Alessio Trois

Bio: Alessio Trois is an academic researcher from INAF. The author has contributed to research in topics: Physics & Pulsar. The author has an hindex of 41, co-authored 184 publications receiving 5533 citations. Previous affiliations of Alessio Trois include University of Cagliari.

Discovery of powerful gamma-ray flares from the Crab Nebula

Abstract: 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 AGILE Mission

Abstract: Context. AGILE is an Italian Space Agency mission dedicated to observing the gamma-ray Universe. The AGILE’s very innovative instrumentation for the first time combines a gamma-ray imager (sensitive in the energy range 30 MeV–50 GeV), a hard X-ray imager (sensitive in the range 18–60 keV), a calorimeter (sensitive in the range 350 keV–100 MeV), and an anticoincidence system. AGILE was successfully launched on 2007 April 23 from the Indian base of Sriharikota and was inserted in an equatorial orbit with very low particle background. Aims. AGILE provides crucial data for the study of active galactic nuclei, gamma-ray bursts, pulsars, unidentified gamma-ray sources, galactic compact objects, supernova remnants, TeV sources, and fundamental physics by microsecond timing. Methods. An optimal sky angular positioning (reaching 0.1 degrees in gamma-rays and 1–2 arcmin in hard X-rays) and very large fields of view (2.5 sr and 1 sr, respectively) are obtained by the use of Silicon detectors integrated in a very compact instrument. Results. AGILE surveyed the gamma-ray sky and detected many Galactic and extragalactic sources during the first months of observations. Particular emphasis is given to multifrequency observation programs of extragalactic and galactic objects. Conclusions. AGILE is a successful high-energy gamma-ray mission that reached its nominal scientific performance. The AGILE Cycle-1 pointing program started on 2007 December 1, and is open to the international community through a Guest Observer Program.

Neutral Pion Emission from Accelerated Protons in the Supernova Remnant W44

Abstract: 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 ({approx}20, 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 {+-}more » 0.1) and a low-energy cut-off at E{sub c} = 6 {+-} 1 GeV. Direct evidence for pion emission is then established in an SNR for the first time.« less

The SUrvey for Pulsars and Extragalactic Radio Bursts – II. New FRB discoveries and their follow-up.

Abstract: We report the discovery of four Fast Radio Bursts (FRBs) in the ongoing SUrvey for Pulsars and Extragalactic Radio Bursts at the Parkes Radio Telescope: FRBs 150610, 151206, 151230 and 160102. Our real-time discoveries have enabled us to conduct extensive, rapid multimessenger follow-up at 12 major facilities sensitive to radio, optical, X-ray, gamma-ray photons and neutrinos on time-scales ranging from an hour to a few months post-burst. No counterparts to the FRBs were found and we provide upper limits on afterglow luminosities. None of the FRBs were seen to repeat. Formal fits to all FRBs show hints of scattering while their intrinsic widths are unresolved in time. FRB 151206 is at low Galactic latitude, FRB 151230 shows a sharp spectral cut-off, and FRB 160102 has the highest dispersion measure (DM = 2596.1 ± 0.3 pc cm^−3) detected to date. Three of the FRBs have high dispersion measures (DM > 1500 pc cm^−3), favouring a scenario where the DM is dominated by contributions from the intergalactic medium. The slope of the Parkes FRB source counts distribution with fluences >2 Jy ms is $$\alpha =-2.2^{+0.6}_{-1.2}$$ and still consistent with a Euclidean distribution (α = −3/2). We also find that the all-sky rate is $$1.7^{+1.5}_{-0.9}\times 10^3$$FRBs/(4π sr)/day above $${\sim }2{\rm \, }\rm {Jy}{\rm \, }\rm {ms}$$ and there is currently no strong evidence for a latitude-dependent FRB sky rate.

Extreme particle acceleration in the microquasar Cygnus X-3

Abstract: Galactic microquasars are binaries with a neutron star or stellar-mass black hole accreting gas from a companion star. They can accelerate particles to relativistic energies and emit radio flares, but the mechanism of jet energization is not known. A survey of the Cygnus region between mid-2007 and mid-2009 has detected four major γ-ray flaring episodes with energies over 100 MeV, each lasting 1 or 2 days, emanating from the microquasar Cygnus X-3. There is a clear pattern of temporal correlation between the γ-ray flares and transitional spectral states of radio and X-ray emission. Particle acceleration occurred a few days before radio jet ejections for two of the flares, suggesting that jet formation involves the production of highly energetic particles. Super-massive black holes in active galaxies can accelerate particles to relativistic energies, producing jets with associated γ-ray emission. Galactic 'microquasars' also produce relativistic jets; however, apart from an isolated event detected in Cygnus X-1, there has hitherto been no systematic evidence for the acceleration of particles to gigaelectronvolt or higher energies in a microquasar. Here, a report of four γ-ray flares with energies above 100 MeV from the microquasar Cygnus X-3 illuminates this important problem. Super-massive black holes in active galaxies can accelerate particles to relativistic energies1, producing jets with associated γ-ray emission. Galactic ‘microquasars’, which are binary systems consisting of a neutron star or stellar-mass black hole accreting gas from a companion star, also produce relativistic jets, generally together with radio flares2. Apart from an isolated event detected3 in Cygnus X-1, there has hitherto been no systematic evidence for the acceleration of particles to gigaelectronvolt or higher energies in a microquasar, with the consequence that we are as yet unsure about the mechanism of jet energization. Here we report four γ-ray flares with energies above 100 MeV from the microquasar Cygnus X-3 (an exceptional X-ray binary4,5,6 that sporadically produces radio jets7,8,9). There is a clear pattern of temporal correlations between the γ-ray flares and transitional spectral states of the radio-frequency and X-ray emission. Particle acceleration occurred a few days before radio-jet ejections for two of the four flares, meaning that the process of jet formation implies the production of very energetic particles. In Cygnus X-3, particle energies during the flares can be thousands of times higher than during quiescent states.

The Observation of Gravitational Waves from a Binary Black Hole Merger

Abstract: On September 14, 2015 at 09:50:45 UTC the two detectors of the Laser Interferometer Gravitational-Wave Observatory simultaneously observed a transient gravitational-wave signal. The signal sweeps upwards in frequency from 35 to 250 Hz with a peak gravitational-wave strain of 1.0×10(-21). It matches the waveform predicted by general relativity for the inspiral and merger of a pair of black holes and the ringdown of the resulting single black hole. The signal was observed with a matched-filter signal-to-noise ratio of 24 and a false alarm rate estimated to be less than 1 event per 203,000 years, equivalent to a significance greater than 5.1σ. The source lies at a luminosity distance of 410(-180)(+160) Mpc corresponding to a redshift z=0.09(-0.04)(+0.03). In the source frame, the initial black hole masses are 36(-4)(+5)M⊙ and 29(-4)(+4)M⊙, and the final black hole mass is 62(-4)(+4)M⊙, with 3.0(-0.5)(+0.5)M⊙c(2) radiated in gravitational waves. All uncertainties define 90% credible intervals. These observations demonstrate the existence of binary stellar-mass black hole systems. This is the first direct detection of gravitational waves and the first observation of a binary black hole merger.

The Large Area Telescope on the Fermi Gamma-ray Space Telescope Mission

Abstract: (Abridged) The Large Area Telescope (Fermi/LAT, hereafter LAT), the primary instrument on the Fermi Gamma-ray Space Telescope (Fermi) mission, is an imaging, wide field-of-view, high-energy gamma-ray telescope, covering the energy range from below 20 MeV to more than 300 GeV. This paper describes the LAT, its pre-flight expected performance, and summarizes the key science objectives that will be addressed. On-orbit performance will be presented in detail in a subsequent paper. The LAT is a pair-conversion telescope with a precision tracker and calorimeter, each consisting of a 4x4 array of 16 modules, a segmented anticoincidence detector that covers the tracker array, and a programmable trigger and data acquisition system. Each tracker module has a vertical stack of 18 x,y tracking planes, including two layers (x and y) of single-sided silicon strip detectors and high-Z converter material (tungsten) per tray. Every calorimeter module has 96 CsI(Tl) crystals, arranged in an 8 layer hodoscopic configuration with a total depth of 8.6 radiation lengths. The aspect ratio of the tracker (height/width) is 0.4 allowing a large field-of-view (2.4 sr). Data obtained with the LAT are intended to (i) permit rapid notification of high-energy gamma-ray bursts (GRBs) and transients and facilitate monitoring of variable sources, (ii) yield an extensive catalog of several thousand high-energy sources obtained from an all-sky survey, (iii) measure spectra from 20 MeV to more than 50 GeV for several hundred sources, (iv) localize point sources to 0.3 - 2 arc minutes, (v) map and obtain spectra of extended sources such as SNRs, molecular clouds, and nearby galaxies, (vi) measure the diffuse isotropic gamma-ray background up to TeV energies, and (vii) explore the discovery space for dark matter.

Multi-messenger Observations of a Binary Neutron Star Merger

Abstract: On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of $\sim$1.7 s with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a sky region of 31 deg$^2$ at a luminosity distance of $40^{+8}_{-8}$ Mpc and with component masses consistent with neutron stars. The component masses were later measured to be in the range 0.86 to 2.26 Msun. An extensive observing campaign was launched across the electromagnetic spectrum leading to the discovery of a bright optical transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC 4993 (at $\sim$40 Mpc) less than 11 hours after the merger by the One-Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The optical transient was independently detected by multiple teams within an hour. Subsequent observations targeted the object and its environment. Early ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a redward evolution over $\sim$10 days. Following early non-detections, X-ray and radio emission were discovered at the transient's position $\sim$9 and $\sim$16 days, respectively, after the merger. Both the X-ray and radio emission likely arise from a physical process that is distinct from the one that generates the UV/optical/near-infrared emission. No ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches. (Abridged)

Multi-messenger observations of a binary neutron star merger

Abstract: On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of $\sim 1.7\,{\rm{s}}$ with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a sky region of 31 deg2 at a luminosity distance of ${40}_{-8}^{+8}$ Mpc and with component masses consistent with neutron stars. The component masses were later measured to be in the range 0.86 to 2.26 $\,{M}_{\odot }$. An extensive observing campaign was launched across the electromagnetic spectrum leading to the discovery of a bright optical transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC 4993 (at $\sim 40\,{\rm{Mpc}}$) less than 11 hours after the merger by the One-Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The optical transient was independently detected by multiple teams within an hour. Subsequent observations targeted the object and its environment. Early ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a redward evolution over ~10 days. Following early non-detections, X-ray and radio emission were discovered at the transient's position $\sim 9$ and $\sim 16$ days, respectively, after the merger. Both the X-ray and radio emission likely arise from a physical process that is distinct from the one that generates the UV/optical/near-infrared emission. No ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches. These observations support the hypothesis that GW170817 was produced by the merger of two neutron stars in NGC 4993 followed by a short gamma-ray burst (GRB 170817A) and a kilonova/macronova powered by the radioactive decay of r-process nuclei synthesized in the ejecta.

Parameter estimation in astronomy through application of the likelihood ratio. [satellite data analysis techniques

Abstract: Many problems in the experimental estimation of parameters for models can be solved through use of the likelihood ratio test. Applications of the likelihood ratio, with particular attention to photon counting experiments, are discussed. The procedures presented solve a greater range of problems than those currently in use, yet are no more difficult to apply. The procedures are proved analytically, and examples from current problems in astronomy are discussed.