[en] We report the Large Area Telescope on board the Fermi Gamma-ray Space Telescope detected a strong γ-ray flare on 2011 May 15 from a source identified as 4C +49.22, a flat spectrum radio quasar (FSRQ) also known as S4 1150+49. This blazar, characterized by a prominent radio–optical–X-ray jet, was in a low γ-ray activity state during the first years of Fermi observations. Simultaneous observations during the quiescent, outburst and post-flare γ-ray states were obtained by Swift, Planck and optical–IR–radio telescopes (Instituto Nacional de Astrofísica, Óptica y Electrónica, Catalina Sky Survey, Very Long Baseline Array [VLBA], Metsähovi). The flare is observed from microwave to X-ray bands with correlated variability and the Fermi, Swift and Planck data for this FSRQ show some features more typical of BL Lac objects, like the synchrotron peak in the optical band that outshines the thermal blue-bump emission, and the X-ray spectral softening. Multi-epoch VLBA observations show the ejection of a new component close in time with the GeV γ-ray flare. In conclusion, the radio-to-γ-ray spectral energy distribution is modelled and fitted successfully for the outburst and the post-flare epochs using either a single flaring blob with two emission processes (synchrotron self-Compton (SSC), and external-radiation Compton), and a two-zone model with SSC-only mechanism.

[en] The Fermi observatory is advancing our knowledge of Gamma-Ray Bursts (GRBs) through pioneering observations at high energies, covering more then 7 decades of energy with the two on-board detectors, the Large Area Telescope (LAT) and the Gamma-ray Burst Monitor (GBM). Here we report on the observation of long GRB 090217 which triggered the GBM and as been independently detected by the on-ground blind search LAT algorithm. We present the GBM and LAT observations, including a temporal profile and time-resolved spectral analysis from 8 keV up to ∼1 GeV. The time-averaged and time-resolved spectra are well reproduced by a Band model, with no substantial spectral evolution. We compare these observations to the other LAT detections of long bursts, and discuss some theoretical implications on GRB high-energy emission.

[en] The Fermi Gamma-ray Space Telescope was successfully launched on June 11, 2008 and has already opened a new era for gamma-ray astronomy. The Large Area Telescope(LAT), the main instrument on board Fermi, with its large field of view and effective area, combined with its excellent timing capabilities, presents a significant improvement in sensitivity over its predecessor EGRET. The preliminary results of the Spectral Energy Distribution Analysis performed on a sample of bright blazars are presented. We have studied the quasi-simultaneous Spectral Energy Distributions (SED) of 48 blazars, detected within the three months of the Fermi LAT Bright AGN Sample (LBAS) data taking period, combining Fermi and Swift data with radio NIR-Optical and hard-X/gamma-ray data. We have used these SEDs to characterize the peak position and intensity of both the low and the high-energy features of blazar spectra. The results have been used to derive empirical relationships that estimate the position of the two peaks from the broad-band colors, i.e. the radio to optical (α_ro) and optical to X-ray (α_ox) spectral slopes, and from the gamma-ray spectral index. Our data show that the synchrotron peak frequency is positioned between 10^12.5 and 10^14.5 Hz in FSRQs and between 10¹³ and 10¹⁷ Hz in BL Lacertae objects. We find that the gamma-ray spectral slope is strongly correlated with the synchrotron peak energy, as expected at first order in synchrotron--inverse Compton scenarios. However, simple homogeneous, one-zone, Synchrotron Self Compton (SSC) models cannot explain most of our SEDs, especially in the case of FSRQs and low energy peaked (LBL) BL Lacs. More complex models involving External Compton Radiation or multiple SSC components are required to reproduce the overall SEDs and the observed spectral variability.

[en] The Minimal Spanning Tree (MST) algorithm is a graph-theoretical method of finding clusters in a given set of points. We apply MST to gamma-ray bidimensional images and associate possible sources with the regions where photons' arrival directions clusterize. Additional filters to select clusters are included to reduce spurious detections. We present some applications to simulated GLAST data

[en] The coexistence of Planck and Fermi satellites in orbit has enabled the exploration of the connection between the (sub-)millimeter and γ-ray emission in a large sample of blazars. We find that the γ-ray emission and the (sub-)mm luminosities are correlated over five orders of magnitude, L_γ∝L_(sub-)mm. However, this correlation is not significant at some frequency bands when simultaneous observations are considered. The most significant statistical correlations, on the other hand, arise when observations are quasi-simultaneous within two months. Moreover, we find that sources with an approximate spectral turnover in the middle of the mm-wave regime are more likely to be strong γ-ray emitters. These results suggest a physical relation between the newly injected plasma components in the jet and the high levels of γ-ray emission.

[en] Blazars are a small fraction of all extragalactic sources but, unlike other objects, they are strong emitters across the entire electromagnetic spectrum. In this study we have conducted a detailed investigation of the broad-band spectral properties of the gamma-ray selected blazars of the Fermi LAT Bright AGN Sample (LBAS). By combining the accurately estimated Fermi gamma-ray spectra with Swift, radio, NIR-Optical and hard-X/gamma-ray data, collected within three months of the LBAS data taking period, we were able to assemble high-quality and quasi-simultaneous Spectral Energy Distributions (SED) for 48 LBAS blazars. Here we show the procedure for the multi wavelength analysis.

[en] Short gamma-ray bursts (GRBs), typically lasting less than 2 s, are a special class of GRBs of great interest. We report the detection by the AGILE satellite of the short GRB 090510 which shows two clearly distinct emission phases: a prompt phase lasting ∼200 ms and a second phase lasting tens of seconds. The prompt phase is relatively intense in the 0.3-10 MeV range with a spectrum characterized by a large peak/cutoff energy near 3 MeV; in this phase, no significant high-energy gamma-ray emission is detected. At the end of the prompt phase, intense gamma-ray emission above 30 MeV is detected showing a power-law time decay of the flux of the type t ^-1.3 and a broadband spectrum remarkably different from that of the prompt phase. It extends from sub-MeV to hundreds of MeV energies with a photon index α ≅ 1.5. GRB 090510 provides the first case of a short GRB with delayed gamma-ray emission. We present the timing and spectral data of GRB 090510 and briefly discuss its remarkable properties within the current models of gamma-ray emission of short GRBs.

[en] We present optical, X-ray and gamma-ray observations of GRB 111209A, observed at a redshift of z = 0.677. We show that this event was active in its prompt phase for about 25000 s, making it the longest burst ever observed. This rare event could have been detected up to z ∼ 1.4 in gamma-rays. Compared to other long gamma-ray bursts (GRBs), GRB 111209A is a clear outlier in the energy-fluence and duration plane. The high-energy prompt emission shows no sign of a strong blackbody component, the signature of a tidal disruption event, or a supernova shock breakout. Given the extreme longevity of this event, and lack of any significant observed supernova signature, we propose that GRB 111209A resulted from the core-collapse of a low-metallicity blue supergiant star. This scenario is favored because of the necessity to supply enough mass to the central engine over a duration of thousands of seconds. Hence, we suggest that GRB 111209A could have more in common with population III stellar explosions, rather than those associated with normal long GRBs.

[en] We use nine years of γ-ray data provided by the Fermi Large Area Telescope (LAT) to systematically study the light curves (LCs) of more than 2000 active galactic nuclei (AGN) included in recent Fermi-LAT catalogs. Ten different techniques are used, which are organized in an automatic periodicity-search pipeline, in order to search for evidence of periodic emission in γ rays. Understanding the processes behind this puzzling phenomenon will provide a better view about the astrophysical nature of these extragalactic sources. However, the observation of temporal patterns in γ-ray LCs of AGN is still challenging. Despite the fact that there have been efforts to characterize the temporal emission of some individual sources, a systematic search for periodicities by means of a full likelihood analysis applied to large samples of sources was missing. Our analysis finds 11 AGN, of which 9 are identified for the first time, showing periodicity at more than 4σ in at least four algorithms. These findings will help in solving questions related to the astrophysical origin of this periodic behavior.

[en] Simultaneous Swift and Fermi observations of gamma-ray bursts (GRBs) offer a unique broadband view of their afterglow emission, spanning more than 10 decades in energy. We present the sample of X-ray flares observed by both Swift and Fermi during the first three years of Fermi operations. While bright in the X-ray band, X-ray flares are often undetected at lower (optical), and higher (MeV to GeV) energies. We show that this disfavors synchrotron self-Compton processes as the origin of the observed X-ray emission. We compare the broadband properties of X-ray flares with the standard late internal shock model, and find that in this scenario, X-ray flares can be produced by a late-time relativistic (Γ > 50) outflow at radii R ∼ 10"1"3-10"1"4 cm. This conclusion holds only if the variability timescale is significantly shorter than the observed flare duration, and implies that X-ray flares can directly probe the activity of the GRB central engine