Sara Cutini

AGILE DETECTION OF A STRONG GAMMA-RAY FLARE FROM THE BLAZAR 3C 454.3

We report the first blazar detection by AGILE. AGILE detected 3C 454.3 during a period of strongly enhanced optical emission in 2007 July. AGILE observed the source with a dedicated repointing during the period 2007 July 24–30 with its two co-aligned imagers, the Gamma-Ray Imaging Detector and the hard X-ray imager SuperAGILE sensitive in the 30 MeV to 50 GeV and 18–60 keV ranges, respectively. Over the entire period, AGILE detected g-ray emission from 3C 454.3 at a significance level of 13.8 j with an average flux ( MeV) of E 1 100 photons cm s . The g-ray flux appears to be variable toward the end of the observation. 8 2 1 (280 40) # 10 No emission was detected by Super-AGILE in the energy range 20–60 keV, with a 3 j upper limit of 2.3 # photons cm s . The g-ray flux level of 3C 454.3 detected by AGILE is the highest ever detected for this 3 2 1 10 quasar and among the most intense g-ray fluxes ever detected from flat-spectrum radio quasars. Subject headings: gamma rays: observations — quasars: individual (3C 454.3)

Gamma-ray burst detection with the AGILE mini-calorimeter

Context. The mini-calorimeter (MCAL) instrument on-board the AGILE satellite is a non-imaging gamma-ray scintillation detector sensitive in the 300 keV–100 MeV energy range with a total on-axis geometrical area of 1400 cm 2 . Gamma-ray bursts (GRBs) are one of the main scientific targets of the AGILE mission and the MCAL design as an independent self-triggering detector makes it a valuable all-sky monitor for GRBs. Furthermore MCAL is one of the very few operative instruments with microsecond timing capabilities in the MeV range. Aims. In this paper the results of GRB detections with MCAL after one year of operation in space are presented and discussed. Methods. A flexible trigger logic implemented in the AGILE payload data-handling unit allows the on-board detection of GRBs. For triggered events, energy and timing information are sent to telemetry on a photon-by-photon basis, so that energy and time binning are limited by counting statistics only. When the trigger logic is not active, GRBs can be detected offline in ratemeter data, although with worse energy and time resolution. Results. Between the end of June 2007 and June 2008 MCAL detected 51 GRBs, with a detection rate of about 1 GRB/week, plus several other events at a few milliseconds timescales. Since February 2008 the on-board trigger logic has been fully active. Comparison of MCAL detected events and data provided by other space instruments confirms the sensitivity and effective area estimations. MCAL also joined the 3rd Inter-Planetary Network, to contribute to GRB localization by means of triangulation.

Swift and Fermi Observations of X-Ray Flares: The Case of Late Internal Shock

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-r ...

A multiwavelength study of Swift GRB 060111B constraining the origin of its prompt optical emission

In this work, we present the results obtained from a multi-wavelength campaign, as well as from the public Swift/BAT, XRT, and UVOT data of GRB 060111B for which a bright optical emission was measured with good temporal resolution during the prompt phase. We identified the host galaxy at R~25 mag; its featureless spectral continuum and brightness, as well as the non-detection of any associated supernova 16 days after the trigger and other independent redshift estimates, converge to z~1-2. From the analysis of the early afterglow SED, we find that non-negligible host galaxy dust extinction, in addition to the Galactic one, affects the observed flux in the optical regime. The extinction-corrected optical-to-gamma-ray spectral energy distribution during the prompt emission shows a flux density ratio

The AGILE observations of the hard and bright GRB 100724B

F_{gamma}/F_{opt}

A year-long AGILE observation of Cygnus X-1 in hard spectral state

=0.01-0.0001 with spectral index

Long-term AGILE monitoring of the puzzling gamma-ray source 3EG J1835+5918

beta_{gamma,opt}> beta_{gamma}

Challenging the one zone SSC model in VHE gamma ray emitting BL lacs: The interesting case of PKS 1424+240

, strongly suggesting a separate origin of the optical and gamma-ray components. This result is supported by the lack of correlated behavior in the prompt emission light curves observed in the two energy domains. The properties of the prompt optical emission observed during GRB 060111B favor interpretation of this optical light as radiation from the reverse shock in a thick shell limit and in the slow cooling regime. The expected peak flux is consistent with the observed one corrected for the host extinction, likely indicating that the starting time of the TAROT observations is very near to or coincident with the peak time. The estimated fireball initial Lorentz factor is >260-360 at z=1-2, similar to the Lorentz factors obtained from other GRBs. GRB 060111B is a rare, good test case of the reverse shock emission mechanism in both the X-ray and optical energy ranges.

The Second LAT AGN Catalogue from the Fermi observatory

The observation of gamma ray bursts (GRBs) in the gamma ray band has been advanced by the AGILE and Fermi satellites after the era of the Compton Gamma-Ray Observatory. AGILE and Fermi are showing that the GeV-bright GRBs share a set of common features, particularly the high fluence from the keV up to the GeV energy bands, the high value of the minimum Lorentz factor, an extended emission of gamma rays, which is often delayed with respect to lower energies, and finally the possible multiple spectral components. GRB 100724B, localised in a joint effort by Fermi and the InterPlanetary Newtork, is the brightest burst detected in gamma rays so far by AGILE. Characteristic features of GRB 100724B are the simultaneous emissions at MeV and GeV, without delayed onset or any time lag as shown by the analysis of the cross correlation function, and the significant spectral evolution in hard X-rays over the event duration. In this paper we show the analysis of the AGILE data of GRB 100724B and discuss its features in the context of the bursts observed so far in gamma rays and the recently proposed models.

Multi-wavelength observations of blazar 4C + 49.22 during flaring state

Context. Cygnus X-1 (Cyg X-1) is a high mass X-ray binary system, known to be a black hole candidate and one of the brightest sources in the X-ray sky, which shows both variability on all timescales and frequent flares. The source spends most of the time in a hard spectral state, dominated by a power-law emission, with occasional transitions to the soft and intermediate states, where a strong blackbody component emerges. Aims. We present the observation of Cyg X-1 in a hard spectral state performed during the AGILE science verification phase and observing cycle 1 in hard X-rays (with SuperAGILE) and gamma rays (with the gamma ray imaging detector) and lasting for about 160 days with a live time of ~6 Ms. Methods. We investigated the variability of Cyg X-1 in hard X-rays on different timescales, from ~300 s up to one day, and we applied different tools of timing analysis, such as the autocorrelation function, the first-order structure function, and the Lomb-Scargle periodogram, to our data (from SuperAGILE) and to the simultaneous data in soft X-rays (from RXTE/ASM). We concluded our investigation with a search for emission in the energy range above 100 MeV with the maximum likelihood technique. Results. In the hard X-ray band, the flux of Cyg X-1 shows its typical erratic fluctuations on all timescales with variations of about a factor of two that do not significantly affect the shape of the energy spectrum. From the first-order structure function, we find that the X-ray emission of Cyg X-1 is characterized by antipersistence (anticorrelation in the time series, with an increase in the emission likely followed by a decrease), indicative of a negative feedback mechanism at work. In the gamma ray data a statistically significant point-like source at the position of Cyg X-1 is not found, and the upper limit on the flux is 5 × 10 -8 ph cm -2 s -1 over the whole observation (160 days). Finally we compared our upper limit in gamma rays with the expectation of various models of the Cyg X-1 emission, both of hadronic and leptonic origin, in the GeV-TeV band. Conclusions. The time history of Cyg X-1 in the hard X-ray band over 13 months (not continuous) is shown. Different analysis tools do not provide fully converging results of the characteristic timescales in the system, suggesting that the timescales found in the structure function are not intrinsic to the physics of the source. While Cyg X-1 is not detected in gamma rays, our upper limit is a factor of two lower than the EGRET one and is compatible with the extrapolation of the flux measured by COMPTEL in the same spectral state.