Strong electric discharges associated with thunderstorms can produce terrestrial gamma-ray flashes (TGFs), i.e., intense bursts of x rays and γ rays lasting a few milliseconds or less. We present in ...this Letter new TGF timing and spectral data based on the observations of the Italian Space Agency AGILE satellite. We determine that the TGF emission above 10 MeV has a significant power-law spectral component reaching energies up to 100 MeV. These results challenge TGF theoretical models based on runaway electron acceleration. The TGF discharge electric field accelerates particles over the large distances for which maximal voltages of hundreds of megavolts can be established. The combination of huge potentials and large electric fields in TGFs can efficiently accelerate particles in large numbers, and we reconsider here the photon spectrum and the neutron production by photonuclear reactions in the atmosphere.
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CMK, CTK, FMFMET, IJS, NUK, PNG, UM
We report the AGILE detection and the results of the multifrequency follow-up observations of a bright γ-ray flare of the blazar 3C 279 in 2015 June. We use AGILE and Fermi gamma-ray data, together ...with Swift X-ray andoptical-ultraviolet data, and ground-based GASP-WEBT optical observations, including polarization information, to study the source variability and the overall spectral energy distribution during the γ-ray flare. The γ-ray flaring data, compared with as yet unpublished simultaneous optical data that will allow constraints on the big blue bump disk luminosity, show very high Compton dominance values of ∼100, with the ratio of γ-ray to optical emission rising by a factor of three in a few hours. The multiwavelength behavior of the source during the flare challenges one-zone leptonic theoretical models. The new observations during the 2015 June flare are also compared with already published data and nonsimultaneous historical 3C 279 archival data.
ABSTRACT We report the results of an extensive search through the AGILE data for a gamma-ray counterpart to the LIGO gravitational-wave (GW) event GW150914. Currently in spinning mode, AGILE has the ...potential of cover 80% of the sky with its gamma-ray instrument, more than 100 times a day. It turns out that AGILE came within a minute of the event time of observing the accessible GW150914 localization region. Interestingly, the gamma-ray detector exposed ∼65% of this region during the 100 s time intervals centered at −100 and +300 s from the event time. We determine a 2 flux upper limit in the band 50 MeV-10 GeV, UL = 1.9 × 10−8 erg cm−2 s−1, obtained ∼300 s after the event. The timing of this measurement is the fastest ever obtained for GW150914, and significantly constrains the electromagnetic emission of a possible high-energy counterpart. We also carried out a search for a gamma-ray precursor and delayed emission over five timescales ranging from minutes to days: in particular, we obtained an optimal exposure during the interval −150/−30 s. In all these observations, we do not detect a significant signal associated with GW150914. We do not reveal the weak transient source reported by Fermi-GBM 0.4 s after the event time. However, even though a gamma-ray counterpart of the GW150914 event was not detected, the prospects for future AGILE observations of GW sources are decidedly promising.
We present the characteristics of 308 terrestrial gamma ray flashes (TGFs) detected by the Minicalorimeter (MCAL) instrument on board the AGILE satellite during the period March 2009–July 2012 in the ...±2.5° latitude band and selected to have the maximum photon energy up to 30 MeV. The characteristics of the AGILE events are analyzed and compared to the observational framework established by the two other currently active missions capable of detecting TGFs from space, RHESSI and Fermi. A detailed model of the MCAL dead time is presented, which is fundamental to properly interpret our observations. The most significant contribution to dead time is due to the anticoincidence shield in its current configuration and not to the MCAL detector itself. Longitude and local time distributions are compatible with previous observations, while the duration distribution is biased toward longer values because of dead time. The intensity distribution is compatible with previous observations, when dead time is taken into account. The TGFs cumulative spectrum supports a low production altitude, in agreement with previous measurements. We also compare our sample to lightning sferics detected by the World Wide Lightning Location Network and suggest a new method to assess quantitatively the consistency of two TGF populations based on the comparison of the associated lightning activity. According to this method, AGILE and RHESSI samples are compatible with the same parent population. The AGILE TGF catalog below 30 MeV is accessible online at the website of the ASI Science Data Center http://www.asdc.asi.it/mcaltgfcat/.
Key Points
Terrestrial gamma ray flashes detected by the AGILE satellite are described
The data set properties provide independent confirmation for key TGF properties
A new technique for comparing different TGF data sets is presented and discussed
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Background: The conditions associated with fatty liver disease presenting with normal liver enzymes and the mechanism involved in its development remain to be fully elucidated. Aims: The aim of the ...present study was to test the hypothesis that fatty liver with normal liver enzymes occurs more frequently in arterial hypertensive patients and to establish whether this condition is associated with insulin resistance. Patients: A total of 55 non-obese, non-diabetic, non-heavy alcohol drinking patients with arterial hypertensive and normal liver enzymes and 55 sex and age matched healthy subjects were enrolled into the study. Methods: Plasma metabolic parameters, body mass index, and the presence of fatty liver were investigated. Insulin resistance was estimated from plasma insulin and glucose as the homeostasis model assessment index. Stepwise logistic regression and multivariate regression analysis were used on the combined sample to identify variables independently associated with fatty liver and insulin resistance. Results: Hypertensive patients had a significantly higher prevalence of fatty liver (30.9% v 12.7%; p<0.041), higher insulin resistance (mean 2.27 (SD 1.81) v 1.56 (0.70); p = 0.022), and slightly higher body mass index (24.9 (3.0) v 24.0 (2.2); p = 0.043) than controls. Multivariate logistic regression identified insulin resistance (odds ratio 1.66 (95% confidence interval (CI) 1.03–2.52)) and body mass index (OR 1.22 (95% CI 1.00–1.49)) as factors independently associated with fatty liver. Multivariate regression analysis showed insulin resistance to be predicted by alanine transaminase (p = 0.002), presence of arterial hypertension (p = 0.029), and body mass index (p = 0.048). Conclusion: The higher prevalence of non-alcoholic fatty liver in non-obese hypertensive patients with normal liver enzymes appears to be related to increases in insulin resistance and body weight.
GRB 080514B is the first gamma ray burst (GRB), since the time of EGRET, for which individual photons of energy above several tens of MeV have been detected with a pair-conversion tracker telescope. ...This burst was discovered with the Italian AGILE gamma-ray satellite. The GRB was localized by a cooperation between AGILE and the interplanetary network (IPN). The gamma-ray imager (GRID) estimate of the position, obtained before the SuperAGILE-IPN localization, is found to be consistent with the burst position. The hard X-ray emission observed by SuperAGILE lasted about 7 s, while there is evidence that the emission above 30 MeV extends for a longer duration (at least 13 s). Similar behavior has been seen from a few other GRBs observed with EGRET. However, during the brightest phases, the latter measurements were affected by instrumental dead time effects, resulting in only lower limits to the burst intensity. Thanks to the small dead time of the AGILE/GRID we could assess that in the case of GRB 080514B the gamma-ray to X-ray flux ratio changes significantly between the prompt and extended emission phase.
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FMFMET, NUK, UL, UM, UPUK
We report on the extremely intense and fast gamma-ray flare above 100 MeV detected by AGILE from the Crab Nebula in mid-April 2011. This event is the fourth of a sequence of reported major gamma-ray ...flares produced by the Crab Nebula in the period 2007/mid-2011. These events are attributed to strong radiative and plasma instabilities in the inner Crab Nebula, and their properties are crucial for theoretical studies of fast and efficient particle acceleration up to 1015 eV. Here we study the very rapid flux and spectral evolution of the event that on 2011 April 16 reached the record-high peak flux of F = (26 ? 5) X 10--6 photons cm--2 s--1 with a rise-time timescale that we determine to be in the range 6-10 hr. The peak flaring gamma-ray spectrum reaches a distinct maximum near 500 MeV with no substantial emission above 1 GeV. The very rapid rise time and overall evolution of the Crab Nebula flare strongly constrain the acceleration mechanisms and challenge MHD models. We briefly discuss the theoretical implications of our observations.
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