Context. The Sunyaev-Zel’dovich (SZ) effect is a powerful tool for studying clusters of galaxies and cosmology. Large mm-wave telescopes are now routinely detecting and mapping the SZ effect in a ...number of clusters, measure their comptonisation parameter and use them as probes of the large-scale structure and evolution of the universe. Aims. We show that estimates of the physical parameters of clusters (optical depth, plasma temperature, peculiar velocity, non-thermal components etc.) obtained from ground-based multi-band SZ photometry can be significantly biased, owing to the reduced frequency coverage, to the degeneracy between the parameters and to the presence of a number of independent components larger than the number of frequencies measured. We demonstrate that low-resolution spectroscopic measurements of the SZ effect that also cover frequencies >270 GHz are effective in removing the degeneracy. Methods. We used accurate simulations of observations with lines-of-sight through clusters of galaxies with different experimental configurations (4-band photometers, 6-band photometer, multi-range differential spectrometer, full coverage spectrometers) and different intracluster plasma stratifications. Results. We find that measurements carried out with ground-based few-band photometers are biased towards high electron temperatures and low optical depths, and require coverage of high frequency and/or independent complementary observations to produce unbiased information; a differential spectrometer that covers 4 bands with a resolution of ~6 GHz eliminates most if not all bias; full-range differential spectrometers are the ultimate resource that allows a full recovery of all parameters.
Context. Populations of high energy electrons can produce hard X-ray (HXR) emission in galaxy clusters by up-scattering CMB photons via the inverse Compton scattering (ICS) mechanism. However, this ...scenario has various astrophysical consequences. Aims. We discuss here the consequences of the presence of a population of high energy particles for the multi-frequency emissivity of the same clusters and the structure of their atmospheres. Methods. We derive predictions for the ICS HXR emission in the specific case of the Ophiuchus cluster (for which an interesting combination of observational limits and theoretical scenarios have been presented) for three main scenarios producing high-E electrons: primary cosmic ray model, secondary cosmic rays model and neutralino DM annihilation scenario. We further discuss the predictions of the Warming Ray model for the cluster atmosphere. Under the assumption to fit the HXR emission observed in Ophiuchus, we explore the consequences that these electron populations induce on the cluster atmosphere. Results. We find that: i) primary electrons can be marginally consistent with the available data provided that the electron spectrum is cutoff at E ≲ 30 and E ≲ 90 MeV for electron spectral index values of 3.5 and 4.4, respectively; ii) secondary electron models from pp collisions are strongly inconsistent with the viable gamma-ray limits, cosmic ray protons produce too much heating of the intracluster (IC) gas and their pressure at the cluster center largely exceeds the thermal one; iii) secondary electron models from DM annihilation are also strongly inconsistent with the viable gamma-ray and radio limits, and electrons produce too much heating of the IC gas at the cluster center, unless the neutralino annihilation cross-section is much lower than the proposed value. In that case, however, these models no longer reproduce the HXR excess in Ophiuchus. Conclusions. We conclude that ICS by secondary electrons from both neutralino DM annihilation and pp collisions cannot be the mechanism responsible for the HXR excess emission; primary electrons are still a marginally viable solution provided that their spectrum has a low-energy cutoff at E ≲ 30-90 MeV. We also find that diffuse radio emission localized at the cluster center is expected in all these models and requires quite low values of the average magnetic field (B ~ 0.1-0.2 μG in primary and secondary-pp models; B ~ 0.055-0.39 μG in secondary-DM models) to agree with the available observations. Finally, the WR model (with B ~ 0.4-2.0 μG) offers, so far, the most accurate description of the cluster in terms of the temperature distribution, heating and pressure and multi-frequency spectral energy distribution. Fermi observations of Ophiuchus will provide further constraints to this model.
Context. The Gamma-ray emission from galaxy clusters hosting active galaxies is a complex combination of diffuse and point-like emission features with different spectral and spatial properties. Aims. ...We discuss in details the case of the Perseus cluster containing the radio-galaxy NGC 1275 that has been recently detected as a bright gamma-ray source by the Fermi-LAT experiment, in order to disentangle the sources of emission. Methods. We provide a detailed study of the gamma-ray emission coming from the core of Perseus by modeling the central AGN emission with a multiple plasma blob model, and the emission from the extended cluster atmosphere with both a warming ray (WR) model and dark matter (DM) neutralino annihilation models. We set constraints on both the central galaxy and cluster SED models by using both archival multi-frequency data and the recent very high energy observations obtained by Fermi and MAGIC. Results. We find that: i) in all the viable models for the cluster gamma-ray emission, the emission detected recently by Fermi from the center of the Perseus cluster is dominated by the active galaxy NGC 1275, that is found in a high-emission state; ii) the diffuse gamma-ray emission of the cluster, in the WR model and in the DM models with the highest allowed normalization, could be detected by Fermi if the central emission from NGC 1275 is in a low-emission state; iii) Fermi can have the possibility to resolve and detect the diffuse gamma-ray flux (predicted by the WR model) coming from the outer corona of the Perseus cluster atmosphere at distances r $\ga$ 800 kpc. These results are consistent with the evidence that in the other frequency bands, the diffuse cluster emission dominates on the central galaxy one at low radio frequencies with ν $\la$ 1 GHz and at X-ray energies of order of E~ keV. Conclusions. Our results show that a simultaneous study of the various emission mechanisms that produce diffuse gamma-rays from galaxy clusters and the study of the emission mechanisms that produce gamma-rays from active galaxies residing in the cluster atmospheres is absolutely crucial first to disentangle the spectral and spatial characteristics of the gamma-ray emission and secondly to assess the optimal observational strategy in the attempt to reveal the still elusive diffuse gamma-ray emission widely predicted for the atmospheres of large-scale structures.
We present the results of a series of optical, UV, X-ray and γ-ray observations of the BL Lac object S50716+714 carried out by the Swift and AGILE satellites in late 2007 when the blazar was flaring ...close to its historical maximum at optical frequencies. We have found that the optical through soft X-ray emission, likely due to synchrotron radiation, was highly variable and displayed a different behavior in the optical UV and soft X-ray bands. The 4-10 keV flux, most probably dominated by the inverse Compton component, instead remained constant. The counting statistics in the relatively short AGILE GRID observation was low and consistent with a constant γ-ray flux at a level similar to the maximum observed by EGRET. An estimate of the γ-ray spectral slope gives a value of the photon index that is close to 2, suggesting that the peak of the inverse Compton component in the Spectral Energy Distribution (SED) is within the AGILE energy band. The different variability behavior observed in different parts of the SED exclude interpretations predicting highly correlated flux variability like changes in the beaming factor or the magnetic field in simple SSC scenarios. The observed SED changes may instead be interpreted as due to the sum of two SSC components, one of which is constant while the other is variable and with a systematically higher synchrotron peak energy.
Aims. We studied the multi-frequency predictions of various annihilating dark matter (DM) scenarios in order to explore the possibility to interpret the still unknown origin of non-thermal phenomena ...in galaxy clusters. Methods. We consider three different DM models with light (9 GeV), intermediate (60 GeV), and high (500 GeV) neutralino mass and study their physical effects in the atmosphere of the Coma cluster. The secondary particles created in the neutralino annihilation processes produce a multi-frequency spectral energy distribution (SED) of non-thermal radiation and also heat the intracluster gas, which we test against the observations available for the Coma cluster from radio to gamma-rays. The various DM-produced SEDs are normalized by the condition to fit the Coma radio halo spectrum, thus obtaining best-fit values of the annihilation cross-section σV and of the central magnetic field B0. Results. We find that it is not possible to interpret all the non-thermal phenomena observed in galaxy clusters in terms of DM annihilation. The light-mass DM model with 9 GeV mass produces too little power at all other frequencies, while the high-mass DM model with 500 GeV produces a large excess power at all other frequencies. The intermediate-mass DM model with 60 GeV and τ ± composition is marginally consistent with the HXR and gamma-ray observations, but narrowly fails to reproduce the EUV and soft X-ray observations. The intermediate-mass DM model with 60 GeV and \hbox{$b{\bar b}$}bb̅ composition is, on the other hand, always below the observed fluxes. We note that the radio halo spectrum of Coma is well fitted only in the \hbox{$b{\bar b}$}bb̅ or light- and intermediate-mass DM models. We also find that the heating produced by the DM annihilation in the centre of the Coma cluster is always larger than the intracluster gas cooling rate for an NFW DM density profile and it is substantially smaller than the cooling rate only for a cored DM density profile in light-mass DM model with 9 GeV. Conclusions. The possibility of interpreting the origin of non-thermal phenomena in galaxy clusters with DM annihilation scenarios requires a low neutralino mass and a cored DM density profile. If we then consider the multi-messenger constraints to the neutralino annihilation cross-section, it turns out that this scenario would also be excluded unless we introduce a substantial boost factor that represents DM substructures. If we relax the condition to fit the Coma radio halo and we consider the EUV and HXR detections as upper limits for the non-thermal emission, together with the gamma-ray limit, then the limits on σV are less stringent than those obtained by the multi-messenger analysis.
We present a new catalogue of blazars based on multifrequency surveys and on an extensive review of the literature. Blazars are classified as BL Lacertae objects, as flat spectrum radio quasars or as ...blazars of uncertain/transitional type. Each object is identified by a root name, coded as BZB, BZQ and BZU for these three subclasses respectively, and by its coordinates. This catalogue is being built as a tool useful for the identification of the extragalactic sources that will be detected by present and future experiments for X and gamma-ray astronomy, like Swift, AGILE, Fermi-GLAST and Simbol-X. An electronic version is available from the ASI Science Data Center web site at http://www.asdc.asi.it/bzcat.
Context.The effective content of cosmic rays (CR) in galaxy clusters remains elusive. The evidence of relativistic electrons (RE) in the subset of clusters endowed with a radio halo remains hardly ...quantitative in the absence of robust estimates of the magnetic field $B(r)$, derived from Faraday Rotation (FR) measurements. The content in relativistic protons (RP) requires a different approach, the only direct one residing in the detection of their collisional production of gamma rays (GR). Aims.Based on the evidence of merging phenomena in clusters, theory predicts a large content of RP, whose energy density could be a large fraction of the thermal energy. This paper aims to estimate a maximum production of both secondary relativistic electrons, SRE, and GR from the RP that have supposedly accumulated throughout the entire history of a cluster. Methods.SRE and GR production is maximized when the RP and the thermal gas share the same radial profile. The production rate is normalized by adopting a reference value of 0.3 for ξ, the ratio of RP to thermal pressure. The SRE content which obtains, when constrained to reproduce the observed radio brightness profile, yields univocally $B(r)$, if the presence of primary RE were negligible. Results.This procedure is applied to four radio-halo clusters (Coma, A2163, A2255, A2319). In these objects, the central value B0 required is consistent with typical, albeit rather uncertain, values derived from FR, although for A2163 and A2319 no reliable FR estimates are available to strengthen this result. On the other hand, $B(r)$ typically increases beyond the thermal core, a hardly acceptable condition. This problem is alleviated by assuming a mix of SRE and of “primary” RE (PRE), with the latter becoming the dominant component beyond the thermal core. These results suggest that in clusters without a radio halo detected so far a diffuse radio-emission should also be observable due to SRE alone, and therefore more centrally condensed, provided that ξ is of the order of 0.3. To encourage deeper radio observations of such clusters, some examples were selected that seem rather promising. Efforts in this direction, if accompanied by FR measurements, could provide highly significant constraints on the CR content in clusters, even before the future GLAST mission will have accomplished the hard task of detecting the GR. A complementary result concerns the excess far UV in the Coma cluster, that some authors have attributed to IC emission from SRE. It is shown that this hypothesis can be excluded, because it requires a RP energy content in excess of the thermal one.
MeerKAT’s large number (64) of 13.5 m diameter antennas, spanning 8 km with a densely packed 1 km core, create a powerful instrument for wide-area surveys, with high sensitivity over a wide range of ...angular scales. The MeerKAT Galaxy Cluster Legacy Survey (MGCLS) is a programme of long-track MeerKAT
L
-band (900−1670 MHz) observations of 115 galaxy clusters, observed for ∼6−10 h each in full polarisation. The first legacy product data release (DR1), made available with this paper, includes the MeerKAT visibilities, basic image cubes at ∼8″ resolution, and enhanced spectral and polarisation image cubes at ∼8″ and 15″ resolutions. Typical sensitivities for the full-resolution MGCLS image products range from ∼3−5 μJy beam
−1
. The basic cubes are full-field and span 2° × 2°. The enhanced products consist of the inner 1.2° × 1.2° field of view, corrected for the primary beam. The survey is fully sensitive to structures up to ∼10′ scales, and the wide bandwidth allows spectral and Faraday rotation mapping. Relatively narrow frequency channels (209 kHz) are also used to provide H
I
mapping in windows of 0 <
z
< 0.09 and 0.19 <
z
< 0.48. In this paper, we provide an overview of the survey and the DR1 products, including caveats for usage. We present some initial results from the survey, both for their intrinsic scientific value and to highlight the capabilities for further exploration with these data. These include a primary-beam-corrected compact source catalogue of ∼626 000 sources for the full survey and an optical and infrared cross-matched catalogue for compact sources in the primary-beam-corrected areas of Abell 209 and Abell S295. We examine dust unbiased star-formation rates as a function of cluster-centric radius in Abell 209, extending out to 3.5
R
200
. We find no dependence of the star-formation rate on distance from the cluster centre, and we observe a small excess of the radio-to-100 μm flux ratio towards the centre of Abell 209 that may reflect a ram pressure enhancement in the denser environment. We detect diffuse cluster radio emission in 62 of the surveyed systems and present a catalogue of the 99 diffuse cluster emission structures, of which 56 are new. These include mini-halos, halos, relics, and other diffuse structures for which no suitable characterisation currently exists. We highlight some of the radio galaxies that challenge current paradigms, such as trident-shaped structures, jets that remain well collimated far beyond their bending radius, and filamentary features linked to radio galaxies that likely illuminate magnetic flux tubes in the intracluster medium. We also present early results from the H
I
analysis of four clusters, which show a wide variety of H
I
mass distributions that reflect both sensitivity and intrinsic cluster effects, and the serendipitous discovery of a group in the foreground of Abell 3365.
The surface brightness produced by synchrotron radiation in clusters of galaxies with a radio-halo sets a degenerate constraint on the magnetic field strength, the relativistic electron density and ...their spatial distributions, $B(r)$ and $n_{\rm rel}(r)$, in the intracluster medium. Using the radio-halo in the Coma Cluster as a case study, with the radio brightness profile and the spectral index as the only constraints, predictions are made for the brightness profiles expected in the 20–80 keV band due to Inverse Compton Scattering (ICS) by the relativistic electrons on the Cosmic Microwave Background. This is done for a range of central values of the magnetic field, B, and models of its radial dependence, $B(r)$ (of which two represent extreme situations, namely a constant value either of B or of $n_{\rm rel}(r)$, the third a more realistic intermediate case). It is shown that the possible presence of scalar fluctuations on small scales in the strength of B tends to systematically depress the electron density required by the radio data, hence to decrease the ICS brightness expected. These predictions should be useful to evaluate the sensitivity required in future imaging HXR instruments, in order to obtain direct information on the spatial distribution and content of relativistic electrons, hence on the magnetic field properties. If compared with the flux in the HXR tail, whose detection has been claimed in the Coma cluster (Fusco-Femiano et al. 2004, ApJ, 602, L73), when interpreted as ICS from within the radius Rh of the radio-halo, as measured so far, the predictions lead to central values B0 that are significantly lower than those which have been obtained (albeit still controversial) from Faraday Rotation measurements. The discrepancy is somewhat reduced if the radio-halo profile is hypothetically extrapolated out to Rvir, that is about three times Rh, or, as suggested by hydrodynamical simulations (Dolag et al. 2002, A&A, 387, 383), if it is assumed that $B(r) \propto n_{\rm th}(r)$. In the latter case $n_{\rm rel}(r)$ has its minimum value at the center of the cluster. If real and from ICS, the bulk of the HXR tail should then be contributed by electrons other than those responsible for the bulk of the radio-halo emission. This case illustrates the need for spatially resolved spectroscopy in the HXR, in order to obtain solid information on the non-thermal content of clusters of galaxies.
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