A. Antonelli

Intrinsic spectra and energetics of BeppoSAX Gamma-Ray Bursts with known redshifts

We present the main results of a study of spectral and energetics properties of twelve gamma-ray bursts (GRBs) with redshift estimates. All GRBs in our sample were detected by BeppoSAX in a broad energy range (2-700 keV). From the redshift estimates and the good-quality BeppoSAX time-integrated spectra we deduce the main properties of GRBs in their cosmological rest frames. All spectra in our sample are satisfactorily represented by the Band model, with no significant soft X-ray excesses or spectral absorptions. We find a positive correlation between the estimated total (isotropic) energies in the 1-10 000 keV energy range (Erad) and redshifts z. Interestingly, more luminous GRBs are characterized also by larger peak energies Ep so f theirEF(E) spectra. Furthermore, more distant GRBs appear to be systematically harder in the X-ray band compared to GRBs with lower redshifts. We discuss how selection and data truncation eects could bias our results and give possible explanations for the correlations that we found.

Extreme particle acceleration in the microquasar Cygnus X-3

Super-massive black holes in active galaxies can accelerate particles to relativistic energies, producing jets with associated γ-ray emission. Galactic ‘microquasars’, which are binary systems consisting of a neutron star or stellar-mass black hole accreting gas from a companion star, also produce relativistic jets, generally together with radio flares. Apart from an isolated event detected in Cygnus X-1, there has hitherto been no systematic evidence for the acceleration of particles to gigaelectronvolt or higher energies in a microquasar, with the consequence that we are as yet unsure about the mechanism of jet energization. Here we report four γ-ray flares with energies above 100 MeV from the microquasar Cygnus X-3 (an exceptional X-ray binary that sporadically produces radio jets). There is a clear pattern of temporal correlations between the γ-ray flares and transitional spectral states of the radio-frequency and X-ray emission. Particle acceleration occurred a few days before radio-jet ejections for two of the four flares, meaning that the process of jet formation implies the production of very energetic particles. In Cygnus X-3, particle energies during the flares can be thousands of times higher than during quiescent states.

Discovery of extreme particle acceleration in the microquasar Cygnus X-3

Super-massive black holes in active galaxies can accelerate particles to relativistic energies, producing jets with associated γ-ray emission. Galactic ‘microquasars’, which are binary systems consisting of a neutron star or stellar-mass black hole accreting gas from a companion star, also produce relativistic jets, generally together with radio flares. Apart from an isolated event detected in Cygnus X-1, there has hitherto been no systematic evidence for the acceleration of particles to gigaelectronvolt or higher energies in a microquasar, with the consequence that we are as yet unsure about the mechanism of jet energization. Here we report four γ-ray flares with energies above 100 MeV from the microquasar Cygnus X-3 (an exceptional X-ray binary that sporadically produces radio jets). There is a clear pattern of temporal correlations between the γ-ray flares and transitional spectral states of the radio-frequency and X-ray emission. Particle acceleration occurred a few days before radio-jet ejections for two of the four flares, meaning that the process of jet formation implies the production of very energetic particles. In Cygnus X-3, particle energies during the flares can be thousands of times higher than during quiescent states.

First AGILE catalog of high-confidence gamma-ray sources

We present the first catalog of high-confidence γ-ray sources detected by the AGILE satellite during observations performed from July 9, 2007 to June 30, 2008. Cataloged sources were detected by merging all the available data over the entire time period. AGILE, launched in April 2007, is an ASI mission devoted to γ-ray observations in the 30 MeV–50 GeV energy range, with simultaneous X-ray imaging capability in the 18–60 keV band. This catalog is based on Gamma-Ray Imaging Detector (GRID) data for energies greater than 100 MeV. For the first AGILE catalog, we adopted a conservative analysis, with a high-quality event filter optimized to select γ-ray events within the central zone of the instrument field of view (radius of 40 ◦ ). This is a significance-limited (4σ) catalog, and it is not a complete flux-limited sample due to the non-uniform first-year AGILE sky coverage. The catalog includes 47 sources, 21 of which are associated with confirmed or candidate pulsars, 13 with blazars (7 FSRQ, 4 BL Lacs, 2 unknown type), 2 with HMXRBs, 2 with SNRs, 1 with a colliding-wind binary system, and 8 with unidentified sources.

Evidence for supernova-synthesized dust from the rising afterglow of GRB 071025 at z∼ 5

We present observations and analysis of the broad-band afterglow of Swift GRB 071025. Using optical and infrared (RIYJHK) photometry, we derive a photometric redshift of 4.4 < z < 5.2; at this redshift our simultaneous multicolour observations begin at ∼30 s after the gamma-ray burst trigger in the host frame, during the initial rising phase of the afterglow. We associate the light-curve peak at ∼580 s in the observer frame with the formation of the forward shock, giving an estimate of the initial Lorentz factor � 0 ∼ 200. The red spectral energy distribution (even in regions not affected by the Lyman α break) provides secure evidence of a large dust column. However, the inferred extinction curve shows a prominent flat component between 2000 and 3000 A in the rest frame, inconsistent with any locally observed template but well fitted by models of dust formed by supernovae. Time-dependent fits to the extinction profile reveal no evidence of dust destruction and limit the decrease in

GRB 081029: a gamma-ray burst with a multi-component afterglow

We present an analysis of the unusual optical light curve of the gamma-ray burst GRB 081029, a long-soft burst with a redshift of z = 3.8479. We combine X-ray and optical observations from the Swift X-Ray Telescope and the Swift UltraViolet/Optical Telescope with ground-based optical and infrared data obtained using the REM, ROTSE, and CTIO 1.3 m telescopes to construct a detailed data set extending from 86 s to similar to 100000 s after the BAT trigger. Our data cover a wide energy range from 10 keV to 0.77 eV (1.24 angstrom-16000 angstrom). The X-ray afterglow shows a shallow initial decay followed by a rapid decay starting at about 18000 s. The optical and infrared afterglow, however, shows an uncharacteristic rise at about 3000 s that does not correspond to any feature in the X-ray light curve. Our data are not consistent with synchrotron radiation from a jet interacting with an external medium, a two-component jet, or continuous energy injection from the central engine. We find that the optical light curves can be broadly explained by a collision between two ejecta shells within a two-component jet. A growing number of gamma-ray-burst afterglows are consistent with complex jets, which suggests that some (or all) gamma-ray-burst jets are complex and will require detailed modeling to fully understand them.

Detection of Gamma-Ray Emission from the Vela Pulsar Wind Nebula with AGILE

Pulsar Wind Nebula Pulsar wind nebulae are the result of the interaction between the wind of relativistic charged particles emitted by pulsars and their surrounding interstellar medium. Using the AGILE satellite, Pellizzoni et al. (p. 663, published online 31 December) detected extended gamma-ray emission between 100 megaelectron volts and 3 gigaelectron volts from the 10,000-year-old Vela pulsar wind nebula. This detection sets constraints on the relativistic particle wind of pulsars, its energetic content, and its interactions with the surrounding medium. It also suggests that pulsar wind nebulae could form a subset of the yet unidentified galactic gamma-ray sources. Pulsar wind nebulae could account for some of the yet unidentified Galactic gamma-ray sources. Pulsars are known to power winds of relativistic particles that can produce bright nebulae by interacting with the surrounding medium. These pulsar wind nebulae are observed by their radio, optical, and x-ray emissions, and in some cases also at TeV (teraelectron volt) energies, but the lack of information in the gamma-ray band precludes drawing a comprehensive multiwavelength picture of their phenomenology and emission mechanisms. Using data from the AGILE satellite, we detected the Vela pulsar wind nebula in the energy range from 100 MeV to 3 GeV. This result constrains the particle population responsible for the GeV emission and establishes a class of gamma-ray emitters that could account for a fraction of the unidentified galactic gamma-ray sources.

BeppoSAX Observations of the Maser Seyfert 2 Galaxy ESO 103‐G35

We have made BeppoSAX observations of the Seyfert 2/1.9 galaxy ESO 103-G35, which contains a nuclear maser source and is known to be heavily absorbed in the X-rays. Analysis of the X-ray spectra observed by BeppoSAX in 1996 October and 1997 October yields a spectral index αE = 0.74 ± 0.07 (Fν ∝ ν), which is typical of Seyfert galaxies and consistent with earlier observations of this source. The strong, soft X-ray absorption has a column density NH of 1.79 ± 0.09 × 1023 cm-2, again consistent with earlier results. The best-fitting spectrum is that of a power law with a high-energy cutoff at 29 ± 10 keV, a cold (E = 6.3 ± 0.1 keV, rest frame), marginally resolved (σ = 0.35 ± 0.14 keV, FWHM ~ 31 ± 12 × 103 km s-1) Fe Kα line with EW 290 eV (1996), and a mildly ionized Fe K edge at 7.37 keV, τ = 0.24. The Kα line and cold absorption are consistent with origin in an accretion disk/torus through which our line of sight passes at a radial distance of ~50 pc. The Fe K edge is mildly ionized, suggesting the presence of ionized gas, probably in the inner accretion disk close to the central source or in a separate warm absorber. The data quality is too low to distinguish between these possibilities, but the edge-on geometry implied by the water maser emission favors the former. Comparison with earlier observations of ESO 103-G35 shows little or no change in spectral parameters while the flux changes by factors of a few on timescales of a few months. The 2-10 keV flux decreased by a factor of ~2.7 between 1996 October and 1997 October with no detectable change in the count rate greater than 20 keV (i.e., the Phoswich Detector System data). Spectral fits to the combined data sets indicate either a significant hardening of the spectrum (αE ~ 0.5) or an approximately constant or delayed response reflection component. The high-energy cutoff (29 ± 10 keV) is lower than the typical ~300 keV values seen in Seyfert galaxies. A significant subset of similar sources would affect current models of the active galactic nucleus contribution to the cosmic X-ray background which generally assume a high-energy cutoff of ~300 keV.

Monitoring the hard X-ray sky with SuperAGILE

Context SuperAGILE is the hard X-ray monitor of the AGILE gamma ray mission, in orbit since 23 April 2007. It is an imaging experiment based on a set of four independent silicon strip detectors, eq ...

The X-ray afterglow of GRB 081109A: clue to the wind bubble structure

We present the prompt Burst Alert Telescope and afterglow X-ray Telescope data of Swift-discovered GRB 081109A up to similar to 5 x 105 s after the trigger, and the early ground-based optical followups. The temporal and spectral indices of the X-ray afterglow emission change remarkably. We interpret this as the gamma-ray burst jet first traversing the freely expanding supersonic stellar wind of the progenitor with density varying as proportional to r-2. Then, after approximately 300 s the jet traverses into a region of apparent constant density similar to that expected in the stalled-wind region of a stellar wind bubble or the interstellar medium. The optical afterglow data are generally consistent with such a scenario. Our best numerical model has a wind density parameter A(*) similar to 0.02, a density of the stalled wind n similar to 0.12 cm-3 and a transition radius similar to 4.5 x 1017 cm. Such a transition radius is smaller than that predicted by numerical simulations of the stellar wind bubbles and may be due to a rapidly evolving wind of the progenitor close to the time of its core-collapse.