arXiv subjects

Francesco Miniati

Publications and source records attributed to Francesco Miniati.

At least 55 records · Page 3Linked to original sources

arXiv2004

The 21 centimeter emission from the reionization epoch: extended and point source foregrounds

Fluctuations in the redshifted 21 centimeter emission from neutral hydrogen probe the epoch of reionization. We examine the observability of this signal and the impact of extragalactic foreground radio sources. We use cosmological simulations to predict the angular correlation functions of intensity fluctuations due to unresolved radio galaxies, cluster radio halos and relics and free-free emission from the interstellar and intergalactic medium at the frequencies and angular scales relevant for the proposed 21cm tomography. In accord with previous findings, the brightness temperature fluctuations due to foreground sources are much larger than those from the primary 21cm signal at all scales. In particular, diffuse cluster radio emission, which has been previously neglected, provides the most significant foreground contamination. However, we show that the contribution to the angular fluctuations at scales θ> 1' is dominated by the spatial clustering of bright foreground sources. This excess can be removed if sources above flux levels S > 0.1 mJy are detected and removed. Hence, efficient source removal may be sufficient to allow the detection of angular fluctuations in the 21cm emission free of extragalactic foregrounds at θ> 1 arcmin. In addition, the removal of sources above S=0.1 mJy also reduces the foreground fluctuations to roughly the same level as the 21cm signal at scales θ< 1 arcmin. This should allow the substraction of the foreground components in frequency space, making it possible to observe in detail the topology and history of reionization.

astro-ph↗

arXiv2003

Ultraviolet Background Radiation from Cosmic Structure Formation

We calculate the contribution to the ultraviolet background (UVB) from thermal emission from gas shock heated by cosmic structure formation. Our main calculation is based on an updated version of Press-Schechter theory. It is consistent with a more empirical estimate based on the observed properties of galaxies and the observed cosmic star formation history. Thermal UVB emission is characterized by a hard spectrum extending well beyond 4 Ry. The bulk of the radiation is produced by objects in the mass range 10^11-13 M_solar, i.e. large galaxies and small groups. We compute a composite UVB spectrum due to QSO, stellar and thermal components. The ratio of the UVB intensities at the H and He Lyman limits increases from 60 at z=2 to more than 300 at z=6. A comparison of the resulting photoionization rates to the observed Gunn-Peterson effect at high redshifts constrains the escape fraction of ionizing photons from galaxies to be less than a few percent. Near 1 Ry, thermal and stellar emission are comparable amounting to about 10 %, 20 % and 35 % of the total flux at redshifts of 3, 4.5 and higher, respectively. However, near the ionization threshold for HeII, the thermal contribution is much stronger. It is comparable to the QSO intensity already at redshift ~3 and dominates at redshifts above 4. Thermal photons alone are enough to produce and sustain HeII reionization already at z~6. We discuss the possible implications of our results for the thermal history of the intergalactic medium, in particular for HeII reionization.

astro-ph↗

arXiv2003

Signatures of Magnetized Large Scale Structure in Ultra-High Energy Cosmic Rays

We investigate the impact of a structured universe in the multi-pole moments, auto-correlation function, and cluster statistics of cosmic rays above 10^19 eV. We compare structured and uniform source distributions with and without magnetic fields obtained from a cosmological simulation. We find that current data marginally favor structured source distributions and magnetic fields reaching a few micro Gauss in galaxy clusters but below 0.1 micro Gauss in our local extragalactic neighborhood. A pronounced GZK cutoff is also predicted in this scenario. Future experiments will make the degree scale auto-correlation function a sensitive probe of micro Gauss fields surrounding the sources.

astro-ph↗

arXiv2003

CHANDRA/VLA Follow-up of TeV J2032+4131, the Only Unidentified TeV Gamma-ray Source

The HEGRA Cherenkov telescope array group recently reported a steady and extended unidentified TeV gamma-ray source lying at the outskirts of Cygnus OB2. This is the most massive stellar association known in the Galaxy, estimated to contain ~2600 OB type members alone. It has been previously argued that the large scale shocks and turbulence induced by the multiple interacting supersonic winds from the many young stars in such associations may play a role in accelerating Galactic cosmic rays. Indeed, Cyg OB2 also coincides with the non-variable MeV-GeV range unidentified EGRET source, 3EG 2033+4118. We report on the near-simultaneous follow-up observations of the extended TeV source region with the CHANDRA X-ray Observatory and the Very Large Array (VLA) radio telescope obtained in order to explore this possibility. Analysis of the CO, HI, and IRAS 100 micron emissions shows that the TeV source region coincides with an outlying sub-group of powerful OB stars which have evacuated or destroyed much of the ambient atomic, molecular and dust material, and which may be related to the very high-energy emissions. An interesting SNR-like structure is also revealed near the TeV source region in the CO, HI and radio emission maps. Applying a numerical simulation which accurately tracks the radio to gamma-ray emission from primary hadrons as well as primary and secondary e+/-, we find that the broadband spectrum of the TeV source region favors a predominantly nucleonic - rather than electronic - origin of the high-energy flux, though deeper X-ray and radio observations are needed to confirm this. A very reasonable, ~0.1%, conversion efficiency of Cyg OB2's extreme stellar wind mechanical luminosity to nucleonic acceleration to ~PeV (10^15 eV) energies is sufficient to explain the multifrequency emissions.

astro-ph↗

arXiv2003

Ultra-High Energy Cosmic Rays in a Structured and Magnetized Universe

We simulate propagation of cosmic ray nucleons above 10^{19} eV in scenarios where both the source distribution and magnetic fields within about 50 Mpc from us are obtained from an unconstrained large scale structure simulation. We find that consistency of predicted sky distributions with current data above 4 x 10^{19} eV requires magnetic fields of ~0.1 microGauss in our immediate environment, and a nearby source density of ~10^{-4}-10^{-3} Mpc^{-3}. Radio galaxies could provide the required sources, but only if both high and low-luminosity radio galaxies are very efficient cosmic ray accelerators. Moreover, at ~10^{19} eV an additional isotropic flux component, presumably of cosmological origin, should dominate over the local flux component by about a factor three in order to explain the observed isotropy. This argues against the scenario in which local astrophysical sources of cosmic rays above ~10^{19} eV reside in strongly magnetized (B~0.1 microGauss) and structured intergalactic medium. Finally we discuss how future large scale full-sky detectors such as the Pierre Auger project will allow to put much more stringent constraints on source and magnetic field distributions.

astro-ph↗

arXiv2003

Cosmic Ray Acceleration by Stellar Associations? The Case of Cygnus OB2

The origins of all extra-solar cosmic rays -- both the ultra high-energy and lower energy Galactic variety -- remain unclear. It has been argued that the large scale shocks and turbulence induced by the multiple interacting supersonic winds from the many hot stars in young stellar associations may play a role in accelerating Galactic cosmic rays -- with or without the associated multiple supernova remnants (eg. Cesarsky & Montmerle, 1983; Bykov 2001). In this context, the report by the HEGRA Cherenkov telescope group of a steady and extended unidentified TeV gamma-ray source lying at the outskirts of Cygnus OB2 is particularly significant. This is the most massive stellar association known in the Galaxy, estimated to contain ~2600 OB type members alone indeed, Cyg OB2 also coincides with the non-variable MeV-GeV range unidentified EGRET source, 3EG 2033+4118. We summarise here the near-simultaneous follow-up observations of the extended TeV source region with the CHANDRA X-ray Observatory and the Very Large Array (VLA) radio telescope. Applying a numerical simulation which accurately tracks the radio to gamma-ray emission from primary hadrons as well as primary and secondary e+/-, we find that the broadband spectrum of the TeV source region favors a predominantly nucleonic rather than electronic origin of the high-energy flux, though deeper X-ray and radio observations will help confirm this. A very reasonable, ~0.1%, conversion efficiency of Cyg OB2's extreme stellar wind mechanical luminosity to nucleonic acceleration to ~PeV (10^15 eV) energies is sufficient to explain the multifrequency emissions.

astro-ph↗

arXiv2003

Numerical Modeling of Gamma Radiation from Galaxy Clusters

We investigate the spatial and spectral properties of non-thermal emission from clusters of galaxies at gamma-ray energies between 10 keV and 10 TeV due to inverse-Compton (IC) emission, pion-decay and non-thermal bremsstrahlung (NTB) from cosmic-ray(CR) ions and electrons accelerated at cosmic shock and secondary e+- from inelastic p-p collisions. We identify two main emission region, namely the core (also bright in thermal X-ray) and the outskirts region where accretion shocks occur. IC emission from shock accelerated CR electrons dominate the emission in the outer regions of galaxy clusters, provided that at least a fraction of a percent of the shock ram pressure is converted into CR electrons. A clear detection of this component and of its spatial distribution will allow us direct probing of cosmic accretion shocks. In the cluster core, gamma-ray emission above 100 MeV is dominated by pion-decay mechanism and, at lower energies, by IC emission from secondary e+-. However, IC emission from shock accelerated electrons projected onto the cluster core will not be negligible. We emphasize the importance of separating these emission components for a correct interpretation of the experimental data and outline a strategy for that purpose. Failure in addressing this issue will produce unsound estimates of the intra-cluster magnetic field strength and CR ion content. According to our estimate future space borne and ground based gamma-ray facilities should be able to measure the whole nonthermal spectrum both in the cluster core and at its outskirts. The importance of such measurements in advancing our understanding of non-thermal processes in the intra-cluster medium is discussed.

astro-ph↗

arXiv2002

Investigating Galaxy Clusters through Gamma-ray Emission

We address the role of gamma-ray astronomy in the investigation of nonthermal processes in the large scale structure of the universe. Based on EGRET upper limits on nearby galaxy clusters (GCs) we constrain the acceleration efficiency of CR electrons at intergalactic shocks to <=1 % than the shock ram pressure. That implies a contribution to the cosmic gamma-ray background from intergalactic shocks of order 25 % of the measured level. We model spatial and spectral properties of nonthermal gamma-ray emission due to shock accelerated cosmic-rays (CRs) in GCs and emphasize the importance of imaging capability of upcoming gamma-ray facilities for a correct interpretation of the observational results. GC observations at this photon energy will help us understand the origin of the radio emitting particles, the possible level of CR pressure and the strength of magnetic fields in intracluster environment and possibly will allow us detect the accretion shocks.

astro-ph↗

arXiv2002

Inter-galactic Shock Acceleration and the Cosmic Gamma-ray Background

We investigate numerically the contribution to the cosmic gamma-ray background from cosmic-rays ions and electrons accelerated at intergalactic shocks associated with cosmological structure formation. We show that the kinetic energy of accretion flows in the low-red-shift inter-galactic medium is thermalized primarily through moderately strong shocks, which allow for an efficient conversion of shock ram pressure into cosmic-ray pressure. Cosmic-rays accelerated at these shocks produce a diffuse gamma-ray flux which is dominated by inverse Compton emission from electrons scattering off cosmic microwave background photons. Decay of neutral pi-mesons generated in p-p inelastic collisions of the ionic cosmic-ray component with the thermal gas contribute about 30% of the computed emission. Based on experimental upper limits on the photon flux above 100 MeV from nearby clusters we constrain the efficiency of conversion of shock ram pressure into relativistic CR electrons to < 1%. Thus, we find that cosmic-rays of cosmological origin can generate an overall significant fraction of order 20% and no more than 30% of the measured gamma-ray background.

astro-ph↗

arXiv2002

Structure Shocks as a Source of Cosmic Rays in Clusters

Shocks are a ubiquitous consequence of cosmic structure formation, and they play an essential role in heating galaxy cluster media. Virtually all of the gas in clusters has been processed by one or more shocks of at least moderate strength. These are collisionless shocks, so likely sites for diffusive shock acceleration of high energy particles. We have carried out numerical simulations of cosmic structure formation that directly include acceleration and transport of nonthermal protons, as well as primary and secondary electrons. Nonthermal emissions have also been computed from the resulting particle spatial and energy distributions. Here we outline some of our current findings, showing that nonthermal protons may contribute a significant pressure in cluster media, and that expected radio, X-ray and $γ$-ray emissions from these populations should be important cluster diagnostics.

astro-ph↗

arXiv2002

Gamma-rays from the Large Scale Structure of the Universe

Gamma-ray astronomy will play a crucial role in the investigation of nonthermal processes in the large scale structure of the universe. Particularly, galaxy clusters (GC) observations at this photon energy will help us understand the origin of radio emitting high energy particles, the possible level of cosmic-ray (CR) pressure in intracluster environment, and the strength of intracluster magnetic fields. In addition here we point out the importance of these observations for a possible detection of cluster accretion shocks and for constraining their CR acceleration efficiency. We model spatial and spectral properties of gamma-ray emission due to shock accelerated CRs in GC and emphasize the importance of imaging capability of upcoming gamma-ray facilities for a correct interpretation of observational results.

astro-ph↗

arXiv2002

Evidence for Shock Acceleration and Intergalactic Magnetic Fields in a Large-Scale Filament of Galaxies ZwCl 2341.1+0000

We report the discovery of large-scale diffuse radio emission from what appears to be a large-scale filamentary network of galaxies in the region of cluster ZwCl 2341.1+0000, and stretching over an area of at least $6 h^{-1}_{50}$ Mpc in diameter. Multicolour CCD observations yield photometric redshifts indicating that a significant fraction of the optical galaxies in this region is at a redshift of z=0.3. This is supported by spectroscopic measurements of 4 galaxies in the SDSS survey at a mean z=0.27. We present VLA images at 20 cm (NVSS) and 90 cm wavelengths, showing the detailed radio structure of the filaments. Comparison with the VLA high resolution FIRST radio survey shows that the diffuse emission is not due to known individual point sources. The diffuse radio-emission has a spectral index $α\lesssim -0.5$, and is most likely synchrotron emission from relativistic charged particles in an inter-galactic magnetic field. Furthermore, this optical/radio structure is detected in X-rays by the ROSAT all-sky survey. It has a 0.1--2.4 keV luminosity of about $10^{44}$ erg s$^{-1}$ and shows an extended highly non-relaxed morphology. These observations suggest that ZwCl 2341.1+0000 is possibly a proto-cluster of galaxies in which we are witnessing the process of structure formation. We show (both analytically and by numerical simulations) that the energetics of accretion shocks generated in forming large-scale structures are sufficient to produce enough high energy cosmic-ray (CR) electrons required to explain the observed radio emission, provided a magnetic field of strength of about 1 micro Gauss is present there.Thus it is the first evidence of cosmic-ray particle acceleration and magnetic fields occuring on a super-cluster scale. (Abridged)

astro-ph↗

arXiv2001

Cosmic Ray Electrons in Groups and Clusters of Galaxies: Primary and Secondary Populations from a Numerical Cosmological Simulation

We study the generation and distribution of high energy electrons in cosmic environment and their observational consequences by carrying out the first cosmological simulation that includes directly cosmic ray (CR) particles. Starting from cosmological initial conditions we follow the evolution of primary and secondary electrons (CRE), CR ions (CRI) and a passive magnetic field. CRIs and primary CREs are injected and accelerated at large scale structure shocks. Secondary CREs are continuously generated through inelastic p-p collisions. We include spatial transport, adiabatic expansion/compression, Coulomb collisions, bremsstrahlung, synchrotron (SE)and inverse Compton (IC) emission. We find that, from the perspective of cosmic shock energy and acceleration efficiency, the few detections of hard X-ray radiation excess could be explained in the framework of IC emission of primary CREs in clusters undergoing high accretion/merger phase. Instead, IC emission from both primary and secondary CREs accounts at most for a small fraction of the radiation excesses detected in the extreme-UV (except for the Coma cluster as reported by Bowyer et al.1999). Next, we calculate the SE after normalizing the magnetic field so that for a Coma-like cluster ^1/2~3 \muG. Our results indicate that the SE from secondary CREs reproduces several general properties of radio halos, including the recently found P_1.4GHz vs T relation, the morphology and polarization of the emitting region and, to some extent, the spectral index. Moreover, SE from primary CREs turns out sufficient to power extended regions resembling radio relics observed at the outskirts of clusters. Again we find striking resemblance between morphology, polarization and spectral index of our synthetic maps and those reported in the literature.

astro-ph↗

arXiv2001

Cosmic Ray Protons Accelerated at Cosmological Shocks and Their Impact on Groups and Clusters of Galaxies

We investigate the production of cosmic ray (CR) protons at cosmological shocks by performing, for the first time, numerical simulations of large scale structure formation that include directly the acceleration, transport and energy losses of the high energy particles. CRs are injected at shocks according to the thermal leakage model and, thereafter, accelerated to a power-law distribution as indicated by the test particle limit of the diffusive shock acceleration theory. The evolution of the CR protons accounts for losses due to adiabatic expansion/compression, Coulomb collisions and inelastic p-p scattering. Our results suggest that CR protons produced at shocks formed in association with the process of large scale structure formation could amount to a substantial fraction of the total pressure in the intra-cluster medium. Their presence should be easily revealed by GLAST through detection of gamma-ray flux from the decay of neutral pions produced in inelastic p-p collisions of such CR protons with nuclei of the intra-cluster gas. This measurement will allow a direct determination of the CR pressure contribution in the intra-cluster medium. We also find that the spatial distribution of CR is typically more irregular than that of the thermal gas because it is more influenced by the underlying distribution of shocks. This feature is reflected in the appearance of our gamma-ray synthetic images. Finally, the average CR pressure distribution appears statistically slightly more extended than the thermal pressure.

astro-ph↗

arXiv2001

COSMOCR: A Numerical Code for Cosmic Ray Studies in Computational Cosmology

We present COSMOCR, a numerical code for the investigation of cosmic ray related studies in computational cosmology. The code follows the diffusive shock acceleration, the mechanical and radiative energy losses and the spatial transport of the supra-thermal particles in cosmic environment. Primary cosmic ray electrons and ions are injected at shocks according to the thermal leakage prescription. Secondary electrons are continuously injected as a results of p-p inelastic collisions of primary cosmic ray ions and thermal background nuclei. The code consists of a conservative, finite volume method with a power-law sub-grid model in momentum space. Two slightly different schemes are implemented depending on the stiffness of the cooling terms. Comparisons of numerical results with analytical solution for a number of tests of direct interest show remarkable performance of the present code.

astro-ph↗

arXiv2000

Winds and Shocks in Galaxy Clusters: Shock Acceleration on an Intergalactic Scale

We review the possible roles of large scale shocks as particle accelerators in clusters of galaxies. Recent observational and theoretical work has suggested that high energy charged particles may constitute a substantial pressure component in clusters. If true that would alter the expected dynamical evolution of clusters and increase the dynamical masses consistent with hydrostatic equilibrium. Moderately strong shocks are probably common in clusters, through the actions of several agents. The most obvious of these agents include winds from galaxies undergoing intense episodes of starbursts, active galaxies and cosmic inflows, such as accretion and cluster mergers. We describe our own work derived from simulations of large scale structure formation, in which we have, for the first time, explicitly included passive components of high energy particles. We find, indeed that shocks associated with these large scale flows can lead to nonthermal particle pressures big enough to influence cluster dynamics. These same simulations allow us also to compute nonthermal emissions from the clusters. Here we present resulting predictions of gamma-ray fluxes.

astro-ph↗

arXiv2000

3-D MHD Numerical Simulations of Cloud-Wind Interactions

We present results from three-dimensional (3-D) numerical simulations investigating the magnetohydrodynamics of cloud-wind interactions. The initial cloud is spherical while the magnetic field is uniform and transverse to the cloud motion. A simplified analytical model that describes the magnetic energy evolution in front of the cloud is developed and compared with simulation results. In addition, it is found the interaction of the cloud with a magnetized interstellar medium (ISM) results in the formation of a highly structured magnetotail. The magnetic flux in the wake of the cloud organizes into flux ropes and a reconnection, current sheet is developed, as field lines of opposite polarity are brought close together near the symmetry axis. At the same time, magnetic pressure is strongly enhanced at the leading edge of the cloud from the stretching of the field lines that occurs there. This has an important dynamical effect on the subsequent evolution of the cloud, since some unstable modes tend to be strongly enhanced.

astro-ph↗

arXiv2000

Acceleration of CR at Large Scale Shocks and Their Cosmological Role for Structure Formation in the Universe

We investigate the dynamical importance of a newly recognized possible source of significant feedback generated during structure formation; namely cosmic ray (CR) pressure. We present evidence for the existence of numerous shocks in the hot gas of galaxy clusters (GCs). We employ for the first time an explicit numerical treatment of CR acceleration and transport in hydro simulations of structure formation. According to our results, CRs provide an important fraction of the total pressure inside GCs, up to several tenths. This was true even at high redshift (z=2), meaning that such non-thermal component could affect the evolution of structure formation.

astro-ph↗