Context. We present the AGILE gamma-ray observations of the field containing the puzzling gamma-ray source 3EG J1835+5918. This source is one of the most remarkable unidentified EGRET sources. Aims. ...An unprecedentedly long AGILE monitoring of this source yields important information on the positional error box, flux evolution, and spectrum. Methods. 3EG J1835+5918 has been in the AGILE field of view several times in 2007 and 2008 for a total observing time of 138 days from 2007 Sep. 04 to 2008 June 30 encompassing several weeks of continuous coverage. Results. With an exposure time approximately twice that of EGRET, AGILE confirms the existence of a prominent gamma-ray source (AGL J1836+5926) at a position consistent with that of EGRET, although with a remarkably lower average flux value for photon energies greater than 100 MeV. A 5-day bin temporal analysis of the whole data set of AGL J1836+5926 shows some evidence for variability of the gamma-ray flux. The source spectrum between 100 MeV and 1 GeV can be fitted with a power law with photon index in the range 1.6–1.7, fully consistent with the EGRET value. Conclusions. The faint X-ray source RX J1836.2+5925 that has been proposed as a possible counterpart of 3EG J1835+5918 is well within the AGILE error box. Future continuous monitoring (both by AGILE and GLAST) is needed to confirm the gamma-ray flux variability and to unveil the source origin, a subject that is currently being pursued through a multiwavelength search for counterparts.
ABSTRACT
We report on 18 months of multiwavelength observations of the blazar 3C 454.3 (
Crazy Diamond
) carried out in the period 2007 July–2009 January. In particular, we show the results of the
...AGILE
campaigns which took place on 2008 May–June, 2008 July–August, and 2008 October–2009 January. During the 2008 May–2009 January period, the source average flux was highly variable, with a clear fading trend toward the end of the period, from an average γ-ray flux
F
E>100 MeV
≳ 200 × 10
−8
photons cm
−2
s
−1
in 2008 May–June, to
F
E>100 MeV
∼ 80 × 10
−8
photons cm
−2
s
−1
in 2008 October–2009 January. The average γ-ray spectrum between 100 MeV and 1 GeV can be fit by a simple power law, showing a moderate softening (from Γ
GRID
∼ 2.0 to Γ
GRID
∼ 2.2) toward the end of the observing campaign. Only 3σ upper limits can be derived in the 20–60 keV energy band with Super-
AGILE
, because the source was considerably off-axis during the whole time period. In 2007 July–August and 2008 May–June, 3C 454.3 was monitored by
Rossi X-ray Timing Explorer
(
RXTE
). The
RXTE
/Proportional Counter Array (PCA) light curve in the 3–20 keV energy band shows variability correlated with the γ-ray one. The
RXTE
/PCA average flux during the two time periods is
F
3-20 keV
= 8.4 × 10
−11
erg cm
−2
s
−1
, and
F
3-20 keV
= 4.5 × 10
−11
erg cm
−2
s
−1
, respectively, while the spectrum (a power law with photon index Γ
PCA
= 1.65 ± 0.02) does not show any significant variability. Consistent results are obtained with the analysis of the
RXTE
/High-Energy X-Ray Timing Experiment quasi-simultaneous data. We also carried out simultaneous
Swift
observations during all
AGILE
campaigns.
Swift
/XRT detected 3C 454.3 with an observed flux in the 2–10 keV energy band in the range (0.9–7.5) × 10
−11
erg cm
−2
s
−1
and a photon index in the range Γ
XRT
= 1.33–2.04. In the 15–150 keV energy band, when detected, the source has an average flux of about 5 mCrab. GASP-WEBT monitored 3C 454.3 during the whole 2007–2008 period in the radio, millimeter, near-IR, and optical bands. The observations show an extremely variable behavior at all frequencies, with flux peaks almost simultaneous with those at higher energies. A correlation analysis between the optical and the γ-ray fluxes shows that the γ-optical correlation occurs with a time lag of τ = −0.4
+0.6
−0.8
days, consistent with previous findings for this source. An analysis of 15 GHz and 43 GHz VLBI core radio flux observations in the period 2007 July–2009 February shows an increasing trend of the core radio flux, anti-correlated with the higher frequency data, allowing us to derive the value of the source magnetic field. Finally, the modeling of the broadband spectral energy distributions for the still unpublished data, and the behavior of the long-term light curves in different energy bands, allow us to compare the jet properties during different emission states, and to study the geometrical properties of the jet on a time-span longer than one year.
The AGILE space mission Tavani, M.; Barbiellini, G.; Argan, A. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
04/2008, Letnik:
588, Številka:
1-2
Journal Article
Recenzirano
AGILE is an Italian Space Agency mission dedicated to the exploration of the gamma-ray Universe. The AGILE, very innovative instrument, combines for the first time a gamma-ray imager (sensitive in ...the range 30MeV–50GeV) and a hard X-ray imager (sensitive in the range 18–60keV). An optimal angular resolution and very large fields of view are obtained by the use of state-of-the-art Silicon detectors integrated in a very compact instrument. AGILE was successfully launched on April 23, 2007 from the Indian base of Sriharikota and was inserted in an optimal low-particle background equatorial orbit. AGILE will provide crucial data for the study of Active Galactic Nuclei, Gamma-Ray Bursts, unidentified gamma-ray sources, galactic compact objects, supernova remnants, TeV sources, and fundamental physics by microsecond timing. The AGILE Cycle-1 pointing program started on 2007 December 1, and is open to the international community through a Guest Observer Program.
We report results from an intensive multiwavelength campaign on the intermediate-frequency-peaked BL Lacertae object W Com (z = 0.102) during a strong outburst of very high energy gamma-ray emission ...in 2008 June. The very high energy gamma-ray signal was detected by VERITAS on 2008 June 7-8 with a flux F(>200 GeV) =(5.7 +/- 0.6) X 10-11 cm-2 s-1, about three times brighter than during the discovery of gamma-ray emission from W Com by VERITAS in 2008 March. The initial detection of this flare by VERITAS at energies above 200 GeV was followed by observations in high-energy gamma rays (AGILE; E Delta *g>= 100 MeV), X-rays (Swift and XMM-Newton), and at UV, and ground-based optical and radio monitoring through the GASP-WEBT consortium and other observatories. Here we describe the multiwavelength data and derive the spectral energy distribution of the source from contemporaneous data taken throughout the flare.
Aims. We present a variability study of a sample of bright γ-ray(30 Mev−50 Gev) sources. This sample is an extension of the first AGILE catalogue of γ-ray sources (1AGL), obtained using the complete ...set of AGILE observations in pointing mode performed during a 2.3 year period from July 9, 2007 until October 30, 2009. Methods. The dataset of AGILE pointed observations covers a long time interval and its γ-ray data archive is useful for monitoring studies of medium-to-high brightness γ-ray sources. In the analysis reported here, we used data obtained with an improved event filter that covers a wider field of view, on a much larger (about 27.5 months) dataset, integrating data on observation block time scales, which mostly range from a few days to thirty days. Results. The data processing resulted in a better characterized source list than 1AGL was, and includes 54 sources, 7 of which are new high galactic latitude (|BII| ≥ 5) sources, 8 are new sources on the galactic plane, and 20 sources from the previous catalogue with revised positions. Eight 1AGL sources (2 high-latitude and 6 on the galactic plane) were not detected in the final processing either because of low OB exposure and/or due to their position in complex galactic regions. We report the results in a catalogue of all the detections obtained in each single OB, including the variability results for each of these sources. In particular, we found that 12 sources out of 42 or 11 out of 53 are variable, depending on the variability index used, where 42 and 53 are the number of sources for which these indices could be calculated. Seven of the 11 variable sources are blazars, the others are Crab pulsar+nebula, LS I +61°303, Cyg X-3, and 1AGLR J2021+4030.
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 repointlng during ...the period 2007 July 24-30 with its two co-aligned imagers, the Gamma-Ray Imaging Detector and the hard X-ray imager Super-AGILE sensitive in the 30 MeV to 50 GeV and 18-60 keV ranges, respectively. Over the entire period, AGILE detected y-ray emission from 3C 454.3 at a significance level of 13.8 a with an average flux (E > 100 MeV) of (280 plus or minus 40) x 10 super(-5) photons cm-2, s-1. The y-ray flux appears to be variable toward the end of the observation. No emission was detected by Super-AGILE in the energy range 20-60 keV, with a 3 o upper limit of 2.3 x 10 super(-3) photons cm-2 S-1. The Y-ray flux level of 3C 454.3 detected by AGILE Is the highest ever detected for this quasar and among the most intense Y-ray fluxes ever detected from fiat-spectrum radio quasars.
We report on the extreme γ-ray activity from the flat spectrum radio quasar (FSRQ) PKS 1510−089 observed by the AGILE satellite in March 2009. In the same period a radio-to-optical monitoring of the ...source was provided by the GASP–WEBT and REM facilities. In the radio band we made use also of multi-epoch 15-GHz Very Long Baseline Array data from the MOJAVE Program to get information on the parsec-scale structure. Moreover, several Swift target of opportunity observations were triggered, adding important information on the source behaviour from optical/UV to hard X-rays. We paid particular attention to the calibration of the Swift/UVOT data to make it suitable to the blazars spectra. Simultaneous observations from radio to γ rays allowed us to study in detail the correlation among the emission variability at differentfrequencies and to investigate the mechanisms at work during this high activity state of the source. In the period 9–30 March 2009, AGILE detected γ-ray emission from PKS 1510−089 at a significance level of 21.5-σ with an average flux over the entire period of (311 ± 21) × 10-8 photons cm-2 s-1 for photon energies above 100 MeV, and a peak level of (702 ± 131) × 10-8 photons cm-2 s-1 on daily integration. The activity detected in γ rays occurred during a period of increasing activity from near-infrared to UV, as monitored by GASP–WEBT, REM and Swift/UVOT. A flaring episode on 26–27 March 2009 was detected from near-IR to UV, suggesting that a single mechanism is responsible for the flux enhancement observed at the end of March. By contrast, Swift/XRT observations seem to show no clear correlation of the X-ray fluxes with the optical and γ-ray ones. However, the X-ray observations show a harder photon index (Γx ≃ 1.3–1.6) with respect to most FSRQs and a hint of harder-when-brighter behaviour, indicating the possible presence of a second emission component at soft X-ray energies. Moreover, the broad band spectrum from radio-to-UV confirmed the evidence of thermal features in the optical/UV spectrum of PKS 1510−089 also during high γ-ray state. On the other hand, during 25–26 March 2009 a flat spectrum in the optical/UV energy band was observed, suggesting an important contribution of the synchrotron emission in this part of the spectrum during the brightest γ-ray flare, therefore a significant shift of the synchrotron peak.
AGILE, Astrorivelatore Gamma ad Immagini LEggero, is a small space mission of the Italian Space Agency (ASI) devoted to observations for astrophysics in the gamma ray energy range 30 MeV-50 GeV with ...a simultaneous window in the X-ray band 15 keV-45 keV. AGILE payload is composed of four scientific detectors: a tungsten-silicon tracker, a CsI mini-calorimeter, a silicon based X-ray imager and an anticoincidence system for particle background rejection. Moreover, the payload is completed by a power supply unit (PSU) and a payload data handling unit (PDHU) and by three ancillary sub-systems (a GPS receiver and two star sensors). The PDHU tasks are: the payload scientific and ancillary sub-systems control, the operational modes management, the scientific data processing and the telemetry and telecommands management. The scientific data processing involves the gamma-ray photons filtering, the X-ray data acquisition and a burst acquisition logic able to perform an on-board burst coordinates determination. The HW and SW design and implementation is in charge of the Alenia Spazio S.p.A. - Laben. In this paper we present a general description of the PDHU and the guidelines for the scientific simulations and the HW and SW testing activities.
Terrestrial gamma-ray flashes (TGFs) are very short bursts of high-energy photons and electrons originating in Earth's atmosphere. We present here a localization study of TGFs carried out at ...gamma-ray energies above 20 MeV based on an innovative event selection method. We use the AGILE satellite Silicon Tracker data that for the first time have been correlated with TGFs detected by the AGILE Mini-Calorimeter. We detect 8 TGFs with gamma-ray photons of energies above 20 MeV localized by the AGILE gamma-ray imager with an accuracy of ∼5-10° at 50 MeV. Remarkably, all TGF-associated gamma rays are compatible with a terrestrial production site closer to the subsatellite point than 400 km. Considering that our gamma rays reach the AGILE satellite at 540 km altitude with limited scattering or attenuation, our measurements provide the first precise direct localization of TGFs from space.
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