We study the astrophysical implications of neutralino dark matter annihilations in galaxy clusters, with a specific application to the Coma cluster. We first address the determination of the dark ...halo models for Coma, starting from structure formation models and observational data, and we discuss in detail the role of sub-halos. We then perform a thorough analysis of the transport and diffusion properties of neutralino annihilation products, and investigate the resulting multi-frequency signals, from radio to gamma-ray frequencies. We also study other relevant astrophysical effects of neutralino annihilations, like the DM-induced Sunyaev-Zel'dovich effect and the intracluster gas heating. As for the particle physics setup, we adopt a two-fold approach, resorting both to model-independent bottom-up scenarios and to benchmark, GUT-motivated frameworks. We show that the Coma radio-halo data (the spectrum and the surface brightness) can be nicely fitted by the neutralino-induced signal for peculiar particle physics models and for magnetic field values, which we outline in detail. Fitting the radio data and moving to higher frequencies, we find that the multi-frequency spectral energy distributions are typically dim at EUV and X-ray frequencies (with respect to the data), but show a non-negligible gamma-ray emission, depending on the amplitude of the Coma magnetic field. A simultaneous fit to the radio, EUV and HXR data is not possible without violating the gamma-ray EGRET upper limit. The best-fit particle physics models yields substantial heating of the intracluster gas, but not sufficient energy injection as to explain the quenching of cooling flows in the innermost region of clusters. Due to the specific multi-frequency features of the DM-induced spectral energy distribution in Coma, we find that supersymmetric models can be significantly and optimally constrained either in the gamma-rays or at radio and microwave frequencies.
We compute the non-thermal emissions produced by relativistic particles accelerated by the shocks driven by the active galactic nucleus (AGN) in NGC 1068, and we compare the model predictions with ...the observed γ-ray and radio spectra. The former is produced by pion decay, inverse Compton scattering, and bremsstrahlung, while the latter is produced by synchrotron radiation. We derive the γ-ray and radio emissions by assuming the standard acceleration theory, and we discuss how our results compare with those corresponding to other commonly assumed sources of γ-ray and radio emissions, like supernova remnants (SNR) or AGN jets. We find that the AGN-driven shocks observed in the circumnuclear molecular disk of NGC 1068 provide a contribution to the γ-ray emission comparable to that provided by the starburst activity when standard particle acceleration efficiencies are assumed, while the shocks can yield the whole γ-ray emission only when the parameters describing the acceleration efficiency and the proton coupling with the molecular gas are tuned to values larger than those assumed in standard, SNR-driven shocks. We discuss the range of acceleration efficiencies (for protons and electrons) and of proton calorimetric fractions required to account for the observed γ-ray emission in the AGN outflow model. We further compare the neutrino flux expected in our model with constraints from current experiments, and we provide predictions for the detections by the upcoming KM3NeT neutrino telescope. This analysis strongly motivates observations of NGC 1068 at ≳TeV energies with current and future Cherenkov telescopes in order to gain insight into the nature of the γ-rays source.
We derive the contribution to the extragalactic gamma-ray background (EGB) from active galactic nuclei (AGN) winds and star-forming galaxies by including a physical model for the γ-ray emission ...produced by relativistic protons accelerated by AGN-driven and supernova-driven shocks into a state-of-the-art semi-analytic model of galaxy formation. This is based on galaxy interactions as triggers of AGN accretion and starburst activity and on expanding blast waves as the mechanism to communicate outwards the energy injected into the interstellar medium by the active nucleus. We compare the model predictions with the latest measurement of the EGB spectrum performed by the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope (Fermi) in the range between 100 MeV and 820 GeV. We find that AGN winds can provide ~35 ± 15% of the observed EGB in the energy interval Eγ = 0.1–1 GeV, for ~73 ± 15% at Eγ = 1–10 GeV, and for ~60 ± 20% at Eγ ≳10 GeV. The AGN wind contribution to the EGB is predicted to be larger by a factor of ~3–5 than that provided by star-forming galaxies (quiescent plus starburst) in the hierarchical clustering scenario. The cumulative γ-ray emission from AGN winds and blazars can account for the amplitude and spectral shape of the EGB, assuming the standard acceleration theory, and AGN wind parameters that agree with observations. We also compare the model prediction for the cumulative neutrino background from AGN winds with the most recent IceCube data. We find that for AGN winds with accelerated proton spectral index p = 2.2–2.3, and taking into account internal absorption of γ-rays, the Fermi-LAT and IceCube data could be reproduced simultaneously.
Abstract
We discuss a new technique to constrain models for the origin of radio relics in galaxy clusters using the correlation between the shock Mach number and the radio power of relics. This ...analysis is carried out using a sample of relics with information on both the Mach numbers derived from X-ray observations,
$\mathcal {M}_X$
, and using spectral information from radio observations of the peak and the average values of the spectral index along the relic,
$\mathcal {M}_R$
. We find that there is a lack of correlation between
$\mathcal {M}_X$
and
$\mathcal {M}_R$
; this result is an indication that the spectral index of the relic is likely not due to the acceleration of particles operated by the shock but it is related to the properties of a fossil electrons population. We also find that the available data on the correlation between the radio power P
1.4 and Mach numbers (
$\mathcal {M}_R$
and
$\mathcal {M}_X$
) in relics indicate that neither the diffusive shock acceleration (DSA) nor the adiabatic compression can simply reproduce the observed
$P_{1.4}{\rm -}\mathcal {M}$
correlations. Furthermore, we find that the radio power is not correlated with
$\mathcal {M}_X$
, whereas it is not possible to exclude a correlation with
$\mathcal {M}_R$
. This also indicates that the relic power is mainly determined by the properties of a fossil electron population rather than by the properties of the shock. Our results require either to consider models of shock (re-)acceleration that go beyond the proposed scenarios of DSA and adiabatic compression at shocks, or to reconsider the origin of radio relics in terms of other physical scenarios.
We study the complex structure of the Bullet cluster radio halo to determine the Dark Matter (DM) contribution to the emission observed in the different subhaloes corresponding to the DM- and ...baryonic-dominated regions. We use different non-thermal models to study the different regions, and we compare our results with the available observations in the radio, X-ray and gamma-ray bands, and the Sunyaev–Zel'dovich effect (SZE) data. We find that the radio emission coming from the main DM subhalo can be produced by secondary electrons produced by DM annihilations. In this scenario, there are however some open issues, like the difficulty to explain the observed flux at 8.8 GHz, the high value of the required annihilation cross-section, and the lack of observed emission coming from the minor DM subhalo. We also find that part of the radio emission originated by DM annihilation could be associated with a slightly extended radio source present near the main DM subhalo. Regarding the baryonic subhaloes, the radio measurements do not allow to discriminate between a primary or secondary origin of the electrons, while the SZE data point towards a primary origin for the non-thermal electrons in the Main subcluster. We conclude that in order to better constrain the properties of the DM subhaloes, it is important to perform detailed measurements of the radio emission in the regions where the DM haloes have their peaks, and that the separation of the complex radio halo in different subhaloes is a promising technique to understand the properties of each specific subhalo.
We derive the Sunyaev–Zel'dovich (SZ) effect arising in radio-galaxy lobes that are filled with high-energy, non-thermal electrons. We provide here quantitative estimates for SZ effect expected from ...the radio-galaxy lobes by normalizing it to the inverse Compton light, observed in the X-ray band, as produced by the extrapolation to low energies of the radio emitting electron spectrum in these radio lobes. We compute the spectral and spatial characteristics of the SZ effect associated to the radio lobes of two distant radio galaxies (3C 294 and 3C 432) recently observed by Chandra, and we further discuss its detectability with the next generation microwave and submm experiments with arcsec and ∼μK sensitivity. We finally highlight the potential use of the SZ effect from radio-galaxy lobes in the astrophysical and cosmological context.
Abstract
A recent stacking analysis of Planck HFI data of galaxy clusters led to the derivation of the cluster temperatures using the relativistic corrections to the Sunyaev–Zel'dovich effect (SZE). ...However, the temperatures of high-temperature clusters, as derived from this analysis, were basically higher than the temperatures derived from X-ray measurements, at a moderate statistical significance of 1.5σ. This discrepancy has been attributed by Hurier to calibration issues. In this paper, we discuss an alternative explanation for this discrepancy in terms of a non-thermal SZE astrophysical component. We find that this explanation can work if non-thermal electrons in galaxy clusters have a low minimum momentum (p
1 ∼ 0.5–1), and if their pressure is of the order of 20–30 per cent of the thermal gas pressure. Both these conditions are hard to obtain if the non-thermal electrons are mixed with the hot gas in the intracluster medium, but can be possibly obtained if the non-thermal electrons are mainly confined in bubbles with a high amount of non-thermal plasma and a low amount of thermal plasma, or are in giant radio lobes/relics in the outskirts of the clusters. To derive more precise results on the properties of the non-thermal electrons in clusters, and in view of more solid detections of a discrepancy between X-ray- and SZE-derived cluster temperatures that cannot be explained in other ways, it would be necessary to reproduce the full analysis done by Hurier by systematically adding the non-thermal component of the SZE.
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