ABSTRACT
The galaxy cluster MS 0735.6+7421 hosts two large X-ray cavities, filled with radio emission, where a decrease of the Sunyaev–Zel’dovich (SZ) effect has been detected, without establishing ...if its origin is thermal (from a gas with very high temperature) or non-thermal. In this paper, we study how thermal and non-thermal contributions to the SZ effect in the cavities are related; in fact, Coulomb interactions with the thermal gas modify the spectrum of low-energy non-thermal electrons, which dominate the non-thermal SZ effect; as a consequence, the intensity of the non-thermal SZ effect is stronger for lower density of the thermal gas inside the cavity. We calculate the non-thermal SZ effect in the cavities as a function of the thermal density, and compare the SZ effects produced by thermal and non-thermal components, and with the one from the external intracluster medium (ICM), searching for the best frequency range where it is possible to disentangle the different contributions. We find that for temperatures inside the cavities higher than ∼1500 keV the non-thermal SZ effect is expected to dominate on the thermal one, particularly at high frequencies (ν > 500 GHz), where it can also be a non-negligible fraction of the SZ effect from the external ICM. We also discuss the possible sources of astrophysical bias (as kinetic SZ effect and foreground emission from Galactic dust) and possible ways to address them, as well as necessary improvements in the modelling of the properties of cavities and the ICM.
ABSTRACT
In this paper, we study the effect of reacceleration provided by turbulences on electrons produced by dark matter (DM) annihilation in the Coma cluster. We use a simplified phenomenological ...model to describe the effect of the turbulences, and explore a limited subset of three possible DM models for neutralino particles with different mass and annihilation channel. We find that, for values of the annihilation cross-section of the order of the upper limits found with Fermi–LAT measurements in astrophysical objects, and for conservative values of the boosting factor due to DM substructures, the reacceleration due to turbulences can enhance the radio emission produced by DM-originated electrons up to the level of the observed flux of the radio halo in Coma, for moderate reacceleration intensity in relatively short times. Therefore, we conclude that, even if it is not possible to distinguish between the fits obtained in this paper because of the scattering present in the radio flux data, the electrons produced by DM annihilation can be possible seed electrons for the reacceleration, as well as secondary electrons of hadronic origin. A possible discriminant between these two classes of models is the flux produced in the gamma ray band, which in the case of DM-originated electrons should be more than two orders of magnitude smaller than the present Fermi–LAT upper limits, whereas in the hadronic case the expected gamma ray flux should be close to the value of present upper limits.
In this paper we study the possibility of probing the low-energy part of the spectrum of non-thermal electrons in galaxy clusters by detecting their non-thermal bremsstrahlung (NTB) emission in the ...soft gamma ray band, using instruments like e-ASTROGAM. Using the Coma cluster as a reference case, we find that, for very low values of the minimum energy of the electrons, in principle the NTB is detectable, but this situation is possible only for conditions that can be maintained only for a short time compared to the cluster lifetime. The possibility of constraining the low energy spectrum of non-thermal electrons through NTB is therefore hard to achieve in next years.
The 5th edition of the
Roma-BZCAT
Multifrequency Catalogue of Blazars is available in a printed version and online at the ASDC website (
http://www.asdc.asi.it/bzcat
); it is also in the NED ...database. It presents several relevant changes with respect to the past editions which are briefly described in this paper.
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.
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.
Context. Giant radio galaxy (GRG) lobes are excellent laboratories for studying the evolution of the particle and magnetic field energetics and the past activity of radiogalaxy jets, as indicated by ...recent results of X-ray observations with Suzaku. However, these results are based on assumptions of the shape and extension of the GRG lobe electron spectrum. Aims. We re-examine the energetics of GRG lobes as derived by inverse Compton scattering of cosmic microwave background (CMB) photons (ICS-CMB) by relativistic electrons in RG lobes to assess the realistic physical conditions of RG lobes, their energetics, and their radiation regime. We consider the steep-spectrum GRG DA 240 recently observed by Suzaku as a reference case and we also discuss other RG lobes observed with Chandra and XMM. Methods. We model the spectral energy distribution (SED) of the GRG DA 240 East lobe to obtain constraints on the shape and on the extension of the electron spectrum by using multi-frequency information from radio to gamma-rays. We use radio and X-ray data to constrain the shape and normalization of the electron spectrum and we then calculate the Sunyaev-Zel’dovich (SZ) effect expected in GRG lobes that is sensitive to the total electron energy density. Results. We show that the electron energy density Ue derived form X-ray observations can yield only a rough lower limit to its actual value and that most of the estimates of Ue based on X-ray measurements have to be increased even by a large factor by considering realistic estimates of the lower electron momentum p1. This moves RG lobes away from the equipartition condition toward a particle-dominated and Compton power dominance regime. We propose to use the distribution of RG lobes in the Ue/UB vs. Ue/UCMB plane as another divide divide between the different physical regimes of particle and field dominance and radiation mechanism dominance in RG lobes. Conclusions. We conclude that the SZ effect produced by ICS-CMB mechanism observable in RG lobes provides reliable estimate of p1 and Ue and is the best tool to determine the total energy density of RG lobes and to assess their physical regime. This observational tool is available with the sensitive high-frequency radio and mm experiments.
Context. The evolution of the Universe during the dark ages (DA) and the epoch of reonization (EoR) marks an important transition in the history of the Universe but it is not yet fully understood. ...Aims. We study here an alternative technique to probe the DA and EoR that makes use of the Comptonization of the CMB spectrum modified by physical effects occurring during this epoch related to the emergence of the 21-cm radiation background. Inverse Compton scattering of 21-cm photon background by thermal and non-thermal electrons residing in the atmospheres of cosmic structures like galaxy clusters, radiogalaxy lobes and galaxy halos, produces a specific form of Sunyaev-Zel’dovich effect (SZE) that we refer to as SZE-21 cm. Methods. We derived the SZE-21 cm in a general relativistic approach, which is required to describe the correct spectral features of this astrophysical effect. We calculated the spectral features of the thermal and non-thermal SZE-21 cm in galaxy clusters and in radiogalaxy lobes, and their dependence on the history of physical mechanisms occurring during the DA and EoR. We studied how the spectral shape of the SZE-21 cm can be used to establish the global features in the mean 21-cm spectrum generated during and prior to the EoR, and how it depends on the properties of the (thermal and non-thermal) plasma in cosmic structures. Results. We found that the thermal and non-thermal SZE-21 cm have peculiar spectral shapes that allow to investigate the physics and history of the EoR and DA. Its spectrum depends on the gas temperature (for the thermal SZE-21 cm) and on the electrons minimum momentum (for the non-thermal SZE-21 cm). The global SZE-21 cm signal can be detected (in ~ 1000 h) by SKA1-low in the frequency range ν ≳ 75−90 MHz, for clusters in the temperature range 5 to 20 keV, and the difference between the SZE-21 cm and the standard SZE can be detected by SKA1 or SKA2 at frequencies depending on the background model and the cluster temperature. Conclusions. We have shown that the detection of the SZE-21 cm can provide unique information on the DA and EoR, and on the cosmic structures that produce the scattering; the frequencies at which the SZE-21 cm shows its main spectral features will indicate the epoch at which the physical processes related to the cosmological 21-cm signal occurred and shed light on the cosmic history during the DA and EoR by using local, well-known cosmic structures like galaxy clusters and radio galaxies.
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