arXiv subjects

G. Piano

Publications and source records attributed to G. Piano.

At least 37 records · Page 2Linked to original sources

arXiv2018

A new gamma-ray source unveiled by AGILE in the region of Orion

Diffuse galactic gamma-ray emission is produced by the interaction of cosmic rays (CRs) with the interstellar environment. The study of gamma-ray emission is therefore a powerful tool to investigate the origin of CRs and the processes through which they are accelerated. We aim to gain deeper insights of the nature of gamma-ray emission in the region of Orion, which is one of the best studied sites of on-going star formation, by analysing data from the AGILE satellite. The diffuse gamma-ray emission expected from the Orion region is relatively high. Its separation from the galactic plane also ensures a very low contribution from foreground or background emission, which makes it an ideal site for studying the processes of particle acceleration in star forming environments. The AGILE data are modelled through a template that quantifies the gamma-ray diffuse emission expected from atomic and molecular hydrogen. Other sources of emission are modelled as an isotropic contribution. Gamma-ray emission exceeding the amount expected by the diffuse emission model is detected with high level of significance. The main excess is in the high-longitude part of Orion A. A thorough analysis of this feature suggests a connection between the observed gamma-ray emission and the B0.5 Ia star k Orionis. The location of the gamma-ray excess is compatible with the site where stellar wind collides with the ISM. Both scattering on dark gas and cosmic-ray acceleration at the shock between the two environments are discussed as possible explanations, with the latter hypothesis being supported by the hardness of the energy spectrum of the emission. If confirmed, this would be the first direct detection of gamma-ray emission from the interaction between ISM and a single star's stellar wind.

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arXiv2017

The hypersoft state of Cygnus X-3 - A key to jet quenching in X-ray binaries?

Cygnus X-3 is a unique microquasar in the Galaxy hosting a Wolf-Rayet companion orbiting a compact object that most likely is a low-mass black hole. The unique source properties are likely due to the interaction of the compact object with the heavy stellar wind of the companion. In this paper, we concentrate on a very specific period of time prior to the massive outbursts observed from the source. During this period, Cygnus X-3 is in a so-called hypersoft state, where the radio and hard X-ray fluxes are found to be at their lowest values (or non-detected), the soft X-ray flux is at its highest values, and sporadic gamma-ray emission is observed. We will utilize multiwavelength observations in order to study the nature of the hypersoft state. We observed Cygnus X-3 during the hypersoft state with Swift and NuSTAR in the X-rays and SMA, AMI-LA, and RATAN-600 in the radio. We also considered X-ray monitoring data from MAXI and $γ$-ray monitoring data from AGILE and Fermi. We found that the spectra and timing properties of the multiwavelength observations can be explained by a scenario where the jet production is turned off or highly diminished in the hypersoft state and the missing jet pressure allows the wind to refill the region close to the black hole. The results provide proof of actual jet quenching in soft states of X-ray binaries.

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arXiv2017

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

The LIGO-Virgo Collaboration (LVC) detected, on 2017 August 17, an exceptional gravitational-wave (GW) event temporally consistent within $\sim\,1.7 \, \rm s$ 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 (e.m.) 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 (GRID) 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 $10^{15} \, \rm G$. Our data are particularly significant during the early stage of evolution of the e.m. remnant.

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arXiv2017

AGILE Observations of the Gravitational Wave Source GW170104

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 $4.4\,σ$ lasting about 32 ms and occurring $0.46\,\pm\,0.05 \,\rm s$ before $T_0$. A study of the MCAL background and of the false-alarm rate of E2 leads to the determinination of a post-trial significance between $2.4\,σ$ and $2.7\,σ$ for a temporal coincidence with GW170104. We note that E2 has characteristics similar to those detected from the weak precursor of the short 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 sec and involving $\sim 10^{-7}$ of the total rest mass energy of the system.

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arXiv2017

AGILE detection of a candidate gamma-ray precursor to the ICECUBE-160731 neutrino event

On July 31st, 2016, the ICECUBE collaboration reported the detection of a high-energy starting event induced by an astrophysical neutrino. We report here about the search for a gamma-ray counterpart of the ICECUBE-160731 event made with the AGILE satellite. No detection was found spanning the time interval of +/- 1 ks around the neutrino event time T0 using the AGILE "burst search" system. Looking for a possible gamma-ray precursor in the results of the AGILE-GRID automatic Quick Look procedure over predefined 48-hours time-bins, we found an excess above 100 MeV between one and two days before T0, positionally consistent with the ICECUBE error circle, having a post-trial significance of about 4 sigma. A refined data analysis of this excess confirms a-posteriori the automatic detection. The new AGILE transient source, named AGL J1418+0008, thus stands as possible ICECUBE-160731 gamma-ray precursor. No other space missions nor ground observatories have reported any detection of transient emission consistent with the ICECUBE event. We show that Fermi-LAT had a low exposure of the ICECUBE region during the AGILE gamma-ray transient. Based on an extensive search for cataloged sources within the error regions of ICECUBE-160731 and AGL J1418+0008, we find a possible common counterpart showing some of the key features associated to the high-energy peaked BL Lac (HBL) class of blazars. Further investigations on the nature of this source using dedicated SWIFT ToO data are presented.

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arXiv2017

High-energy gamma-ray activity from V404 Cygni detected by AGILE during the 2015 June outburst

The AGILE satellite detected transient high-energy gamma-ray emission from the X-ray binary V404 Cygni, during the June 2015 outburst observed in radio, optical, X-ray and soft gamma-ray frequencies. The activity was observed by AGILE in the 50-400 MeV energy band, between 2015-06-24 UT 06:00:00 and 2015-06-26 UT 06:00:00 (MJD 57197.25 - 57199.25), with a detection significance of ${\sim}4.3σ$. The gamma-ray detection, consistent with a contemporaneous observation by Fermi-LAT, is correlated with a bright flare observed at radio and hard X-ray frequencies, and with a strong enhancement of the 511 keV line emission, possibly indicating plasmoid ejections in a lepton-dominated transient jet. The AGILE observations of this binary system are compatible with a microquasar scenario in which transient jets are responsible for the high-energy gamma-ray emission.

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arXiv2017

The e-ASTROGAM gamma-ray space mission

The e-ASTROGAM is a gamma-ray space mission to be proposed as the M5 Medium-size mission of the European Space Agency. It is dedicated to the observation of the Universe with unprecedented sensitivity in the energy range 0.2 - 100 MeV, extending up to GeV energies, together with a groundbreaking polarization capability. It is designed to substantially improve the COMPTEL and Fermi sensitivities in the MeV-GeV energy range and to open new windows of opportunity for astrophysical and fundamental physics space research. e-ASTROGAM will operate as an open astronomical observatory, with a core science focused on (1) the activity from extreme particle accelerators, including gamma-ray bursts and active galactic nuclei and the link of jet astrophysics to the new astronomy of gravitational waves, neutrinos, ultra-high energy cosmic rays, (2) the high-energy mysteries of the Galactic center and inner Galaxy, including the activity of the supermassive black hole, the Fermi Bubbles, the origin of the Galactic positrons, and the search for dark matter signatures in a new energy window; (3) nucleosynthesis and chemical evolution, including the life cycle of elements produced by supernovae in the Milky Way and the Local Group of galaxies. e-ASTROGAM will be ideal for the study of high-energy sources in general, including pulsars and pulsar wind nebulae, accreting neutron stars and black holes, novae, supernova remnants, and magnetars. And it will also provide important contributions to solar and terrestrial physics. The e-ASTROGAM telescope is optimized for the simultaneous detection of Compton and pair-producing gamma-ray events over a large spectral band. It is based on a very high technology readiness level for all subsystems and includes many innovative features for the detectors and associated electronics.

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arXiv2016

The gamma-ray source AGL J2241+4454 as the possible counterpart of MWC 656

The AGILE satellite discovered the transient source AGL J2241+4454 in 2010 which triggered the study of the associated field allowing for the discovery of the first Be/black hole binary system: MWC 656. This binary was suggested to be the counterpart of AGL J2241+4454, but this is still not a firm association. In this work we explore the archival AGILE and Fermi/LAT data in order to find more transient events originating in the field of AGL J2241+4454 and address the possibility to link them to the accretion/ejection processes of MWC 656. We found a total of 9 other transient events with AGILE compatible with the position of AGL J2241+4454, besides the one from 2010. We folded this events with the period of the binary system and we did not find significant results that allow us to associate the gamma-ray activity to any particular orbital phase. By stacking the 10 transient events we obtained a spectrum that extends between 100 MeV and 1 GeV, and we fitted it with a power law with photon index Γ = 2.3 +/- 0.2. We searched into the Fermi /LAT data in order to complement the gamma-ray information provided by AGILE but no significant results arose. In order to investigate this apparent contradiction between the two gamma-ray telescopes, we studied the exposure of the field of AGL J2241+4454 in both instruments finding significant differences. In particular, AGILE exposed for longer time and at lower off-axis angle distance the field of AGL J2241+4454. This fact, together with the energy dependent sensitivity of both instruments, and the soft spectrum found in the stacking analysis, might explain why AGILE observed the transient events not seen by Fermi/LAT.

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arXiv2016

AGILE Observations of the Gravitational Wave Event GW150914

We report the results of an extensive search in the AGILE data for a gamma-ray counterpart of the LIGO gravitational wave event GW150914. Currently in spinning mode, AGILE has the potential of covering with its gamma-ray instrument 80 % of the sky more than 100 times a day. It turns out that AGILE came within a minute from the event time of observing the accessible GW150914 localization region. Interestingly, the gamma-ray detector exposed about 65 % of this region during the 100 s time intervals centered at -100 s and +300 s from the event time. We determine a 2-sigma flux upper limit in the band 50 MeV - 10 GeV, $UL = 1.9 \times 10^{-8} \rm \, erg \, cm^{-2} \, s^{-1}$ obtained about 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 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 gravitational wave sources are decidedly promising.

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arXiv2015

Data Reduction of Multi-wavelength Observations

Multi-messenger astronomy is becoming the key to understanding the Universe from a comprehensive perspective. In most cases, the data and the technology are already in place, therefore it is important to provide an easily-accessible package that combines datasets from multiple telescopes at different wavelengths. In order to achieve this, we are working to produce a data analysis pipeline that allows the data reduction from different instruments without needing detailed knowledge of each observation. Ideally, the specifics of each observation are automatically dealt with, while the necessary information on how to handle the data in each case is provided by a tutorial that is included in the program. We first focus our project on the study of pulsars and their wind nebulae (PWNe) at radio and gamma-ray frequencies. In this way, we aim to combine time-domain and imaging datasets at two extremes of the electromagnetic spectrum. In addition, the emission has the same non-thermal origin in pulsars at radio and gamma-ray frequencies, and the population of electrons is believed to be the same at these energies in PWNe. The final goal of the project will be to unveil the properties of these objects by tracking their behaviour using all of the available multi-wavelength data.

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arXiv2015

Study of the Gamma-ray performance of the GAMMA-400 Calorimeter

GAMMA-400 is a new space mission, designed as a dual experiment, capable to study both high energy gamma rays (from $\sim$100 MeV to few TeV) and cosmic rays (electrons up to 20 TeV and nuclei up to $\sim$10$^{15}$ eV). The full simulation framework of GAMMA-400 is based on the Geant4 toolkit. The details of the gamma-ray reconstruction pipeline in the pre-shower and calorimeter will be outlined. The performance of GAMMA-400 (PSF, effective area) have been obtained using this framework. The most updated results on them will be shown.

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arXiv2013

Hadronic Gamma-Ray and Neutrino Emission from Cygnus X-3

Cygnus X-3 (Cyg X-3) is a remarkable Galactic microquasar (X-ray binary) emitting from radio to $γ$-ray energies. In this paper, we consider hadronic model of emission of $γ$-rays above 100 MeV and their implications. We focus here on the joint $γ$-ray and neutrino production resulting from proton-proton interactions within the binary system. We find that the required proton injection kinetic power, necessary to explain the $γ$-ray flux observed by AGILE and Fermi-LAT, is $L_p \sim 10^{38}\:\rm{erg\:s^{-1}}$, a value in agreement with the average bolometric luminosity of the hypersoft state (when Cygnus X-3 was repeatedly observed to produce transient $γ$-ray activity). If we assume an increase of the wind density at the superior conjunction, the asymmetric production of $γ$-rays along the orbit can reproduce the observed modulation. According to observational constraints and our modelling, a maximal flux of high-energy neutrinos would be produced for an initial proton distribution with a power-law index $α=2.4$. The predicted neutrino flux is almost two orders of magnitude less than the 2-month IceCube sensitivity at $\sim$1 TeV. If the protons are accelerated up to PeV energies, the predicted neutrino flux for a prolonged "soft X-ray state" would be a factor of about 3 lower than the 1-year IceCube sensitivity at $\sim$10 TeV. This study shows that, for a prolonged soft state (as observed in 2006) possibly related with $γ$-ray activity and a hard distribution of injected protons, Cygnus X-3 might be close to being detectable by cubic-kilometer neutrino telescopes such as IceCube.

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arXiv2013

An updated list of AGILE bright gamma-ray sources and their variability in pointing mode

We present a variability study of a sample of bright gamma-ray (30 MeV -- 50 GeV) sources. This sample is an extension of the first AGILE catalogue of gamma-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. The dataset of AGILE pointed observations covers a long time interval and its gamma-ray data archive is useful for monitoring studies of medium-to-high brightness gamma-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. 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.

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arXiv2013

Calibration of AGILE-GRID with In-Flight Data and Monte Carlo Simulations

Context: AGILE is a gamma-ray astrophysics mission which has been in orbit since 23 April 2007 and continues to operate reliably. The gamma-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 gamma-ray energies and incident angles, including background rejection filters and Kalman filter-based gamma-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 Centre since November 25, 2011, while the changes in the analysis software will be distributed in an upcoming release.

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arXiv2013

Transient Gamma-ray Emission from Cygnus X-3 Detected by AGILE: Leptonic and Hadronic Emission Models

The AGILE satellite detected several episodes of transient gamma-ray emission from Cygnus X-3. Cross-correlating the AGILE light curve with both X-ray and radio monitoring data, we found that the main events of gamma-ray activity were detected while the system was in soft spectral X-ray states, that coincide with local and often sharp minima of the hard X-ray flux, a few days before intense radio outbursts. This repetitive temporal coincidence between the gamma-ray transient emission and spectral state changes of the source turns out to be the spectral signature of high-energy activity from this microquasar. The gamma-ray differential spectrum of Cygnus X-3 (100 MeV - 3 GeV), which was obtained by averaging the data collected by AGILE during the gamma-ray events, is consistent with a power law of photon index α = 2.0 +/- 0.2. Finally, we examined leptonic and hadronic emission models for the gamma-ray activity and found that both scenarios are valid. In particular, 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.

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arXiv2013

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

We present the AGILE-GRID 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 gamma-ray activity coincides with the same repetitive pattern of multiwavelength emission and to analyze in depth the overall gamma-ray spectrum by assuming both leptonic and hadronic scenarios. Seven intense gamma-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 gamma-ray emitting particles, implying we see continuous acceleration rather than the result of an impulsive event such as the ejection of a single plasmoid which then cools as it propagates outwards. Cross-correlating the AGILE-GRID light curve with X-ray and radio monitoring data, we find that the main events of gamma-ray activity have been detected while the system was in soft spectral X-ray states (RXTE/ASM count rate > 3 counts/s), that coincide with local and often sharp minima of the hard X-ray flux (Swift/BAT count rate < 0.02 counts/cm^2/s), a few days before intense radio outbursts. [...] These gamma-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 radio activity. [...] Finally, we examine leptonic and hadronic emission models for the gamma-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 the Wolf-Rayet companion star and from 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 sometimes seen during special soft X-ray states in Cygnus X-3.

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arXiv2013

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 carried out for which includes a remarkably prolonged `soft state' phase (June 2010 -- May 2011). Previous 1--10 MeV observations of Cyg X-1 in this state hinted at a possible existence of a non-thermal 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 F_soft < 20 x 10^{-8} ph/cm^2/s, 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 October 2009 during the hard state, we report a weak but important candidate for enhanced emission which occurred at the end of June 2010 (2010-06-30 10:00 - 2010-07-02 10:00 UT) exactly in coincidence 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.

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arXiv2013

Variable gamma-ray emission from the Crab Nebula: short flares and long "waves"

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 Sept. 2007 and Sept. 2012, 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 Sept. - Oct. 2007 gamma-ray enhancement episode detected by AGILE that shows both "wave" and flaring behavior. We extend our analysis to the publicly available Fermi-LAT dataset and show that several additional "wave" episodes can be identified. We discuss the spectral properties of the September 2007 "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 \sim 10^{16} cm and enhanced magnetic fields B \sim (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_w \sim 10^{42} erg, where E_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 spindown 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.

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