Rossella Cassano - Academia.edu (original) (raw)

Papers by Rossella Cassano

Astronomy & Astrophysics, 2010

Aims. Giant radio halos are mega-parsec scale synchrotron sources detected in a fraction of massi... more Aims. Giant radio halos are mega-parsec scale synchrotron sources detected in a fraction of massive and merging galaxy clusters. Radio halos provide one of the most important pieces of evidence for non-thermal components in large scale structure. Statistics of their properties can be used to discriminate among various models for their origin. Therefore, theoretical predictions of the occurrence of radio halos are important as several new radio telescopes are about to begin to survey the sky at low frequencies with unprecedented sensitivity. Methods. In this paper we carry out Monte Carlo simulations to model the formation and evolution of radio halos in a cosmological framework. We extend previous works on the statistical properties of radio halos in the context of the turbulent re-acceleration model. Results. First we compute the fraction of galaxy clusters that show radio halos and derive the luminosity function of radio halos. Then, we derive differential and integrated number count distributions of radio halos at low radio frequencies with the main goal to explore the potential of the upcoming LOFAR surveys. By restricting to the case of clusters at redshifts < 0.6, we find that the planned LOFAR all sky survey at 120 MHz is expected to detect about 350 giant radio halos. About half of these halos have spectral indices larger than 1.9 and substantially brighten at lower frequencies. If detected they will allow for a confirmation that turbulence accelerates the emitting particles. We expect that also commissioning surveys, such as MS 3 , have the potential to detect about 60 radio halos in clusters of the ROSAT Brightest Cluster Sample and its extension (eBCS). These surveys will allow us to constrain how the rate of formation of radio halos in these clusters depends on cluster mass.

Astronomy & Astrophysics, 2008

In this letter we present a morphological comparison between giant radio halos and radio mini-hal... more In this letter we present a morphological comparison between giant radio halos and radio mini-halos in galaxy clusters based on radio--X-ray luminosity, P_{1.4}-L_X, and radio luminosity-size, P_{1.4}-R_H, correlations. We report evidence that P_{1.4}-L_X and P_{1.4}-R_H trends may also exist for mini--halos: mini--halo clusters share the same region of giant halo clusters in the (P_{1.4},L_X) plane, whereas they are clearly separated in the (P_{1.4},R_H) plane. The synchrotron emissivity of mini-halos is found to be more than 50 times larger than that of giant halos, implying a very efficient process for their origins. By assuming a scenario of sporadical turbulent particle re-acceleration for both giant and mini halos, we discuss basic physical differences between these sources. Regardless of the origin of the turbulence, a more efficient source of injection of particles, which eventually takes part in the re-acceleration process, is required in mini-halos, and this may result from the central radio galaxy or from proton-proton collisions in the dense cool core regions.

Monthly Notices of The Royal Astronomical Society, 2003

There is a growing evidence that extended radio halos are most likely generated by electrons reac... more There is a growing evidence that extended radio halos are most likely generated by electrons reaccelerated via some kind of turbulence generated in the cluster volume during major mergers. It is well known that Alfv\'en waves channel most of their energy flux in the acceleration of relativistic particles. Much work has been done recently to study this phenomenon and its consequences for the explanation of the observed non-thermal phenomena in clusters of galaxies. We investigate here the problem of particle-wave interactions in the most general situation in which relativistic electrons, thermal protons and relativistic protons exist within the cluster volume. The interaction of all these components with the waves, as well as the turbulent cascading and damping processes of Alfv\'en waves, are treated in a fully time-dependent way. This allows us to calculate the spectra of electrons, protons and waves at any fixed time. The {\it Lighthill} mechanism is invoked to couple the fluid turbulence, supposedly injected during cluster mergers, to MHD turbulence. We find that present observations of non-thermal radiation from clusters of galaxies are well described within this approach, provided the fraction of relativistic hadrons in the intracluster medium (ICM) is smaller than 5-10 %.

Giant radio halos are mega-parsec scale synchrotron sources detected in a fraction of massive and... more Giant radio halos are mega-parsec scale synchrotron sources detected in a fraction of massive and merging galaxy clusters. Radio halos provide one of the most important pieces of evidence for non-thermal components in large scale structure. Statistics of their properties can be used to discriminate among various models for their origin. Therefore, theoretical predictions of the occurrence of radio halos are important as several new radio telescopes are about to begin to survey the sky at low frequencies with unprecedented sensitivity. In this paper we carry out Monte Carlo simulations to model the formation and evolution of radio halos in a cosmological framework. We extend previous works on the statistical properties of radio halos in the context of the turbulent re-acceleration model. First we compute the fraction of galaxy clusters that show radio halos and derive the luminosity function of radio halos. Then, we derive differential and integrated number count distributions of radio halos at low radio frequencies with the main goal to explore the potential of the upcoming LOFAR surveys. By restricting to the case of clusters at redshifts <0.6, we find that the planned LOFAR all sky survey at 120 MHz is expected to detect about 350 giant radio halos. About half of these halos have spectral indices larger than 1.9 and substantially brighten at lower frequencies. If detected they will allow for a confirmation that turbulence accelerates the emitting particles. We expect that also commissioning surveys, such as MSSS, have the potential to detect about 60 radio halos in clusters of the ROSAT Brightest Cluster Sample and its extension (eBCS). These surveys will allow us to constrain how the rate of formation of radio halos in these clusters depends on cluster mass.

Journal of The Korean Astronomical Society, 2004

With the aim to investigate the statistical properties and the connection between thermal and non... more With the aim to investigate the statistical properties and the connection between thermal and non-thermal properties of the ICM in galaxy clusters, we have developed a statistical magneto-turbulent model which describes, at the same time, the evolution of the thermal and non-thermal emission from galaxy clusters. In particular, starting from the cosmological evolution of clusters, we follow cluster mergers, calculate the spectrum of the magnetosonic waves generated in the ICM during these mergers, the evolution of relativistic electrons and the resulting synchrotron and Inverse Compton spectra. We show that the broad band (radio and hard x-ray) non-thermal spectral properties of galaxy clusters can be well accounted for by our model for viable values of the parameters.

Nature, 2008

Clusters of galaxies are the largest gravitationally bound objects in the Universe, containing ab... more Clusters of galaxies are the largest gravitationally bound objects in the Universe, containing about 10^15 solar masses of hot (10^8 K) gas, galaxies and dark matter in a typical volume of about 10 Mpc^3. Magnetic fields and relativistic particles are mixed with the gas as revealed by giant radio haloes, which arise from diffuse, megaparsec-scale synchrotron radiation at cluster center. Radio haloes require that the emitting electrons are accelerated in situ (by turbulence), or are injected (as secondary particles) by proton collisions into the intergalactic medium. They are found only in a fraction of massive clusters that have complex dynamics, which suggests a connection between these mechanisms and cluster mergers. Here we report a radio halo at low frequencies associated with the merging cluster Abell 521. This halo has an extremely steep radio spectrum, which implies a high frequency cut-off; this makes the halo difficult to detect with observations at 1.4 GHz (the frequency at which all other known radio haloes have been best studied). The spectrum of the halo is inconsistent with a secondary origin of the relativistic electrons, but instead supports turbulent acceleration, which suggests that many radio haloes in the Universe should emit mainly at low frequencies.

Monthly Notices of The Royal Astronomical Society, 2006

We present a study of the turbulent velocity fields in the intracluster medium (ICM) of a sample ... more We present a study of the turbulent velocity fields in the intracluster medium (ICM) of a sample of 21 galaxy clusters simulated by the smoothed particle hydrodynamics code gadget2, using a new numerical scheme where the artificial viscosity is suppressed outside shocks. The turbulent motions in the ICM of our simulated clusters are detected with a novel method devised to better disentangle laminar bulk motions from chaotic ones. We focus on the scaling law between the turbulent energy content of the gas particles and the total mass, and find that the energy in the form of turbulence scales approximately with the thermal energy of clusters. We follow the evolution with time of the scaling laws and discuss the physical origin of the observed trends. The simulated data are in agreement with independent semi-analytical calculations, and the combination between the two methods allows one to constrain the scaling law over more than two decades in cluster mass.

Monthly Notices of The Royal Astronomical Society, 2006

The most important evidence of non-thermal phenomena in galaxy clusters comes from giant radio ha... more The most important evidence of non-thermal phenomena in galaxy clusters comes from giant radio haloes (GRHs), spectacular synchrotron radio sources extended over ≥Mpc scales, detected in the central regions of a growing number of massive galaxy clusters. A promising possibility to explain these sources is given by in situ stochastic reacceleration of relativistic electrons by turbulence generated in the cluster volume during merger events. Cassano and Brunetti have recently shown that the expected fraction of clusters with radio haloes and the increase of such a fraction with cluster mass can be reconciled with present observations provided that a fraction of 20–30 per cent of the turbulence in clusters is in the form of compressible modes.In this work, we extend the above-mentioned analysis by including a scaling of the magnetic field strength with cluster mass. We show that, in the framework of the reacceleration model, the observed correlations between the synchrotron radio power of a sample of 17 GRHs and the X-ray properties of the hosting clusters are consistent with, and actually predicted by a magnetic field dependence on the virial mass of the form B∝Mbv, with b≳ 0.5 and typical μG strengths of the average B intensity. The occurrence of GRHs as a function of both cluster mass and redshift is obtained: the evolution of such a probability depends on the interplay between synchrotron and inverse Compton losses in the emitting volume, and it is maximized in clusters for which the two losses are comparable.The most relevant findings are that the predicted luminosity functions of GRHs are peaked around a power P1.4 GHz∼ 1024 W Hz−1, and severely cut off at low radio powers due to the decrease of the electron reacceleration in smaller galaxy clusters, and that the occurrence of GRHs at 1.4 GHz beyond a redshift z∼ 0.7 appears to be negligible. As a related check, we also show that the predicted integral radio source counts within a limited volume (z≤ 0.2) are consistent with present observational constraints. Extending the source counts beyond z= 0.2, we estimate that the total number of GRHs to be discovered at ∼ mJy radio fluxes could be ∼100 at 1.4 GHz. Finally, the occurrence of GRHs and their number counts at 150 MHz are estimated in view of the forthcoming operation of low-frequency observatories (LOFAR, LWA) and compared with those at higher radio frequencies.

Monthly Notices of The Royal Astronomical Society, 2010

We use the results from a constrained, cosmological magnetohydrodynamic simulation of the Local U... more We use the results from a constrained, cosmological magnetohydrodynamic simulation of the Local Universe to predict the radio halo and the γ-ray flux from the Coma cluster and compare it to current observations. The simulated magnetic field within the Coma cluster is the result of turbulent amplification of the magnetic field during the build-up of the cluster. The magnetic seed field originates from starburst driven, galactic outflows. The synchrotron emission is calculated assuming a hadronic model. We follow four approaches with different distributions for the cosmic ray proton population within galaxy clusters. The radial profile of the radio halo can only be reproduced with a radially increasing energy fraction within the cosmic ray proton population, reaching >100 per cent of the thermal-energy content at ≈1 Mpc, for example the edge of the radio-emitting region. Additionally, the spectral steepening of the observed radio halo in Coma cannot be reproduced, even when accounting for the negative flux from the thermal Sunyaev–Zeldovich effect at high frequencies. Therefore, the hadronic models are disfavoured from the present analysis. The emission of γ-rays expected from our simulated Coma is still below the current observational limits (by a factor of ∼6) but would be detectable by FERMI observations in the near future.

Astrophysical Journal, 2007

Clusters of galaxies are sites of acceleration of charged particles and sources of non-thermal ra... more Clusters of galaxies are sites of acceleration of charged particles and sources of non-thermal radiation. We report on new constraints on the population of cosmic rays in the Intra Cluster Medium (ICM) obtained via radio observations of a fairly large sample of massive, X-ray luminous, galaxy clusters in the redshift interval 0.2--0.4. The bulk of the observed galaxy clusters does not show any hint of Mpc scale synchrotron radio emission at the cluster center (Radio Halo). We obtained solid upper limits to the diffuse radio emission and discuss their implications for the models for the origin of Radio Halos. Our measurements allow us to derive also a limit to the content of cosmic ray protons in the ICM. Assuming spectral indices of these protons delta =2.1-2.4 and microG level magnetic fields, as from Rotation Measures, these limits are one order of magnitude deeper than present EGRET upper limits, while they are less stringent for steeper spectra and lower magnetic fields.

Monthly Notices of The Royal Astronomical Society, 2007

In this paper we derive new expected scaling relations for clusters with giant radio halos in the... more In this paper we derive new expected scaling relations for clusters with giant radio halos in the framework of the re-acceleration scenario in a simplified, but physically motivated, form, namely: radio power (P R ) vs size of the radio emitting region (R H ), and P R vs total cluster mass (M H ) contained in the emitting region and cluster velocity dispersion (σ H ) in this region.

Astronomy & Astrophysics, 2007

Aims. We present the first results of an ongoing project devoted to the search of giant radio hal... more Aims. We present the first results of an ongoing project devoted to the search of giant radio halos in galaxy clusters located in the redshift range z=0.2--0.4. One of the main goals of our study is to measure the fraction of massive galaxy clusters in this redshift interval hosting a radio halo, and to constrain the expectations of the particle re--acceleration model for the origin of non--thermal radio emission in galaxy clusters. Methods. We selected 27 REFLEX clusters and here we present Giant Metrewave Radio Telescope (GMRT) observations at 610 MHz for 11 of them. The sensitivity (1$\sigma$) in our images is in the range 35--100~$\mu$Jy beam$^{-1}$ for all clusters. Results. We found three new radio halos, doubling the number of halos known in the selected sample. In particular, giant radio halos were found in A 209 and RXCJ 2003.5--2323, and one halo (of smaller size) was found in RXCJ 1314.4--2515. Candidate extended emission on smaller scale was found around the central galaxy in A 3444 which deserves further investigation. Furthermore, a radio relic was found in A 521, and two relics were found in RXCJ 1314.5--2515. The remaining six clusters observed do not host extended emission of any kind.

Astronomische Nachrichten, 2006

There are several possibilities to constrain the value of the magnetic field in the ICM, the most... more There are several possibilities to constrain the value of the magnetic field in the ICM, the most direct ones being the combination of inverse Compton and synchrotron observations, and the Faraday rotation measures. Here we discuss on the possibility to provide constraints on the magnetic field in the ICM from the analysis of the statistical properties of the giant radio halos, Mpc--scale diffuse radio emission in galaxy clusters. Present observations of a few well studied radio halos can be interpreted under the hypothesis that the emitting relativistic electrons are re-accelerated on their way out. By using statistical calculations carried out in the framework of the re-acceleration model we show that the observed radio--power vs cluster mass correlation in radio halos can be reproduced only by assuming mu\mumuG fields in the ICM and a scaling of the magnetic field with cluster mass BproptoMvbB \propto M_v^bBproptoMvb, with bgeq0.6b \geq 0.6bgeq0.6. We also show that the expected occurrence of radio halos with mass and redshift, and their number counts are sensitive to the magnetic field intensity in massive galaxy clusters and to the scaling of B with cluster mass. Thus future deep surveys of radio halos would provide constraints on B in galaxy clusters.

Astronomy & Astrophysics, 2009

Aims. We present a combined radio, X-ray and optical study of the galaxy cluster RXCJ 2003.5-2323... more Aims. We present a combined radio, X-ray and optical study of the galaxy cluster RXCJ 2003.5-2323. The cluster hosts one of the largest, most powerful and distant giant radio halos known to date, suggesting that it may be undergoing a strong merger process. The aim of our multiwavelength study is to investigate the radio-halo cluster merger scenario. Methods. We studied the radio properties of the giant radio halo in RXCJ 2003.5-2323 by means of new radio data obtained at 1.4 GHz with the Very Large Array, and at 240 MHz with the Giant Metrewave Radio Telescope, in combination with previously published GMRT data at 610 MHz. The dynamical state of the cluster was investigated by means of X-ray Chandra observations and optical ESO-NTT observations. Results. Our study confirms that RXCJ 2003.5-2323 is an unrelaxed cluster. The unusual filamentary and clumpy morphology of the radio halo could be due to a combination of the filamentary structure of the magnetic field and turbulence in the inital stage of a cluster merger.

Monthly Notices of The Royal Astronomical Society, 2004

There is growing evidence that extended radio haloes are most likely generated by electrons reacc... more There is growing evidence that extended radio haloes are most likely generated by electrons reaccelerated via some kind of turbulence generated in the cluster volume during major mergers. It is well known that Alfvén waves channel most of their energy flux in the acceleration of relativistic particles. Much work has been done recently to study this phenomenon and its consequences for the explanation of the observed non-thermal phenomena in clusters of galaxies. We investigate here the problem of particle–wave interactions in the most general situation in which relativistic electrons, thermal protons and relativistic protons exist within the cluster volume. The interaction of all these components with the waves, as well as the turbulent cascading and damping processes of Alfvén waves, are treated in a fully time-dependent way. This allows us to calculate the spectra of electrons, protons and waves at any fixed time. The Lighthill mechanism is invoked to couple the fluid turbulence, supposedly injected during cluster mergers, to MHD turbulence. We find that present observations of non-thermal radiation from clusters of galaxies are well described within this approach, provided the fraction of relativistic hadrons in the intracluster medium (ICM) is smaller than 5–10 per cent.

Monthly Notices of The Royal Astronomical Society, 2005

There is now firm evidence that the ICM consists of a mixture of hot plasma, magnetic fields and ... more There is now firm evidence that the ICM consists of a mixture of hot plasma, magnetic fields and relativistic particles. The most important evidences for non-thermal phenomena in galaxy clusters comes from the spectacular synchrotron radio emission diffused over Mpc scale observed in a growing number of massive clusters and, more recently, in the hard X-ray tails detected in a few cases in excess of the thermal bremmstrahlung spectrum. A promising possibility to explain giant radio halos is given by the presence of relativistic electrons reaccelerated by some kind of turbulence generated in the cluster volume during merger events. With the aim to investigate the connection between thermal and non-thermal properties of the ICM, in this paper we develope a statistical magneto-turbulent model which describes in a self-consistent way the evolution of the thermal ICM and that of the non-thermal emission from clusters. Making use of the extended Press & Schechter formalism, we follow cluster mergers and estimate the injection rate of the fluid turbulence generated during these energetic events. We then calculate the evolution of the spectrum of the relativistic electrons in the ICM during the cluster life by taking into account both the electron-acceleration due to the merger-driven turbulence and the relevant energy losses of the electrons. We end up with a synthetic population of galaxy clusters for which the evolution of the ICM and of the non-thermal spectrum emitted by the accelerated electrons is calculated. The generation of detectable non-thermal radio and hard X-ray emission in the simulated clusters is found to be possible during major merger events for reliable values of the model parameters. In addition the occurrence of radio halos as a function of the mass of the parent clusters is calculated and compared with observations. In this case it is found that the model expectations are in good agreement with observations: radio halos are found in about 30% of the more massive clusters in our synthetic population (M > ∼ 1.8 × 10 15 M ⊙ ) and in about 4% of the intermediate massive clusters (9 × 10 14 < M < 1.8 × 10 15 M ⊙ ), while the radio halo phenomenon is found to be extremely rare in the case of the smaller clusters.

Astronomy & Astrophysics, 2010

In this paper we present a detailed study of the giant radio halo in the galaxy cluster Abell 697... more In this paper we present a detailed study of the giant radio halo in the galaxy cluster Abell 697, with the aim to constrain its origin and connection with the cluster dynamics. We performed high sensitivity GMRT observations at 325 MHz, which showed that the radio halo is much brighter and larger at this frequency, compared to previous 610 MHz observations. In order to derive the integrated spectrum in the frequency range 325 MHz--1.4 GHz, we re--analysed archival VLA data at 1.4 GHz and made use of proprietary GMRT data at 610 MHz. {Our multifrequency analysis shows that the total radio spectrum of the giant radio halo in A\,697 is very steep, with alpharm3˜25MHzrm1˜.4GHzapprox1.7−1.8\alpha_{\rm~325 MHz}^{\rm~1.4 GHz} \approx 1.7-1.8alpharm3˜25MHzrm1˜.4GHzapprox1.7−1.8. %\pm0.1$. Due to energy arguments, a hadronic origin of the halo is disfavoured by such steep spectrum. Very steep spectrum halos in merging clusters are predicted in the case that the emitting electrons are accelerated by turbulence, observations with the upcoming low frequency arrays will be able to test these expectations.}

Monthly Notices of The Royal Astronomical Society, 2006

The most important evidence of non-thermal phenomena in galaxy clusters comes from giant radio ha... more The most important evidence of non-thermal phenomena in galaxy clusters comes from giant radio haloes (GRHs), spectacular synchrotron radio sources extended over Mpc scales, detected in the central regions of a growing number of massive galaxy clusters. A promising possibility to explain these sources is given by in situ stochastic reacceleration of relativistic electrons by turbulence generated in the cluster volume during merger events. Cassano and Brunetti have recently shown that the expected fraction of clusters with radio haloes and the increase of such a fraction with cluster mass can be reconciled with present observations provided that a fraction of 20-30 per cent of the turbulence in clusters is in the form of compressible modes.

Astronomy & Astrophysics, 2008

Results. We find a significant statistical evidence (at 3.7 sigma) of an increase of the fraction... more Results. We find a significant statistical evidence (at 3.7 sigma) of an increase of the fraction of clusters with Radio Halos with the X-ray luminosity (mass) of the parent clusters and show that this increase is in line with statistical calculations based on the re-acceleration scenario. We argue that a fundamental expectation of this scenario is that the probability to have radio halos emitting at hundred MHz is larger than that at GHz frequencies and thus that future radio interferometers operating at low frequencies, such as LOFAR and LWA, should detect a larger number of radio halos with respect to that caught by present GHz observations. We also show that the expected increase of the fraction of clusters with radio halos with the cluster mass as measured with future LOFAR and LWA surveys should be less strong than that in present surveys.

New Astronomy, 2006

We present high sensitivity radio observations of the merging cluster A 521, at a mean redsfhit z... more We present high sensitivity radio observations of the merging cluster A 521, at a mean redsfhit z=0.247. The observations were carried out with the GMRT at 610 MHz and cover a region of ∼1 square degree, with a sensitivity limit of 1σ = 35

Astronomy & Astrophysics, 2010

Aims. Giant radio halos are mega-parsec scale synchrotron sources detected in a fraction of massi... more Aims. Giant radio halos are mega-parsec scale synchrotron sources detected in a fraction of massive and merging galaxy clusters. Radio halos provide one of the most important pieces of evidence for non-thermal components in large scale structure. Statistics of their properties can be used to discriminate among various models for their origin. Therefore, theoretical predictions of the occurrence of radio halos are important as several new radio telescopes are about to begin to survey the sky at low frequencies with unprecedented sensitivity. Methods. In this paper we carry out Monte Carlo simulations to model the formation and evolution of radio halos in a cosmological framework. We extend previous works on the statistical properties of radio halos in the context of the turbulent re-acceleration model. Results. First we compute the fraction of galaxy clusters that show radio halos and derive the luminosity function of radio halos. Then, we derive differential and integrated number count distributions of radio halos at low radio frequencies with the main goal to explore the potential of the upcoming LOFAR surveys. By restricting to the case of clusters at redshifts < 0.6, we find that the planned LOFAR all sky survey at 120 MHz is expected to detect about 350 giant radio halos. About half of these halos have spectral indices larger than 1.9 and substantially brighten at lower frequencies. If detected they will allow for a confirmation that turbulence accelerates the emitting particles. We expect that also commissioning surveys, such as MS 3 , have the potential to detect about 60 radio halos in clusters of the ROSAT Brightest Cluster Sample and its extension (eBCS). These surveys will allow us to constrain how the rate of formation of radio halos in these clusters depends on cluster mass.

Astronomy & Astrophysics, 2008

In this letter we present a morphological comparison between giant radio halos and radio mini-hal... more In this letter we present a morphological comparison between giant radio halos and radio mini-halos in galaxy clusters based on radio--X-ray luminosity, P_{1.4}-L_X, and radio luminosity-size, P_{1.4}-R_H, correlations. We report evidence that P_{1.4}-L_X and P_{1.4}-R_H trends may also exist for mini--halos: mini--halo clusters share the same region of giant halo clusters in the (P_{1.4},L_X) plane, whereas they are clearly separated in the (P_{1.4},R_H) plane. The synchrotron emissivity of mini-halos is found to be more than 50 times larger than that of giant halos, implying a very efficient process for their origins. By assuming a scenario of sporadical turbulent particle re-acceleration for both giant and mini halos, we discuss basic physical differences between these sources. Regardless of the origin of the turbulence, a more efficient source of injection of particles, which eventually takes part in the re-acceleration process, is required in mini-halos, and this may result from the central radio galaxy or from proton-proton collisions in the dense cool core regions.

Monthly Notices of The Royal Astronomical Society, 2003

There is a growing evidence that extended radio halos are most likely generated by electrons reac... more There is a growing evidence that extended radio halos are most likely generated by electrons reaccelerated via some kind of turbulence generated in the cluster volume during major mergers. It is well known that Alfv\'en waves channel most of their energy flux in the acceleration of relativistic particles. Much work has been done recently to study this phenomenon and its consequences for the explanation of the observed non-thermal phenomena in clusters of galaxies. We investigate here the problem of particle-wave interactions in the most general situation in which relativistic electrons, thermal protons and relativistic protons exist within the cluster volume. The interaction of all these components with the waves, as well as the turbulent cascading and damping processes of Alfv\'en waves, are treated in a fully time-dependent way. This allows us to calculate the spectra of electrons, protons and waves at any fixed time. The {\it Lighthill} mechanism is invoked to couple the fluid turbulence, supposedly injected during cluster mergers, to MHD turbulence. We find that present observations of non-thermal radiation from clusters of galaxies are well described within this approach, provided the fraction of relativistic hadrons in the intracluster medium (ICM) is smaller than 5-10 %.

Giant radio halos are mega-parsec scale synchrotron sources detected in a fraction of massive and... more Giant radio halos are mega-parsec scale synchrotron sources detected in a fraction of massive and merging galaxy clusters. Radio halos provide one of the most important pieces of evidence for non-thermal components in large scale structure. Statistics of their properties can be used to discriminate among various models for their origin. Therefore, theoretical predictions of the occurrence of radio halos are important as several new radio telescopes are about to begin to survey the sky at low frequencies with unprecedented sensitivity. In this paper we carry out Monte Carlo simulations to model the formation and evolution of radio halos in a cosmological framework. We extend previous works on the statistical properties of radio halos in the context of the turbulent re-acceleration model. First we compute the fraction of galaxy clusters that show radio halos and derive the luminosity function of radio halos. Then, we derive differential and integrated number count distributions of radio halos at low radio frequencies with the main goal to explore the potential of the upcoming LOFAR surveys. By restricting to the case of clusters at redshifts <0.6, we find that the planned LOFAR all sky survey at 120 MHz is expected to detect about 350 giant radio halos. About half of these halos have spectral indices larger than 1.9 and substantially brighten at lower frequencies. If detected they will allow for a confirmation that turbulence accelerates the emitting particles. We expect that also commissioning surveys, such as MSSS, have the potential to detect about 60 radio halos in clusters of the ROSAT Brightest Cluster Sample and its extension (eBCS). These surveys will allow us to constrain how the rate of formation of radio halos in these clusters depends on cluster mass.

Journal of The Korean Astronomical Society, 2004

With the aim to investigate the statistical properties and the connection between thermal and non... more With the aim to investigate the statistical properties and the connection between thermal and non-thermal properties of the ICM in galaxy clusters, we have developed a statistical magneto-turbulent model which describes, at the same time, the evolution of the thermal and non-thermal emission from galaxy clusters. In particular, starting from the cosmological evolution of clusters, we follow cluster mergers, calculate the spectrum of the magnetosonic waves generated in the ICM during these mergers, the evolution of relativistic electrons and the resulting synchrotron and Inverse Compton spectra. We show that the broad band (radio and hard x-ray) non-thermal spectral properties of galaxy clusters can be well accounted for by our model for viable values of the parameters.

Nature, 2008

Clusters of galaxies are the largest gravitationally bound objects in the Universe, containing ab... more Clusters of galaxies are the largest gravitationally bound objects in the Universe, containing about 10^15 solar masses of hot (10^8 K) gas, galaxies and dark matter in a typical volume of about 10 Mpc^3. Magnetic fields and relativistic particles are mixed with the gas as revealed by giant radio haloes, which arise from diffuse, megaparsec-scale synchrotron radiation at cluster center. Radio haloes require that the emitting electrons are accelerated in situ (by turbulence), or are injected (as secondary particles) by proton collisions into the intergalactic medium. They are found only in a fraction of massive clusters that have complex dynamics, which suggests a connection between these mechanisms and cluster mergers. Here we report a radio halo at low frequencies associated with the merging cluster Abell 521. This halo has an extremely steep radio spectrum, which implies a high frequency cut-off; this makes the halo difficult to detect with observations at 1.4 GHz (the frequency at which all other known radio haloes have been best studied). The spectrum of the halo is inconsistent with a secondary origin of the relativistic electrons, but instead supports turbulent acceleration, which suggests that many radio haloes in the Universe should emit mainly at low frequencies.

Monthly Notices of The Royal Astronomical Society, 2006

We present a study of the turbulent velocity fields in the intracluster medium (ICM) of a sample ... more We present a study of the turbulent velocity fields in the intracluster medium (ICM) of a sample of 21 galaxy clusters simulated by the smoothed particle hydrodynamics code gadget2, using a new numerical scheme where the artificial viscosity is suppressed outside shocks. The turbulent motions in the ICM of our simulated clusters are detected with a novel method devised to better disentangle laminar bulk motions from chaotic ones. We focus on the scaling law between the turbulent energy content of the gas particles and the total mass, and find that the energy in the form of turbulence scales approximately with the thermal energy of clusters. We follow the evolution with time of the scaling laws and discuss the physical origin of the observed trends. The simulated data are in agreement with independent semi-analytical calculations, and the combination between the two methods allows one to constrain the scaling law over more than two decades in cluster mass.

Monthly Notices of The Royal Astronomical Society, 2006

The most important evidence of non-thermal phenomena in galaxy clusters comes from giant radio ha... more The most important evidence of non-thermal phenomena in galaxy clusters comes from giant radio haloes (GRHs), spectacular synchrotron radio sources extended over ≥Mpc scales, detected in the central regions of a growing number of massive galaxy clusters. A promising possibility to explain these sources is given by in situ stochastic reacceleration of relativistic electrons by turbulence generated in the cluster volume during merger events. Cassano and Brunetti have recently shown that the expected fraction of clusters with radio haloes and the increase of such a fraction with cluster mass can be reconciled with present observations provided that a fraction of 20–30 per cent of the turbulence in clusters is in the form of compressible modes.In this work, we extend the above-mentioned analysis by including a scaling of the magnetic field strength with cluster mass. We show that, in the framework of the reacceleration model, the observed correlations between the synchrotron radio power of a sample of 17 GRHs and the X-ray properties of the hosting clusters are consistent with, and actually predicted by a magnetic field dependence on the virial mass of the form B∝Mbv, with b≳ 0.5 and typical μG strengths of the average B intensity. The occurrence of GRHs as a function of both cluster mass and redshift is obtained: the evolution of such a probability depends on the interplay between synchrotron and inverse Compton losses in the emitting volume, and it is maximized in clusters for which the two losses are comparable.The most relevant findings are that the predicted luminosity functions of GRHs are peaked around a power P1.4 GHz∼ 1024 W Hz−1, and severely cut off at low radio powers due to the decrease of the electron reacceleration in smaller galaxy clusters, and that the occurrence of GRHs at 1.4 GHz beyond a redshift z∼ 0.7 appears to be negligible. As a related check, we also show that the predicted integral radio source counts within a limited volume (z≤ 0.2) are consistent with present observational constraints. Extending the source counts beyond z= 0.2, we estimate that the total number of GRHs to be discovered at ∼ mJy radio fluxes could be ∼100 at 1.4 GHz. Finally, the occurrence of GRHs and their number counts at 150 MHz are estimated in view of the forthcoming operation of low-frequency observatories (LOFAR, LWA) and compared with those at higher radio frequencies.

Monthly Notices of The Royal Astronomical Society, 2010

We use the results from a constrained, cosmological magnetohydrodynamic simulation of the Local U... more We use the results from a constrained, cosmological magnetohydrodynamic simulation of the Local Universe to predict the radio halo and the γ-ray flux from the Coma cluster and compare it to current observations. The simulated magnetic field within the Coma cluster is the result of turbulent amplification of the magnetic field during the build-up of the cluster. The magnetic seed field originates from starburst driven, galactic outflows. The synchrotron emission is calculated assuming a hadronic model. We follow four approaches with different distributions for the cosmic ray proton population within galaxy clusters. The radial profile of the radio halo can only be reproduced with a radially increasing energy fraction within the cosmic ray proton population, reaching >100 per cent of the thermal-energy content at ≈1 Mpc, for example the edge of the radio-emitting region. Additionally, the spectral steepening of the observed radio halo in Coma cannot be reproduced, even when accounting for the negative flux from the thermal Sunyaev–Zeldovich effect at high frequencies. Therefore, the hadronic models are disfavoured from the present analysis. The emission of γ-rays expected from our simulated Coma is still below the current observational limits (by a factor of ∼6) but would be detectable by FERMI observations in the near future.

Astrophysical Journal, 2007

Clusters of galaxies are sites of acceleration of charged particles and sources of non-thermal ra... more Clusters of galaxies are sites of acceleration of charged particles and sources of non-thermal radiation. We report on new constraints on the population of cosmic rays in the Intra Cluster Medium (ICM) obtained via radio observations of a fairly large sample of massive, X-ray luminous, galaxy clusters in the redshift interval 0.2--0.4. The bulk of the observed galaxy clusters does not show any hint of Mpc scale synchrotron radio emission at the cluster center (Radio Halo). We obtained solid upper limits to the diffuse radio emission and discuss their implications for the models for the origin of Radio Halos. Our measurements allow us to derive also a limit to the content of cosmic ray protons in the ICM. Assuming spectral indices of these protons delta =2.1-2.4 and microG level magnetic fields, as from Rotation Measures, these limits are one order of magnitude deeper than present EGRET upper limits, while they are less stringent for steeper spectra and lower magnetic fields.

Monthly Notices of The Royal Astronomical Society, 2007

In this paper we derive new expected scaling relations for clusters with giant radio halos in the... more In this paper we derive new expected scaling relations for clusters with giant radio halos in the framework of the re-acceleration scenario in a simplified, but physically motivated, form, namely: radio power (P R ) vs size of the radio emitting region (R H ), and P R vs total cluster mass (M H ) contained in the emitting region and cluster velocity dispersion (σ H ) in this region.

Astronomy & Astrophysics, 2007

Aims. We present the first results of an ongoing project devoted to the search of giant radio hal... more Aims. We present the first results of an ongoing project devoted to the search of giant radio halos in galaxy clusters located in the redshift range z=0.2--0.4. One of the main goals of our study is to measure the fraction of massive galaxy clusters in this redshift interval hosting a radio halo, and to constrain the expectations of the particle re--acceleration model for the origin of non--thermal radio emission in galaxy clusters. Methods. We selected 27 REFLEX clusters and here we present Giant Metrewave Radio Telescope (GMRT) observations at 610 MHz for 11 of them. The sensitivity (1$\sigma$) in our images is in the range 35--100~$\mu$Jy beam$^{-1}$ for all clusters. Results. We found three new radio halos, doubling the number of halos known in the selected sample. In particular, giant radio halos were found in A 209 and RXCJ 2003.5--2323, and one halo (of smaller size) was found in RXCJ 1314.4--2515. Candidate extended emission on smaller scale was found around the central galaxy in A 3444 which deserves further investigation. Furthermore, a radio relic was found in A 521, and two relics were found in RXCJ 1314.5--2515. The remaining six clusters observed do not host extended emission of any kind.

Astronomische Nachrichten, 2006

There are several possibilities to constrain the value of the magnetic field in the ICM, the most... more There are several possibilities to constrain the value of the magnetic field in the ICM, the most direct ones being the combination of inverse Compton and synchrotron observations, and the Faraday rotation measures. Here we discuss on the possibility to provide constraints on the magnetic field in the ICM from the analysis of the statistical properties of the giant radio halos, Mpc--scale diffuse radio emission in galaxy clusters. Present observations of a few well studied radio halos can be interpreted under the hypothesis that the emitting relativistic electrons are re-accelerated on their way out. By using statistical calculations carried out in the framework of the re-acceleration model we show that the observed radio--power vs cluster mass correlation in radio halos can be reproduced only by assuming mu\mumuG fields in the ICM and a scaling of the magnetic field with cluster mass BproptoMvbB \propto M_v^bBproptoMvb, with bgeq0.6b \geq 0.6bgeq0.6. We also show that the expected occurrence of radio halos with mass and redshift, and their number counts are sensitive to the magnetic field intensity in massive galaxy clusters and to the scaling of B with cluster mass. Thus future deep surveys of radio halos would provide constraints on B in galaxy clusters.

Astronomy & Astrophysics, 2009

Aims. We present a combined radio, X-ray and optical study of the galaxy cluster RXCJ 2003.5-2323... more Aims. We present a combined radio, X-ray and optical study of the galaxy cluster RXCJ 2003.5-2323. The cluster hosts one of the largest, most powerful and distant giant radio halos known to date, suggesting that it may be undergoing a strong merger process. The aim of our multiwavelength study is to investigate the radio-halo cluster merger scenario. Methods. We studied the radio properties of the giant radio halo in RXCJ 2003.5-2323 by means of new radio data obtained at 1.4 GHz with the Very Large Array, and at 240 MHz with the Giant Metrewave Radio Telescope, in combination with previously published GMRT data at 610 MHz. The dynamical state of the cluster was investigated by means of X-ray Chandra observations and optical ESO-NTT observations. Results. Our study confirms that RXCJ 2003.5-2323 is an unrelaxed cluster. The unusual filamentary and clumpy morphology of the radio halo could be due to a combination of the filamentary structure of the magnetic field and turbulence in the inital stage of a cluster merger.

Monthly Notices of The Royal Astronomical Society, 2004

There is growing evidence that extended radio haloes are most likely generated by electrons reacc... more There is growing evidence that extended radio haloes are most likely generated by electrons reaccelerated via some kind of turbulence generated in the cluster volume during major mergers. It is well known that Alfvén waves channel most of their energy flux in the acceleration of relativistic particles. Much work has been done recently to study this phenomenon and its consequences for the explanation of the observed non-thermal phenomena in clusters of galaxies. We investigate here the problem of particle–wave interactions in the most general situation in which relativistic electrons, thermal protons and relativistic protons exist within the cluster volume. The interaction of all these components with the waves, as well as the turbulent cascading and damping processes of Alfvén waves, are treated in a fully time-dependent way. This allows us to calculate the spectra of electrons, protons and waves at any fixed time. The Lighthill mechanism is invoked to couple the fluid turbulence, supposedly injected during cluster mergers, to MHD turbulence. We find that present observations of non-thermal radiation from clusters of galaxies are well described within this approach, provided the fraction of relativistic hadrons in the intracluster medium (ICM) is smaller than 5–10 per cent.

Monthly Notices of The Royal Astronomical Society, 2005

There is now firm evidence that the ICM consists of a mixture of hot plasma, magnetic fields and ... more There is now firm evidence that the ICM consists of a mixture of hot plasma, magnetic fields and relativistic particles. The most important evidences for non-thermal phenomena in galaxy clusters comes from the spectacular synchrotron radio emission diffused over Mpc scale observed in a growing number of massive clusters and, more recently, in the hard X-ray tails detected in a few cases in excess of the thermal bremmstrahlung spectrum. A promising possibility to explain giant radio halos is given by the presence of relativistic electrons reaccelerated by some kind of turbulence generated in the cluster volume during merger events. With the aim to investigate the connection between thermal and non-thermal properties of the ICM, in this paper we develope a statistical magneto-turbulent model which describes in a self-consistent way the evolution of the thermal ICM and that of the non-thermal emission from clusters. Making use of the extended Press & Schechter formalism, we follow cluster mergers and estimate the injection rate of the fluid turbulence generated during these energetic events. We then calculate the evolution of the spectrum of the relativistic electrons in the ICM during the cluster life by taking into account both the electron-acceleration due to the merger-driven turbulence and the relevant energy losses of the electrons. We end up with a synthetic population of galaxy clusters for which the evolution of the ICM and of the non-thermal spectrum emitted by the accelerated electrons is calculated. The generation of detectable non-thermal radio and hard X-ray emission in the simulated clusters is found to be possible during major merger events for reliable values of the model parameters. In addition the occurrence of radio halos as a function of the mass of the parent clusters is calculated and compared with observations. In this case it is found that the model expectations are in good agreement with observations: radio halos are found in about 30% of the more massive clusters in our synthetic population (M > ∼ 1.8 × 10 15 M ⊙ ) and in about 4% of the intermediate massive clusters (9 × 10 14 < M < 1.8 × 10 15 M ⊙ ), while the radio halo phenomenon is found to be extremely rare in the case of the smaller clusters.

Astronomy & Astrophysics, 2010

In this paper we present a detailed study of the giant radio halo in the galaxy cluster Abell 697... more In this paper we present a detailed study of the giant radio halo in the galaxy cluster Abell 697, with the aim to constrain its origin and connection with the cluster dynamics. We performed high sensitivity GMRT observations at 325 MHz, which showed that the radio halo is much brighter and larger at this frequency, compared to previous 610 MHz observations. In order to derive the integrated spectrum in the frequency range 325 MHz--1.4 GHz, we re--analysed archival VLA data at 1.4 GHz and made use of proprietary GMRT data at 610 MHz. {Our multifrequency analysis shows that the total radio spectrum of the giant radio halo in A\,697 is very steep, with alpharm3˜25MHzrm1˜.4GHzapprox1.7−1.8\alpha_{\rm~325 MHz}^{\rm~1.4 GHz} \approx 1.7-1.8alpharm3˜25MHzrm1˜.4GHzapprox1.7−1.8. %\pm0.1$. Due to energy arguments, a hadronic origin of the halo is disfavoured by such steep spectrum. Very steep spectrum halos in merging clusters are predicted in the case that the emitting electrons are accelerated by turbulence, observations with the upcoming low frequency arrays will be able to test these expectations.}

Monthly Notices of The Royal Astronomical Society, 2006

The most important evidence of non-thermal phenomena in galaxy clusters comes from giant radio ha... more The most important evidence of non-thermal phenomena in galaxy clusters comes from giant radio haloes (GRHs), spectacular synchrotron radio sources extended over Mpc scales, detected in the central regions of a growing number of massive galaxy clusters. A promising possibility to explain these sources is given by in situ stochastic reacceleration of relativistic electrons by turbulence generated in the cluster volume during merger events. Cassano and Brunetti have recently shown that the expected fraction of clusters with radio haloes and the increase of such a fraction with cluster mass can be reconciled with present observations provided that a fraction of 20-30 per cent of the turbulence in clusters is in the form of compressible modes.

Astronomy & Astrophysics, 2008

Results. We find a significant statistical evidence (at 3.7 sigma) of an increase of the fraction... more Results. We find a significant statistical evidence (at 3.7 sigma) of an increase of the fraction of clusters with Radio Halos with the X-ray luminosity (mass) of the parent clusters and show that this increase is in line with statistical calculations based on the re-acceleration scenario. We argue that a fundamental expectation of this scenario is that the probability to have radio halos emitting at hundred MHz is larger than that at GHz frequencies and thus that future radio interferometers operating at low frequencies, such as LOFAR and LWA, should detect a larger number of radio halos with respect to that caught by present GHz observations. We also show that the expected increase of the fraction of clusters with radio halos with the cluster mass as measured with future LOFAR and LWA surveys should be less strong than that in present surveys.

New Astronomy, 2006

We present high sensitivity radio observations of the merging cluster A 521, at a mean redsfhit z... more We present high sensitivity radio observations of the merging cluster A 521, at a mean redsfhit z=0.247. The observations were carried out with the GMRT at 610 MHz and cover a region of ∼1 square degree, with a sensitivity limit of 1σ = 35