Extragalactic Γ-ray Background Research Articles

The extragalactic γ-ray background: imprints from the physical properties and evolution of star-forming galaxy populations

ABSTRACT Star-forming galaxies (SFGs) harbour an abundant reservoir of cosmic rays (CRs). At GeV energies, these CRs undergo interactions with their environment to produce γ-rays, and the unresolved γ-ray emission from populations of SFGs form a component of the isotropic extragalactic γ-ray background (EGB). In this work, we investigate the contribution to the 0.01–50 GeV EGB from SFG populations located up to redshift z = 3. We find this is dominated by starbursts, while the contribution from main-sequence SFGs is marginal at all energies. We also demonstrate that most of the γ-ray contribution from SFGs emanates from low-mass galaxies, with over 80 per cent of the emission originating from galaxies with stellar masses below $10^8 \, {\rm M}_{\odot }$. Many of these galaxies are located at relatively high redshift, with their peak EGB contribution arising ∼700 Myr before the noon of cosmic star formation. We find that the precise redshift distributions of EGB sources at different energies imprint intensity signatures at different angular scales, which may allow their contribution to be distinguished using analyses of small-scale EGB intensity anisotropies, particularly if the diffuse EGB is dominated by hadronic CR-driven γ-ray emission from SFGs. We show that the EGB is sensitive to the evolution of low-mass populations of galaxies, particularly around z ∼ 2.5, and that it provides a new means to probe the engagement of CRs in these galaxies before the high noon of cosmic star formation.

Characterizing the signatures of star-forming galaxies in the extragalactic γ-ray background

ABSTRACT Galaxies experiencing intense star-formation episodes are expected to be rich in energetic cosmic rays (CRs). These CRs undergo hadronic interactions with the interstellar gases of their host to drive γ-ray emission, which has already been detected from several nearby starbursts. Unresolved γ-ray emission from more distant star-forming galaxies (SFGs) is expected to contribute to the extragalactic γ-ray background (EGB). However, despite the wealth of high-quality all-sky data from the Fermi–LAT γ-ray space telescope collected over more than a decade of operation, the exact contribution of such SFGs to the EGB remains unsettled. We investigate the high-energy γ-ray emission from SFGs up to redshift z = 3 above a GeV, and assess the contribution they can make to the EGB. We show the γ-ray emission spectrum from an SFG population can be determined from just a small number of key parameters, from which we model a range of possible EGB realizations. We demonstrate that populations of SFGs leave anisotropic signatures in the EGB, and that these can be accessed using the spatial power spectrum. Moreover, we show that such signatures will be accessible with ongoing operation of current γ-ray instruments, and detection prospects will be greatly improved by the next generation of γ-ray observatories, in particular the Cherenkov Telescope Array.

Source-count Distribution of Gamma-Ray Blazars

Abstract With 10 years of operation and an exceptional data set, the Fermi-Large Area Telescope allows us to unveil the detailed composition of the extragalactic γ-ray sky above 100 MeV. In this paper, we derive the intrinsic source-count distribution (logN–logS) of extragalactic sources (i.e., blazars) at via the efficiency correction method. With this approach, we are able to measure the distribution down to a photon flux of and to an energy flux of ∼10−12 . In both cases, the logN–logS becomes flatter at low fluxes. Moreover, we show that this logN–logS is representative of the blazar population (assuming the majority of unassociated sources are blazars) and allows us to constrain its evolution quite effectively. Among recently proposed evolutionary models, we find that the Pure Density Evolution model best describes the evolutionary properties of the blazar population and that their integrated emission accounts for of the total extragalactic γ-ray background.

The γ-Ray Emission of Star-forming Galaxies

Abstract A majority of the γ-ray emission from star-forming galaxies is generated by the interaction of high-energy cosmic rays with the interstellar gas and radiation fields. Star-forming galaxies are expected to contribute to both the extragalactic γ-ray background and the IceCube astrophysical neutrino flux. Using roughly 10 yr of γ-ray data taken by the Fermi Large Area Telescope, in this study we constrain the γ-ray properties of star-forming galaxies. We report the detection of 11 bona fide γ-ray-emitting galaxies and 2 candidates. Moreover, we show that the cumulative γ-ray emission of below-threshold galaxies is also significantly detected at ∼5σ confidence. The γ-ray luminosity of resolved and unresolved galaxies is found to correlate with the total (8–1000 μm) infrared luminosity as previously determined. Above 1 GeV, the spectral energy distribution of resolved and unresolved galaxies is found to be compatible with a power law with a photon index of ≈2.2–2.3. Finally, we find that star-forming galaxies account for roughly 5% and 3% of the extragalactic γ-ray background and the IceCube neutrino flux, respectively.

A geomagnetic filter for the Fermi-LAT background

ABSTRACT One of the unsolved questions in γ-ray astronomy is whether the extragalactic γ-ray background is of the discrete-source origin. To respond to this question, one first needs to reduce the data by differentiating charged particles from γ-rays. This procedure is usually performed on the basis of the detector responses. In this paper, we showed that the geomagnetic shielding effect at GeV energies can, to some extent, be used for this purpose for γ-ray telescopes in a low Earth orbit. We illustrated this method by applying it to the Fermi Large Area Telescope data. To partially decompose the charge-filtered background, we examined the contribution from star-forming galaxies by implying a radio/γ-ray connection in consideration of next-generation radio surveys.

Using the Extragalactic Gamma-Ray Background to Constrain the Hubble Constant and Matter Density of the Universe

Abstract The attenuation produced by extragalactic background light (EBL) in γ-ray spectra of blazars has been used to constrain the Hubble constant (H 0) and matter density (Ωm) of the universe. We propose to estimate H 0 and Ωm using the well-measured >10 GeV extragalactic γ-ray background (EGB). This suggestion is based on the fact that the >10 GeV EGB is totally explained by the emissions from blazars, and an EBL-absorption cutoff occurs at ∼50 GeV in the EGB spectrum. We fit the >10 GeV EGB data with modeled EGB spectrum. This results in and . Note that the uncertainties may be underestimated due to the limit of our realization for EBL model. H 0 and Ωm are degenerate in our method. Independent determination of Ωm by other methods would improve the constraint on H 0.

IceCube neutrinos from hadronically powered gamma-ray galaxies

In this work we use a multi-messenger approach to determine if the high energy diffuse neutrino flux observed by the IceCube Observatory can originate from γ-ray sources powered by Cosmic Rays interactions with gas. Typical representatives of such sources are Starburst and Ultra-Luminous Infrared Galaxies. Using the three most recent calculations of the non-blazar contribution to the extragalactic γ-ray background measured by the Fermi-LAT collaboration, we find that a hard power-law spectrum with spectral index α ⩽ 2.12 is compatible with all the estimations for the allowed contribution from non-blazar sources, within 1σ. Using such a spectrum we are able to interpret the IceCube results, showing that various classes of hadronically powered γ-ray galaxies can provide the dominant contribution to the astrophysical signal above 100 TeV and about half of the contribution to the energy flux between 10–100 TeV . With the addition of neutrinos from the Galactic plane, it is possible to saturate the IceCube signal {at high energy}. Our result shows that these sources are still well motivated candidates.

Fermi-LAT Stacking Analysis Technique: An Application to Extreme Blazars and Prospects for their CTA Detection

Abstract We present a likelihood profile stacking technique based on the Fermi-Large Area Telescope (LAT) data to explore the γ-ray characteristics of Fermi-LAT undetected astrophysical populations. The pipeline is applied to a sample of γ-ray unresolved extreme blazars, i.e., sources with the highest synchrotron peak frequencies ( ), and we report a cumulative γ-ray detection with more than 32σ confidence for 2 degrees of freedom. Comparing the generated stacked γ-ray spectrum with the sensitivity limits of the upcoming Cherenkov Telescope Array (CTA), we find that the Fermi-LAT undetected population of such extreme blazars, on average, may remain well below the CTA detection threshold due to their faintness and extragalactic background light (EBL) absorption. However, γ-ray detected blazars belonging to the same class are promising candidates for CTA observations. The EBL-corrected stacked spectra of these sources do not show any softening up to 1 TeV. This finding suggests the inverse Compton peak of extreme blazars lies above 1 TeV, thus indicating a hard intrinsic TeV spectrum. Our analysis also predicts that at 100 GeV, at least ∼10% of the diffuse extragalactic γ-ray background originates from the γ-ray undetected extreme blazars. These results highlight the effectiveness of the developed stacking technique to explore the uncharted territory of γ-ray undetected astrophysical objects.

The Extragalactic Gamma-Ray Background from Core-dominated Radio Galaxies

Abstract Recent radio surveys have discovered a large number of low-luminosity core-dominated radio galaxies that are much more abundant than those at higher luminosities. These objects will be too faint in γ-rays to be detected individually by Fermi. Nevertheless, they may contribute significantly to the unresolved extragalactic γ-ray background. We consider here the possible contribution of these core-dominated radio galaxies to the diffuse extragalactic γ-ray background. Using published data available for all 45 of the radio galaxies listed as detected counterparts in the Fermi FL8Y source list update to the 3FGL catalog, we have searched for radio maps that can resolve the core flux from the total source flux. Using high-resolution radio maps we were able to obtain core fluxes for virtually every source. We then derived a relation between core radio flux and γ-ray flux that we extrapolated to sources with low radio luminosities that are known to be highly core-dominated. We then employed a very recent determination of the luminosity function for core-dominated radio galaxies in order to obtain the contribution of all possible γ-ray-emitting radio galaxies to the unresolved extragalactic γ-ray background. We find this contribution to be possibly non-negligible, 4%–18% of the unresolved γ-ray background observed using the Fermi-LAT telescope.

Cumulative Neutrino and Gamma-Ray Backgrounds from Halo and Galaxy Mergers

Abstract The merger of dark matter halos and the gaseous structures embedded in them, such as protogalaxies, galaxies, and groups and clusters of galaxies, results in strong shocks that are capable of accelerating cosmic rays (CRs) to ≳10 PeV. These shocks will produce high-energy neutrinos and γ-rays through inelastic pp collisions. In this work, we study the contributions of these halo mergers to the diffuse neutrino flux and to the nonblazar portion of the extragalactic γ-ray background. We formulate the redshift dependence of the shock velocity, galactic radius, halo gas content, and galactic/intergalactic magnetic fields over the dark matter halo distribution up to a redshift z = 10. We find that high-redshift mergers contribute a significant amount of the CR luminosity density, and the resulting neutrino spectra could explain a large part of the observed diffuse neutrino flux above 0.1 PeV up to several PeV. We also show that our model can somewhat alleviate tensions with the extragalactic γ-ray background. First, since a larger fraction of the CR luminosity density comes from high redshifts, the accompanying γ-rays are more strongly suppressed through γγ annihilations with the cosmic microwave background and the extragalactic background light. Second, mildly radiative-cooled shocks may lead to a harder CR spectrum with spectral indices of 1.5 ≲ s ≲ 2.0. Our study suggests that halo mergers, a fraction of which may also induce starbursts in the merged galaxies, can be promising neutrino emitters without violating the existing Fermi γ-ray constraints on the nonblazar component of the extragalactic γ-ray background.

Deriving the Contribution of Blazars to the Fermi-LAT Extragalactic γ-ray Background at E > 10 GeV with Efficiency Corrections and Photon Statistics

Abstract The Fermi Large Area Telescope (LAT) Collaboration has recently released the Third Catalog of Hard Fermi-LAT Sources (3FHL), which contains 1556 sources detected above 10 GeV with seven years of Pass 8 data. Building upon the 3FHL results, we investigate the flux distribution of sources at high Galactic latitudes ( ), which are mostly blazars. We use two complementary techniques: (1) a source-detection efficiency correction method and (2) an analysis of pixel photon count statistics with the one-point probability distribution function (1pPDF). With the first method, using realistic Monte Carlo simulations of the γ-ray sky, we calculate the efficiency of the LAT to detect point sources. This enables us to find the intrinsic source-count distribution at photon fluxes down to 7.5 × 10−12 ph cm−2 s−1. With this method, we detect a flux break at (3.5 ± 0.4) × 10−11 ph cm−2 s−1 with a significance of at least 5.4σ. The power-law indexes of the source-count distribution above and below the break are 2.09 ± 0.04 and 1.07 ± 0.27, respectively. This result is confirmed with the 1pPDF method, which has a sensitivity reach of ∼10−11 ph cm−2 s−1. Integrating the derived source-count distribution above the sensitivity of our analysis, we find that (42 ± 8)% of the extragalactic γ-ray background originates from blazars.

Search for sharp and smooth spectral signatures of μνSSM gravitino dark matter with Fermi-LAT

The μνSSM solves the μ problem of supersymmetric models and reproduces neutrino data, simply using couplings with right-handed neutrinos ν's. Given that these couplings break explicitly R parity, the gravitino is a natural candidate for decaying dark matter in the μνSSM. In this work we carry out a complete analysis of the detection of μνSSM gravitino dark matter through γ-ray observations. In addition to the two-body decay producing a sharp line, we include in the analysis the three-body decays producing a smooth spectral signature. We perform first a deep exploration of the low-energy parameter space of the μνSSM taking into account that neutrino data must be reproduced. Then, we compare the γ-ray fluxes predicted by the model with Fermi-LAT observations. In particular, with the 95% CL upper limits on the total diffuse extragalactic γ-ray background using 50 months of data, together with the upper limits on line emission from an updated analysis using 69.9 months of data. For standard values of bino and wino masses, gravitinos with masses larger than about 4 GeV, or lifetimes smaller than about 1028 s, produce too large fluxes and are excluded as dark matter candidates. However, when limiting scenarios with large and close values of the gaugino masses are considered, the constraints turn out to be less stringent, excluding masses larger than 17 GeV and lifetimes smaller than 4 × 1025 s.

Planck Lensing and Cosmic Infrared Background Cross-correlation with Fermi-LAT: Tracing Dark Matter Signals in the Gamma-Ray Background

Abstract The extragalactic γ-ray background and its spatial anisotropy could potentially contain a signature of dark matter (DM) annihilation or particle decay. Astrophysical foregrounds, such as blazars and star-forming galaxies (SFGs), however, dominate the γ-ray background, precluding an easy detection of the signal associated with the DM annihilation or decay in the background intensity spectrum. The DM imprint on the γ-ray background is expected to be correlated with large-scale structure tracers. In some cases, such a cross-correlation is even expected to have a higher signal-to-noise ratio than the auto-correlation. One reliable tracer of the DM distribution in the large-scale structure is lensing of the cosmic microwave background (CMB), and the cosmic infrared background (CIB) is a reliable tracer of SFGs. We analyze Fermi-LAT data taken over 92 months and study the cross-correlation with Planck CMB lensing, Planck CIB, and Fermi-γ maps. We put upper limits on the DM annihilation cross-section from the cross-power spectra with the γ-ray background anisotropies. The unbiased power spectrum estimation is validated with simulations that include cross-correlated signals. We also provide a set of systematic tests and show that no significant contaminations are found for the measurements presented here. Using γ-ray background map from data gathered over 92 months, we find the best constraint on the DM annihilation with a 1 σ confidence level upper limit of 10−25–10−24 cm3 s−1, when the mass of DM particles is between 20 and 100 GeV.

Evidence against Star-forming Galaxies as the Dominant Source of Icecube Neutrinos

Abstract The cumulative emission resulting from hadronic cosmic-ray interactions in star-forming galaxies (SFGs) has been proposed as the dominant contribution to the astrophysical neutrino flux at TeV to PeV energies reported by IceCube. The same particle interactions also inevitably create γ-ray emission that could be detectable as a component of the extragalactic γ-ray background (EGB), which is now measured with the Fermi-LAT in the energy range from 0.1 to 820 GeV. New studies of the blazar flux distribution at γ-ray energies above 50 GeV place an upper bound on the residual non-blazar component of the EGB. We show that these results are in strong tension with models that consider SFGs as the dominant source of the diffuse neutrino backgrounds. A characteristic spectral index for parent cosmic rays in starburst galaxies of ΓSB ≃ 2.3 for dN / dE ∝ E − Γ SB is consistent with the observed scaling relation between γ-ray and IR luminosity for SFGs, the bounds from the non-blazar EGB, and the observed γ-ray spectra of individual starbursts, but underpredicts the IceCube data by approximately an order of magnitude.

Cumulative neutrino background from quasar-driven outflows

Quasar-driven outflows naturally account for the missing component of the extragalactic γ-ray background through neutral pion production in interactions between protons accelerated by the forward outflow shock and interstellar protons. We study the simultaneous neutrino emission by the same protons. We adopt outflow parameters that best fit the extragalactic γ-ray background data and derive a cumulative neutrino background of ∼ 10−7 GeV cm−2 s−1 sr−1 at neutrino energies Eν ≳ 10 TeV, which naturally explains the most recent IceCube data without tuning any free parameters. The link between the γ-ray and neutrino emission from quasar outflows can be used to constrain the high-energy physics of strong shocks at cosmological distances.

Isotropic diffuse and extragalactic γ-ray background: emission from extragalactic sources vs dark matter annihilating particles

The isotropic diffuse γ-ray background (IGRB) has been detected by various experiments and recently the Fermi-LAT Collaboration has precisely measured its spectrum in a wide energy range. The origin of the IGRB is still unclear and we show in this paper the significative improvements that have been done, thanks to the new Fermi-LAT catalogs, to solve this mystery. We demonstrate that the γ-ray intensity and spectrum of the IGRB is fully consistent with the unresolved emission from extragalactic point sources, namely Active Galactic Nuclei and Star Forming Galaxies. We show also that the IGRB can be employed to derive sever constraints for the γ-ray emission from diffuse processes such as annihilation of Dark Matter (DM) particles. Our method is able to provide low bounds for the thermal annihilation cross section for a wide range of DM masses.

EVIDENCE FOR TWO LOGNORMAL STATES IN MULTI-WAVELENGTH FLUX VARIATION OF FSRQ PKS 1510-089

ABSTRACT We present a systematic characterization of multi-wavelength emission from blazar PKS 1510-089 using well-sampled data at near-infrared (NIR), optical, X-ray, and γ-ray energies. The resulting flux distributions, except at X-rays, show two distinct lognormal profiles corresponding to a high and a low flux level. The dispersions exhibit energy-dependent behavior except in the LAT γ-ray and optical B-band. During the low level flux states, it is higher toward the peak of the spectral energy distribution, with γ-ray being intrinsically more variable followed by IR and then optical, consistent with mainly being a result of varying bulk Lorentz factor. On the other hand, the dispersions during the high state are similar in all bands except the optical B-band, where thermal emission still dominates. The centers of distributions are a factor of ∼4 apart, consistent with anticipation from studies of extragalactic γ-ray background with the high state showing a relatively harder mean spectral index compared to the low state.

Tomographic-spectral approach for dark matter detection in the cross-correlation between cosmic shear and diffuse γ-ray emission

We recently proposed to cross-correlate the diffuse extragalactic γ-ray background with the gravitational lensing signal of cosmic shear. This represents a novel and promising strategy to search for annihilating or decaying particle dark matter (DM) candidates. In the present work, we demonstrate the potential of a tomographic-spectral approach: measuring the cross-correlation in separate bins of redshift and energy significantly improves the sensitivity to a DM signal. Indeed, the technique proposed here takes advantage of the different scaling of the astrophysical and DM components with redshift and, simultaneously of their different energy spectra and different angular extensions. The sensitivity to a particle DM signal is extremely promising even when the DM-induced emission is quite faint. We first quantify the prospects of detecting DM by cross-correlating the Fermi Large Area Telescope (LAT) diffuse γ-ray background with the cosmic shear expected from the Dark Energy Survey. Under the hypothesis of a significant subhalo boost, such a measurement can deliver a 5σ detection of DM, if the DM particle is lighter than 300 GeV and has a thermal annihilation rate. We then forecast the capability of the European Space Agency Euclid satellite (whose launch is planned for 2020), in combination with an hypothetical future γ-ray detector with slightly improved specifications compared to current telescopes. We predict that the cross-correlation of their data will allow a measurement of the DM mass with an uncertainty of a factor of 1.5–2, even for moderate subhalo boosts, for DM masses up to few hundreds of GeV and thermal annihilation rates.

Detecting Particle Dark Matter Signatures via Cross-Correlation of Gamma-Ray Anisotropies and Cosmic Shear

AbstractSimilarly to gravitational lensing effects like cosmic shear, cosmological γ-ray emission too is to some extent a tracer of the distribution of dark matter (DM) in the Universe. Intervening DM structures source gravitational lensing distortions of distant galaxy images, and those same galaxies can emit γ rays, either because they host astrophysical sources, or directly by particle DM annihilations or decays occurring in the galactic halo. If such γ rays exhibit correlation with the cosmic shear signal, this will provide novel information on the composition of the extragalactic γ-ray background.

Conservative upper limits on WIMP annihilation cross section from Fermi-LAT γ-rays

The spectrum of an isotropic extragalactic γ-ray background (EGB) has been measured by the Fermi-LAT telescope at high latitudes. Two new models for the EGB are derived from the subtraction of unresolved point sources and extragalactic diffuse processes, which could explain from 30% to 70% of the Fermi-LAT EGB. Within the hypothesis that the two residual EGBs are entirely due to the annihilation of dark matter (DM) particles in the Galactic halo, we obtain conservative upper limits on their annihilation cross section ⟨σν⟩. Severe bounds on a possible Sommerfeld enhancement of the annihilation cross section are set as well. Finally, would ⟨σν⟩ be inversely proportional to the WIMP velocity, very severe limits are derived for the velocity-independent term of the annihilation cross section.