arXiv subjects

M. Giroletti

Publications and source records attributed to M. Giroletti.

At least 73 records · Page 4Linked to original sources

arXiv2021

The First Fermi-LAT Solar Flare Catalog

We present the first Fermi - Large Area Telescope (LAT) solar flare catalog covering the 24 th solar cycle. This catalog contains 45 Fermi -LAT solar flares (FLSFs) with emission in the gamma-ray energy band (30 MeV - 10 GeV) detected with a significance greater than 5 sigma over the years 2010-2018. A subsample containing 37 of these flares exhibit delayed emission beyond the prompt-impulsive hard X-ray phase with 21 flares showing delayed emission lasting more than two hours. No prompt-impulsive emission is detected in four of these flares. We also present in this catalog the observations of GeV emission from 3 flares originating from Active Regions located behind the limb (BTL) of the visible solar disk. We report the light curves, spectra, best proton index and localization (when possible) for all the FLSFs. The gamma-ray spectra is consistent with the decay of pions produced by >300 MeV protons. This work contains the largest sample of high-energy gamma-ray flares ever reported and provides the unique opportunity to perform population studies on the different phases of the flare and thus allowing to open a new window in solar physics.

astro-ph.HE↗

arXiv2020

Jet collimation in NGC 315 and other nearby AGN

Aims. The collimation of relativistic jets in galaxies is a poorly understood process. Detailed radio studies of the jet collimation region have been performed so far in few individual objects, providing important constraints for jet formation models. However, the extent of the collimation zone as well as the nature of the external medium possibly confining the jet are still debated. Methods. In this article we present a multi-frequency and multi-scale analysis of the radio galaxy NGC 315, including the use of mm-VLBI data up to 86 GHz, aimed at revealing the evolution of the jet collimation profile. We then consider results from the literature to compare the jet expansion profile in a sample of 27 low-redshift sources, mainly comprising radio galaxies and BL Lacs, classified based on the accretion properties as low-excitation (LEG) and high-excitation (HEG) galaxies. Results.The jet collimation in NGC 315 is completed on sub-parsec scales. A transition from a parabolic to conical jet shape is detected at $z_{t}=0.58\pm0.28$ parsecs or ${\sim}5\times 10^3$ Schwarzschild radii ($R_{S}$) from the central engine, a distance which is much smaller than the Bondi radius, $r_{B}{\sim}92$ $\rm pc$, estimated based on X-ray data. The jet in this and in few other LEGs in our sample may be initially confined by a thick disk extending out to ${\sim}10^3$-$10^4$ $R_{S}$. A comparison between the mass-scaled jet expansion profiles of all sources indicates that jets in HEGs are surrounded by thicker disk-launched sheaths and collimate on larger scales with respect to jets in LEGs. These results suggest that disk winds play an important role in the jet collimation mechanism, particularly in high-luminosity sources. The impact of winds for the origin of the FRI/FRII dichotomy in radio galaxies is also discussed.

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arXiv2020

Investigating the mini and giant radio flare episodes of Cygnus X-3

The microquasar Cygnus X-3 underwent a giant radio flare in April 2017, reaching a maximum flux of $\sim 16.5$ Jy at 8.5 GHz. We present results from a long monitoring campaign carried out with Medicina at 8.5, 18.6 and 24.1 GHz, in parallel to the Metsähovi radio telescope at 37 GHz, from 4 to 11 April 2017. We observe a spectral steepening from $α= 0.2$ to 0.5 (with $S_ν \propto ν^{-α}$) within 6 h around the epoch of the peak maximum of the flare, and rapid changes in the spectral slope in the following days during brief enhanced emission episodes while the general trend of the radio flux density indicated the decay of the giant flare. We further study the radio orbital modulation of Cyg X-3 emission associated with the 2017 giant flare and with six mini-flares observed in 1983, 1985, 1994, 1995, 2002 and 2016. The enhanced emission episodes observed during the decline of the giant flare at 8.5 GHz coincide with the orbital phase $ϕ\sim 0.5$ (orbital inferior conjunction). On the other hand the light curves of the mini-flares observed at $15-22$ GHz peak at $ϕ\sim 0$, except for the 2016 light curve which is shifted of 0.5 w.r.t. the other ones. We attribute the apparent phase shift to the variable location of the emitting region along the bent jet. This might be explained by the different accretion states of the flaring episodes (the 2016 mini-flare occurred in the hypersoft X-ray state).

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arXiv2020

Parsec-scale properties of the radio brightest jetted AGN at z > 6

We present Director's Discretionary Time multi-frequency observations obtained with the Jansky Very Large Array (VLA) and the Very Long Baseline Array (VLBA) of the blazar PSO J030947.49+271757.31 (hereafter PSO J0309+27) at $z = 6.10\pm0.03$. The milliarcsecond angular resolution of our VLBA observations at 1.5, 5 and 8.4 GHz unveils a bright one-sided jet extended for $\sim500$ parsecs in projection. This high-z radio-loud AGN is resolved into multiple compact sub-components, embedded in a more diffuse and faint radio emission, which enshrouds them in a continuous jet structure. We derive limits on some physical parameters directly from the observable quantities, such as viewing angle, Lorentz and Doppler factors. If PSO J0309+27 is a genuine blazar, as suggested by its X-ray properties, then we find that its bulk Lorentz factor must be relatively low (less than 5). Such value would be in favour of a scenario currently proposed to reconcile the paucity of high-z blazars with respect to current predictions. Nevertheless, we cannot exclude that PSO J0309+27 is seen under a larger viewing angle, which would imply that the X-ray emission must be enhanced, for example, by inverse Compton with the Cosmic Microwave Background. More stringent constraints on the bulk Lorentz factor in PSO J0309+27 and the other high-z blazars are necessary to test whether their properties are intrinsically different with respect to the low-z blazar population.

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arXiv2020

Linear polarization in the nucleus of M87 at 7 mm and 1.3 cm

We report on high angular resolution polarimetric observations of the nearby radio galaxy M87 using the Very Long Baseline Array at 24 GHz ($λ=$1.3 cm) and 43 GHz ($λ=$7 mm) in 2017-2018. New images of the linear polarization substructure in the nuclear region are presented, characterized by a two-component pattern of polarized intensity and smooth rotation of the polarization plane around the 43 GHz core. From a comparison with an analogous dataset from 2007, we find that this global polarization pattern remains stable on a time interval of 11 yr, while showing smaller month-scale variability. We discuss the possible Faraday rotation toward the M87 nucleus at centimeter to millimeter wavelengths. These results can be interpreted in a scenario where the observed polarimetric pattern is associated with the magnetic structure in the confining magnetohydrodynamic wind, which also serves as the source of the observed Faraday rotation.

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arXiv2020

Blazars at the Cosmic Dawn

The uncharted territory of the high-redshift ($z\gtrsim3$) Universe holds the key to understand the evolution of quasars. In an attempt to identify the most extreme members of the quasar population, i.e., blazars, we have carried out a multi-wavelength study of a large sample of radio-loud quasars beyond $z=3$. Our sample consists of 9 $γ$-ray detected blazars and 133 candidate blazars selected based on the flatness of their soft X-ray spectra (0.3$-$10 keV photon index $\leq1.75$), including 15 with NuSTAR observations. The application of the likelihood profile stacking technique reveals that the high-redshift blazars are faint $γ$-ray emitters with steep spectra. The high-redshift blazars host massive black holes ($\langle \log~M_{\rm BH,~M{\odot}} \rangle>9$) and luminous accretion disks ($\langle L_{\rm disk} \rangle>10^{46}$ erg s$^{-1}$). Their broadband spectral energy distributions are found to be dominated by high-energy radiation indicating their jets to be among the most luminous ones. Focusing on the sources exhibiting resolved X-ray jets (as observed with the Chandra satellite), we find the bulk Lorentz factor to be larger with respect to other $z>3$ blazars, indicating faster moving jets. We conclude that the presented list of the high-redshift blazars may act as a reservoir for follow-up observations, e.g., with NuSTAR, to understand the evolution of relativistic jets at the dawn of the Universe.

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arXiv2020

Methods for detection and analysis of weak radio sources with single-dish radio telescopes

The detection of mJy/sub-mJy point sources is a significant challenge for single-dish radio telescopes. Detection or upper limits on the faint afterglow from GRBs or other sources at cosmological distances are important means of constraining the source modeling. Using the Sardinia Radio Telescope (SRT), we compare the sensitivity and robustness of three methods applied to the detection of faint radio sources from raster maps around a known source position: the smart quick-look method, the source extraction method (typical of high-energy astronomy), and the fit with a 2-D Gaussian. We developed a Python code specific for the analysis of point-like radio sources applied to the SRT C-band (6.9 GHz) observations of both undetected sources (GRB afterglows of 181201A and 190114C) and the detected Galactic X-ray binary GRS 1915+105. Our comparative analysis of the different detection methods made extensive use of simulations as a useful complement to actual radio observations. The best method for the SRT data analysis is the fit with a 2-D Gaussian, as it pushes down the sensitivity limits of single-dish observations -- with respect to more traditional techniques -- to ~ 1.8 mJy, improving by ~ 40 % compared with the initial value. This analysis shows that -- especially for faint sources -- good maps of the scanned region pre- or post-outburst are essential.

astro-ph.IM↗

arXiv2020

Very Long Baseline Interferometry imaging of the advancing ejecta in the first gamma-ray nova V407 Cyg

In 2010/3, the Large Area Telescope on board Fermi revealed a transient gamma-ray source, positionally coincident with the optical nova in the symbiotic binary, V407Cyg. This event marked the first discovery of gamma-ray emission from a nova. We aimed to obtain resolved radio imaging of the material involved in the nova event; to determine the ejecta geometry and advance velocity directly in the image plane; to constrain the physical conditions of the system. We observed the source with the EVN and the VLBA over 16 epochs, between 20 days and 6 months after the optical discovery. The source is initially very dim but it later shows a substantial increase in brightness and a resolved shell-like structure 40 to 90 days after the optical event. The shell has a projected elliptical shape and is asymmetric in brightness and spectral index, being brighter and characterised by a rising spectrum at the S-E edge. We determine a projected velocity of ~3500 km/s in the initial phase, and ~2100 km/s between day 20 and 91. We also found an emitting feature about 350 mas (940 AU) to the N-W, advancing at a projected velocity of ~700 km/s along the polar axis of the binary. The total flux density in the VLBI images is significantly lower than that previously reported at similar epochs and over much wider angular scales with the VLA. Optical spectra demonstrated that in 2010 we were viewing V407Cyg along the equatorial plane and from behind the Mira. Our radio observations image the bipolar flow of the ejecta perpendicular to the orbital plane, where deceleration is much lower than through the equatorial plane probed by the truncated profile of optical emission lines. The separated polar knot at 350 mas and the bipolar flow strictly resemble the similar arrangement seen in Hen 2-104. The observed ~700 km/s expansion constrains the launch-date of the polar knot around 2004. [Abridged]

astro-ph.SR↗

arXiv2020

NGC 3894: a young radio galaxy seen by Fermi-LAT

Context. According to radiative models, radio galaxies may produce gamma-ray emission from the first stages of their evolution. However, very few such galaxies have been detected by the Fermi Large Area Telescope (LAT) so far. Aims. NGC 3894 is a nearby (z = 0.0108) object that belongs to the class of compact symmetric objects (CSOs, i.e., the most compact and youngest radio galaxies), which is associated with a gamma-ray counterpart in the Fourth Fermi-LAT source catalog. Here we present a study of the source in the gamma-ray and radio bands aimed at investigating its high-energy emission and assess its young nature. Methods. We analyzed 10.8 years of Fermi-LAT data between 100 MeV and 300 GeV and determined the spectral and variability characteristics of the source. Multi-epoch very long baseline array (VLBA) observations between 5 and 15 GHz over a period of 35 years were used to study the radio morphology of NGC 3894 and its evolution. Results. NGC 3894 is detected in gamma-rays with a significance >9 sigma over the full period, and no significant variability has been observed in the gamma-ray flux on a yearly time-scale. The spectrum is modeled with a flat power law ($Γ$ = 2.0$\pm$0.1) and a flux on the order of 2.2 $\times$ 10$^{-9}$ ph cm$^{-2}$ s$^{-1}$. For the first time, the VLBA data allow us to constrain with high precision the apparent velocity of the jet and counter-jet side to be $β_{\mathrm{app,NW}}$ = 0.132$\pm$0.004 and $β_{\mathrm{app,SE}}$ = 0.065$\pm$0.003, respectively. Conclusions. Fermi-LAT and VLBA results favor the youth scenario for the inner structure of this object, with an estimated dynamical age of 59$\pm$5 years. The estimated range of viewing angle (10° < $θ$ < 21°) does not exclude a possible jet-like origin of the gamma-ray emission.

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arXiv2020

Fermi and Swift Observations of GRB 190114C: Tracing the Evolution of High-Energy Emission from Prompt to Afterglow

We report on the observations of gamma-ray burst (GRB) 190114C by the Fermi Gamma-ray Space Telescope and the Neil Gehrels Swift Observatory. The early-time observations reveal multiple emission components that evolve independently, with a delayed power-law component that exhibits significant spectral attenuation above 40 MeV in the first few seconds of the burst. This power-law component transitions to a harder spectrum that is consistent with the afterglow emission observed at later times. This afterglow component is clearly identifiable in the GBM and BAT light curves as a slowly fading emission component on which the rest of the prompt emission is superimposed. As a result, we are able to constrain the transition from internal shock to external shock dominated emission. We find that the temporal and spectral evolution of the broadband afterglow emission can be well modeled as synchrotron emission from a forward shock propagating into a wind-like circumstellar environment and find that high-energy photons observed by Fermi LAT are in tension with the theoretical maximum energy that can be achieved through synchrotron emission from a shock. These violations of the maximum synchrotron energy are further compounded by the detection of very high energy (VHE) emission above 300 GeV by MAGIC concurrent with our observations. We conclude that the observations of VHE photons from GRB 190114C necessitates either an additional emission mechanism at very high energies that is hidden in the synchrotron component in the LAT energy range, an acceleration mechanism that imparts energy to the particles at a rate that is faster than the electron synchrotron energy loss rate, or revisions of the fundamental assumptions used in estimating the maximum photon energy attainable through the synchrotron process.

astro-ph.HE↗

arXiv2020

A parsec-scale wobbling jet in the high-synchrotron peaked blazar PG 1553+113

PG 1553+113 is the first blazar showing an approximately two-year quasi-periodic pattern in its gamma-ray light curve. Such quasi-periodicity might have a geometrical origin, possibly related to the precessing nature of the jet, or could be intrinsic to the source and related to pulsational accretion flow instabilities. By means of a ~2yr very long baseline array (VLBA) monitoring at 15, 24, and 43 GHz we investigate the source pc-scale properties during an entire cycle of gamma-ray activity in the period 2015-2017. In contrast to the well-defined periodicity in the gamma-ray emission, at radio frequencies no clear periodic pattern can be recognized. The jet position angle, constrained by means of the total intensity ridge line, varies across the different observing epochs in the range 40-60 deg. We also investigate the time evolution of the source polarization properties, including the rotation measure. The brightness temperature is found to decrease as the frequency increases with an intrinsic value of ~1.5 x 10^10 K and the estimated Doppler factor is ~1.4.

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arXiv2019

Apparent superluminal core expansion and limb brightening in the candidate neutrino blazar TXS 0506+056

IceCube has reported a very-high-energy neutrino (IceCube-170922A) in a region containing the blazar TXS 0506+056. Correlated γ-ray activity has led to the first high-probability association of a high-energy neutrino with an extragalactic source. This blazar has been found to be in a radio outburst during the neutrino event. We have performed target-of-opportunity VLBI imaging observations at 43 GHz frequency with the VLBA two and eight months, respectively, after the neutrino event. We produced two images of TXS 0506+056 with angular resolutions of (0.2x1.1) mas and (0.2x0.5) mas, respectively. The source shows a compact, high brightness temperature core (albeit not approaching the equipartition limit) and a bright and originally very collimated inner jet. Beyond about 0.5 mas from the mm-VLBI core, the jet loses this tight collimation and expands rapidly. During the months after the neutrino event associated with this source, the overall flux density is rising. This flux density increase happens solely within the core. The core expands in size with apparent superluminal velocity during these six months so that the brightness temperature drops by a factor of three in spite of the strong flux density increase. The radio jet of TXS 0506+056 shows strong signs of deceleration and/or a spine-sheath structure within the inner 1 mas (corresponding to about 70 pc to 140 pc in deprojected distance) from the mm-VLBI core. This structure is consistent with theoretical models that attribute the neutrino and γ-ray production to interactions of electrons and protons in the highly-relativistic jet spine with external photons originating from a slower-moving jet region. Proton loading due to jet-star interactions in the inner host galaxy is suggested as the possible cause of deceleration

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arXiv2019

Radio VLBA polarization and multi-band monitoring of the high-redshift quasar S5 0836+710 during a high activity period

We report on results of a multi-band monitoring campaign from radio to gamma rays of the high-redshift flat spectrum radio quasar S5 0836+710 during a high activity period detected by the Large Area Telescope on board the Fermi Gamma-ray Space Telescope. Two major flares were detected, in 2015 August and November. In both episodes, the apparent isotropic gamma-ray luminosity exceeds 10^50 erg/s, with a doubling time scale of about 3 hours. The high gamma-ray activity may be related to a superluminal knot that emerged from the core in 2015 April at the peak of the radio activity and is moving downstream along the jet. The low variability observed in X-rays may indicate that X-ray emission is produced by the low-energy tail of the same electron population that produces the gamma-ray emission. The analysis of full-polarization pc-scale radio observations suggests the presence of a limb-brightened polarization structure at about 1 mas from the core in which a rotation measure gradient with a sign change is observed transverse to the jet direction. These characteristics are consistent with a scenario in which Faraday rotation is produced by a sheath of thermal electrons with a toroidal magnetic field surrounding the emitting jet.

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arXiv2019

Detection statistics of the RadioAstron AGN survey

The largest Key Science Program of the RadioAstron space VLBI mission is a survey of active galactic nuclei (AGN). The main goal of the survey is to measure and study the brightness of AGN cores in order to better understand the physics of their emission while taking interstellar scattering into consideration. In this paper we present detection statistics for observations on ground-space baselines of a complete sample of radio-strong AGN at the wavelengths of 18, 6, and 1.3 cm. Two-thirds of them are indeed detected by RadioAstron and are found to contain extremely compact, tens to hundreds of $μ$as structures within their cores.

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arXiv2019

Unresolved Gamma-Ray Sky through its Angular Power Spectrum

The gamma-ray sky has been observed with unprecedented accuracy in the last decade by the Fermi large area telescope (LAT), allowing us to resolve and understand the high-energy Universe. The nature of the remaining unresolved emission (unresolved gamma-ray background, UGRB) below the LAT source detection threshold can be uncovered by characterizing the amplitude and angular scale of the UGRB fluctuation field. This work presents a measurement of the UGRB autocorrelation angular power spectrum based on eight years of Fermi LAT Pass 8 data products. The analysis is designed to be robust against contamination from resolved sources and noise systematics. The sensitivity to subthreshold sources is greatly enhanced with respect to previous measurements. We find evidence (with $\sim$3.7$σ$ significance) that the scenario in which two classes of sources contribute to the UGRB signal is favored over a single class. A double power law with exponential cutoff can explain the anisotropy energy spectrum well, with photon indices of the two populations being 2.55 $\pm$ 0.23 and 1.86 $\pm$ 0.15.

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arXiv2019

From radio-quiet to radio-silent: low luminosity Seyfert radio cores

A strong effort has been devoted to understand the physical origin of radio emission from low-luminosity AGN (LLAGN), but a comprehensive picture is still missing. We used high-resolution ($\le$1 arcsec), multi-frequency (1.5, 5.5, 9 and 14 GHz) NSF's Karl G. Jansky Very Large Array (VLA) observations to characterise the state of the nuclear region of ten Seyfert nuclei, which are the faintest members of a complete, distance-limited sample of 28 sources. With the sensitivity and resolution guaranteed by the VLA-A configuration, we measured radio emission for six sources (NGC3185, NGC3941, NGC4477, NGC4639, NGC4698 and NGC4725), while for the remaining four (NGC0676, NGC1058, NGC2685 and NGC3486) we put upper limits at tens uJy/beam level, below the previous 0.12 mJy/beam level of Ho&Ulvestad (2001), corresponding to luminosities down to L$\le10^{19}$ W/Hz at 1.5 GHz for the highest RMS observation. Two sources, NGC4639 and NGC4698, exhibit spectral slopes compatible with inverted spectra ($α\le$0, $S_ν\,\propto\,ν^{-α}$), hint for radio emission from an optically-thick core, while NGC4477 exhibits a steep (+0.52$\pm$0.09) slope. The detected sources are mainly compact on scales $\le$ arcseconds, predominantly unresolved, except NGC3185 and NGC3941, in which the resolved radio emission could be associated to star-formation processes. A significant X-ray - radio luminosities correlation is extended down to very low luminosities, with slope consistent with inefficient accretion, expected at such low Eddington ratios. Such sources will be one of the dominant Square Kilometre Array (SKA) population, allowing a deeper understanding of the physics underlying such faint AGN.

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arXiv2019

Compact radio emission indicates a structured jet was produced by a binary neutron star merger

The binary neutron star merger event GW170817 was detected through both electromagnetic radiation and gravitational waves. Its afterglow emission may have been produced by either a narrow relativistic jet or an isotropic outflow. High spatial resolution measurements of the source size and displacement can discriminate between these scenarios. We present Very Long Baseline Interferometry observations, performed 207.4 days after the merger, using a global network of 32 radio telescopes. The apparent source size is constrained to be smaller than 2.5 milliarcseconds at the 90% confidence level. This excludes the isotropic outflow scenario, which would have produced a larger apparent size, indicating that GW170817 produced a structured relativistic jet. Our rate calculations show that at least 10% of neutron star mergers produce such a jet.

astro-ph.HE↗

arXiv2019

Probing restarting activity in hard X-ray selected giant radio galaxies

With their sizes larger than 0.7 Mpc, Giant Radio Galaxies (GRGs) are the largest individual objects in the Universe. To date, the reason why they reach such enormous extensions is still unclear. One of the proposed scenarios suggests that they are the result of multiple episodes of jet activity. Cross-correlating the INTEGRAL+Swift AGN population with radio catalogues (NVSS, FIRST, SUMSS), we found that 22% of the sources are GRG (a factor four higher than those selected from radio catalogues). Remarkably, 80% of the sample shows signs of restarting radio activity. The X-ray properties are consistent with this scenario, the sources being in a high-accretion, high-luminosity state with respect to the previous activity responsible for the radio lobes.

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