15836
Hard X-Ray Emission from the Galaxy Cluster A2256.
After the positive detection by BeppoSAX of hard X-ray radiation up to approximately 80 keV in the Coma Cluster spectrum, we present evidence for nonthermal emission from A2256 in excess of thermal emission at a 4.6 sigma confidence level. In addition to this power-law component, a second nonthermal component already detected by ASCA could be present in the X-ray spectrum of the cluster, which is not surprising given the complex radio morphology of the cluster central region. The spectral index of the hard tail detected by the Phoswich Detection System on board BeppoSAX is marginally consistent with that expected for the inverse Compton model. A value of approximately 0.05 µG is derived for the intracluster magnetic field of the extended radio emission in the northern regions of the cluster, while a higher value of approximately 0.5 µG could be present in the central radio halo, which is likely related to the hard tail detected by ASCA.
17366780
The magnetic field in the coma cluster
The polarization data of the radio galaxy NGC4869, belonging to the Coma cluster and located in its central region, allow us to obtain information on the structure of magnetic field associated with the cluster itself. A magnetic field of $\sim$ 8.5 $\mu$G, tangled on scales of the order of less than 1 kpc, is required to explain the observed fluctuations of the rotation measure. This magnetic field is more than one order of magnitude stronger than the equipartition value obtained for Coma C. This implies that the halo source Coma C may not be at the equipartition. Moreover, the need of efficient reacceleration mechanisms for the electrons radiating in Coma C is stronger. The energy supply to the Coma C radiating electrons is probably provided by the cluster merger process.
15779112
The giant radio galaxy 8C0821+695 and its environment
We present new VLA and Effelsberg observations of the radio galaxy 8C0821+695. We have obtained detailed images in total intensity and polarization of this 2 Mpc sized giant. The magnetic field has a configuration predominantly parallel to the source main axis. We observe Faraday rotation at low frequencies, most probably produced by an ionized medium external to the radio source. The spectral index distribution is that typical of FR II radio galaxies, with spectral indices gradually steepening from the source extremes towards the core. Modeling the spectrum in the lobes using standard synchrotron loss models yields the spectral age of the source and the mean velocity of the jet-head with respect to the lobe material. The existence of a possible backflow in the lobe is considered to relate spectral with dynamical determinations of the age and the velocity with respect to the external medium. Through a very simple model, we obtain a physical characterization of the jets and the external medium in which the radio galaxy expands. The results in 8C0821+695 are consistent with a relativistic jet nourishing the lobes which expand in a hot, low density halo. We infer a deceleration of the source expansion velocity which we explain through a progressive increase in the hot-spot size.
119258617
Exploring the central sub-pc region of the \gamma-ray bright radio galaxy 3C 84 with the VLBA at 43 GHz in the period of 2002-2008
Following the discovery of a new radio component right before the GeV \gamma-ray detection since 2008 August by Fermi Gamma-ray Space Telescope, we present a detailed study of the kinematics and lightcurve on the central sub-pc scale of 3C 84 using the archival VLBA 43-GHz data covering the period between 2002 January to 2008 November. We find that the new component "C3", previously reported by the observations with the VLBI Exploration of Radio Astrometry (VERA), was already formed in 2003. The flux density of C3 increases moderately until 2008, and then it becomes brighter rapidly after 2008. The radio core, C1, also shows a similar trend. The apparent speed of C3 with reference to the core C1 shows moderate acceleration from 0.10c to 0.47c between 2003 November to 2008 November, but is still sub-relativistic. We further try to fit the observed broadband spectrum by the one-zone synchrotron self-Compton (SSC) model using the measured apparent speed of C3. The fit can reproduce the observed \gamma-ray emission, but does not agree with the observed radio spectral index between 22 and 43 GHz.
55923178
Deep Chandra observation of the galaxy cluster WARPJ1415.1+3612 at z =1 - An evolved cool-core cluster at high redshift
Using the deepest (370 ksec) Chandra observation of a high-redshift galaxy cluster, we perform a detailed characterization of the intra-cluster medium (ICM) of WARPJ1415.1+3612 at z=1.03. We also explore the connection between the ICM core properties and the radio/optical properties of the brightest cluster galaxy (BCG). We perform a spatially resolved analysis of the ICM to obtain temperature, metallicity and surface brightness profiles. Using the deprojected temperature and density profiles we accurately derive the cluster mass at different overdensities. In addition to the X-ray data, we use archival radio VLA imaging and optical GMOS spectroscopy of the central galaxy to investigate the feedback between the central galaxy and the ICM. The X-ray spectral analysis shows a significant temperature drop towards the cluster center, with a projected value of Tc = 4.6 \pm 0.4 keV, and a remarkably high central iron abundance peak, Zc= 3.6 Zsun. The central cooling time is shorter than 0.1 Gyr and the entropy is equal to 9.9 keV cm2. We detect a strong [OII] emission line in the optical spectra of the BCG with an equivalent width of -25 \AA, for which we derive a star formation rate within the range 2 - 8 Msun/yr. The VLA data reveals a central radio source coincident with the BCG and a faint one-sided jet-like feature with an extent of 80 kpc. The analysis presented shows that WARPJ1415 has a well developed cool core with ICM properties similar to those found in the local Universe. Its properties and the clear sign of feedback activity found in the central galaxy in the optical and radio bands, show that feedback processes are already established at z~1. In addition, the presence of a strong metallicity peak shows that the central regions have been promptly enriched by star formation processes in the central galaxy already at z > 1.
15317243
Electron ageing and polarization in tailed radio galaxies
High-frequency observations of the tailed radio galaxies IC 310, NGC 1265, 3C 129, and 3C 465 have been performed with the Effelsberg 100-m telescope. For the radio galaxies IC 310, NGC 1265 and 3C 465, radio data obtained at low frequencies with the Westerbork Synthesis Radio Telescope are also available. These new radio data allow us to map the extended structure of the radio galaxies and obtain spectral and polarization information in the outermost regions. The multi-frequency spectra were used to study the synchrotron ageing of relativistic electrons with increasing distance from the active nucleus. We found that the spectrum in each radio galaxy progressively steepens with distance, and at each location it is steeper at high frequencies. The spectra are fitted by models involving synchrotron energy losses and the critical frequency is obtained at increasing distance from the core. Assuming that the magnetic field is the equipartition value, we obtain the radiating electron lifetimes and consequently their drift velocities. Our results imply the existence of reacceleration processes or bulk motions along the tails. The polarization data at 10.6 GHz give information on the intrinsic degree of polarized flux and the orientation of the magnetic field. We find that the polarization percentage increases along the tails, reflecting an increase of the degree of ordering of the magnetic field. The magnetic field in the tails is longitudinal.
14486067
Primordial magnetic fields from string cosmology.
Sufficiently large seeds for generating the observed (inter)galactic magnetic fields emerge naturally in string cosmology from the amplification of electromagnetic vacuum fluctuations due to a dynamical dilaton background. The success of the mechanism depends crucially on two features of the so-called pre-big-bang scenario, an early epoch of dilaton-driven inflation at very small coupling, and a sufficiently long intermediate stringy era preceding the standard radiation-dominated evolution.
6344337
Squeezed thermal vacuum and the maximum scale for inflation.
We consider the stimulated emission of gravitons from an initial state of thermal equilibrium, under the action of the cosmic gravitational background field. We find that the low-energy graviton spectrum is enhanced if compared with spontaneous creation from the vacuum; as a consequence, the scale of inflation must be lowered, in order not to exceed the observed CMB quadrupole anisotropy. This effect is particularly important for models based on a symmetry-breaking transition which require, as initial condition, a state of thermal equilibrium at temperatures of the order of the inflation scale.
14851892
Metric perturbations in dilaton-driven inflation.
We compute the spectrum of scalar and tensor metric perturbations generated, as amplified vacuum fluctuations, during an epoch of dilaton-driven inflation of the type occurring naturally in string cosmology. In the tensor case the computation is straightforward while, in the scalar case, it is made delicate by the appearance of a growing mode in the familiar longitudinal gauge. In spite of this, a reliable perturbative calculation of perturbations far outside the horizon can be performed by resorting either to appropriate gauge invariant variables, or to a new coordinate system in which the growing mode can be “gauged down”. The simple outcome of this complicated analysis is that both scalar and tensor perturbations exhibit nearly Planckian spectra, whose common “temperature” is related to some very basic parameters of the string-cosmology background.
12975064
Dilaton contributions to the cosmic gravitational wave background.
We consider the cosmological amplification of a metric perturbation propagating in a higher-dimensional Brans-Dicke background, including a nontrivial dilaton evolution. We discuss the properties of the spectral energy density of the produced gravitons (as well as of the associated squeezing parameter), and we show that the present observational bounds on the graviton spectrum provide significant information on the dynamical evolution of the early Universe.