Searcharxiv⌕ Search

arXiv subjects

I. Donnarumma

Publications and source records attributed to I. Donnarumma.

At least 55 records · Page 3Linked to original sources
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.

astro-ph.HE↗
arXiv2015

Search of MeV-GeV counterparts of TeV sources with AGILE in pointing mode

Context. Known TeV sources detected by major Cerenkov telescopes are investigated to identify possible MeV-GeV gamma-ray counterparts. Aims. A systematic study of the known sources in the web-based TeVCat Catalog has been performed to search for possible gamma-ray counterpart on the AGILE data collected during the first period of operations in observing pointing mode. Methods. For each TeV source, a search for a possible gamma-ray counterpart based on a multi-source Maximum Likelihood algorithm is performed on the AGILE data taken with the GRID instrument from July 2007 to October 2009. Results. In case of high-significance detection, the average gamma-ray flux is estimated. For the cases of low-significance detection the 95 % Confidence Level (C.L.) flux upper limit is given. 52 TeV sources out of 152 (corresponding to ~34 % of the analysed sample) show a significant excess in the AGILE data covering the pointing observation period. Conclusions. This analysis found 26 new AGILE sources with respect to the AGILE reference catalogs, 15 of which are Galactic, 7 are extragalactic and 4 are unidentified. Detailed tables with all available information on the analysed sources are presented.

astro-ph.HE↗
arXiv2015

On the Angular Resolution of the AGILE gamma-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.

astro-ph.HE↗
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.

astro-ph.IM↗
arXiv2015

Radio-X-ray Synergy to discover and Study Jetted Tidal Disruption Events

Observational consequences of tidal disruption of stars (TDEs) by supermassive black holes (SMBHs) can enable us to discover quiescent SMBHs, constrain their mass function, study formation and evolution of transient accretion disks and jet formation. A couple of jetted TDEs have been recently claimed in hard X-rays, challenging jet models, previously applied to $γ$-ray bursts and active galactic nuclei. It is therefore of paramount importance to increase the current sample. In this paper, we find that the best strategy is not to use up-coming X-ray instruments alone, which will yield between several (e-Rosita) and a couple of hundreds (Einstein Probe) events per year below redshift one. We rather claim that a more efficient TDE hunter will be the Square Kilometer Array (SKA) operating {\it in survey mode} at 1.4 GHz. It may detect up to several hundreds of events per year below $z \sim 2.5$ with a peak rate of a few tens per year at $z\approx 0.5$. Therefore, even if the jet production efficiency is {\it not } $100\%$ as assumed here, the predicted rates should be large enough to allow for statistical studies. The characteristic TDE decay of $t^{-5/3}$, however, is not seen in radio, whose flux is quite featureless. {\it Identification} therefore requires localization and prompt repointing by higher energy instruments. If radio candidates would be repointed within a day by future X-ray observatories (e.g. Athena and LOFT-like missions), it will be possible to detect up to $\approx 400$ X-ray counterparts, almost up to redshift $2$. The shortcome is that only for redshift below $\approx 0.4$ the trigger times will be less than 10 days from the explosion. In this regard the X-ray surveys are better suited to probe the beginning of the flare, and are therefore complementary to SKA.

astro-ph.HE↗
arXiv2015

SKA as a powerful hunter of jetted Tidal Disruption Events

Observational consequences of the tidal disruption of stars by supermassive black holes (SMBHs) can enable us to discover quiescent SMBHs and constrain their mass function. Moreover, observing jetted TDEs (from previously non-active galaxies) provides us with a new means of studying the early phases of jet formation and evolution in an otherwise "pristine" environment. Although several (tens) TDEs have been discovered since 1999, only two jetted TDEs have been recently discovered in hard X-rays, and only one, Swift J1644+57, has a precise localization which further supports the TDE interpretation. These events alone are not sufficient to address those science issues, which require a substantial increase of the current sample. Despite the way they were discovered, the highest discovery potential for {\em jetted} TDEs is not held by current and up-coming X-ray instruments, which will yield only a few to a few tens events per year. In fact, the best strategy is to use the Square Kilometer Array to detect TDEs and trigger multi-wavelength follow-ups, yielding hundreds candidates per year, up to $z \sim 2$. Radio and X-ray synergy, however, can in principle constrain important quantities such as the absolute rate of jetted TDEs, their jet power, bulk Lorentz factor, the black hole mass function, and perhaps discover massive black holes (MBH) with $<10^{5} M_{\odot}$. Finally, when comparing SKA results with information from optical surveys like LSST, one can more directly constrain the efficiency of jet production.

astro-ph.HE↗
arXiv2015

The physics of accretion-ejection with LOFT

This is a White Paper in support of the mission concept of the Large Observatory for X-ray Timing (LOFT), proposed as a medium-sized ESA mission. We discuss the potential of LOFT for the study of the physics of accretion and ejection around compact objects. For a summary, we refer to the paper.

astro-ph.HE↗
arXiv2015

LOFT as a discovery machine for jetted Tidal Disruption Events

This is a White Paper in support of the mission concept of the Large Observatory for X-ray Timing (LOFT), proposed as a medium-sized ESA mission. We discuss the potential of LOFT for the study of jetted tidal disruption events. For a summary, we refer to the paper.

astro-ph.HE↗
arXiv2014

The Large Observatory For x-ray Timing

The Large Observatory For x-ray Timing (LOFT) was studied within ESA M3 Cosmic Vision framework and participated in the final down-selection for a launch slot in 2022-2024. Thanks to the unprecedented combination of effective area and spectral resolution of its main instrument, LOFT will study the behaviour of matter under extreme conditions, such as the strong gravitational field in the innermost regions of accretion flows close to black holes and neutron stars, and the supra-nuclear densities in the interior of neutron stars. The science payload is based on a Large Area Detector (LAD, 10 m 2 effective area, 2-30 keV, 240 eV spectral resolution, 1 deg collimated field of view) and a WideField Monitor (WFM, 2-50 keV, 4 steradian field of view, 1 arcmin source location accuracy, 300 eV spectral resolution). The WFM is equipped with an on-board system for bright events (e.g. GRB) localization. The trigger time and position of these events are broadcast to the ground within 30 s from discovery. In this paper we present the status of the mission at the end of its Phase A study.

astro-ph.IM↗
arXiv2014

The Large Area Detector of LOFT: the Large Observatory for X-ray Timing

LOFT (Large Observatory for X-ray Timing) is one of the five candidates that were considered by ESA as an M3 mission (with launch in 2022-2024) and has been studied during an extensive assessment phase. It is specifically designed to perform fast X-ray timing and probe the status of the matter near black holes and neutron stars. Its pointed instrument is the Large Area Detector (LAD), a 10 m 2 -class instrument operating in the 2-30keV range, which holds the capability to revolutionise studies of variability from X-ray sources on the millisecond time scales. The LAD instrument has now completed the assessment phase but was not down-selected for launch. However, during the assessment, most of the trade-offs have been closed leading to a robust and well documented design that will be re- proposed in future ESA calls. In this talk, we will summarize the characteristics of the LAD design and give an overview of the expectations for the instrument capabilities.

astro-ph.IM↗
arXiv2014

Instrumental and scientific simulations of the LOFT Wide Field Monitor

The Large Observatory for X-ray Timing (LOFT) is one of the five candidates that were considered by ESA as an M3 mission (with launch in 2022-2024). It is specifically designed to exploit the diagnostics of very rapid X-ray flux and spectral variability that directly probe the motion of matter down to distances very close to black holes and neutron stars, as well as the physical state of ultradense matter. The LOFT scientific payload is composed of the Large Area Detector (LAD), devoted to spectral-timing observation, and the Wide Field Monitor (WFM), whose primary goal it is to monitor the X-ray sky for transient events that need to be followed up with the LAD, and to measure the long-term variability of galactic X-ray sources and localize gamma-ray bursts. Here we describe the simulations carried out to optimize the WFM design and to characterize the instrument response to both isolated sources and crowded fields in the proximity of the galactic bulge.

astro-ph.IM↗
arXiv2014

The design of the wide field monitor for LOFT

LOFT (Large Observatory For x-ray Timing) is one of the ESA M3 missions selected within the Cosmic Vision program in 2011 to carry out an assessment phase study and compete for a launch opportunity in 2022-2024. The phase-A studies of all M3 missions were completed at the end of 2013. LOFT is designed to carry on-board two instruments with sensitivity in the 2-50 keV range: a 10 m 2 class Large Area Detector (LAD) with a <1° collimated FoV and a wide field monitor (WFM) making use of coded masks and providing an instantaneous coverage of more than 1/3 of the sky. The prime goal of the WFM will be to detect transient sources to be observed by the LAD. However, thanks to its unique combination of a wide field of view (FoV) and energy resolution (better than 500 eV), the WFM will be also an excellent monitoring instrument to study the long term variability of many classes of X-ray sources. The WFM consists of 10 independent and identical coded mask cameras arranged in 5 pairs to provide the desired sky coverage. We provide here an overview of the instrument design, configuration, and capabilities of the LOFT WFM. The compact and modular design of the WFM could easily make the instrument concept adaptable for other missions.

astro-ph.IM↗
arXiv2014

Exploring the blazar zone in High Energy flares of FSRQs

The gamma-ray emission offers a powerful diagnostic tool to probe jets and their surroundings in flat spectrum radio quasars (FSRQ). In particular, sources emitting at high energies (>10 GeV) give us the strongest constraints. This motivates us to start a systematic study of flares with bright emission above 10 GeV, examining archival data of Fermi-LAT gamma-ray telescope. At the same time, we began to trigger Target of Opportunity observations to the Swift observatory at the occurrence of high-energy flares, obtaining a wide coverage of the spectral energy distributions for several FSRQs during flares. Among the others we investigate the SED of a peculiar flare of 3C 454.3, showing a remarkable hard gamma-ray spectrum, quite different from the brightest flares of this source, and a bright flare of CTA 102. We modeled the SED in the framework of the one--zone leptonic model, using also archival optical spectroscopic data to derive the luminosity of the broad lines and thus estimate the disk luminosity, from which the structural parameters of the FSRQ nucleus can be inferred. The model allowed us to evaluate the magnetic field intensity in the blazar zone, and to locate the emitting region of gamma rays in the particular case in which gamma-ray spectra show neither absorption from the BLR, nor the Klein-Nishina curvature expected in leptonic models assuming the BLR as source of seed photons for the External Compton. For FSRQs bright above 10 GeV, we where able to identify short periods lasting less than 1 day characterized by high rate of high energy gamma rays, and hard gamma-ray spectra. We discussed the observed spectra and variability timescales in terms of injection and cooling of energetic particles, arguing that these flares could be triggered by magnetic reconnections events or turbulence in the flow.

astro-ph.HE↗
arXiv2013

A new insight into the innermost jet regions: probing extreme jet variability with LOFT

Blazars are highly variable sources over timescales that can be as low as minutes. This is the case of the High Energy Peaked BL Lac (HBL) objects showing strong variability in X-rays, which highly correlate with that of the TeV emission. The degree of this correlation is still debated, particularly when the flaring activity is followed down to very short time scales. This correlation could challenge the synchrotron-self-Compton scenario in which one relativistic electron population dominates the entire radiative output. We argue that the LOFT Large Area Detector (10 m$^2$, LAD), thanks to its unprecedented timing capability, will allow us to detect the X-ray counterpart (2-50 keV) of the very fast variability observed at TeV energies, sheding light on the nature of X-TeV connection. We will discuss the test case of PKS 2155-304, showing as it would be possible to look for any X-ray variability occurring at very short timescales, never explored so far. This will put strong constraints on the size and the location of any additional electron population in the multi-zone scenario. Under this perspective, LOFT and the CTA observatories, planned to operate in the same time frame, will allow us to investigate in depth the connection between X-ray and TeV emissions. We also discuss the potentialities of LOFT in measuring the change in spectral curvature of the synchrotron spectra in HBLs which will make possible to directly study the mechanism of acceleration of highly energetic electrons. LOFT timing capability will be also promising in the study of Flat Spectrum Radio Quasars (FSRQs) with flux $\ge 1$ mCrab. Constraints to the location of the high energy emission will be given by temporal investigation on second timescale and spectral trend analysis on minute timescales. This represents a further link with CTA because of the rapid (unexpected) TeV emission recently detected in some FSRQs.

astro-ph.HE↗
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.

astro-ph.HE↗
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.

astro-ph.IM↗
arXiv2013

IPN localizations of Konus short gamma-ray bursts

Between the launch of the \textit{GGS Wind} spacecraft in 1994 November and the end of 2010, the Konus-\textit{Wind} experiment detected 296 short-duration gamma-ray bursts (including 23 bursts which can be classified as short bursts with extended emission). During this period, the IPN consisted of up to eleven spacecraft, and using triangulation, the localizations of 271 bursts were obtained. We present the most comprehensive IPN localization data on these events. The short burst detection rate, $\sim$18 per year, exceeds that of many individual experiments.

astro-ph.HE↗