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.
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.
We present the AGILE gamma-ray observations in the energy range 50 MeV-10 GeV of the supernova remnant (SNR) W44, one of the most interesting systems for studying cosmic-ray production. W44 is an ...intermediate-age SNR (~2,000 years) and its ejecta expand in a dense medium as shown by a prominent radio shell, nearby molecular clouds, and bright S II emitting regions. We extend our gamma-ray analysis to energies substantially lower than previous measurements which could not conclusively establish the nature of the radiation. We find that gamma-ray emission matches remarkably well both the position and shape of the inner SNR shocked plasma. Furthermore, the gamma-ray spectrum shows a prominent peak near 1 GeV with a clear decrement at energies below a few hundreds of MeV as expected from neutral pion decay. Here we demonstrate that (1) hadron-dominated models are consistent with all W44 multiwavelength constraints derived from radio, optical, X-ray, and gamma-ray observations; (2) ad hoc lepton-dominated models fail to explain simultaneously the well-constrained gamma-ray and radio spectra, and require a circumstellar density much larger than the value derived from observations; and (3) the hadron energy spectrum is well described by a power law (with index s = 3.0 ? 0.1) and a low-energy cut-off at Ec = 6 ? 1 GeV. Direct evidence for pion emission is then established in an SNR for the first time.
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.
The LIGO/Virgo Collaboration (LVC) detected on 2017 January 4 a significant gravitational-wave (GW) event (now named GW170104). We report in this Letter the main results obtained from the analysis of ...hard X-ray and gamma-ray data of the AGILE mission that repeatedly observed the GW170104 localization region (LR). At the LVC detection time T0 AGILE observed about 36% of the LR. The gamma-ray imaging detector did not reveal any significant emission in the energy range 50 MeV-30 GeV. Furthermore, no significant gamma-ray transients were detected in the LR that was repeatedly exposed over timescales of minutes, hours, and days. We also searched for transient emission using data near T0 of the omnidirectional detector MCAL operating in the energy band 0.4-100 MeV. A refined analysis of MCAL data shows the existence of a weak event (that we call "E2") with a signal-to-noise ratio of 4.4 lasting about 32 ms and occurring 0.46 0.05 s before T0. A study of the MCAL background and of the false-alarm rate of E2 leads to the determination of a post-trial significance between 2.4 and 2.7 for a temporal coincidence with GW170104. We note that E2 has characteristics similar to those detected from the weak precursor of GRB 090510. The candidate event E2 is worth consideration for simultaneous detection by other satellites. If associated with GW170104, it shows emission in the MeV band of a short burst preceding the final coalescence by 0.46 s and involving ∼10−7 of the total rest mass energy of the system.
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
The supernova remnant (SNR) IC 443 is an intermediate-age remnant well known for its radio, optical, X-ray, and gamma-ray energy emissions. In this Letter, we study the gamma-ray emission above 100 ...MeV from IC 443 as obtained by the AGILE satellite. A distinct pattern of diffuse emission in the energy range 100 MeV-3 GeV is detected across the SNR with its prominent maximum (source "A") localized in the northeastern shell with a flux F = (47 +/- 10) x 10(-8) photons cm(-2) s(-1) above 100 MeV. This location is the site of the strongest shock interaction between the SNR blast wave and the dense circumstellar medium. Source "A" is not coincident with the TeV source located 0.4. away and associated with a dense molecular cloud complex in the SNR central region. From our observations, and from the lack of detectable diffuse TeV emission from its northeastern rim, we demonstrate that electrons cannot be the main emitters of gamma rays in the range 0.1-10 GeV at the site of the strongest SNR shock. The intensity, spectral characteristics, and location of the most prominent gamma-ray emission together with the absence of cospatial detectable TeV emission are consistent only with a hadronic model of cosmic-ray acceleration in the SNR. A high-density molecular cloud (cloud "E") provides a remarkable "target" for nucleonic interactions of accelerated hadrons; our results show enhanced gamma-ray production near the molecular cloud/shocked shell interaction site. IC 443 provides the first unambiguous evidence of cosmic-ray acceleration by SNRs.
AGILE Observations of Fast Radio Bursts Verrecchia, F.; Casentini, C.; Tavani, M. ...
The Astrophysical journal,
07/2021, Letnik:
915, Številka:
2
Journal Article
Recenzirano
Odprti dostop
Abstract
We report on a systematic search for hard X-ray and
γ
-ray emission in coincidence with fast radio bursts (FRBs) observed by the AGILE satellite. We used 13 yr of AGILE archival data ...searching for time coincidences between exposed FRBs and events detectable by the MCAL (0.4–100 MeV) and GRID (50 MeV–30 GeV) detectors at timescales ranging from milliseconds to days/weeks. The current AGILE sky coverage allowed us to extend the search for high-energy emission preceding and following the FRB occurrence. We considered all FRB sources currently included in catalogs and identified a subsample (15 events) for which a good AGILE exposure with either MCAL or GRID was obtained. In this paper we focus on nonrepeating FRBs, compared to a few nearby repeating sources. We did not detect significant MeV or GeV emission from any event. Our hard X-ray upper limits (ULs) in the MeV energy range were obtained for timescales from submillisecond to seconds, and in the GeV range from minutes to weeks around event times. We focus on a subset of five nonrepeating and two repeating FRB sources whose distances are most likely smaller than that of 180916.J0158+65 (150 Mpc). For these sources, our MeV ULs translate into ULs on the isotropically emitted energy of about 3 × 10
46
erg, comparable to that observed in the 2004 giant flare from the Galactic magnetar SGR 1806–20. On average, these nearby FRBs emit radio pulses of energies significantly larger than the recently detected SGR 1935+2154 and are not yet associated with intense MeV flaring.
InterPlanetary Network (IPN) data are presented for the gamma-ray bursts in the second Fermi Gamma-Ray Burst Monitor (GBM) catalog. Of the 462 bursts in that catalog between 2010 July 12 and 2012 ...July 11, 428, or 93%, were observed by at least 1 other instrument in the 9-spacecraft IPN. Of the 428, the localizations of 165 could be improved by triangulation. For these bursts, triangulation gives one or more annuli whose half-widths vary between about 2 3° and 16°, depending on the peak flux, fluence, time history, arrival direction, and the distance between the spacecraft. We compare the IPN localizations with the GBM 1 , 2 , and 3 error contours and find good agreement between them. The IPN 3 error boxes have areas between about 8 square arcminutes and 380 square degrees, and are an average of 2500 times smaller than the corresponding GBM 3 localizations. We identify four bursts in the IPN/GBM sample whose origins were given as "uncertain," but may in fact be cosmic. This leads to an estimate of over 99% completeness for the GBM catalog.
The LIGO-Virgo Collaboration (LVC) detected, on 2017 August 17, an exceptional gravitational-wave (GW) event temporally consistent within with the GRB 1708117A observed by Fermi-GBM and INTEGRAL. The ...event turns out to be compatible with a neutron star-neutron star (NS-NS) coalescence that subsequently produced a radio/optical/X-ray transient detected at later times. We report the main results of the observations by the AGILE satellite of the GW170817 localization region (LR) and its electromagnetic (EM) counterpart. At the LVC detection time T0, the GW170817 LR was occulted by the Earth. The AGILE instrument collected useful data before and after the GW/GRB event because in its spinning observation mode it can scan a given source many times per hour. The earliest exposure of the GW170817 LR by the gamma-ray imaging detector started about 935 s after T0. No significant X-ray or gamma-ray emission was detected from the LR that was repeatedly exposed over timescales of minutes, hours, and days before and after GW170817, also considering Mini-calorimeter and Super-AGILE data. Our measurements are among the earliest ones obtained by space satellites on GW170817 and provide useful constraints on the precursor and delayed emission properties of the NS-NS coalescence event. We can exclude with high confidence the existence of an X-ray/gamma-ray emitting magnetar-like object with a large magnetic field of . Our data are particularly significant during the early stage of evolution of the EM remnant.
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