Axion-like particles (ALPs) provide a feasible explanation for the observed lower TeV opacity of the Universe. If the anomaly TeV transparency is caused by ALPs, then the fluxes of distant extragalactic sources will be enhanced at photon energies beyond TeV, resulting in an enhancement of the observed extragalactic gamma-ray background (EGB) spectrum. In this work, we have investigated the ALP modulation on the EGB spectrum at TeV energies. Our results show that in the most optimistic case, the existence of ALPs can cause the EGB spectrum to greatly deviate from the prediction of a pure extragalactic-background-light (EBL) absorption scenario. The deviation occurs at approximately $\gtrsim$1 TeV, and the current EGB measurements by Fermi-LAT cannot identify such an effect. We also find that most of the sensitive ALP parameters have been ruled out by existing constraints, leaving only a small region of unrestricted parameters that can be probed using the EGB effect investigated in this work. Observations from forthcoming very-high-energy instruments like LHAASO and CTA may be beneficial for the study of this effect.
Primordial Black Holes as a Factory of Axions: Extragalactic Photons from Axions
Yonsei University（延世大学）；Weizmann Institute of Science（魏茨曼科学研究所）；Chungnam National University（忠南国立大学）；Institute for Basic Science (IBS)（基础科学研究院）；Korea Institute for Advanced Study（韩国高等研究院）
Detecting Extragalactic Axion-like Dark Matter with Polarization Measurements of Fast Radio Bursts
Purple Mountain Observatory, Chinese Academy of Sciences（中国科学院紫金山天文台）；School of Astronomy and Space Sciences, University of Science and Technology of China（中国科学技术大学天文与空间科学学院）
Very-high energy photons emitted by distant cosmic sources are absorbed on the extragalactic background light (EBL) during their propagation. This effect can be characterized in terms of a photon transfer function at Earth. The presence of extragalactic magnetic fields could also induce conversions between very high-energy photons and hypothetical axion-like particles (ALPs). The turbulent structure of the extragalactic magnetic fields would produce a stochastic behaviour in these conversions, leading to a statistical distribution of the photon transfer functions for the different realizations…
Indirect constraints on the intensity of the Extragalactic Background Light (EBL) were provided by recent studies of extragalactic sources emitting sub-TeV to multi-TeV photons. These constraints are provided thanks to the absorption of gamma rays by soft photons from the EBL (UV/optical/IR) via pair production by gamma - gamma interactions. This paper provides an overview of recent results that have led to substantially reduced uncertainties on the EBL intensity over a wide range of wavelengths from 0.1 to 15 micron.
In addition to gamma-rays, dark matter annihilation products can include energetic electrons which inverse Compton scatter with the cosmic microwave background to produce a diffuse extragalactic background of gamma-rays and X-rays. In models in which the dark matter particles annihilate primarily to electrons or muons, the measurements of EGRET and COMPTEL can provide significant constraints on the annihilation cross section. The Fermi Gamma-Ray Space Telescope will likely provide an even more stringent test of such scenarios.
Very high energy (VHE) gamma-rays from extragalactic sources, interacting by gamma-gamma collisions with diffuse intergalactic radiation fields, provide an alternative way to constrain the diffuse background light, completely independent of direct measurements. The limits depend however on our knowledge of the physics of the gamma-ray sources. After clarifying the interplay between background light and VHE spectra, I summarize the extent and validity of the obtainable limits, and where future improvements can be expected.
Large-scale extragalactic magnetic fields may induce conversions between very-high-energy photons and axionlike particles (ALPs), thereby shielding the photons from absorption on the extragalactic background light. However, in simplified "cell" models, used so far to represent extragalactic magnetic fields, this mechanism would be strongly suppressed by current astrophysical bounds. Here we consider a recent model of extragalactic magnetic fields obtained from large-scale cosmological simulations. Such simulated magnetic fields would have large enhancement in the filaments of matter. As a resu…
The intergalactic magnetic field (IGMF) may leave an imprint on the angular anisotropy of the extragalactic gamma-ray background through its effect on electromagnetic cascades triggered by interactions between very high energy photons and the extragalactic background light. A strong IGMF will deflect secondary particles produced in these cascades and will thus tend to isotropize lower energy cascade photons, thereby inducing a modulation in the anisotropy energy spectrum of the gamma-ray background. Here we present a simple, proof-of-concept calculation of the magnitude of this effect and demo…
We study the origin of the extragalactic diffuse gamma-ray background using the data from the Fermi telescope. To estimate the background level, we count photons at high Galactic latitudes |b|>60 degrees. Subtracting photons associated to known sources and the residual cosmic ray and Galactic diffuse backgrounds, we estimate the Extragalactic Gamma-ray Background (EGB) flux. We find that the spectrum of EGB in the very-high-energy (VHE) band above 30 GeV follows the stacked spectrum of BL Lacs. LAT data reveal the positive (1+z)^k, 1<k<4 cosmological evolution of the BL Lac source population c…
A list of 205 gamma-ray strong objects was reported recently as a result of a 3-month integration with the Large Area Telescope on board the Fermi Gamma-Ray Space Telescope. We attempted identification of these objects, cross-correlating the gamma-ray positions with VLBI positions of a large all-sky sample of extragalactic radio sources selected on the basis of their parsec-scale flux density. The original associations reported by the Fermi team are confirmed and six new identifications are suggested. A Monte-Carlo analysis shows that the fraction of chance associations in our analysis is less…
High-energy photons from cosmological emitters suffer attenuation due to pair production interactions with the extragalactic background light (EBL). The collective emission of any high-energy emitting cosmological population will exhibit an absorption feature at the highest energies. We calculate this absorption feature in the collective emission of blazars for various models of the blazar gamma-ray luminosity function (GLF) and the EBL. We find that models of the blazar GLF that predict higher relative contributions of high-redshift blazars to the blazar collective spectrum result in emission…
Most extragalactic jets in radio-loud Active Galactic Nuclei are bright and variable gamma-ray sources, which are continuously monitored with Fermi/LAT. We present the gamma-ray properties of the MOJAVE and TANAMI AGN samples of radio-loud AGN. Both programs provide properties of the parsec-scale radio jets using Very Long Baseline Interferometry (VLBI) techniques. This information is important to understand the broad-band emission mechanism of these sources. In this work we compare the radio and gamma -ray properties of the two samples and present upper limits on the gamma-ray flux of the rad…
Recent observations of blazars at high energy (HE, 0.1-100 GeV) and very high energy (VHE, >0.1 TeV) have provided important constraints on the intensity and spectrum of the diffuse Extragalactic Background Light (EBL), shedding light on its main origin. Several issues remain open, however, in particular in the mid- and far-infrared bands and in the blazar emission at multi-TeV energies. This review summarizes the observational and theoretical progress in the study of the EBL with gamma-rays and the most promising future improvements, which are mainly expected from spectra in the multi-TeV ran…
Probing Extragalactic Ultralight Axion Dark Matter with Decade-long Blazar Optical Polarimetry of 1ES 1959+650
Purple Mountain Observatory, Chinese Academy of Sciences（中国科学院紫金山天文台）；School of Astronomy and Space Sciences, University of Science and Technology of China（中国科学技术大学天文与空间科学学院）
The light emitted by stars and accreting compact objects through the history of the Universe is encoded in the intensity of the extragalactic background light (EBL). Knowledge of the EBL is important to understand the nature of star formation and galaxy evolution, but direct measurements of the EBL are lim- ited by Galactic and other foreground emissions. Here we report an absorption feature seen in the combined spectra of a sample of gamma-ray blazars out to a redshift of z$\sim$1.6. This feature is caused by attenuation of gamma rays by the EBL at optical to UV frequencies, and allowed us to…
The Cherenkov Telescope Array (CTA) is about to enter construction phase and one of its main key science projects is to perform an unbiased survey in search of extragalactic sources. We make use of both the latest blazar gamma--ray luminosity function and spectral energy distribution to derive the expected number of detectable sources for both the planned Northern and Southern arrays of the CTA observatory. We find that a shallow, wide survey of about 0.5 hour per field of view would lead to the highest number of blazar detections. Furthermore, we investigate the effect of axion-like particles…
The global contribution of unresolved gamma-ray point sources to the extragalactic gamma-ray background has been recently measured down to gamma-ray fluxes lower than those reached with standard source detection techniques, and by employing the statistical properties of the observed gamma-ray counts. We investigate and exploit the complementarity of the information brought by the one-point statistics of photon counts (using more than 10 years of Fermi-LAT data) and by the recent measurement of the angular power spectrum of the unresolved gamma-ray background (based on 8 years of Fermi-LAT data…
Primary gamma rays emitted by extragalactic sources, such as blazars, will generate electromagnetic cascades in intergalactic space. These cascades proceed via electron-positron pair production and inverse Compton scattering on cosmic background radiation, mainly the cosmic microwave background (CMB) and extragalactic background light (EBL) fields. The existence of an extragalactic magnetic field (EGMF) could deflect electron-positron pair trajectories and scatter the cascade photons, possibly creating a halo around the source while suppressing the cascade flux collected by a detector. We deve…
The origin of the extragalactic gamma-ray background (EGRB) radiation has been a mystery in astrophysics for a long time. Recently the Fermi gamma-ray satellite (Fermi) has revealed that ~22% of the unresolved EGRB would be explained by blazars, which are one population of beamed active galactic nuclei (AGNs). The remaining ~78% of the unresolved EGRB is still unknown. We estimate the contribution of gamma-ray loud radio galaxies, which are misaligned radio loud AGNs recently detected by Fermi, to EGRB using the radio luminosity function of radio-loud AGNs with the correlation between the radi…
The origin of the extragalactic $γ$-ray background permeating throughout the Universe remains a mystery forty years after its discovery. The extrapolated population of blazars can account for only half of the background radiation at the energy range of ~ 0.1-10 GeV. Here we show that quasar-driven outflows generate relativistic protons that produce the missing component of the extragalactic $γ$-ray background and naturally match its spectral fingerprint, with a generic break above ~ 1 GeV. The associated $γ$-ray sources are too faint to be detected individually, explaining why they had not bee…
As very high energy (VHE) photons propagate through the extragalactic background light (EBL), they interact with the soft photons and initiate electromagnetic cascades of lower energy photons and electrons. The collective intensity of a cosmological population emitting at VHEs will be attenuated at the highest energies through interactions with the EBL and enhanced at lower energies by the resulting cascade. We calculate the cascade radiation created by VHE photons produced by blazars and investigate the effects of cascades on the collective intensity of blazars and the resulting effects on th…
The TANAMI program has been monitoring the parsec-scale radio jets of southern gamma-ray bright AGN with VLBI techniques simultaneously with Fermi/LAT monitoring of their gamma-ray emission. Here we present the gamma-ray properties of the TANAMI sources based on an analysis of the preliminary 1-year LAT source list. We present upper limits on the gamma-ray flux for TANAMI sources not detected by LAT.
Origin of extragalactic gamma-ray background (EGRB) is still a matter of debate. EGRB can either have truly diffuse or unresolved discrete point sources origin. Majority of the Fermi and EGRET detected identified sources are blazar. So, they are expected to be a significant contributors to the EGRB. In order to estimate their contribution to the EGRB one needs to construct their luminosity functions. Here, we examine and construct the evolution and luminosity function of blazars using first LAT AGN catalog. We consider both pure luminosity and pure density evolution models for flat spectrum ra…
As very-high--energy photons propagate through the extragalactic background light (EBL), they interact with the soft photons and initiate electromagnetic cascades of lower energy photons and electrons. The collective intensity of a cosmological population emitting at very-high energies (VHE) will be attenuated at the highest energies through interactions with the EBL and enhanced at lower energies by the resulting cascade. We calculate the cascade radiation created by VHE photons produced by blazars and investigate the effects of cascades on the collective intensity of blazars and the resultin…
We estimate the contribution to the extragalactic gamma-ray background (EGRB) from both intrinsic and cascade emissions produced by blazars using a simple semi- analysis method for two models of the blazar gamma-ray luminosity function (GLF). For the cascade emission, we consider two possible contributions: one is due to that the flux of the cascade emission is lower than the Fermi LAT sensitivity (case I), which is independent on the extragalactic magnetic field (EGMF), another is due to the fact that the flux of the cascade emission is larger than the Fermi LAT sensitivity but the emission a…
The extragalactic background light (EBL) from the infrared to the ultraviolet is difficult to measure directly, but can be constrained with a variety of methods. EBL photons absorb gamma-rays from distant blazars, allowing one to use blazar spectra from atmospheric Cherenkov telescopes (ACTs) to put upper limits on the EBL by assuming a blazar source spectrum. Here we apply a simple technique, similar to the one developed by Schroedter (2005), to the most recent very-high energy (VHE) gamma-ray observations of blazars to put upper limits on the EBL energy density. This technique is independent…
During a core-collapse supernova (SN), axionlike particles (ALPs) could be produced through the Primakoff process and subsequently convert into $γ$ rays in the magnetic field of the Milky Way. We do not find evidence for such a $γ$-ray burst in observations of extragalactic SNe with the Fermi Large Area Telescope (LAT). The SN explosion times are estimated from optical light curves and we find a probability of about $\sim$90% that the LAT observed at least one SN at the time of the core collapse. Under the assumption that at least one SN was contained within the LAT field of view, we exclude p…
The imprint of cosmic backgrounds in the gamma ray spectra of blazars has recently been detected by H.E.S.S. and Fermi-LAT, opening the way to studies of gamma-ray propagation on cosmological scales. This proceeding discusses the current constraints on the extragalactic background light (EBL), the effort to increase the collection of blazars detected at TeV energies, and a crucial part of the science case of next-generation instruments: gamma-ray cosmology.
High-energetic gamma rays from astrophysical targets constitute a unique probe for annihilation or decay of heavy particle dark matter (DM). After several decades, diverse null detections have resulted in strong constraints for DM particle masses up to the TeV scale. While the gamma-ray signature is expected to be universal from various targets, uncertainties of astrophysical origin strongly affect and weaken the limits. At the same time, spurious signals may originate from non-DM related processes. The many gamma-ray targets in the extragalactic sky being searched for DM play a crucial role t…
The TANAMI (Tracking AGN with Austral Milliarcsecond Interferometry) and associated programs provide comprehensive radio monitoring of extragalactic gamma-ray sources south of declination -30 degrees. Joint quasi-simultaneous observations between the Fermi Gamma-ray Space Telescope and ground based observatories allow us to discriminate between competing theoretical blazar emission models. High resolution VLBI observations are the only way to spatially resolve the sub-parsec level emission regions where the high-energy radiation originates. The gap from radio to gamma-ray energies is spanned w…
The Major Atmospheric Gamma-ray Imaging Cherenkov (MAGIC) experiment is an array of two 17-meter telescopes located in the Canary Island of La Palma that observes the very-high energy (VHE) gamma-ray sky in stereoscopic mode since 2009. MAGIC is distinguished by its low-energy threshold of approximately 50 GeV, which grants the system a unique potential in the study of distant extragalactic sources whose gamma-ray emission is significantly attenuated due to absorption by the extragalactic background light (EBL). The observation of non-thermal gamma rays in the GeV-TeV range from extragalactic …
We discuss the influence of extragalactic magnetic fields on the intensity of gamma-ray emission produced in electromagnetic cascades from ultra-high energy cosmic rays propagating in extragalactic space. Both cosmic rays and cascade particles propagate mostly out of galaxies, galactic clusters, and large scale structures as their relative volume is small. Therefore their magnetic fields weakly affect emission produced in cascades. Yet estimates of this influence can be useful searching for dark matter particles, when components of extragalactic gamma-ray background should be known, including …
Axion Like Particles (ALPs) are predicted to couple with photons in the presence of magnetic fields. This effect may lead to a significant change in the observed spectra of gamma-ray sources such as AGNs. Here we carry out a detailed study that for the first time simultaneously considers in the same framework both the photon/axion mixing that takes place in the gamma-ray source and that one expected to occur in the intergalactic magnetic fields. An efficient photon/axion mixing in the source always means an attenuation in the photon flux, whereas the mixing in the intergalactic medium may resu…
Data from (non-) attenuation of gamma rays from active galactic nuclei (AGN) and gamma ray bursts (GRBs) give upper limits on the extragalactic background light (EBL) from the UV to the mid-IR that are only a little above the lower limits from observed galaxies. These upper limits now rule out some EBL models and purported observations, with improved data likely to provide even stronger constraints. We present EBL calculations both based on multiwavelength observations of thousands of galaxies and also based on semi-analytic models, and show that they are consistent with these lower limits fro…
We derive the maximum fraction of energy emitted in the form of massive (Kaluza- Klein) gravitons by core collapse supernovae, and the corresponding minimal extra-dimensional Planck mass M* in the ADD gravity framework at TeV scales. Our constraints arise: a) from the extragalactic gamma ray background observed by Fermi-LAT after astrophysical sources have been removed, and b) via the residual galactic emission left after astrophysical and potentially dark matter emission have been removed. We focus on a number of extra dimensions 3 and 4, since M* is then in the TeV range, where astrophysical…
Ultracompact minihalo (UCMH) is a special type of dark matter halo with a very steep density profile which may form in the early universe seeded by an overdense region or a primordial black hole. Constraints on its abundance give valuable information on the power spectrum of primordial perturbation. In this work, we update the constraints on the UCMH abundance in the universe using the extragalactic gamma-ray background (EGB) observation. Comparing to previous works, we adopt the updated Fermi-LAT EGB measurement and derive constraints based on a full consideration of the astrophysical contrib…
We consider possible experimental tests of recent hypotheses suggesting that TeV photons survive the pair production interaction with extragalactic background light over cosmological distances by converting to axion-like particles (ALPs) in galactic magnetic fields. We show that proposed giant ultra-low background scintillation detectors will even have a difficult time reaching the present CAST sensitivity, which is one to two orders of magnitude less sensitive than necessary for a meaningful test of the ALP-photon oscillation hypothesis. Potential alternative tests are briefly discussed.
The extragalactic background light (EBL) is one of the fundamental observational quantities in cosmology. All energy releases from resolved and unresolved extragalactic sources, and the light from any truly diffuse background, excluding the cosmic microwave background (CMB), contribute to its intensity and spectral energy distribution. It therefore plays a crucial role in cosmological tests for the formation and evolution of stellar objects and galaxies, and for setting limits on exotic energy releases in the universe. The EBL also plays an important role in the propagation of very high energy…
Fermi has been instrumental in constraining the luminosity function and redshift evolution of gamma-ray bright blazars. This includes limits upon the spectrum and anisotropy of the extragalactic gamma-ray background (EGRB), redshift distribution of nearby Fermi active galactic nuclei (AGN), and the construction of a log(N)-log(S) relation. Based upon these, it has been argued that the evolution of the gamma-ray bright blazar population must be much less dramatic than that of other AGN. However, critical to such claims is the assumption that inverse Compton cascades reprocess emission above a T…
The launch of the Fermi gamma-ray space telescope and the imaging air Cerenkov telescopes H.E.S.S., MAGIC, and VERITAS have substantially transformed our knowledge of gamma-ray sources in the last decade. The extragalactic gamma-ray sky is teeming with blazars, which are active galactic nuclei whose jet is directed at us. Additionally, there are radio galaxies, starburst and spiral galaxies, and gamma-ray bursts, albeit with smaller numbers. Galaxy clusters have not yet been observed in gamma rays. Here, I will introduce the different gamma-ray emission processes and review what they may tell …
Recent high-energy data from Fermi-LAT on the diffuse gamma-ray background have been used to set among the best constraints on annihilating TeV cold dark matter candidates. In order to assess the robustness of these limits, we revisit and update the calculation of the isotropic extragalactic gamma-ray intensity from dark matter annihilation. The emission from halos with masses $\geq10^{10}\,M_{\odot}$ provides a robust lower bound on the predicted intensity. The intensity including smaller halos whose properties are extrapolated from their higher mass counterparts is typically 5 times higher, …
We discuss ultra-high energy (UHE) cosmic rays (CR) from minor sources and their possible contribution to the extragalactic diffuse gamma-ray emission. As an illustration of minor sources we consider possible specific type of active galactic nuclei (AGN) in which supermassive black hole is surrounded by a super strong magnetic field of 10^(10)-10^(11) Gs. In this model we have calculated CR energy spectra at the Earth and intensity of cascade quanta produced by CRs in extragalactic space. Proceeding from numerical results and the data by Pierre Auger Observatory and Telescope Array it is shown…
Axion Like Particles (ALPs), postulated to solve the strong-CP problem, are predicted to couple with photons in the presence of magnetic fields, which may lead to a significant change in the observed spectra of gamma-ray sources such as AGNs. Here we simultaneously consider in the same framework both the photon/axion mixing that takes place in the gamma-ray source and that one expected to occur in the intergalactic magnetic fields. We show that photon/axion mixing could explain recent puzzles regarding the observed spectra of distant gamma-ray sources as well as the recently published lower li…
Axion Like Particles (ALPs), postulated to solve the strong-CP problem, are predicted to couple with photons in the presence of magnetic fields, which may lead to a significant change in the observed spectra of gamma-ray sources such as AGNs. Here we simultaneously consider in the same framework both the photon/axion mixing that takes place in the gamma-ray source and that one expected to occur in the intergalactic magnetic fields. We show that photon/axion mixing could explain recent puzzles regarding the observed spectra of distant gamma-ray sources as well as the recently published lower li…
Axion-like particles (ALPs) are attracting increasing interest since, among other things, they are a prediction of many extensions of the standard model of elementary particles physics and in particular of superstrings and superbranes. Remarkably, depending on the set of their parameter space, they strongly increase the photon transparency in the very-high energy band. The recent discovery of photon dispersion on the CMB requires a substantial modification of the previous picture: this is indeed the goal of the present paper. We compute the photon survival probability from a blazar to us exact…
We compute the contribution of blazars (Flat Spectrum Radio Quasars and BL-Lacs) to the Fermi-LAT Extragalactic gamma-ray Background, assuming a gamma-ray luminosity function proportional to the luminosity function of radio galaxies and the Spectral Energy Distribution of blazars given by the so-called blazar sequence. We find that blazars account for the 45% of the total Fermi-LAT EGB. However, at low (<10 GeV) and high (>50 GeV) energies the blazar contribution falls short to account for the observed background light. At E<10 GeV, the Extragalactic gamma-ray Background can be fully explained…
We analyze 9 years of PASS 8 $\textit{Fermi}$-LAT data in the 60$-$500 MeV range and determine flux upper limits (UL) for 17 gamma-ray dark pulsars as a probe of axions produced by nucleon-nucleon Bremsstrahlung in the pulsar core. Using a previously published axion decay gamma-ray photon flux model for pulsars which relies on a high core temperature of 20 MeV, we improve the determination of the UL axion mass ($m_a$), at 95 percent confidence level, to 9.6 $\times$ 10$^{-3}$ eV, which is a factor of 8 improvement on previous results. We show that the axion emissivity (energy loss rate per vol…
The observed lightcurves and estimated sky rate of fast extragalactic x-ray transients (XRTs) discovered in archival Chandra data indicate that they belong to two distinct XRT populations. The first population of relatively short duration pulses, which typically last less than few minutes seems to be pulses of x-ray flashes (XRFs), which are nearby long duration gamma ray bursts viewed from far off axis. The second population of much longer pulses, which typically last hours, seems to be the early time afterglows of short gamma ray bursts (SGRBs) which are beamed away from Earth, as was shown …
In the paper, we continually use the method of image stacking to study the origin of the extragalactic gamma-ray background (EGB) at GeV bands, and find that the Faint Images of the Radio Sky at Twenty centimeters (FIRST) sources undetected by the Large Area Telescope on the Fermi Gamma-ray Space Telescope can contribute about (56{plus minus}6)% of the EGB. Because the FIRST is a flux limited sample of radio sources with incompleteness at the faint limit, we consider that the point-sources, including blazars, non-blazar AGNs,starburst galaxies, could produce a much larger fraction of the EGB.
The angular, spectral and temporal features of the highest energy protons and accompanying them secondary neutrinos and synchrotron gamma-rays propagating through the intergalactic magnetic and radiation fields are studied using the analytical solutions of the Boltzmann transport equation obtained in the limit of the small-angle and continuous-energy-loss approximation.
Using data from the WISE all-sky survey we discovered that the non-thermal infrared (IR) emission of blazars, the largest known population of extragalactic gamma-ray sources, has peculiar spectral properties. In this work, we confirm and strengthen our previous analyses using the latest available releases of both the WISE and the Fermi source catalogs. We also show that there is a tight correlation between the mid-IR colors and the gamma-ray spectral index of Fermi blazars. We name this correlation "the infrared--gamma-ray connection". We discuss how this connection links both the emitted powe…
D. Wouters and P. Brun in Phys. Rev. D 86, 043005 (2012) claim that an observable effect in the spectra of distant very-high-energy blazars arises as a consequence of oscillations of photons into axion-like particles (ALPs) in the presence of turbulent extra-galactic magnetic fields. The main objective of this comment is to demonstrate that such a result is physically incorrect. We also show that a physically correct treatment of the same issue leads to a much less relevant conclusion, which makes the effect pointed out by WB likely unobservable with the present capabilities.
Pseudoscalar particles like axion-like particles generically couple to two photons, giving rise to the possibility of oscillations with photons in an external magnetic field. These oscillations could lead to measurable imprints in the electromagnetic spectrum of astrophysical sources if the coupling to photons is strong enough. One possible signature is the presence of irregularities in a limited energy range of the spectrum. In this study, anomalous irregularities are searched for in the spectrum of the bright extragalactic TeV emitter PKS 2155-304 as measured by H.E.S.S. to put constraints o…
We present new theoretical estimates of the relative contributions of unresolved blazars and star-forming galaxies to the extragalactic gamma-ray background (EGB) and discuss constraints on the contributions from alternative mechanisms such as dark matter annihilation and truly diffuse gamma-ray production. We find that the Fermi source count data do not rule out a scenario in which the EGB is dominated by emission from unresolved blazars, though unresolved star-forming galaxies may also contribute significantly to the background, within order-of-magnitude uncertainties. In addition, we find t…
We consider gamma-ray signals of dark matter annihilation in extragalactic halos in the case where dark matter annihilates from a $p$-wave or $d$-wave state. In these scenarios, signals from extragalactic halos are enhanced relative to other targets, such as the Galactic Center or dwarf spheroidal galaxies, because the typical relative speed of the dark matter is larger in extragalactic halos. We perform a mock data analysis of gamma rays produced by dark matter annihilation in halos detected by the Sloan Digital Sky Survey. We include a model for uncorrelated galactic and extragalactic gamma …
A tight correlation between gamma-ray and radio emission is found for a sample of BL Lacertae (BL Lac) objects detected by Fermi Gamma-ray Space Telescope (Fermi) and the Energetic Gamma-Ray Experiment Telescope (EGRET). The gamma-ray emission of BL Lac objects exhibits strong variability, and the detection rate of gamma-ray BL Lac objects is low, which may be related to the gamma-ray duty cycle of BL Lac objects. We estimate the gamma-ray duty cycle ~ 0.11, for BL Lac objects detected by EGRET and Fermi. Using the empirical relation of gamma-ray emission with radio emission and the estimated …
The recent discovery of the gamma-ray emission from the lobes of the closest radio galaxy Centaurus A by Fermi implies the presence of high-energy electrons at least up to gamma ~ 10^5 - 10^6. These high-energy electrons are required to interpret the observed gamma-ray radiation in terms of inverse Compton emission off the cosmic microwave background (IC/CMB); the widely accepted scenario to describe the X-ray emission of radio galaxy lobes. In this Letter, we consider the giant radio lobes of FR II radio galaxies showing that it is possible to maintain electrons at energies gamma ~ 10^5 - 10^…
Axions and axion-like particles (ALPs) are hypothetical particles that occur in extensions of the Standard Model and are candidates for cold dark matter. They could be detected through their oscillations into photons in the presence of external electromagnetic fields. gammaALPs is an open-source python framework that computes the oscillation probability between photons and axions/ALPs. In addition to solving the photon-ALP equations of motion, gammaALPs includes models for magnetic fields in different astrophysical environments such as jets of active galactic nuclei, intra-cluster and intergal…
