arXiv subjects

S. Vercellone

Publications and source records attributed to S. Vercellone.

At least 37 records · Page 2Linked to original sources

arXiv2020

Gamma-Ray and X-Ray Observations of the Periodic-Repeater FRB 180916 During Active Phases

FRB 180916 is a most intriguing source at 150 Mpc distance capable of producing repeating fast radio bursts with a periodic 16.35 day temporal pattern. We report on the X-ray and $γ$-ray observations of FRB 180916 obtained by AGILE and Swift. We focused on the recurrent 5-day time intervals of active radio bursting and present results obtained on Feb. 3 - 8; Feb. 25; Mar. 5 - 10; Mar. 22 - 28, 2020 during a multiwavelength campaign involving high-energy and radio observations. We also searched for temporal coincidences at millisecond timescales between all known radio bursts of FRB 180916 and X-ray and MeV events detectable by AGILE. We do not detect any simultaneous event or any extended X-ray and $γ$-ray emission on timescales of hours/days/weeks. Our cumulative X-ray (0.3-10 keV) flux upper limit of $5 \times\,10^{-14} \rm \, erg \, cm^{-2} s^{-1}$ (obtained during 5-day active intervals) translates into an isotropic luminosity upper limit of $L_{X,UL} \sim 1.5 \times\, 10^{41} \rm erg \, s^{-1}$. Observations above 100 MeV over a many-year timescale provide an average luminosity upper limit one order of magnitude larger. These results provide the so-far most stringent limits on high-energy emission from FRB 180916 and constrain the dissipation of magnetic energy from a magnetar-like source of radius $R_m$, internal magnetic field $B_m$ and dissipation timescale $τ_d$ to satisfy the relation $R_{m,6}^3 B_{m,16}^2 τ_{d,8}^{-1} \lesssim 1$, where $R_{m,6}$ is $R_m$ in units of $10^6$ cm, $B_{m,16}$ is $B_m$ in units of $10^{16}$ G, and $τ_{d,8}$ in units of $10^8$ s.

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arXiv2020

Swift observations of Mrk\,421 in selected epochs. iii. Extreme x-ray timing/spectral properties and multiwavelength lognormality in 2015\,December--2018\,April

We present the results from the timing and spectral study of Mrk\,421 based mainly on the \emph{Swift} data in the X-ray energy range obtained during the time interval 2015\,December--2018\,April. The most extreme X-ray flaring activity on the long-term, daily and intraday timescales was observed during the 2-month period which started in 2017\,December when the 0.3--10\,keV flux exceeded a level of 5$\times$10$^{-9}$erg\,cm$^{-2}$s$^{-1}$, recorded only twice previously. While the TeV-band and X-ray variabilities mostly were correlated, the source often varied in a complex manner in the MeV--GeV and radio--UV energy ranges, indicating that the multifrequency emission of Mrk\,421 could not be always generated in a single zone. The longer-term flares at X-rays and $γ$-rays showed a lognormal character, possibly indicating a variability imprint of the accretion disk onto the jet. A vast majority of the 0.3--10\,keV spectra were consistent with the log-parabolic model, showing relatively low spectral curvature and correlations between the different spectral parameters, predicted in the case of the first and second-order Fermi processes. The position of the synchrotron spectral energy distribution (SED) peak showed an extreme variability on diverse timescales between the energies $E_{\rm p}$$<$0.1\,keV and $E_{\rm p}$$>$15\,keV, with 15\% of the spectra peaking at hard X-rays and was related to the peak height as $S_{\rm p}$$\varpropto$$E^α_{\rm p}$ with $α$$\sim$0.6, which is expected for the transition from Kraichnan-type turbulence into the \textquotedblleft hard-sphere\textquotedblright~ one. The 0.3--300\,GeV spectra showed the features of the hadronic contribution, jet-star interaction and upscatter in the Klein-Nishina regime in different time intervals.

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arXiv2020

Unveiling the monster heart: unbeamed properties of blazar 4C 71.07

4C 71.07 is a high-redshift blazar whose optical radiation is dominated by quasar-like nuclear emission. We here present the results of a spectroscopic monitoring of the source to study its unbeamed properties. We obtained 24 optical spectra at the Nordic Optical Telescope (NOT) and William Herschel Telescope (WHT) and 3 near-infrared spectra at the Telescopio Nazionale Galileo (TNG). They show no evidence of narrow emission lines. The estimate of the systemic redshift from the H$β$ and H$α$ broad emission lines leads to $z_{\rm sys}=2.2130 \pm 0.0004$. Notwithstanding the nearly face-on orientation of the accretion disc, the high-ionization emission lines present large broadening as well as noticeable blueshifts, which increase with the ionizing energy of the corresponding species. This is a clear indication of strong ionized outflows. Line broadening and blueshift appear correlated. We applied scaling relationships to estimate the mass of the supermassive black hole from the Balmer and C IV lines, taking into account the prescriptions to correct for outflow. They give $M_{\rm BH} \sim 2 \times 10^9 \, M_\odot$. We derived an Eddington luminosity $L_{\rm Edd} \sim 2.5 \times 10^{47} \rm \, erg \, s^{-1}$ $\sim L_{\rm disc}$, and a broad line region luminosity $L_{\rm BLR} \sim 1.5 \times 10^{46} \rm \, erg \, s^{-1}$. The line fluxes do not show significant variability in time. In particular, there is no line reaction to the jet flaring activity detected in 2015 October and November. This implies that the jet gives no contribution to the photoionization of the broad line region in the considered period.

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arXiv2020

Prospects for gamma-ray observations of narrow-line Seyfert 1 galaxies with the Cherenkov Telescope Array II. Gamma-gamma absorption in the broad-line region radiation fields

Gamma-ray emitting narrow-line Seyfert 1 ($γ$-NLS1) galaxies possibly harbour relatively low-mass black holes (10$^6$-10$^8$ M$_{\odot}$) accreting close to the Eddington limit, and share many characteristics with their sibling sources, flat-spectrum radio quasars. Although they have been detected in the MeV--GeV band with Fermi-LAT, they have never been seen in the very high energy band with current imaging atmospheric Cherenkov telescopes (IACTs). Thus, they are key targets for the next-generation IACT, the Cherenkov Telescope Array (CTA). In a previous work we selected, by means of extensive simulations, the best candidates for a prospective CTA detection (SBS 0846$+$513, PMN J0948$+$0022, and PKS 1502$+$036) taking into account the effects of both the intrinsic absorption (approximated with a cut-off at 30 GeV), and the extra-galactic background light on the propagation of $γ$-rays. In this work we simulate the spectra of these three sources by adopting more realistic broad-line region (BLR) absorption models. In particular, we consider the detailed treatment of $γ$-$γ$ absorption in the radiation fields of the BLR as a function of the location of the $γ$-ray emission region with parameters inferred from observational constraints. We find that, due to the energy range extent and its sensitivity, CTA is particularly well suited to locate the $γ$-ray emitting region in $γ$-NLS1. In particular CTA will be able not only to distinguish whether the $γ$-ray emitting region is located inside or outside the BLR, but also where inside the BLR it may be.

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arXiv2020

AGILE Observations of Two Repeating Fast Radio Bursts with Small Intrinsic Dispersion Measures

We focus on two repeating fast radio bursts (FRBs) recently detected by the CHIME/FRB experiment in 2018--2019 (Source 1: 180916.J0158+65, and Source 2: 181030.J1054+73). These sources have low excess dispersion measures (DMs) ($ < 100 \rm \, pc \, cm^{-3}$ and $ < 20 \rm \, pc \, cm^{-3}$, respectively), implying relatively small maximal distances. They were repeatedly observed by AGILE in the MeV--GeV energy range. We do not detect prompt emission simultaneously with these repeating events. This search is particularly significant for the submillisecond and millisecond integrations obtainable by AGILE. The sources are constrained to emit a MeV-fluence in the millisecond range below $F'_{MeV} = 10^{-8} \, \rm erg \, cm^{-2}$ corresponding to an isotropic energy near $E_{MeV,UL} \simeq 2 \times 10^{46}\,$erg for a distance of 150 Mpc (applicable to Source 1). We also searched for $γ$-ray emission for time intervals up to 100 days, obtaining 3$\,σ$ upper limits (ULs) for the average isotropic luminosity above 50 MeV, $L_{γ,UL} \simeq \,$(5-10)$\,\times 10^{43} \rm \, erg \, s^{-1}$. For a source distance near 100 kpc (possibly applicable to Source 2), our ULs imply $E_{MeV,UL}\simeq10^{40} \rm erg$, and $L_{γ,UL} \simeq \,$2$\,\times 10^{37} \rm \, erg \, s^{-1}$. Our results are significant in constraining the high-energy emission of underlying sources such as magnetars, or other phenomena related to extragalactic compact objects, and show the prompt emission to be lower than the peak of the 2004 magnetar outburst of SGR 1806-20 for source distances less than about 100 Mpc.

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arXiv2020

First detection of the Crab Nebula at TeV energies with a Cherenkov telescope in a dual-mirror Schwarzschild-Couder configuration: the ASTRI-Horn telescope

We report on the first detection of very high-energy (VHE) gamma-ray emission from the Crab Nebula by a Cherenkov telescope in dual-mirror Schwarzschild-Couder (SC) configuration. The result has been achieved by means of the 4 m size ASTRI-Horn telescope, operated on Mt. Etna (Italy) and developed in the context of the Cherenkov Telescope Array Observatory preparatory phase. The dual-mirror SC design is aplanatic and characterized by a small plate scale, allowing us to implement large field of view cameras with small-size pixel sensors and a high compactness. The curved focal plane of the ASTRI camera is covered by silicon photo-multipliers (SiPMs), managed by an unconventional front-end electronics based on a customized peak-sensing detector mode. The system includes internal and external calibration systems, hardware and software for control and acquisition, and the complete data archiving and processing chain. The observations of the Crab Nebula were carried out in December 2018, during the telescope verification phase, for a total observation time (after data selection) of 24.4 h, equally divided into on- and off-axis source exposure. The camera system was still under commissioning and its functionality was not yet completely exploited. Furthermore, due to recent eruptions of the Etna Volcano, the mirror reflection efficiency was reduced. Nevertheless, the observations led to the detection of the source with a statistical significance of 5.4 sigma above an energy threshold of ~3 TeV. This result provides an important step towards the use of dual-mirror systems in Cherenkov gamma-ray astronomy. A pathfinder mini-array based on nine large field-of-view ASTRI-like telescopes is under implementation.

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arXiv2019

Calibrating the power of relativistic jets

There are several methods to calculate the radiative and kinetic power of relativistic jets, but their results can differ by one or two orders of magnitude. Therefore, it is necessary to perform a calibration of the jet power, to understand the reasons for these differences (whether wrong hypotheses or intrinsic source variability), and if it is possible to converge to a reliable measurement of this physical quantity. We present preliminary results of a project aimed at calibrating the power of relativistic jets in active galactic nuclei (AGN) and X-ray binaries (XRB). We started by selecting all the AGN associations with known redshift in the Fourth Fermi LAT Gamma-Ray Catalog (4FGL). We then calculated the radiative and/or kinetic powers from available data or we extracted this information from literature. We compare the values obtained for overlapping samples and highlight early conclusions.

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arXiv2019

Properties of flat-spectrum radio-loud Narrow-Line Seyfert 1 Galaxies

We have conducted a multiwavelength survey of 42 radio loud narrow-1ine Seyfert 1 galaxies (RLNLS1s), selected by searching among all the known sources of this type and omitting those with steep radio spectra. We analyse data from radio frequencies to X-rays, and supplement these with information available from online catalogs and the literature in order to cover the full electromagnetic spectrum. This is the largest known multiwavelength survey for this type of source. We detected 90% of the sources in X-rays and found 17% at gamma rays. Extreme variability at high energies was also found, down to timescales as short as hours. In some sources, dramatic spectral and flux changes suggest interplay between a relativistic jet and the accretion disk. The estimated masses of the central black holes are in the range $\sim 10^{6-8}M_{\odot}$, smaller than those of blazars, while the accretion luminosities span a range from $\sim 0.01$ to $\sim 0.49$ times the Eddington limit, similar to those of quasars. The distribution of the calculated jet power spans a range from $\sim 10^{42.6}$ to $\sim 10^{45.6}$ erg s$^{-1}$, generally lower than quasars and BL Lac objects, but partially overlapping with the latter. Once normalised by the mass of the central black holes, the jet power of the three types of active galactic nuclei are consistent with each other, indicating the scalability of the jet. Despite the observational differences, the central engine of RLNLS1s is apparently quite similar to that of blazars. The historical difficulties in finding radio-loud narrow-line Seyfert 1 galaxies might be due to their low power and to intermittent jet activity.

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arXiv2019

Overview of non-transient gamma-ray binaries and prospects for the Cherenkov Telescope Array

We simulate the spectral behaviour of the non-transient gamma-ray binaries using archival observations as a reference. With this we test the CTA capability to measure the sources' spectral parameters and detect variability on various time scales. We review the known properties of gamma-ray binaries and the theoretical models that have been used to describe their spectral and timing characteristics. We show that CTA is capable of studying these sources on time scales comparable to their characteristic variability time scales. For most of the binaries, the unprecedented sensitivity of CTA will allow the spectral evolution to be studied on a time scale as short as 30 min. This will enable a direct comparison of the TeV and lower energy (radio to GeV) properties of these sources from simultaneous observations. We also review the source-specific questions that can be addressed with such high-accuracy CTA measurements.

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arXiv2019

The beamed jet and quasar core of the distant blazar 4C 71.07

The object 4C 71.07 is a high-redshift blazar whose spectral energy distribution shows a prominent big blue bump and a strong Compton dominance. We present the results of a two-year multiwavelength campaign led by the Whole Earth Blazar Telescope (WEBT) to study both the quasar core and the beamed jet of this source. The WEBT data are complemented by ultraviolet and X-ray data from Swift, and by gamma-ray data by Fermi. The big blue bump is modelled by using optical and near-infrared mean spectra obtained during the campaign, together with optical and ultraviolet quasar templates. We give prescriptions to correct the source photometry in the various bands for the thermal contribution, in order to derive the non-thermal jet flux. The role of the intergalactic medium absorption is analysed in both the ultraviolet and X-ray bands. We provide opacity values to deabsorb ultraviolet data, and derive a best-guess value for the hydrogen column density through the analysis of X-ray spectra. We estimate the disc and jet bolometric luminosities, accretion rate, and black hole mass. Light curves do not show persistent correlations among flux changes at different frequencies. We study the polarimetric behaviour and find no correlation between polarisation degree and flux, even when correcting for the dilution effect of the big blue bump. Similarly, wide rotations of the electric vector polarisation angle do not seem to be connected with the source activity.

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arXiv2019

The Second AGILE Catalog of Gamma-Ray Sources

Aims. We present the second AGILE-GRID Catalog (2AGL) of γ-ray sources in the 100 MeV-10 GeV energy range. Methods. With respect to previous AGILE-GRID catalogs, the current 2AGL Catalog is based on the first 2.3 years of science data from the AGILE mission (the so called 'pointing mode') and incorporates more data and several analysis improvements, including better calibrations at the event reconstruction level, an updated model for the Galactic diffuse gamma-ray emission, a refined procedure for point-like source detection, and the inclusion of a search for extended γ-ray sources. Results. The 2AGL Catalog includes 175 high-confidence sources (above 4σ significance) with their location regions and spectral properties, and a variability analysis with 4-day light curves for the most significant ones. Relying on the error region of each source position, including systematic uncertainties, 121 sources are considered as positionally associated with known couterparts at different wavelengths or detected by other γ-ray instruments. Among the identified or associated sources, 62 are Active Galactic Nuclei (AGNs) of the blazar class. Pulsars represent the largest Galactic source class, with 40 associated pulsars, 7 of them with detected pulsation; 8 Supernova Remnants and 4 high-mass X-ray binaries have also been identified. A substantial number of 2AGL sources are unidentified: for 54 sources no known counterpart is found at different wavelengths. Among these sources, we discuss a sub-class of 29 AGILE-GRID-only γ-ray sources that are not present in 1FGL, 2FGL or 3FGL catalogs; the remaining sources are unidentified in both 2AGL and 3FGL Catalogs. We also present an extension of the analysis of 2AGL sources detected in the 50-100 MeV energy range.

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arXiv2019

A Cloud-based architecture for the Cherenkov Telescope Array observation simulations. Optimisation, design, and results

Simulating and analysing detailed observations of astrophysical sources for very high energy (VHE) experiments, like the Cherenkov Telescope Array (CTA), can be a demanding task especially in terms of CPU consumption and required storage. In this context, we propose an innovative cloud computing architecture based on Amazon Web Services (AWS) aiming to decrease the amount of time required to simulate and analyse a given field by distributing the workload and exploiting the large computational power offered by AWS. We detail how the various services offered by the Amazon online platform are jointly used in our architecture and we report a comparison of the execution times required for simulating observations of a test source with the CTA, by a single machine and the cloud-based approach. We find that, by using AWS, we can run our simulations more than 2 orders of magnitude faster than by using a general purpose workstation for the same cost. We suggest to consider this method when observations need to be simulated, analysed, and concluded within short timescales.

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arXiv2018

Blazars and Fast Radio Bursts with LSST

The aim of this white paper is to discuss the observing strategies for the LSST Wide-Fast-Deep that would improve the study of blazars (emission variability, census, environment) and Fast Radio Bursts (FRBs). For blazars, these include the adoption of: i) a reference filter to allow reconstruction of a well-sampled light curve not affected by colour changes effects; ii) two snapshots/visit with different exposure times to avoid saturation during flaring states; iii) a rolling cadence to get better-sampled light curves at least in some time intervals. We also address the potential importance of Target of Opportunity (ToO) observations of blazar neutrino sources, and the advantages of a Minisurvey with a star trail cadence (see white paper by David Thomas et al.) for both the blazar science and the detection of possible very fast optical counterparts of FRBs.

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arXiv2018

AGILE, Fermi, Swift, and GASP-WEBT multi-wavelength observations of the high-redshift blazar 4C $+$71.07 in outburst

The flat-spectrum radio quasar 4C $+$71.07 is a high-redshift ($z=2.172$), $γ$-loud blazar whose optical emission is dominated by the thermal radiation from accretion disc. 4C $+$71.07 has been detected in outburst twice by the AGILE $γ$-ray satellite during the period end of October - mid November 2015, when it reached a $γ$-ray flux of the order of $F_{\rm E>100\,MeV} = (1.2 \pm 0.3)\times 10^{-6}$ photons cm$^{-2}$ s$^{-1}$ and $F_{\rm E>100\,MeV} = (3.1 \pm 0.6)\times 10^{-6}$ photons cm$^{-2}$ s$^{-1}$, respectively, allowing us to investigate the properties of the jet and of the emission region. We investigated its spectral energy distribution by means of almost simultaneous observations covering the cm, mm, near-infrared, optical, ultra-violet, X-ray and $γ$-ray energy bands obtained by the GASP-WEBT Consortium, the Swift and the AGILE and Fermi satellites. The spectral energy distribution of the second $γ$-ray flare (the one whose energy coverage is more dense) can be modelled by means of a one-zone leptonic model, yielding a total jet power of about $4\times10^{47}$ erg s$^{-1}$. During the most prominent $γ$-ray flaring period our model is consistent with a dissipation region within the broad-line region. Moreover, this class of high-redshift, large-mass black-hole flat-spectrum radio quasars might be good targets for future $γ$-ray satellites such as e-ASTROGAM.

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arXiv2018

AGILE detection of gamma-ray sources coincident with cosmic neutrino events

The origin of cosmic neutrinos is still largely unknown. Using data obtained by the gamma-ray imager on board of the AGILE satellite, we systematically searched for transient gamma-ray sources above 100 MeV that are temporally and spatially coincident with ten recent high-energy neutrino IceCube events. We find three AGILE candidate sources that can be considered possible counterparts to neutrino events. Detecting 3 gamma-ray/neutrino associations out of 10 IceCube events is shown to be unlikely due to a chance coincidence. One of the sources is related to the BL Lac source TXS 0506+056. For the other two AGILE gamma-ray sources there are no obvious known counterparts, and both Galactic and extragalactic origin should be considered.

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arXiv2018

Putting the hadron beam scenario for extreme blazars to the test with the Cherenkov Telescope Array

Hadron beams are invoked to explain the peculiar properties of a subclass of BL Lac objects, the so-called extreme BL Lacs (EHBLs). This scenario predicts a quite distinctive feature for the high-energy gamma-ray spectrum of these sources, namely a hard energy tail extending up to $\sim100$ TeV. It has been proposed that the detection of this tail can offer an unambiguous way to distinguish between the hadron beam scenario and the standard one, which instead assumes gamma-ray emission from the jet strongly depleted at the highest energies ($E>30$ TeV) because of the interaction with the optical-IR cosmic radiation field. We present dedicated simulations of observations through the presently under construction Cherenkov Telescope Array (CTA) of the very-high energy spectrum of the prototypical EHBL 1ES 0229+200 assuming the two alternative models. We demonstrate that, considering 50 hours of observations from the southern site of CTA (the most sensitive at the highest energies), in the case of the hadron beam model it is possible to detect the source up to 100 TeV. This, together with the non detection of the source above 10 TeV in the standard case, ensures that CTA observations can be effectively used to unambiguously confirm or rule out the hadron beam scenario.

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arXiv2018

Prospects for gamma-ray observations of narrow-line Seyfert 1 galaxies with the Cherenkov Telescope Array

Gamma-ray emitting narrow-line Seyfert 1 ($γ$-NLSy1) galaxies are thought to harbour relatively low-mass black holes (10$^6$-10$^8$ M$_{\odot}$) accreting close to the Eddington limit. They show characteristics similar to those of blazars, such as flux and spectral variability in the gamma-ray energy band and radio properties which point toward the presence of a relativistic jet. These characteristics make them an intriguing class of sources to be investigated with the Cherenkov Telescope Array (CTA), the next-generation ground-based gamma-ray observatory. We present our extensive set of simulations of all currently known $γ$-ray emitters identified as NLS1s (20 sources),investigating their detections and spectral properties, taking into account the effect of both the extra-galactic background light in the propagation of gamma-rays and intrinsic absorption components.We find that the prospects for observations of $γ$-NLSy1 with CTA are promising. In particular, the brightest sources of our sample, SBS 0846+513, PMN J0948+0022, and PKS 1502+036 can be detected during high/flaring states, the former two even in the case in which the emission occurs within the highly opaque central regions, which prevent $γ$ rays above few tens of GeV to escape. In this case the low-energy threshold of CTA will play a key role. If, on the other hand, high-energy emission occurs outside the broad line region, we can detect the sources up to several hundreds of GeV-depending on the intrinsic shape of the emitted spectrum. Therefore, CTA observations will provide valuable information on the physical conditions and emission properties of their jets.

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arXiv2018

Calibration of AGILE-GRID with on-ground data and Monte Carlo simulations

AGILE is a mission of the Italian Space Agency (ASI) Scientific Program dedicated to gamma-ray astrophysics, operating in a low Earth orbit since April 23, 2007. It is designed to be a very light and compact instrument, capable of simultaneously detecting and imaging photons in the 18 keV to 60 keV X-ray energy band and in the 30 MeV{50 GeV gamma-ray energy with a good angular resolution (< 1 deg at 1 GeV). The core of the instrument is the Silicon Tracker complemented with a CsI calorimeter and a AntiCoincidence system forming the Gamma Ray Imaging Detector (GRID). Before launch, the GRID needed on-ground calibration with a tagged gamma-ray beam to estimate its performance and validate the Monte Carlo simulation. The GRID was calibrated using a tagged gamma-ray beam with energy up to 500 MeV at the Beam Test Facilities at the INFN Laboratori Nazionali di Frascati. These data are used to validate a GEANT3 based simulation by comparing the data and the Monte Carlo simulation by measuring the angular and energy resolutions. The GRID angular and energy resolutions obtained using the beam agree well with the Monte Carlo simulation. Therefore the simulation can be used to simulate the same performance on-light with high reliability.

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