M. Giusti

Discovery of powerful gamma-ray flares from the Crab Nebula.

Gamma-ray observations of the Crab Nebula by two different space telescopes challenge particle acceleration theory. 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 BRIGHTEST GAMMA-RAY FLARING BLAZAR IN THE SKY: AGILE AND MULTI-WAVELENGTH OBSERVATIONS OF 3C 454.3 DURING 2010 NOVEMBER

Since 2005, the blazar 3C 454.3 has shown remarkable flaring activity at all frequencies, and during the last four years it has exhibited more than one ?-ray flare per year, becoming the most active ?-ray blazar in the sky. We present for the first time the multi-wavelength AGILE, Swift, INTEGRAL, and GASP-WEBT data collected in order to explain the extraordinary ?-ray flare of 3C 454.3 which occurred in 2010 November. On 2010 November 20 (MJD 55520), 3C 454.3 reached a peak flux (E >100?MeV) of Fp ? = (6.8 ? 1.0) ? 10?5?photons cm?2 s?1? on a timescale of about 12 hr, more than a factor of six higher than the flux of the brightest steady ?-ray source, the Vela pulsar, and more than a factor of three brighter than its previous super-flare on 2009 December 2-3. The multi-wavelength data make possible a thorough study of the present event: the comparison with the previous outbursts indicates a close similarity to the one that occurred in 2009. By comparing the broadband emission before, during, and after the ?-ray flare, we find that the radio, optical, and X-ray emission varies within a factor of 2-3, whereas the ?-ray flux by a factor of 10. This remarkable behavior is modeled by an external Compton component driven by a substantial local enhancement of soft seed photons.

THE EXTRAORDINARY GAMMA-RAY FLARE OF THE BLAZAR 3C 454.3

We present the gamma-ray data of the extraordinary flaring activity above 100?MeV from the flat spectrum radio quasar 3C 454.3 detected by AGILE during the month of 2009 December. 3C 454.3, which has been among the most active blazars of the FSRQ type since 2007, has been detected in the gamma-ray range with a progressively rising flux since 2009 November 10. The gamma-ray flux reached a value comparable with that of the Vela pulsar on 2009 December 2. Remarkably, between 2009 December 2 and 3, the source more than doubled its gamma-ray emission and became the brightest gamma-ray source in the sky with a peak flux of F ?,p = (2000 ? 400) ? 10?8 ph cm?2 s?1 for a 1 day integration above 100?MeV. The gamma-ray intensity decreased in the following days with the source flux remaining at large values near F ? (1000 ? 200) ? 10?8 ph cm?2 s?1 for more than a week. This exceptional gamma-ray flare dissipated among the largest ever detected intrinsic radiated power in gamma-rays above 100?MeV (L ?,source,peak 3 ? 1046 erg s?1, for a relativistic Doppler factor of ? 30). The total isotropic irradiated energy of the month-long episode in the range 100?MeV-3?GeV is E ?,iso 1056 erg. We report the intensity and spectral evolution of the gamma-ray emission across the flaring episode. We briefly discuss the important theoretical implications of our detection.

GAMMA-RAY OBSERVATIONS OF CYGNUS X-1 ABOVE 100 MeV IN THE HARD AND SOFT STATES

We present the results of multi-year gamma-ray observations by the AGILE satellite of the black hole binary system Cygnus X-1. In a previous investigation we focused on gamma-ray observations of Cygnus X-1 in the hard state during the period mid-2007/2009. Here we present the results of the gamma-ray monitoring of Cygnus X-1 during the period 2010/mid-2012 which includes a remarkably prolonged “soft state” phase (2010 June–2011 May). Previous 1–10 MeV observations of Cyg X-1 in this state hinted at a possible existence of a nonthermal particle component with substantial modifications of the Comptonized emission from the inner accretion disk. Our AGILE data, averaged over the mid-2010/mid-2011 soft state of Cygnus X-1, provide a significant upper limit for gamma-ray emission above 100 MeV of Fsoft < 20 × 10 −8 photons cm −2 s −1 , excluding the existence of prominent non-thermal emission above 100 MeV during the soft state of Cygnus X-1. We discuss theoretical implications of our findings in the context of high-energy emission models of black hole accretion. We also discuss possible gamma-ray flares detected by AGILE. In addition to a previously reported episode observed by AGILE in 2009 October during the hard state, we report a weak but important candidate for enhanced emission which occurred at the end of 2010 June (2010 June 30 10:00–2010 July 2 10:00 UT) exactly coinciding with a hard-to-soft state transition and before an anomalous radio flare. An appendix summarizes all previous high-energy observations and possible detections of Cygnus X-1 above 1 MeV.

The AGILE monitoring of Cygnus X-3: transient gamma-ray emission and spectral constraints

We present the AGILE-GRID (Astro-rivelatore Gamma a Immagini LEggero – Gamma-Ray Imaging Detector) monitoring of Cygnus X-3, during the period between November 2007 and July 2009. We report here the whole AGILE-GRID monitoring of Cygnus X-3 in the AGILE “pointing” mode data-taking, to confirm that the γ-ray activity coincides with the same repetitive pattern of multiwavelength emission and analyze in depth the overall γ-ray spectrum by assuming both leptonic and hadronic scenarios. Seven intense γ-ray events were detected in this period, with a typical event lasting one or two days. These durations are longer than the likely cooling times of the γ-ray emitting particles, implying we see continuous acceleration rather than the result of an impulsive event such as the ejection of a single plasmoid that then cools as it propagates outwards. Cross-correlating the AGILE-GRID light curve with both X-ray and radio monitoring data, we find that the main events of γ-ray activity were detected while the system was in soft spectral X-ray states (RXTE/ASM (Rossi X-ray Timing Explorer/All-Sky Monitor) count rate in the 3−5 keV band 3 counts s −1 ), that coincide with local and often sharp minima of the hard X-ray flux (Swift/BAT (Burst Alert Telescope) count rate 0.02 counts cm −2 s −1 ), a few days before intense radio outbursts. This repetitive temporal coincidence between the γ-ray transient emission and spectral state changes of the source turns out to be the spectral signature of γ-ray activity from this microquasar. These γ-ray events may thus reflect a sharp transition in the structure of the accretion disk and its corona, which leads to a rebirth of the microquasar jet and subsequent enhanced activity in the radio band. The γ-ray differential spectrum of Cygnus X-3 (100 MeV–3 GeV), which was obtained by averaging the data collected by the AGILE-GRID during the γ-ray events, is consistent with a power law of photon index α = 2.0 ± 0.2. Finally, we examine leptonic and hadronic emission models for the γ-ray events and find that both scenarios are valid. In the leptonic model – based on inverse Compton scatterings of mildly relativistic electrons on soft photons from both the Wolf-Rayet companion star and the accretion disk – the emitting particles may also contribute to the overall hard X-ray spectrum, possibly explaining the hard non-thermal power-law tail seen during special soft X-ray states in Cygnus X-3.

AGILE detection of Cygnus X-3 gamma-ray active states during the period mid-2009/mid-2010

Context. Cygnus X-3 (Cyg X-3) is a well-known microquasar producing variable emission at all wavelengths. Cyg X-3 is a prominent X-ray binary producing relativistic jets, and studying its high energy emission is crucial for the understanding of the fundamental acceleration processes in accreting compact objects. Aims. Our goal is to study extreme particle acceleration and γ-ray production above 100 MeV during special spectral states of Cyg X-3 usually characterized by a low hard X-ray flux and enhanced soft X-ray states. Methods. We observed Cyg X-3 with the AGILE satellite in extended time intervals from 2009 Jun.–Jul., and 2009 Nov.–2010 Jul. We report here the results of the AGILE γ-ray monitoring of Cyg X-3 as well as the results from extensive multiwavelength campaigns involving radio (RATAN-600, AMI-LA and Metsahovi Radio Observatories) and X-ray monitoring data (XTE and Swift). Results. We detect a series of repeated γ-ray flaring activity from Cyg X-3 that correlate with the soft X-ray states and episodes of decreasing or non-detectable hard X-ray emission. Furthermore, we detect γ-ray enhanced emission that tends to be associated with radio flares greater than 1 Jy at 15 GHz, confirming a trend already detected in previous observations. The source remained active above 100 MeV for an extended period of time (almost 1.5 months in 2009 Jun.–Jul. and 1 month in 2010 May). We study in detail the short timescale γ-ray flares that occurred before or near the radio peaks. Conclusions. Our results confirm the transient nature of the extreme particle acceleration from the microquasar Cyg X-3. A series of repeated γ-ray flares shows correlations with radio and X-ray emission confirming a well established trend of emission. We compare our results with Fermi-LAT and MAGIC TeV observations of Cyg X-3.

Study of the γ-ray source 1AGL J2022+4032 in the Cygnus region

Context. Identification of -ray-emitting Galactic sources is a long-standing problem in astrophysics. One such source, 1AGL J2022+4032, coincident with the interior of the radio shell of the supernova remnant Gamma Cygni (SNR G78.2+2.1) in the Cygnus Region, has recently been identified by Fermi as a -ray pulsar, LAT PSR J2021+4026. Aims. We present long-term observations of 1AGL J2022+4032 with the AGILE -ray telescope, measuring its flux and light curve. Methods. We compare the light curve of 1AGL J2022+4032 with that of 1AGL J2021+3652 (PSR J2021+3651), showing that the flux variability of 1AGL J2022+4032 appears to be greater than the level predicted from statistical and systematic e ects and producing detailed simulations to estimate the probability of the apparent observed variability. Results. We evaluate the possibility that the -ray emission may be due to the superposition of two or more point sources, some of which may be variable, considering a number of possible counterparts. Conclusions. We consider the possibility of a nearby X-ray quiet microquasar contributing to the flux of 1AGL J2022+4032 to be more likely than the hypotheses of a background blazar or intrinsic -ray variabilty of LAT PSR J2021+4026.

Calibration of AGILE-GRID with in-flight data and Monte Carlo simulations

Context. AGILE is a γ-ray astrophysics mission which has been in orbit since 23 April 2007 and continues to operate reliably. The γ-ray detector, AGILE-GRID, has observed Galactic and extragalactic sources, many of which were collected in the first AGILE Catalog. Aims. We present the calibration of the AGILE-GRID using in-flight data and Monte Carlo simulations, producing instrument response functions (IRFs) for the effective area (A eff), energy dispersion probability (EDP), and point spread function (PSF), each as a function of incident direction in instrument coordinates and energy. Methods. We performed Monte Carlo simulations at different γ-ray energies and incident angles, including background rejection filters and Kalman filter-based γ-ray reconstruction. Long integrations of in-flight observations of the Vela, Crab and Geminga sources in broad and narrow energy bands were used to validate and improve the accuracy of the instrument response functions. Results. The weighted average PSFs as a function of spectra correspond well to the data for all sources and energy bands. Conclusions. Changes in the interpolation of the PSF from Monte Carlo data and in the procedure for construction of the energy-weighted effective areas have improved the correspondence between predicted and observed fluxes and spectra of celestial calibration sources, reducing false positives and obviating the need for post-hoc energy-dependent scaling factors. The new IRFs have been publicly available from the AGILE Science Data Center since November 25, 2011, while the changes in the analysis software will be distributed in an upcoming release.

The characterization of the distant blazar GB6 J1239+0443 from flaring and low activity periods

In 2008, AGILE and Fermi detected gamma-ray flaring activity from the unidentified EGRET source 3EG J1236+0457, recently associated with a flat spectrum radio quasar (GB6

Upper limits on the high-energy emission from gamma-ray bursts observed by AGILE-GRID

Context. The detection and the characterization of the high energy emission component from individual gamma-ray bursts (GRBs) is one of the key science objectives of the currently operating gamma-ray satellite AGILE, launched in April 2007. In its first two years of operation AGILE detected three GRBs with photons of energy larger than 30 MeV. One more GRB was detected in AGILE third operation year, while operating in spinning mode. Aims. For the 64 other GRBs localized during the period July 2007 to October 2009 in the field of view of the AGILE Gamma-Ray Imaging Detector (GRID), but not detected by this instrument, we estimate the count and flux upper limits on the GRB high energy emission in the AGILE-GRID energy band (30 MeV− 3G eV). Methods. To calculate the count upper limits, we adopted a Bayesian approach. The flux upper limits are derived using several assumptions on the high-energy spectral behavior. For 28 GRBs with available prompt spectral information, a flux upper limit and the comparison with the expected flux estimated from spectral extrapolation of the Band spectrum to the 30 MeV−3 GeV band are provided. Moreover, upper limits on the flux under the assumption of an extra power law component dominating the 30 MeV−3 GeV band are calculated for all GRBs and considering four different values for the spectral photon index. Finally, we performed a likelihood upper limit on the possible delayed emission up t o1ha fter the GRB. Results. The estimated flux upper limits range between 1 × 10 −4 and ∼2 × 10 −2 photons cm −2 s −1 and generally lie above the flux estimated from the extrapolation of the prompt emission in the 30 MeV−3 GeV band. A notable case is GRB 080721, where the AGILE-GRID upper limit suggests a steeper spectral index or the presence of a cut-off in the high energy part of the Band prompt spectrum. The four GRBs detected by AGILE-GRID show high-energy (30 MeV− 3G eV) to low-energy (1 keV−10 MeV) fluence ratios similar to those estimated in this paper for the 64 GRBs without GRID detection, favoring the possibility that AGILE-GRID detected only high-fluence, hard GRBs. From the flux upper limits derived in this work we put some constraint on high-energy radiation from the afterglow emission and from synchrotron self Compton emission in internal shocks.