The BL Lac PG 1553+113 has been continuously monitored in gamma-rays with Fermi-LAT for over 9 years. Its updated light curve now includes five iterations of a main pattern comprising a high peak and ...a longer trough, with a period . Our analysis of 2015-2017 data confirms the occurrence in 2017 January of a new peak fitting in with the previous trend. In addition, we identify secondary peaks ("twin peaks") that occur in closely symmetric pairs on both sides of most main peaks, including the last one; their occurrence is supported by correlated X-ray outbursts. We stress that the above features strongly point to binary dynamics in a system of two black holes (BHs) of some 108 and . At periastron the smaller BH periodically stresses the jet j1 launched by the heavier companion, and triggers MHD-kinetic tearing instabilities. These lead to magnetic reconnections and to acceleration of electrons that produce synchrotron emission from the optical to X-ray bands, and inverse Compton scattering into the GeV range. We discuss two possible origins of the twin peaks : a single-jet model, based on added instabilities induced in j1 by the smaller companion BH on its inner orbital arc; and a two-jet model with the smaller BH supporting its own, precessing jet j2 that contributes lower, specific GeV emissions. Such behaviors combining time stability with amplitude variations betray plasma instabilities driven in either jet by binary dynamics, and can provide a double signature of the long-sought supermassive BH binaries.
The First AGILE Solar Flare Catalog Ursi, A.; Parmiggiani, N.; Messerotti, M. ...
The Astrophysical journal. Supplement series,
07/2023, Letnik:
267, Številka:
1
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Abstract
We report the Astro-rivelatore Gamma a Immagini LEggero (AGILE) observations of solar flares, detected by the onboard anticoincidence system in the 80–200 keV energy range, from 2007 May 1 ...to 2022 August 31. In more than 15 yr, AGILE detected 5003 X-ray, minute-lasting transients, compatible with a solar origin. A cross-correlation of these transients with the Geostationary Operational Environmental Satellites (GOES) official solar flare database allowed us to associate an intensity class (i.e., B, C, M, or X) to 3572 of them, for which we investigated the main temporal and intensity parameters. The AGILE data clearly revealed the solar activity covering the last stages of the 23rd cycle, the whole 24th cycle, and the beginning of the current 25th cycle. In order to compare our results with other space missions operating in the high-energy range, we also analyzed the public lists of solar flares reported by RHESSI and Fermi Gamma-ray Burst Monitor. This catalog reports 1424 events not contained in the GOES official data set, which, after statistical comparisons, are compatible with low-intensity, short-duration solar flares. Besides providing a further data set of solar flares detected in the hard X-ray range, this study allowed to point out two main features: a longer persistence of the decay phase in the high-energy regime, with respect to the soft X-rays, and a tendency of the flare maximum to be reached earlier in the soft X-rays with respect to the hard X-rays. Both these aspects support a two-phase acceleration mechanism of electrons in the solar atmosphere.
The search in active galactic nuclei (AGNs) for supermassive binary systems with total mass M ∼ 108-109 M is intertwined with the observational efforts aimed at their contributions to the stochastic ...background of gravitational waves (GWs). Here we discuss and compare two classes of AGNs that are outstanding in electromagnetic bands: bright, relatively distant and massive quasars, and closer jetted and less massive BL Lac objects (BL Lacs). Among the latter, we focus on the source PG 1553+113 at redshift z 0.5, one of the best periodic candidates so far, that features a five-fold repetitive pattern of period P = 2.2 yr recurring in its gamma-ray light curve, as continuously monitored by Fermi-LAT for over 9 yr and updated here to 2018 March 15. We discuss this object as an interesting candidate for hosting a supermassive binary source of GWs, and explore the possible contributions to the GW stochastic background from it and similar sources in the BL Lac class. We find the possible contributions of such sources to lie below the limits set by the current Pulsar Timing Arrays, but well within the reach of the upcoming Square Kilometer Array.
The well-known Crab Nebula is at the center of the SN1054 supernova remnant. It consists of a rotationally powered pulsar interacting with a surrounding nebula through a relativistic particle wind. ...The emissions originating from the pulsar and nebula have been considered to be essentially stable. Here, we report the detection of strong gamma-ray (100 mega-electron volts to 10 giga-electron volts) flares observed by the AGILE satellite in September 2010 and October 2007. In both cases, the total gamma-ray flux increased by a factor of three compared with the non-flaring flux. The flare luminosity and short time scale favor an origin near the pulsar, and we discuss Chandra Observatory x-ray and Hubble Space Telescope optical follow-up observations of the nebula. Our observations challenge standard models of nebular emission and require power-law acceleration by shock-driven plasma wave turbulence within an approximately 1-day time scale.
FRB 180916 is a most intriguing source capable of producing repeating fast radio bursts with a periodic 16.3 day temporal pattern. The source is well positioned in a star-forming region in the ...outskirts of a nearby galaxy at 150 Mpc distance. In this Letter we report on the X-ray and γ-ray observations of FRB 180916 obtained by AGILE and Swift. We focused especially on the recurrent 5 day time intervals of enhanced radio bursting. In particular, we report on the results obtained in the time intervals 2020 February 3-8, 2020 February 25, 2020 March 5-10, and 2020 March 22-28 during a multiwavelength campaign involving high-energy and radio observations of FRB 180916. We also searched for temporal coincidences at millisecond timescales between the 32 known radio bursts of FRB 180916 and X-ray and MeV events detectable by AGILE. We do not detect any simultaneous event or any extended X-ray and γ-ray emission on timescales of hours/days/weeks. Our cumulative X-ray (0.3-10 keV) flux upper limit of 5 × 10−14 erg cm−2 s−1 (obtained during 5 day active intervals from several 1-2 ks integrations) translates into an isotropic luminosity upper limit of LX,UL ∼ 1.5 × 1041 erg s−1. Deep γ-ray observations above 100 MeV over a many-year timescale provide an average luminosity upper limit one order of magnitude larger. These results provide the so-far most stringent upper limits on high-energy emission from the FRB 180916 source. Our results constrain the dissipation of magnetic energy from a magnetar-like source of radius Rm, internal magnetic field Bm, and dissipation timescale τd to satisfy the relation , where Rm,6 is Rm in units of 106 cm, Bm,16 is Bm in units of 1016 G, and τd,8 in units of 108 s.
Gamma-ray emission from the Crab Nebula has been recently shown to be unsteady. In this paper, we study the flux and spectral variability of the Crab above 100 MeV on different timescales ranging ...from days to weeks. In addition to the four main intense and day-long flares detected by AGILE and Fermi-LAT between 2007 September and 2012 September, we find evidence for week-long and less intense episodes of enhanced gamma-ray emission that we call "waves." Statistically significant "waves" show timescales of 1-2 weeks, and can occur by themselves or in association with shorter flares. We present a refined flux and spectral analysis of the 2007 September-October gamma-ray enhancement episode detected by AGILE that shows both "wave" and flaring behavior. We extend our analysis to the publicly available Fermi-LAT data set and show that several additional "wave" episodes can be identified. We discuss the spectral properties of the 2007 September "wave"/flare event and show that the physical properties of the "waves" are intermediate between steady and flaring states. Plasma instabilities inducing "waves" appear to involve spatial distances l ~ 10 super(16) cm and enhanced magnetic fields B ~ (0.5-1) mG. Day-long flares are characterized by smaller distances and larger local magnetic fields. Typically, the deduced total energy associated with the "wave" phenomenon (E sub(w) ~ 10 super(42) erg, where E sub(w) is the kinetic energy of the emitting particles) is comparable with that associated to the flares, and can reach a few percent of the total available pulsar spin-down energy. Most likely, flares and waves are the product of the same class of plasma instabilities that we show acting on different timescales and radiation intensities.
The AGILE Mission Barbiellini, G.; Argan, A.; Boffelli, F. ...
Astronomy and astrophysics (Berlin),
08/2009, Letnik:
502, Številka:
3
Journal Article
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Context. AGILE is an Italian Space Agency mission dedicated to observing the gamma-ray Universe. The AGILE's very innovative instrumentation for the first time combines a gamma-ray imager (sensitive ...in the energy range 30 MeV–50 GeV), a hard X-ray imager (sensitive in the range 18–60 keV), a calorimeter (sensitive in the range 350 keV–100 MeV), and an anticoincidence system. AGILE was successfully launched on 2007 April 23 from the Indian base of Sriharikota and was inserted in an equatorial orbit with very low particle background. Aims. AGILE provides crucial data for the study of active galactic nuclei, gamma-ray bursts, pulsars, unidentified gamma-ray sources, galactic compact objects, supernova remnants, TeV sources, and fundamental physics by microsecond timing. Methods. An optimal sky angular positioning (reaching 0.1 degrees in gamma-rays and 1–2 arcmin in hard X-rays) and very large fields of view (2.5 sr and 1 sr, respectively) are obtained by the use of Silicon detectors integrated in a very compact instrument. Results. AGILE surveyed the gamma-ray sky and detected many Galactic and extragalactic sources during the first months of observations. Particular emphasis is given to multifrequency observation programs of extragalactic and galactic objects. Conclusions. AGILE is a successful high-energy gamma-ray mission that reached its nominal scientific performance. The AGILE Cycle-1 pointing program started on 2007 December 1, and is open to the international community through a Guest Observer Program.
We study the capability of the AGILE gamma ray space mission in detecting magnetospheric particles (mostly electrons) in the energy range 10–100 MeV. Our measurements focus on the inner magnetic ...shells with
L≲1.2 in the magnetic equator. The instrument characteristics and a quasi‐equatorial orbit of ∼500 km altitude make it possible to address several important properties of the particle populations in the inner magnetosphere. We review the on board trigger logic and study the acceptance of the AGILE instrument for particle detection. We find that the AGILE effective geometric factor (acceptance) is R≃50 cm2 sr for particle energies in the range 10–100 MeV. Particle event reconstruction allows to determine the particle pitch angle with the local magnetic field with good accuracy. We obtain the pitch angle distributions for both the AGILE “pointing” phase (July 2007 to October 2009) and the “spinning” phase (November 2009 to present). In spinning mode, the whole range (0–180 degrees) is accessible every 7 min. We find a pitch angle distribution of the “dumbbell” type with a prominent depression near α = 90° which is typical of wave‐particle resonant scattering and precipitation in the inner magnetosphere. Most importantly, we show that AGILE is not affected by solar particle precipitation events in the magnetosphere. The satellite trajectory intersects magnetic shells in a quite narrow range (
1.0≲L≲1.2); AGILE then has a high exposure to a magnetospheric region potentially rich of interesting phenomena. The large particle acceptance in the 10–100 MeV range, the pitch angle determination capability, the L shell exposure, and the solar‐free background make AGILE a unique instrument for measuring steady and transient particle events in the inner magnetosphere.
Key Points
AGILE has the currently largest particle acceptance for 10–100 MeV electrons in the inner belt with L approximately less than 1.2
Measurement of the pitch angle distribution and particle flux which is unaffected by solar activity
Detection of a stable "dumbbell" pitch angle distribution for high‐energy electrons and positrons in the inner magnetosphere
The origin of cosmic neutrinos is still largely unknown. Using data obtained by the gamma-ray imager on board the Astro-rivelatore Gamma a Immagini Leggero (AGILE) satellite, we systematically ...searched for transient gamma-ray sources above 100 MeV that are temporally and spatially coincident with 10 recent high-energy neutrino IceCube events. We found three AGILE candidate sources that can be considered possible counterparts to neutrino events. Detecting three gamma-ray/neutrino associations out of 10 IceCube events is shown to be unlikely due to a chance coincidence. One of the sources is related to the BL Lac source TXS 0506+056. For the other two AGILE gamma-ray sources there are no obvious known counterparts, and both Galactic and extragalactic origin should be considered.
At the end of March 2015 the onboard software configuration of the Astrorivelatore Gamma a Immagini Leggero (AGILE) satellite was modified in order to disable the veto signal of the anticoincidence ...shield for the minicalorimeter instrument. The motivation for such a change was the understanding that the dead time induced by the anticoincidence prevented the detection of a large fraction of Terrestrial Gamma‐Ray Flashes (TGFs). The configuration change was highly successful resulting in an increase of one order of magnitude in TGF detection rate. As expected, the largest fraction of the new events has short duration (<100 μs), and part of them has simultaneous association with lightning sferics detected by the World Wide Lightning Location Network. The new configuration provides the largest TGF detection rate surface density (TGFs/km2/yr) to date, opening prospects for improved correlation studies with lightning and atmospheric parameters on short spatial and temporal scales along the equatorial region.
Key Points
Tenfold improvement in TGF detection rate by AGILE is obtained after a major configuration change
The sample is dead time‐free, with TGF durations as short as 20 μs
The relationship between TGF duration and likelihood of association with lightning is confirmed
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