Diffuse Gamma-ray Research Articles

A new perspective on the diffuse gamma-ray emission excess* *Supported by the National Natural Science Foundation of China (12105292, 12175248, 12393853)

The Large High-Altitude Air Shower Observatory (LHAASO) recently published measurements of diffuse Galactic gamma-ray emission (DGE) in the 10−1000 TeV energy range. The measured DGE flux is significantly higher than the expectation from hadronic interactions between cosmic rays (CRs) and the interstellar medium. This excess has been proposed to originate from unknown extended sources produced by electron radiation, such as pulsar wind nebulae or pulsar halos (PWNe/halos). In this paper, we propose a new perspective to explain the DGE excess observed by LHAASO. The masking regions used in the LHAASO DGE measurement may not fully encompass the extended signals of PWNe/halos. By employing a two-zone diffusion model for electrons around pulsars, we find that the DGE excess in most regions of the Galactic plane can be well explained by the signal leakage model under certain parameters. Our results indicate that this signal leakage from known sources and contributions from unresolved sources should be considered as complementary in explaining the DGE excess.

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.

On the Source Contribution to the Galactic Diffuse Gamma Rays above 398 TeV Detected by the Tibet ASγ Experiment

Potential contribution from gamma-ray sources to the Galactic diffuse gamma rays observed above 100 TeV (sub-PeV energy range) by the Tibet ASγ experiment is an important key to interpreting recent multimessenger observations. This paper reveals a surprising fact: none of the 23 Tibet ASγ diffuse gamma-ray events above 398 TeV within the Galactic latitudinal range of ∣b∣ < 10° come from the 43 sub-PeV gamma-ray sources reported in the 1LHAASO catalog, which proves that these sources are not the origins of the Tibet ASγ diffuse gamma-ray events. No positional overlap between the Tibet ASγ diffuse gamma-ray events and the sub-PeV LHAASO sources currently supports the diffusive nature of the Tibet ASγ diffuse gamma-ray events, although their potential origin in the gamma-ray sources yet unresolved in the sub-PeV energy range cannot be ruled out.

Searching for temporary gamma-ray dark blazars associated with IceCube neutrinos

Context. Tensions between the diffuse gamma-ray sky observed by the Fermi Large Area Telescope (Fermi-LAT) and the diffuse, high-energy neutrino sky detected by the IceCube South Pole Neutrino Observatory raise questions about our knowledge of high-energy neutrino sources in the gamma-ray regime. While blazars are among the most energetic persistent particle accelerators in the Universe, studies suggest that they could account for up to 10–30% of the neutrino flux measured by IceCube. Aims. Our recent results highlight that the associated IceCube neutrinos arrived in a local gamma-ray minimum (dip) of three strong neutrino point-source candidates. Here, we increase the sample of neutrino-source candidates in order to study their gamma-ray light curves. Methods. We generated the one-year Fermi-LAT light curve for eight neutrino-source candidate blazars (RBS 0958, GB6 J1040+0617, PKS 1313-333, TXS 0506+056, PKS 1454-354, NVSS J042025-374443, PKS 0426-380, and PKS 1502+106), centered on the detection time of the associated IceCube neutrinos. We applied the Bayesian block algorithm to the light curves to characterize their variability. Results. Our results indicate that GB6 J1040+0617 was in a phase of high gamma-ray activity, while none of the other seven neutrino-source candidates were statistically bright during the detection of the corresponding neutrinos; indeed, most of the time neutrinos arrived in a faint gamma-ray phase of the light curves. This suggests either that the eight source candidate blazars (associated with seven neutrino events) in our reduced sample are not the sources of the corresponding IceCube neutrinos, or that an in-source effect (e.g., the suppression of gamma rays due to high gamma-gamma opacity) complicates the multimessenger scenario of neutrino emission for these blazars.

Extragalactic electromagnetic cascades and cascade gamma-ray emission in magnetic fields of various strength

We discuss the magnetic field influence on diffuse gamma-ray emission from extragalactic electromagnetic cascades initiated by ultra-high energy cosmic rays. Regions in space vary considerably in field strength: it is possibly of 10−12 G and lower in voids, of ∼10−6 G inside galaxies, galactic clusters and groups, of ∼10−7 G around them, and of ∼10−8–10−9 G in filaments. Structures having fields higher than in voids occupy comparatively small fraction of the Universe, so they affect weakly on cascade emission. Still knowledge of this influence may be relevant studying large-scale component of the extragalactic magnetic field and to the search for exotic particles, as in the latter case contribution of all components to extragalactic gamma-ray background should be known, one of which is cascade emission. To study magnetic field effect we simulate particle propagation in homogeneous magnetic field of ∼10−6, 10−9, and 10−12 G and lower. It is found that in fields of ∼10−9 G and lower the spectra of diffuse cascade gamma-rays at energies E ≤ 1017 eV coincide. Thus no specific models of EGMF are required to study contribution of cascade emission in the extragalactic gamma-ray background at E ≤ 1017 eV. In the case of uniform field of 10−6 G (which seems to be unrealistic), this inference is valid in the energy range of ∼107109 eV. Results obtained can be also used studying large-scale component of the extragalactic magnetic field.

Constraints on the e± pair injection of pulsar halos: Implications from the Galactic diffuse multi-TeV gamma-ray emission

Diffuse gamma-ray emission (DGE) has been discovered over the Galactic disk in the energy range from sub-GeV to sub-PeV. While it is believed to be dominated by the pionic emission of cosmic ray hadrons via interactions with interstellar medium, unresolved gamma-ray sources may also be potential contributors. TeV gamma-ray halos around middle-aged pulsars have been proposed as such sources. Their contribution to DGE, however, highly depends on the injection rate of electrons and the injection spectral shape, which are not well determined based on current observations. The measured fluxes of DGE can thus provide constraints on the ${e}^{\ifmmode\pm\else\textpm\fi{}}$ injection of the pulsar halo population in turn. In this paper, we estimate the contribution of pulsar halos to DGE based on the Australia Telescope National Facility pulsar samples with taking into account the off-beam pulsars. The recent measurement on DGE by Tibet $\mathrm{AS}\ensuremath{\gamma}$ and an early measurement by Multiple Institution Los Alamos Gamma Ray Observatory (MILAGRO) are used to constrain the pair injection parameters of the pulsar halo population. Our result may be used to distinguish different models for pulsar halos.

Diffuse Emission of Galactic High-energy Neutrinos from a Global Fit of Cosmic Rays

In the standard picture of Galactic cosmic rays, a diffuse flux of high-energy gamma rays and neutrinos is produced from inelastic collisions of cosmic-ray nuclei with the interstellar gas. The neutrino flux is a guaranteed signal for high-energy neutrino observatories such as IceCube but has not been found yet. Experimental searches for this flux constitute an important test of the standard picture of Galactic cosmic rays. Both observation and nonobservation would allow important implications for the physics of cosmic-ray acceleration and transport. We present CRINGE, a new model of Galactic diffuse high-energy gamma rays and neutrinos, fitted to recent cosmic-ray data from AMS-02, DAMPE, IceTop, as well as KASCADE. We quantify the uncertainties for the predicted emission from the cosmic-ray model but also from the choice of source distribution, gas maps, and cross sections. We consider the possibility of a contribution from unresolved sources. Our model predictions exhibit significant deviations from older models. Our fiducial model is available at https://doi.org/10.5281/zenodo.7859442 .

The diffuse gamma-ray flux from clusters of galaxies

The origin of the diffuse gamma-ray background (DGRB), the one that remains after subtracting all individual sources from observed gamma-ray sky, is unknown. The DGRB possibly encompasses contributions from different source populations such as star-forming galaxies, starburst galaxies, active galactic nuclei, gamma-ray bursts, or galaxy clusters. Here, we combine cosmological magnetohydrodynamical simulations of clusters of galaxies with the propagation of cosmic rays (CRs) using Monte Carlo simulations, in the redshift range z ≤ 5.0, and show that the integrated gamma-ray flux from clusters can contribute up to 100% of the DGRB flux observed by Fermi-LAT above 100 GeV, for CRs spectral indices α = 1.5 − 2.5 and energy cutoffs {E}_{max }=1{0}^{16}-1{0}^{17} eV. The flux is dominated by clusters with masses 1013 ≲ M/M⊙ ≲ 1015 and redshift z ≲ 0.3. Our results also predict the potential observation of high-energy gamma rays from clusters by experiments like the High Altitude Water Cherenkov (HAWC), the Large High Altitude Air Shower Observatory (LHAASO), and potentially the upcoming Cherenkov Telescope Array (CTA).

A new multimessenger study of Starburst galaxies: a closer look to expecting neutrinos

Considering the high star formation rate (up to 100 M⊙/year) of starburst galaxies (SBGs), they are well posed between the guaranteed “factories” of high energy neutrinos, since they can contain accelerated cosmic rays in the central region where the high-density gas is present. A more accurate description of their possible hadronic emission could help to better explain the diffuse astrophysical flux measured by IceCube as well as the observed point-like excess, like the case of NGC1068. With this in mind we report here a multi-messenger study, looking at diffuse and resolved gamma-ray and neutrino measurements, that explain the very-high-energy (VHE) emission through a calorimetric scenario. For the analysis of the diffuse component we perform a blending of the available spectral indexes and produce a multi-component study of extragalactic background light (EGB), high energy starting events (HESE) and high-energy cascade IceCube data. In contrast to common prototype scenarios, the spectral index blending leads to a non negligible diffuse neutrino component from SBGs, accounting up to 40% of the HESE events, at 95.4% CL. This scenario privileges also a maximal energy within tens of PeV for the accelerated charge particles inside these galaxies. For the point-like study we report the proposed calorimetric description for the gamma-ray resolved SBGs within 100 Mpc, taking into account the star formation rate derived from their infrared emission. These neutrino expectations are then compared with the sensitivity of IceCube, IceCube/Gen2 and the incoming KM3NeT.

Upper Limits on the Isotropic Diffuse Flux of Cosmic PeV Photons from Carpet-2 Observations

Isotropic diffuse gamma-ray flux in the PeV energy band is an important tool for multimessenger tests of models of the origin of high-energy astrophysical neutrinos and for new-physics searches. So far, this flux has not yet been observed. Carpet-2 is an air-shower experiment capable of detecting astrophysical gamma rays with energies above 0.1 PeV. Here we report the upper limits on the isotropic gamma-ray flux from Carpet-2 data obtained in 1999-2011 and 2018-2022. These results, obtained with the new statistical method based on the shape of the muon-number distribution, summarize Carpet-2 observations as the upgraded installation, Carpet-3, starts its operation.

Binary collisions of dark matter blobs

We describe the model-independent mechanism by which dark matter and dark matter structures heavier than ~ 8 × 1011 GeV form binary pairs in the early Universe that spin down and merge both in the present and throughout the Universe’s history, producing potentially observable signals. Sufficiently dense dark objects will dominantly collide through binary mergers instead of random collisions. We detail how one would estimate the merger rate accounting for finite size effects, multibody interactions, and friction with the thermal bath. We predict how mergers of dark dense objects could be detected through gravitational and electromagnetic signals, noting that such mergers could be a unique source of high frequency gravitational waves. We rule out objects whose presence would contradict observations of the CMB and diffuse gamma-rays.

A simulation study for the expected performance of Sharjah-Sat-1 payload improved X-Ray Detector (iXRD) in the orbital background radiation

Sharjah-Sat-1 is a 3U cubesat with a CdZnTe based hard X-ray detector, called iXRD (improved X-ray Detector) as a scientific payload with the primary objective of monitoring bright X-ray sources in the galaxy. We investigated the effects of the in-orbit background radiation on the iXRD based on Geant4 simulations. Several background components were included in the simulations such as the cosmic diffuse gamma-rays, galactic cosmic rays (protons and alpha particles), trapped protons and electrons, and albedo radiation arising from the upper layer of the atmosphere. The most dominant component is the albedo photon radiation which contributes at low and high energies alike in the instrument energy range of 20 keV - 200 keV. On the other hand, the cosmic diffuse gamma-ray contribution is the strongest between 20 keV and 60 keV in which most of the astrophysics source flux is expected. The third effective component is the galactic cosmic protons. The radiation due to the trapped particles, the albedo neutrons, and the cosmic alpha particles are negligible when the polar regions and the South Atlantic Anomaly region are excluded in the analysis. The total background count rates are ~0.36 and ~0.85 counts/s for the energy bands of 20 - 60 keV and 20 - 200 keV, respectively. We performed charge transportation simulations to determine the spectral response of the iXRD and used it in sensitivity calculations as well. The simulation framework was validated with experimental studies. The estimated sensitivity of 180 mCrab between the energy band of 20 keV - 100 keV indicates that the iXRD could achieve its scientific goals.

Gamma-ray spectroscopy of galactic nucleosynthesis

Diffuse gamma-ray emission from the decay of radioactive 26Al is a messenger from the nucleosynthesis activity in our current-day galaxy. Because this material is attributed to ejections from massive stars and their supernovae, the gamma-ray signal includes information about nucleosynthesis in massive star interiors as it varies with evolutionary stages, and about their feedback on the surrounding interstellar medium. Our method of population synthesis of massive-star groups has been refined as a diagnostic tool for this purpose. It allows to build a bottom-up prediction of the diffuse gamma-ray sky when known massive star group distributions and theoretical models of stellar evolution and core-collapse supernova explosions are employed. We find general consistency of an origin in such massive-star groups, in particular we also find support for the clumpy distribution of such source regions across the Galaxy, and characteristics of large cavities around these. A discrepancy in the integrated 26Al gamma-ray flux is interpreted as an indication for excess 26Al emission from nearby, distributed in cavities that extend over major regions of the sky.

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.

Galactic Contribution to the High-energy Neutrino Flux Found in Track-like IceCube Events

Astrophysical sources of neutrinos detected by large-scale neutrino telescopes remain uncertain. While there exist statistically significant observational indications that a part of the neutrino flux is produced by blazars, numerous theoretical studies suggest also the presence of potential Galactic point sources. Some of them have been observed in gamma rays above 100 TeV. Moreover, cosmic-ray interactions in the Galactic disk guarantee a diffuse neutrino flux. However, these Galactic neutrinos have not been unambiguously detected so far. Here we examine whether such a Galactic component is present among the observed neutrinos of the highest energies. We analyze public track-like IceCube events with estimated neutrino energies above 200 TeV. We examine the distribution of arrival directions of these neutrinos in the Galactic latitude b with the help of a simple unbinned, nonparametric test statistics, the median ∣b∣ over the sample. This distribution deviates from that implied by the null hypothesis of the neutrino flux isotropy, and is shifted toward lower ∣b∣ with the p-value of 4 × 10−5, corresponding to the statistical significance of 4.1σ. There exists a significant component of the high-energy neutrino flux of Galactic origin, matching well the multimessenger expectations from Tibet-ASγ observations of diffuse Galactic gamma rays at hundreds of TeV. Together with the previously established extragalactic associations, the Galactic component we report here implies that the neutrino sky is rich and is composed of contributions from various classes of sources.

Constraining axion-like particles with the diffuse gamma-ray flux measured by the Large High Altitude Air Shower Observatory

The detection of very high-energy neutrinos by IceCube experiment supports the existence of a comparable gamma-ray counterpart from the same cosmic accelerators. Under the likely assumption that the sources of these particles are of extragalactic origin, the emitted photon flux would be significantly absorbed during its propagation over cosmic distances. However, in the presence of photon mixing with ultra-light axion-like-particles (ALPs), this expectation would be strongly modified. Notably, photon-ALP conversions in the host galaxy would produce an ALP flux which propagates unimpeded in the extragalactic space. Then, the back-conversion of ALPs in the Galactic magnetic field leads to a diffuse high-energy photon flux. In this context, the recent detection of the diffuse high-energy photon flux by the Large High Altitude Air Shower Observatory (LHAASO) allows us to exclude at the 95% CL an ALP-photon coupling g_{agamma } > rsim 3.9–7.8 times 10^{-11}~mathrm {GeV^{-1}} for m_{a}lesssim 4times 10^{-7}~textrm{eV}, depending on the assumptions on the magnetic fields and on the original gamma-ray spectrum. This new bound is complementary with other ALP constraints from very-high-energy gamma-ray experiments and sensitivities of future experiments.

First constraints on axionlike particles from Galactic sub-PeV gamma rays

Experimental refinements and technical innovations in the field of extensive air shower telescopes have enabled measurements of Galactic cosmic-ray interactions in the sub-PeV range, providing new avenues for the search for new physics and dark matter. For the first time, we exploit sub-PeV (1 TeV -- 1 PeV) observations of Galactic diffuse gamma rays by HAWC and Tibet AS$\gamma$ to search for an axion-like-particle (ALP) induced gamma-ray signal directly linked to the origin of the IceCube extragalactic high-energy neutrino flux. Indeed, the production of high-energy neutrinos in extragalactic sources implies the concomitant production of gamma rays at comparable energies. Within the magnetic field of the neutrino emitting sources, gamma rays may efficiently convert into ALPs, escape their host galaxy un-attenuated, propagate through intergalactic space, and reconvert into gamma rays in the magnetic field of the Milky Way. Such a scenario creates an all-sky diffuse high-energy gamma-ray signal in the sub-PeV range. Accounting for the guaranteed Galactic astrophysical gamma-ray contributions from cosmic-ray interactions with gas and radiation and from sub-threshold sources, we set competitive upper limits on the photon-ALP coupling constant $g_{a\gamma\gamma}$. We find $g_{a\gamma\gamma} < 2.1\times10^{-11}$ GeV$^{-1}$ for ALP masses $m_a \leq 2\times10^{-7}$ eV at a 95\% confidence level. Our results are comparable to previous limits on ALPs derived from the TeV gamma-ray domain and progressively close the mass gap towards ADMX limits. The code and data to reproduce the results of this study are available on GitHub \url{https://github.com/ceckner/subPeVALPs}.

Is the θ[over ¯] Parameter of QCD Constant?

Testing the cosmological variation of fundamental constants of nature can provide valuable insights into new physics scenarios. While many such constraints have been derived for standard model coupling constants and masses, the θ[over ¯] parameter of QCD has not been as extensively examined. In this Letter, we discuss potentially promising paths to investigate the time dependence of the θ[over ¯] parameter. While laboratory searches for CP-violating signals of θ[over ¯] yield the most robust bounds on today's value of θ[over ¯], we show that CP-conserving effects provide constraints on the variation of θ[over ¯] over cosmological timescales. We find no evidence for a variation of θ[over ¯] that could have implied an "iron-deficient" Universe at higher redshifts. By converting recent atomic clock constraints on a variation of constants, we infer d(θ[over ¯]^{2})/dt≤6×10^{-15} yr^{-1}, at 1σ. Finally, we also sketch an axion model that results in a varying θ[over ¯] and could lead to excess diffuse gamma ray background, from decays of axions produced in high redshift supernova explosions.

Gamma-ray pulsar halos in the Galaxy

Pulsar halos are extended gamma-ray structures generated by electrons and positrons escaping from pulsar wind nebulae (PWNe), considered a new class of gamma-ray sources. They are ideal indicators for cosmic-ray propagation in localized regions of the Galaxy and particle escape process from PWNe. The cosmic-ray diffusion coefficient inferred from pulsar halos is more than two orders of magnitude smaller than the average value in the Galaxy, which has been arousing extensive discussion. We review the recent advances in the study of pulsar halos, including the characteristics of this class of sources, the known pulsar halos, the possible mechanisms of the extremely slow diffusion, the critical roles of pulsar halos in the studies of cosmic-ray propagation and electron injection from PWNe, and the implications on the problems of the cosmic positron excess and the diffuse TeV gamma-ray excess. Finally, we give prospects for the study in this direction based on the expectation of a larger sample of pulsar halos and deeper observations for bright sources.

Observable signatures of cosmic rays transport in Starburst Galaxies on gamma-ray and neutrino observations

ABSTRACT The gamma-ray emission from Starburst and Star-forming Galaxies (SBGs and SFGs) strongly suggests a correlation between star-forming activity and gamma-ray luminosity. However, the very nature of cosmic ray (CR) transport and the degree of their confinement within SBG cores are still open questions . We aim at probing the imprints left by CR transport on gamma-ray and neutrino observations of point-like SFGs and SBGs, looking into quantitative ways to discriminate among different transport models. We analyse the 10-yr Fermi-LAT spectral energy distributions of 13 nearby galaxies with two different CR transport models, taking into account the corresponding IR and UV observations. We also generate mock gamma-ray data to simulate the CTA performance in detecting these sources. In this way, we propose a test to discriminate between the two CR models, quantifying the statistical confidence at which one model can be preferred over the other. We point out that the current data already give a slight preference to CR models that are dominated by advection. Moreover, we show that CTA will allow us to firmly disfavour models dominated by diffusion over self-induced turbulence, compared to advection-dominated models, with Bayes factors, which can be as large as 107 for some of the SBGs. Finally, we estimate the diffuse gamma-ray and neutrino fluxes of SFGs and SBGs, showing that they can explain $25{{\,\rm per\ cent}}$ of the diffuse HESE data while remaining consistent with gamma-ray limits on non-blazar sources.