Blazars are radio-loud active galactic nuclei well known for their non thermal emission spanning a wide range of frequencies. The Roma-BZCAT is, to date, the most comprehensive list of these sources. We performed the cross-match of several catalogs o
btained from recent surveys at different frequencies to search for new blazars. We cross-matched the 1$^{st}$ Swift-XRT Point Source catalog with the spectroscopic sample of the 9$^{th}$ Data Release of the Sloan Digital Sky Survey. Then, we performed further cross-matches with the catalogs corresponding to the Faint Images of the Radio Sky at Twenty cm survey and to the AllWISE Data release, focusing on sources with infrared colors similar to those of confirmed $gamma$-ray blazars included in the Second Fermi-LAT catalog. As a result, we obtained a preliminary list of objects with all the elements needed for a proper blazar classification according to the prescriptions of the Roma-BZCAT. We carefully investigated additional properties such as their morphology and the slope of their spectral energy distribution in the radio domain, the features shown in their optical spectrum, and the luminosity in the soft X rays to exclude generic active galactic nuclei and focus on authentic blazar-like sources. At the end of our screening we obtained a list of 15 objects with firmly established blazar properties.
Long-duration Gamma-Ray Bursts (GRBs) are an extremely rare outcome of the collapse of massive stars, and are typically found in the distant Universe. Because of its intrinsic luminosity ($Lsim 3 times 10^{53}$ erg s$^{-1}$) and its relative proximit
y ($z=0.34$), GRB 130427A was a unique event that reached the highest fluence observed in the gamma-ray band. Here we present a comprehensive multiwavelength view of GRB 130427A with Swift, the 2-m Liverpool and Faulkes telescopes and by other ground-based facilities, highlighting the evolution of the burst emission from the prompt to the afterglow phase. The properties of GRB 130427A are similar to those of the most luminous, high-redshift GRBs, suggesting that a common central engine is responsible for producing GRBs in both the contemporary and the early Universe and over the full range of GRB isotropic energies.
We searched for gamma-ray blazar candidates among the 382 unidentified hard X-ray sources of the 3rd Palermo BAT Catalog (3PBC) obtained from the analysis of 66 months of SWIFT-BAT survey data and listing 1586 sources. We adopted a recently developed
association method based on the peculiar infrared colors which characterize the gamma-ray blazars included in the second catalog of active galactic nuclei detected by the Fermi Large Area Telescope (2LAC). We used this method exploiting the data of the all-sky survey performed by the Wide-Field Infrared Survey Explorer (WISE) to establish correspondences between unidentified 3PBC sources and WISE gamma-ray blazar candidates located within the BAT positional uncertainty region at 99% confidence level. We obtained a preliminary list of candidates for which we analysed all the available data in the SWIFT archive to complement the information in the literature and in the radio, infrared and optical catalogs with the information on their optical-UV and soft X-ray emission. Requiring the presence of radio and soft X-ray counterparts consistent with the infrared positions of the selected WISE sources, as well as a blazar-like radio morphology, we finally obtained a list of 24 gamma-ray blazar candidates.
We discuss the feasibility of the detection of the 21cm forest in the diffuse IGM with the radio telescope LOFAR. The optical depth to the 21cm line has been derived using simulations of reionization which include detailed radiative transfer of ioniz
ing photons. We find that the spectra from reionization models with similar total comoving hydrogen ionizing emissivity but different frequency distribution look remarkably similar. Thus, unless the reionization histories are very different from each other (e.g. a predominance of UV vs. x-ray heating) we do not expect to distinguish them by means of observations of the 21cm forest. Because the presence of a strong x-ray background would make the detection of 21cm line absorption impossible, the lack of absorption could be used as a probe of the presence/intensity of the x-ray background and the thermal history of the universe. Along a random line of sight LOFAR could detect a global suppression of the spectrum from z>12, when the IGM is still mostly neutral and cold, in contrast with the more well-defined, albeit broad, absorption features visible at lower redshift. Sharp, strong absorption features associated with rare, high density pockets of gas could be detected also at z~7 along preferential lines of sight.
We investigate the effect of helium on hydrogen reionisation using a hydrodynamical simulation combined with the cosmological radiative transfer code CRASH. The simulations are run in a 35.12/h comoving Mpc box using a variety of assumptions for the
amplitude and power-law extreme-UV (EUV) spectral index, alpha, of the ionising emissivity. We use an empirically motivated prescription for ionising sources which ensures all of the models are consistent with constraints on the Thomson scattering optical depth and the hydrogen photo-ionisation rate at z=6. The inclusion of helium slightly delays reionisation due to the small number of ionising photons which reionise neutral helium instead of hydrogen. However, helium has a significant impact on the thermal state of the IGM. Models with alpha=3 produce IGM temperatures at the mean density at z=6 which are about 20 % higher compared to models without helium photo-heating. Harder EUV indices produce even larger IGM temperature boosts. A comparison to recent observational estimates of the IGM temperature at z=5 - 6 suggests that hydrogen reionisation was primarily driven by pop-II stellar sources with a soft EUV index, alpha<3. We also find that faint, as yet undetected galaxies, characterised by a luminosity function with a steepening faint-end slope and an increasing Lyman continuum escape fraction (fesc=0.5), are required to reproduce the ionising emissivity used in our simulations at z>6. Finally, we note there is some tension between recent observational constraints which indicate the IGM is > 10% neutral by volume z=7, and estimates of the ionising emissivity at z=6 which indicate only between 1 and 3 ionising photons are emitted per hydrogen atom over a Hubble time. This tension may be alleviated by either a lower neutral fraction at z=7 or an IGM which still remains a few % neutral by volume at z=6.
We present the results of our analysis devoted to the research of sources emitting in the energy bands surveyed by both the Swift-BAT and the Fermi-LAT telescopes. We cross-correlate the Fermi-LAT 1-year point source catalogue (1FGL) of {gamma}-ray s
ources and the second Palermo BAT catalogue (2PBC) of hard X-ray sources, establishing a correspondence between sources when their error boxes overlap. We also extract the significance value in the BAT 15-150 keV map, obtained using a dedicated software for the reduction of BAT data, in the direction of the 1FGL sources and take into account those above the significance threshold {sigma} = 3. We obtain a sample of common sources emitting in both the hard X- and the {gamma}-ray energy bands and evaluate its content in galactic and extragalactic objects. We assess the fraction of unidentified sources and describe in greater detail the properties of two of them, 1FGL J0137.8+5814 and 1FGL J2056.7+4938, supporting their classification as blazars after the analysis of their broad-band spectral energy distribution. We discuss the blazar content of the collected 1FGL-2PBC sources: we build its redshift distibution and compare it with that of the whole blazar population as reported in the second edition of the BZCAT blazar catalogue.
We propose the Apparent Shrinking Criterion (ASC) to interpret the spatial extent, R_w, of transmitted flux windows in the absorption spectra of high-z quasars. The ASC can discriminate between the two regimes in which R_w corresponds either to the p
hysical size, R_HII, of the quasar HII region, or to the distance, R^{max}_w, at which the transmitted flux drops to =0.1 and a Gunn-Peterson (GP) trough appears. In the first case (HR regime), one can determine the IGM mean HI fraction, x_HI; in the second (PR regime), the value of R_w allows to measure the local photoionization rate and the local enhancement of the photoionization rate, Gamma_G, due to nearby/intervening galaxies. The ASC has been tested against radiative transfer+SPH numerical simulations, and applied to 15 high-z (z>5.8) quasars sample from Fan et al. (2006). All sample quasars are found to be in the PR regime; hence, their observed spectral properties (inner flux profile, extent of transmission window) cannot reliably constrain the value of x_HI. Four sample quasars show evidence for a local enhancement (up to 50%) in the local photoionization rate possibly produced by a galaxy overdensity. We discuss the possible interpretations and uncertainties of this result.
In this paper we report on the improvements implemented in the cosmological radiative transfer code CRASH. In particular we present a new multi-frequency algorithm for spectra sampling which makes use of colored photon packets: we discuss the need fo
r the multi-frequency approach, describe its implementation and present the improved CRASH performance in reproducing the effects of ionizing radiation with an arbitrary spectrum. We further discuss minor changes in the code implementation which allow for more efficient performance and an increased precision.
In this paper we present CRASH_alpha, the first radiative transfer code for cosmological application that follows the parallel propagation of Ly_alpha and ionizing photons. CRASH_alpha is a version of the continuum radiative transfer code CRASH with
a new algorithm to follow the propagation of Ly_alpha photons through a gas configuration whose ionization structure is evolving. The implementation introduces the time evolution for Ly_alpha photons (a feature commonly neglected in line radiative transfer codes) and, to reduce the computational time needed to follow each scattering, adopts a statistical approach to the Ly_alpha treatment by making extensive use of pre-compiled tables. With this statistical approach we experience a drastic increase of the computational speed and, at the same time, an excellent agreement with the full Ly_alpha radiative transfer computations of the code MCLy_alpha. We find that the emerging spectra keep memory of the ionization history which generates a given ionization configuration of the gas and, to properly account for this effect, a self-consistent joint evolution of line and ionizing continuum radiation as implemented in CRASH_alpha is necessary. A comparison between the results from our code and from Ly_alpha scattering alone on a fixed HI density field shows that the extent of the difference between the emerging spectra depends on the particular configuration considered, but it can be substantial and can thus affect the physical interpretation of the problem at hand. These differences should furthermore be taken into account when computing the impact of the Ly_alpha radiation on e.g. the observability of the 21 cm line from neutral hydrogen at epochs preceeding complete reionization.
We studied the temporal and spectral evolution of the synchrotron emission from the high energy peaked BL Lac object 1E 1207.9+3945. Two recent observations have been performed by the XMM-Newton and Swift satellites; we carried out X-ray spectral ana
lysis for both of them, and photometry in optical-ultraviolet filters for the Swift one. Combining the results thus obtained with archival data we built the long-term X-ray light curve, spanning a time interval of 26 years, and the Spectral Energy Distribution (SED) of this source. The light curve shows a large flux increasing, about a factor of six, in a time interval of a few years. After reaching its maximum in coincidence with the XMM-Newton pointing in December 2000 the flux decreased in later years, as revealed by Swift. The very good statistics available in the 0.5-10 keV XMM-Newton X-ray spectrum points out a highly significant deviation from a single power law. A log-parabolic model with a best fit curvature parameter of 0.25 and a peak energy at ~1 keV describes well the spectral shape of the synchrotron emission. The simultaneous fit of Swift UVOT and XRT data provides a milder curvature (b~0.1) and a peak at higher energies (~15 keV), suggesting a different state of source activity. In both cases UVOT data support the scenario of a single synchrotron emission component extending from the optical/UV to the X-ray band. New X-ray observations are important to monitor the temporal and spectral evolution of the source; new generation gamma-ray telescopes like AGILE and GLAST could for the first time detect its inverse Compton emission.
