Gamma-ray Imaging Detector Research Articles

Multiwavelength observations of the lensed quasar PKS 1830-211 during the 2019 γ-ray flare

ABSTRACT PKS 1830 -211 is a γ-ray emitting, high-redshift (z =2.507 ± 0.002), lensed flat-spectrum radio quasar. During the period 2019 mid-February to mid-April, this source underwent a series of strong γ-ray flares that were detected by both AGILE-GRID (Gamma-Ray Imaging Detector) and Fermi Large Area Telescope (Fermi-LAT), reaching a maximum γ-ray flux of $F_{\rm E\gt 100\, MeV}\approx 2.3\times 10^{-5}$ photons cm−2 s−1. Here, we report on a coordinated campaign from both on-ground [Medicina, Owens Valley Radio Observatory (OVRO), Rapid Eye Mount (REM), and Sardinia Radio Telescope (SRT)] and orbiting facilities (AGILE, Fermi, INTEGRAL, NuSTAR, Swift, and Chandra), with the aim of investigating the multiwavelength properties of PKS 1830-211 through nearly simultaneous observations presented here for the first time. We find a possible break in the radio spectra in different epochs above 15 GHz, and a clear maximum of the 15 GHz data approximately 110 d after the γ-ray main activity periods. The spectral energy distribution shows a very pronounced Compton dominance (> 200) which challenges the canonical one-component emission model. Therefore, we propose that the cooled electrons of the first component are re-accelerated to a second component by, for example, kink or tearing instability during the γ-ray flaring periods. We also note that PKS 1830-211 could be a promising candidate for future observations with both Compton satellites [e.g. enhanced ASTROGAM (e-ASTROGAM)] and Cherenkov arrays [Cherenkov Telescope Array Observatory (CTAO)] which will help, thanks to their improved sensitivity, in extending the data availability in energy bands currently uncovered.

Simulation design and performance evaluation of the wide-energy range gamma-ray imaging detectors

康普顿成像是直观感知核辐射的探测技术之一, 但因受限于探测器体积和成像事例的类型, 能量范围通常为0.3~3 MeV, 难以在对天然核素γ成像的基础上同时兼顾能量分布在3~7 MeV的瞬发γ成像应用. 覆盖0.3~7 MeV范围的宽能域γ成像是扩展核辐射探测与成像应用领域的重要发展方向之一, 基于两次散射的三层探测器结构是一种极具潜力的实现方案, 其中探测器的设计与构建是该方案的基础与关键核心. 考虑系统成本和扩展灵活性等因素, 本文选用硅光电倍增管(SiPM)阵列和钆镓铝石榴石(GAGG:Ce)阵列作为探测器材料. 根据三层探测器的成像事例要求, 分析每层探测器内的作用事例类型特征; 并基于Geant4蒙特卡罗模拟平台构建包含材料光学参数的仿真模型, 通过设置物理过程与两种信息提取方式, 对比研究了不同几何参数情形下的探测器位置散点图和探测器内作用事例类型统计结果. 根据仿真设计, 制备了由像素数为15×15, 像素大小为3.2×3.2 mm², 厚度分别为3 mm和10 mm的GAGG阵列和1.25 mm厚度光导组成的两种规格SiPM探测器模块, 并基于电子学读出系统评估了探测器模块的性能. 结果表明, 3 mm厚度的散射探测器与10 mm厚度的吸收探测器均可清晰分辨15×15像素上的作用位置, 平均峰谷比分别为9.77和11.85; 全部像素的662 keV平均能量分辨率分别可达7.21%和8.35%, 具有较好的位置与能量分辨能力. 本文所建立的仿真模型, 分析结果以及所研制的探测器模块, 为三层探测器系统构建提供了重要依据, 并为实现宽能域γ成像奠定了科学技术基础.

Development of a gamma-ray imaging detector based on ceramic scintillators with SiPM readout

Depth of interaction (DOI) measurements can provide additional position information when long scintillator bars or scintillator pixels with relatively large thickness are employed in gamma-ray imaging detectors. In this study, commercially available GGAG ceramic scintillators were used to fabricate a gamma-ray imaging detector. The light attenuation, DOI resolution, and spectroscopic performance of 3.0× 3.0× 48.0 mm3 ceramic bars were evaluated using double-side SiPM readout. The measured full width at half maximum (FWHM) energy resolution values ranged between 7.6% and 8.7% at 511 keV. The DOI resolution was approximately 4 mm (FWHM). A prototype two-layer Compton imaging detector was fabricated based on 32 ceramic bars and the signals were readout using four 16-ch application-specific integrated circuits. The lutetium background was evaluated and a 22Na point source (0.19 μSv/h 511 keV gamma-ray dose rate at 50 cm) was used to verify the feasibility of using the commercial GGAG ceramic material in nuclear security applications. The source can be located within 120 s using a simple back projection method. The angular resolution measure was estimated as 15∘ (FWHM). A filtered back projection based algorithm improved the angular resolution (FWHM) to 9.2∘.

A Deep Learning Method for AGILE-GRID Gamma-Ray Burst Detection

Abstract The follow-up of external science alerts received from gamma-ray burst (GRB) and gravitational wave detectors is one of the AGILE Team’s current major activities. The AGILE team developed an automated real-time analysis pipeline to analyze AGILE Gamma-Ray Imaging Detector (GRID) data to detect possible counterparts in the energy range 0.1–10 GeV. This work presents a new approach for detecting GRBs using a convolutional neural network (CNN) to classify the AGILE-GRID intensity maps by improving the GRB detection capability over the Li & Ma method, currently used by the AGILE team. The CNN is trained with large simulated data sets of intensity maps. The AGILE complex observing pattern due to the so-called “spinning mode” is studied to prepare data sets to test and evaluate the CNN. A GRB emission model is defined from the second Fermi-LAT GRB catalog and convoluted with the AGILE observing pattern. Different p-value distributions are calculated, evaluating, using the CNN, millions of background-only maps simulated by varying the background level. The CNN is then used on real data to analyze the AGILE-GRID data archive, searching for GRB detections using the trigger time and position taken from the Swift-BAT, Fermi-GBM, and Fermi-LAT GRB catalogs. From these catalogs, the CNN detects 21 GRBs with a significance of ≥3σ, while the Li & Ma method detects only two GRBs. The results shown in this work demonstrate that the CNN is more effective in detecting GRBs than the Li & Ma method in this context and can be implemented into the AGILE-GRID real-time analysis pipeline.

Row–Column Readout Method to Mitigate Radiographic-Image Blurring From Multipixel Events in a Coded-Aperture Imaging System

In order to improve the image quality of a coded-aperture-based gamma-ray imaging apparatus, a method for removing Compton scattered and other multipixel events is presented that removes inaccurate position estimations from the data stream. In coded-aperture gamma-ray imaging detectors, scattering events that are shared between different partitions in the active medium can result in the inaccurate determination of the initial gamma-ray interaction position, the result of which is distortion in the angular reconstruction mapping. For coded-aperture system in which the rapid localization and identification of the emitting source is important, the distortion due to scattered radiation can increase the integration time of the system. We show, in both software and hardware implementations using a cerium-doped gadolinium aluminum gallium garnet pixelated scintillator array, that filtering the data to remove multipixel events can improve the image quality despite the efficiency loss associated with not utilizing all of the interactions within the detector.

Improving the spatial resolution of a pixelated LaBr3(Ce) scintillator coupled with a multi-pixel photon counter array for boron neutron capture therapy

Fundamental preclinical research with mice to improve boron neutron capture therapy requires a prompt gamma-ray imaging detector that can chart the real-time accumulation of 10B in tumors. This study aimed to improve the spatial resolution of our previous detector, which was based on a slab-shape LaBr3(Ce) scintillator, by changing to a pixelated 8 × 8 array scintillator.The difference between the scintillation-light detection processes, such as Compton scattering and the photoelectron effect, affects energy resolution. This was experimentally revealed and evaluated with Monte Carlo simulations. The events caused by Compton scattering were eliminated, and our detector obtained the desired energy resolution for 511 keV gamma rays at 5.0 ± 0.4 %. The spatial resolution was evaluated using collimated gamma rays and was found to be better than that of the previous system because the scintillation light did not spread over the pixelated scintillator. As a result, the proposed detection system had the energy resolution to discriminate between 478- and 511-keV gamma rays to obtain the 10B concentration. The lateral and vertical spatial resolutions FWHM of the improved system were 5.9 mm and 5.2 mm, respectively, which were better than the value of 8.0 mm provided by the previous system to visualize the two-dimensional distribution in real time.

AGILESim: Monte Carlo Simulation of the AGILE Gamma-Ray Telescope

Abstract The accuracy of Monte Carlo simulations in reproducing the scientific performance of space telescopes (e.g., angular resolution) is mandatory for a correct design of the mission. A brand-new Monte Carlo simulator of the Astrorivelatore Gamma ad Immagini LEggero (AGILE)/Gamma-Ray Imaging Detector (GRID) space telescope, AGILESim, is built using the customizable Bologna Geant4 Multi-Mission Simulator (BoGEMMS) architecture and the latest Geant4 library to reproduce the instrument performance of the AGILE/GRID instrument. The Monte Carlo simulation output is digitized in the BoGEMMS postprocessing pipeline, according to the instrument electronic readout logic, then converted into the onboard data handling format, and finally analyzed by the standard mission on-ground reconstruction pipeline, including the Kalman filter, as a real observation in space. In this paper we focus on the scientific validation of AGILESim, performed by reproducing (i) the conversion efficiency of the tracker planes, (ii) the tracker charge readout distribution measured by the on-ground assembly, integration, and verification activity, and (iii) the point-spread function of in-flight observations of the Vela pulsar in the 100 MeV–1 GeV energy range. We measure an in-flight angular resolution (full width at half-maximum) for Vela-like point sources of and in the 100–300 and 300–1000 MeV energy bands, respectively. The successful cross-comparison of the simulation results with the AGILE on-ground and in-space performance validates the BoGEMMS framework for its application to future gamma-ray trackers (e.g., e-ASTROGAM and AMEGO).

Evaluation of the energy resolution of a prompt gamma-ray imaging detector using LaBr3(Ce) scintillator and 8 × 8 array MPPC for an animal study of BNCT

To develop boron neutron capture therapy (BNCT), it is desired to measure 10B concentration and obtain a two-dimensional 10B distribution in animal studies. In this research, we develop a prompt gamma-ray imaging detector to measure 10B distribution using a 50 mm × 50 mm x 10 mm LaBr3(Ce) scintillator and a multi-pixel photon counter (MPPC). To measure a two-dimensional 10B distribution, the 478 keV gamma-ray emitted from 10B(n,α)7Li reaction should be measured with the discrimination from 511 keV background gamma rays in each MPPC. Furthermore, as a characteristic of the detector, it is necessary to investigate whether the 478 keV events individually incident on the MPPC can be measured in two dimensions. In this study, we evaluated the energy resolution and performed a two-dimensional distribution measurement using a thermal neutron beam and prompt gamma rays from a boron sample.This system was able to obtain energy resolution as full width at half maximum at 511 keV of 5.0 ± 0.2% in all MPPC pixels, better than the 6.5% energy resolution required to discriminate between 478 and 511 keV gamma rays.When the region of interest (ROI) was set up from -3σ to – σ (first ROI) and -3σ to the median (second ROI) for the Gaussian distribution of a 478 keV gamma-ray peak using a 6.25 ppm sample, the detector count rate of the 478 keV gamma rays was 0.03 and 0.11 cps, respectively, without a collimator. Moreover, the effect due to the overlapping 511 keV gamma ray peak was approximately 2.0% in the first ROI and approximately 3.2% in the second ROI. In addition, the counts of 478 keV gamma rays were visualized in two-dimensional.

Effect of deep level defects on space charge distribution in CdZnTe crystals

CdZnTe recently emerged as a leading semiconductor crystal for fabricating room-temperature x- and gamma-ray imaging detectors, due to its excellent energy resolution and sensitivity. However, its wide deployment is hampered by the low availability of high-quality CdZnTe crystals. As-grown CdZnTe crystals generally encounter the problems arising from the impurities and defects, especially deep level defects. The presence of impurities and defects leads to severe charge trapping, which significantly affects detector performance. Especially for high counting rate imaging detector used in medical imaging and tomography, the accumulation of space charge at deep levels significantly deforms the electric field distribution and subsequently reduces the charge collection efficiency. Therefore, a considerable interest is focused on the investigation of the space charge accumulation effect in CdZnTe crystal, which is the key factor to improve the performance of high counting rate imaging detector. Thus, the goal of this work is to investigate the effects of deep level defects on space charge distribution and internal electric field in CdZnTe detector. In order to reveal the major problem therein, Silvaco TCAD technique is used to simulate the space charge and electric field distribution profile in CdZnTe detector with considering the typical deep level defects <inline-formula><tex-math id="Z-20201111102325-1">\begin{document}$ \rm Te_{Cd}^{++} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20200553_Z-20201111102325-1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20200553_Z-20201111102325-1.png"/></alternatives></inline-formula>in CdZnTe crystals with activation energy of <i>E</i><sub>v</sub> + 0.86 eV and concentration of 1 × 10<sup>12</sup> cm<sup>–3</sup> at room temperature. The simulation results demonstrate that the Au/ CdZnTe /Au energy band tilts intensively with the increase of applied bias, which makes the deep level ionization fraction increase. The space charge concentration also increases in the crystal. Meanwhile, the dead layer of electric field distribution decreases, which is of benefit to the carrier collection of CdZnTe detector. In addition, under the premiseof the high resistivity of CdZnTe crystal, the reduction of deep level defect concentration located at <i>E</i><sub>v</sub> + 0.86 eV can narrow the internal dead layer moderately. The deep level defect located at <i>E</i><sub>v</sub> + 0.8 eV can also reduce the space charge concentration near the cathode, which flattens the electric field distribution with narrower dead layer, thus significantly improving the carrier collection efficiency of CdZnTe detector. These simulation results will provide meaningful theoretical guidance for further optimizing the CdZnTe crystal growth, device design and fabrication.

Calibration of AGILE-GRID with On-ground Data and Monte Carlo Simulations

AGILE is a mission of the Italian Space Agency (ASI) Scientific Program dedicated to γ-ray astrophysics, and has operated in a low Earth orbit since 2007 April 23. It is designed to be a very light and compact instrument, capable of simultaneously detecting and imaging photons in the 18–60 keV X-ray energy band and in the 30 MeV–50 GeV γ-ray energy range with a good angular resolution (≈1◦ @ 1 GeV). The core of the instrument is the Silicon Tracker, supplemented by a CsI calorimeter and an AntiCoincidence system, which form the Gamma Ray Imaging Detector (GRID). Before launch, the GRID needed on-ground calibration with a tagged γ-ray beam to estimate its performance and validate the Monte Carlo simulation. The GRID was calibrated using a tagged γ-ray beam with energy up to 500 MeV at the Beam Test Facilities at the INFN Laboratori Nazionali di Frascati. These data are used to validate a GEANT 3-based simulation by comparing the data and the Monte Carlo simulation by measuring the angular and energy resolutions. The GRID angular and energy resolutions obtained using the beam agree well with the Monte Carlo simulation. Therefore the simulation, can be used to simulate the same performance on-flight with high reliability.

AGILE Observations of the Gravitational-wave Source GW170817: Constraining Gamma-Ray Emission from an NS–NS Coalescence

Abstract 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 T 0, 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 T 0. 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.

A high-resolution CdTe imaging detector with multi-pinhole optics for in-vivo molecular imaging

High-resolution CdTe semiconductor detectors with both fine position resolution and high energy resolution hold great promise to improve measurement in various hard X-ray and gamma-ray imaging experiments. In the field of in-vivo molecular imaging, where the molecules labeled with radioisotopes are injected into a small animal and their distribution in a body is detected by gamma-ray imaging detectors externally, a large detection area is also required to obtain a number of projections from multiple pinholes. We have developed a prototype imaging system consisting of a 32 mm-wide CdTe double-sided strip detector equipped with multi-pinhole optics. We describe its imaging capability, as studied through experiments with radioisotopes for medical use (125I and 111In) and a 3D phantom.

Feasibility of Using Crystal Geometry for a DOI Scintillation Detector.

We have used simulations and measurements to investigate the feasibility of using slanted scintillator crystal geometries as means to provide depth-of-interaction (DOI) information for a pixelated gamma ray imaging detector. The simulations were performed to estimate the fraction of scintillation light detected by the photodetector as a function of interaction location along the height of crystals with different geometries. In addition, physical measurements of the light output for these crystal geometries were obtained from individual crystals coupled to a solid state photodetector (Philips digital-SiPM DPC-3200). In agreement with previous work, we found a change in light output in the slanted region of the crystals compared to the rectangular region. The results from this study indicate the potential of using slanted crystals to gather DOI information based on light output changes as a function of the location of interaction. An examination of the measured energy spectra for the geometries evaluated here, suggests that for BGO crystals somewhere between 2 or 3 DOI bins could be implemented. Based on these results, we conceived a design for a DOI detector module that consists of two slanted crystals, each read-out by separate SiPM pixels.

AGILE Observations of the Gravitational-wave Source GW170104

Abstract 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 T 0 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 T 0 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 4.4σ lasting about 32 ms and occurring 0.46 ± 0.05 s before T 0. A study of the MCAL background and of the false-alarm rate of E2 leads to the determination of a post-trial significance between 2.4σ and 2.7σ 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.

Gamma-Ray Observations of Active Galactic Nuclei

This article reviews the recent observational results regarding γ-ray emission from active galaxies. The most numerous discrete extragalactic γ-ray sources are AGNs dominated by relativistic jets pointing in our direction (commonly known as blazars), and they are the main subject of the review. They are detected in all observable energy bands and are highly variable. The advent of the sensitive γ-ray observations, afforded by the launch and continuing operation of the Fermi Gamma-ray Space Telescope and the AGILE Gamma-ray Imaging Detector, as well as by the deployment of current-generation Air Cerenkov Telescope arrays such as VERITAS, MAGIC, and HESS-II, continually provides sensitive γ-ray data over the energy range of ∼100 MeV to multi-TeV. Importantly, it has motivated simultaneous, monitoring observations in other bands, resulting in unprecedented time-resolved broadband spectral coverage. After an introduction, in Sections 3, 4, and 5, we cover the current status and highlights of γ-ray observations with (mainly) Fermi but also AGILE and put those in the context of broadband spectra in Section 6. We discuss the radiation processes operating in blazars in Section 7, and we discuss the content of their jets and the constraints on the location of the energy dissipation regions in, respectively, Sections 8 and 9. Section 10 covers the current ideas for particle acceleration processes in jets, and Section 11 discusses the coupling of the jet to the accretion disk in the host galaxy. Finally, Sections 12, 13, and 14 cover, respectively, the contribution of blazars to the diffuse γ-ray background, the utility of blazars to study the extragalactic background light, and the insight they provide for study of populations of supermassive black holes early in the history of the Universe.

Hard-X and gamma-ray imaging detector for astrophysics based on pixelated CdTe semiconductors

Stellar explosions are astrophysical phenomena of great importance and interest. Instruments with high sensitivities are essential to perform detailed studies of cosmic explosions and cosmic accelerators. In order to achieve the needed performance, a hard-X and gamma-ray imaging detector with mm spatial resolution and large enough efficiency is required. We present a detector module which consists of a single CdTe crystal of 12.5 × 12.5mm2 and 2mm thick with a planar cathode and with the anode segmented in an 11x11 pixel array with a pixel pitch of 1 mm attached to the readout chip. Two possible detector module configurations are considered: the so-called Planar Transverse Field (PTF) and the Parallel Planar Field (PPF). The combination of several modules in PTF or PPF configuration will achieve the desired performance of the imaging detector. The sum energy resolution of all pixels of the CdTe module measured at 122 keV and 356 keV is 3.8% and 2% respectively, in the following operating conditions: PPF irradiation, bias voltage −500 V and temperature −10̂C.

ON THE ANGULAR RESOLUTION OF THEAGILEGAMMA-RAY IMAGING DETECTOR

We present a study of the Angular Resolution of the AGILE gamma-ray imaging detector (GRID) that is operational in space since April 2007. The AGILE instrument is made of an array of 12 planes each equipped with a Tungsten converter and Silicon micros trip detectors and is sensitive in the energy range 50 MeV - 10 GeV. Among the space instruments devoted to gamma-ray astrophysics, AGILE uniquely exploits an analog readout system with dedicated electronics coupled with Silicon detectors. We show the results of Monte Carlo simulations carried out to reproduce the gamma-ray detection by the GRID, and we compare them to in-flight data. We use the Crab (pulsar + Nebula) system for discussion of real data performance, since its E^{-2} energy spectrum is representative of the majority of gamma-ray sources. For Crab-like spectrum sources, the GRID angular resolution (FWHM of ~4deg at 100 MeV; ~0.8deg at 1 GeV; ~0.9deg integrating the full energy band from 100 MeV to tens of GeV) is stable across a large field of view, being characterized by a flat response up to 30deg off-axis. A comparison of the angular resolution obtained by the two operational gamma-ray instruments, AGILE-GRID and Fermi-LAT, is interesting in view of future gamma-ray missions, that are currently under study. The two instruments exploit different detector configurations affecting the angular resolution: the former being optimized in the readout and track reconstruction especially in the low-energy band, the latter in terms of converter thickness and power consumption. We show that, despite these differences, the angular resolution of both instruments is very similar between 100 MeV and a few GeV.

THEAGILEALERT SYSTEM FOR GAMMA-RAY TRANSIENTS

In recent years, a new generation of space missions offered great opportunities of discovery in high-energy astrophysics. In this article we focus on the scientific operations of the Gamma-Ray Imaging Detector (GRID) onboard the AGILE space mission. The AGILE-GRID, sensitive in the energy range of 30 MeV-30 GeV, has detected many gamma-ray transients of galactic and extragalactic origins. This work presents the AGILE innovative approach to fast gamma-ray transient detection, which is a challenging task and a crucial part of the AGILE scientific program. The goals are to describe: (1) the AGILE Gamma-Ray Alert System, (2) a new algorithm for blind search identification of transients within a short processing time, (3) the AGILE procedure for gamma-ray transient alert management, and (4) the likelihood of ratio tests that are necessary to evaluate the post-trial statistical significance of the results. Special algorithms and an optimized sequence of tasks are necessary to reach our goal. Data are automatically analyzed at every orbital downlink by an alert pipeline operating on different timescales. As proper flux thresholds are exceeded, alerts are automatically generated and sent as SMS messages to cellular telephones, e-mails, and push notifications of an application for smartphones and tablets. These alerts are crosschecked with the results of two pipelines, and a manual analysis is performed. Being a small scientific-class mission, AGILE is characterized by optimization of both scientific analysis and ground-segment resources. The system is capable of generating alerts within two to three hours of a data downlink, an unprecedented reaction time in gamma-ray astrophysics.

On-ground calibration of AGILE-GRID with a photon beam: results and lessons for the future

On the AGILE satellite, there is the Gamma Ray Imaging Detector (GRID) consisting of a Silicon Tracker (ST), a Cesium Iodide Mini-Calorimeter and an Anti-Coincidence system of plastic scintillator bars. The ST needs a calibration with a γ-ray beam to validate the simulation used to calculate the detector response versus the energy and the direction of the γ rays. A tagged γ-ray beam line was designed at the Beam Test Facility of the Laboratori Nazionali of Frascati, generated by an electron beam through bremsstrahlung in a position-sensitive target. The γ-ray energy is deduced by the difference with the post-bremsstrahlung electron energy [P. W. Cattaneo, et al., Characterization of a tagged γ-ray beam line at the daΦne beam test facility, Nucl. Instr. and Meth. A 674 (2012) 55–66; P. W. Cattaneo, et al., First results about on-ground calibration of the silicon tracker for the agile satellite, Nucl. Instr. and Meth. A 630(1) (2011) 251–257.]. The electron energy is measured by a spectrometer consisting of a dipole magnet and an array of position sensitive silicon strip detectors, the Photon Tagging System (PTS). In this paper the setup and the calibration of AGILE performed in 2005 are described.

Characterization of a tagged [formula omitted] beam line at the [formula omitted] Beam Test Facility

At the core of the AGILE scientific instrument, designed to operate on a satellite, there is the Gamma Ray Imaging Detector (GRID) consisting of a Silicon Tracker (ST), a Cesium Iodide Mini-Calorimeter and an Anti-Coincidence system of plastic scintillator bars. The ST needs an on-ground calibration with a γ-ray beam to validate the simulation used to calculate the energy response function and the effective area versus the energy and the direction of the γ rays. A tagged γ-ray beam line was designed at the Beam Test Facility (BTF) of the INFN Laboratori Nazionali of Frascati (LNF), based on an electron beam generating γ-rays through bremsstrahlung in a position-sensitive target. The γ-ray energy is deduced by difference with the post-bremsstrahlung electron energy [1,2]. The electron energy is measured by a spectrometer consisting of a dipole magnet and an array of position sensitive silicon strip detectors, the Photon Tagging System (PTS). The use of the combined BTF-PTS system as tagged photon beam requires understanding the efficiency of γ-ray tagging, the probability of fake tagging, the energy resolution and the relation of the PTS hit position versus the γ-ray energy. This paper describes this study comparing data taken during the AGILE calibration occurred in 2005 with simulation.