fascinating puzzle.

On the fraction of particles involved in magneto-centrifugally generated ultra-high energy electrons in the Crab pulsar

The earthward journey of ultra high energy electrons (∼600TeV) produced in the Pulsar atmosphere by Landau damping of magneto-centrifugally excited Langmuir waves (drawing energy form the rotational slowdown) on primary electrons, is charted. It is shown, that just as they escape the light cylinder zone, the ultra-high energy particles, interacting with the medium of the Crab nebula, rapidly loose their energy via the quantum synchrotron process, producing highly energetic gamma rays ∼0.6 PeV. Interacting with the cosmic background radiation in the interstellar medium, only a tiny fraction of these ultra high energy photons (via the γγ channel) are, then transformed into electron–positron pairs. Detected flux of these photons imposes an upper limit on the fraction (4×10−7) of the magnetospheric particles involved in the process of generation of ultra-high energy photons (up to 600TeV).

Probing Spacetime Foam with Extragalactic Sources of High-Energy Photons

Quantum fluctuations can endow spacetime with a foamy structure. In this review article, we discuss our various proposals to observationally constrain models of spacetime foam. One way is to examine if the light wave-front from a distant quasar or GRB can be noticeably distorted by spacetime-foam-induced phase incoherence. As the phase fluctuations are proportional to the distance to the source but inversely proportional to the wavelength, ultra-high energy photons (>1 TeV) from distant sources are particularly useful. We elaborate on several proposals, including the possibility of detecting spacetime foam by observing “seeing disks” in the images of distant quasars and active galactic nuclei. We also discuss the appropriate distance measure for calculating the expected angular broadening. In addition, we discuss our more recent work in which we investigate whether wave-front distortions on small scales (due to spacetime foam) can cause distant objects become undetectable because the phase fluctuations have accumulated to the point at which image formation is impossible. Another possibility that has recently become accessible is to use interferometers to observe cosmologically distant sources, thereby giving a large baseline perpendicular to the local wave vector over which the wave front could become corrugated and thus distorted, reducing or eliminating its fringe visibility. We argue that all these methods ultimately depend on the availability of ways (if any) to carry out proper averaging of contributions from different light paths from the source to the telescope.

Cosmic ray ensembles as signatures of ultra-high energy photons interacting with the solar magnetic field

Propagation of ultra-high energy photons in the solar magnetosphere gives rise to cascades comprising thousands of photons. We study the cascade development using Monte Carlo simulations and find that the photons in the cascades are spatially extended over millions of kilometers on the plane distant from the Sun by 1 AU. We estimate the chance of detection considering upper limits from current cosmic rays observatories in order to provide an optimistic estimate rate of 0.002 events per year from a chosen ring-shaped region around the Sun. We compare results from simulations which use two models of the solar magnetic field, and show that although signatures of such cascades are different for the models used, for practical detection purpose in the ground-based detectors, they are similar.

Ultrahigh-energy γ-Rays from Past Explosions in Our Galaxy

Abstract The discovery of the sources of ultrahigh-energy photons in our Galaxy by the High-Altitude Water Cherenkov Gamma-ray Observatory and the Large High Altitude Air Shower Observatory (LHAASO) has revolutionized the field of gamma-ray astronomy in the last few years. These emissions are sometimes found in the vicinity of powerful pulsars or supernova remnants (SNRs) associated with giant molecular clouds (GMCs). Inverse Compton emission by shock-accelerated electrons emitted by pulsars and proton–proton interactions of shock-accelerated protons emitted by SNRs with cold protons in molecular clouds are often identified as the causes of these emissions. In this paper we have selected two ultrahigh-energy photon sources, LHAASO J2108+5157 and LHAASO J0341+5258, which are associated with GMCs, but no powerful pulsar or SNR has been detected in their vicinity. We have proposed a scenario where shock-accelerated electrons and protons are injected in the local environment of these sources from past explosions, which happened thousands of years ago. We show that the observed ultrahigh-energy photon flux can be explained with the secondary gamma rays produced by the time-evolved relativistic electron and proton spectra.

Multimessenger Astronomy of Transient Point Sources at the Pierre Auger Observatory

One of the key challenges in astroparticle physics is the identification of the sources of cosmic rays at the highest energies (above 1018 eV). In this context, the search for neutral messenger particles in the ultra-high energy (UHE) regime is of high interest. The sources of the gravitational waves (GWs) that can be observed with the current generation of GW detectors provide extreme astrophysical environments that are most likely to be unique in the universe. Another extraordinary source candidate is the anomalous blazar TXS 0506+056 which has been found to be coincident with two periods of enhanced high-energy neutrino flux in 2014/15 and 2017 as reported by the IceCube Collaboration. Due to their distance and transient nature, the capabilities of these sources to produce UHE radiation can only be studied through neutral messengers like photons and neutrinos. The Pierre Auger Observatory near Malargue, Argentina, is the largest air shower experiment for the detection of ultra-high energy cosmic rays. With its surface detector, consisting of a grid of 1660 water-Cherenkov detectors covering an area of 3000 km2, it has a unique exposure to UHE photons and neutrinos and has published first constraints on these particles from GW sources and TXS 0506+056.

Constraining superheavy decaying dark matter with directional ultra-high energy gamma-ray limits

We present constraints on the lifetime of the superheavy decaying dark matter branching to the qq̅ channel in the mass range 1019–1025 eV based on the directional limits on the ultra-high-energy (UHE) gamma rays from dwarf spheroidal galaxies (dSphs) and the Milky Way (MW) centre obtained by the Pierre Auger Observatory and the Telescope Array experiment. Attenuation effects during the propagation of UHE photons towards Earth are taken into account. The strongest constraints are derived for the MW centre and have an order of 1020 yr. We conclude that the UHE diffuse gamma-ray limits provide more efficient signature for the superheavy DM search than the directional gamma-ray limits.

Ultrahigh-energy photons from LHAASO as probes of Lorentz symmetry violations

The Large High Altitude Air Shower Observatory (LHAASO) is one of the most sensitive gamma-ray detector arrays currently operating at TeV and PeV energies. Recently the LHAASO experiment detected ultra-high-energy (UHE; ${E}_{\ensuremath{\gamma}}\ensuremath{\gtrsim}100\text{ }\text{ }\mathrm{TeV}$) photon emissions up to 1.4 PeV from twelve astrophysical gamma-ray sources. We point out that the detection of cosmic photons at such energies can constrain the photon self-decay motivated by superluminal Lorentz symmetry violation (LV) to a higher level, thus can put strong constraints to certain LV frameworks. Meanwhile, we suggest that the current observation of the PeV-scale photon with LHAASO may provide hints to permit a subluminal type of Lorentz violation in the proximity of the Planckian regime, and may be compatible with the light speed variation at the scale of $3.6\ifmmode\times\else\texttimes\fi{}{10}^{17}\text{ }\text{ }\mathrm{GeV}$ recently suggested from gamma-ray burst (GRB) time delays. We further propose detecting PeV photons coming from extragalactic sources with future experiments, based on LV-induced threshold anomalies of ${e}^{+}{e}^{\ensuremath{-}}$ pair-production, as a crucial test of subluminal Lorentz violation. We comment that these observations are consistent with a D-brane/string-inspired quantum-gravity framework, the space-time foam model.

Threshold anomalies of ultra-high energy cosmic photons due to Lorentz invariance violation

From special relativity, photon annihilation process γγ→e+e− prevents cosmic photons with energies above a threshold to propagate a long distance in cosmic space due to their annihilation with low energy cosmic background photons. However, modifications of the photon dispersion relation from Lorentz invariance violation (LIV) can cause novel phenomena beyond special relativity to happen. In this paper, we point out that these phenomena include optical transparency, threshold reduction and reappearance of ultra-high energy photons in cosmic space. The recent observation of near and above PeV photon events by the LHAASO Collaboration reveals the necessity to consider the threshold anomalies. Future observations of above threshold photons from extragalactic sources can testify LIV properties of photons.