arXiv subjects

F. Tavecchio

Publications and source records attributed to F. Tavecchio.

At least 37 records · Page 2Linked to original sources

arXiv2023

Probing AGN variability with the Cherenkov Telescope Array

Relativistic jets launched by Active Galactic Nuclei are among the most powerful particle accelerators in the Universe. The emission over the entire electromagnetic spectrum of these relativistic jets can be extremely variable with scales of variability from less than few minutes up to several years. These variability patterns, which can be very complex, contain information about the acceleration processes of the particles and the area(s) of emission. Thanks to its sensitivity, five-to twenty-times better than the current generation of Imaging Atmospheric Cherenkov Telescopes depending on energy, the Cherenkov Telescope Array will be able to follow the emission from these objects with a very accurate time sampling and over a wide spectral coverage from 20 GeV to > 20 TeV and thus reveal the nature of the acceleration processes at work in these objects. We will show the first results of our lightcurve simulations and long-term behavior of AGN as will be observed by CTA, based on state-of-art particle acceleration models.

astro-ph.HE↗

arXiv2023

X-ray polarimetry of X-ray pulsar X Persei: another orthogonal rotator?

X Persei is a persistent low-luminosity X-ray pulsar of period of $\sim$835 s in a Be binary system. The field strength at the neutron star surface is not known precisely, but indirect signs indicate a magnetic field above $10^{13}$ G, which makes the object one of the most magnetized known X-ray pulsars. Here we present the results of observations X Persei performed with the Imaging X-ray Polarimetry Explorer (IXPE). The X-ray polarization signal was found to be strongly dependent on the spin phase of the pulsar. The energy-averaged polarization degree in 3-8 keV band varied from several to $\sim$20 per cent over the pulse with a positive correlation with the pulsed X-ray flux. The polarization angle shows significant variation and makes two complete revolutions during the pulse period resulting in nearly nil pulse-phase averaged polarization. Applying the rotating vector model to the IXPE data we obtain the estimates for the rotation axis inclination and its position angle on the sky as well as for the magnetic obliquity. The derived inclination is close to the orbital inclination reported earlier for X Persei. The polarimetric data imply a large angle between the rotation and magnetic dipole axes, which is similar to the result reported recently for the X-ray pulsar GRO J1008$-$57. After eliminating the effect of polarization angle rotation over the pulsar phase using the best-fitting rotating vector model, the strong dependence of the polarization degree with energy was discovered with its value increasing from 0% at $\sim$2 keV to 30% at 8 keV.

astro-ph.HE↗

arXiv2022

Accretion geometry of the neutron star low mass X-ray binary Cyg X-2 from X-ray polarization measurements

We report spectro-polarimetric results of an observational campaign of the bright neutron star low-mass X-ray binary Cyg X-2 simultaneously observed by IXPE, NICER and INTEGRAL. Consistently with previous results, the broad-band spectrum is characterized by a lower-energy component, attributed to the accretion disc with $kT_{\rm in} \approx$ 1 keV, plus unsaturated Comptonization in thermal plasma with temperature $kT_{\rm e} = 3$ keV and optical depth $τ\approx 4$, assuming a slab geometry. We measure the polarization degree in the 2-8 keV band $P=1.8 \pm 0.3$ per cent and polarization angle $ϕ= 140^{\circ} \pm 4^{\circ}$, consistent with the previous X-ray polarimetric measurements by OSO-8 as well as with the direction of the radio jet which was earlier observed from the source. While polarization of the disc spectral component is poorly constrained with the IXPE data, the Comptonized emission has a polarization degree $P =4.0 \pm 0.7$ per cent and a polarization angle aligned with the radio jet. Our results strongly favour a spreading layer at the neutron star surface as the main source of the polarization signal. However, we cannot exclude a significant contribution from reflection off the accretion disc, as indicated by the presence of the iron fluorescence line.

astro-ph.HE↗

arXiv2022

Mapping the circumnuclear regions of the Circinus galaxy with the Imaging X-ray Polarimetry Explorer

We report on the Imaging X-ray Polarimetry Explorer (IXPE) observation of the closest and X-ray brightest Compton-thick active galactic nucleus (AGN), the Circinus galaxy. We find the source to be significantly polarized in the 2--6 keV band. From previous studies, the X-ray spectrum is known to be dominated by reflection components, both neutral (torus) and ionized (ionization cones). Our analysis indicates that the polarization degree is $28 \pm 7$ per cent (at 68 per cent confidence level) for the neutral reflector, with a polarization angle of $18° \pm 5°$, roughly perpendicular to the radio jet. The polarization of the ionized reflection is unconstrained. A comparison with Monte Carlo simulations of the polarization expected from the torus shows that the neutral reflector is consistent with being an equatorial torus with a half-opening angle of 45°-55°. This is the first X-ray polarization detection in a Seyfert galaxy, demonstrating the power of X-ray polarimetry in probing the geometry of the circumnuclear regions of AGNs, and confirming the basic predictions of standard Unification Models.

astro-ph.HE↗

arXiv2022

ASTRI Mini-Array Core Science at the Observatorio del Teide

The ASTRI (Astrofisica con Specchi a Tecnologia Replicante Italiana) Project led by the Italian National Institute for Astrophysics (INAF) is developing and will deploy at the Observatorio del Teide a mini-array (ASTRI Mini-Array) composed of nine telescopes similar to the small-size dual-mirror Schwarzschild-Couder telescope (ASTRI-Horn) currently operating on the slopes of Mt. Etna in Sicily. The ASTRI Mini-Array will surpass the current Cherenkov telescope array differential sensitivity above a few tera-electronvolt (TeV), extending the energy band well above hundreds of TeV. This will allow us to explore a new window of the electromagnetic spectrum, by convolving the sensitivity performance with excellent angular and energy resolution figures. In this paper we describe the Core Science that we will address during the first four years of operation, providing examples of the breakthrough results that we will obtain when dealing with current open questions, such as the acceleration of cosmic rays, cosmology and fundamental physics and the new window, for the TeV energy band, of the time-domain astrophysics.

astro-ph.HE↗

arXiv2022

Extragalactic Observatory Science with the ASTRI Mini-Array at the Observatorio del Teide

The ASTRI Mini-Array is a next-generation system of nine imaging atmospheric Cherenkov telescopes that is going to be built at the Observatorio del Teide site. After a first phase, in which the instrument will be operated as an experiment prioritizing a schedule of primary science cases, an observatory phase is foreseen in which other significant targets will be pointed. We focus on the observational feasibility of extragalactic sources and on astrophysical processes that best complement and expand the ASTRI Mini-Array core science, presenting the most relevant examples that are at reach of detection over long-term time scales and whose observation can provide breakthrough achievements in the very-high energy extragalactic science. Such examples cover a wide range of $γ$-ray emitters, including the study of AGN low states in the multi-TeV energy range, the possible detection of Seyfert galaxies with long exposures and the searches of dark matter lines above 10 TeV. Simulations of the presented objects show that the instrument performance will be competitive at multi-TeV energies with respect to current arrays of Cherenkov telescopes.

astro-ph.HE↗

arXiv2022

Extreme blazars: the result of unstable recollimated jets?

Extreme BL Lacs (EHBL) form a subclass of blazars which challenge standard emission scenarios. In a recent study it has been argued that their peculiar properties can be explained if emitting electrons are accelerated in a series of oblique shocks induced by the recollimation of the relativistic jet. However, new 3D simulations of recollimated, weakly magnetized jets reveal that, in correspondence with the first recollimation shock, the flow develops a rapidly growing instability, becomes highly turbulent and decelerates, effectively hampering the formation of the multiple shock structure routinely observed in 2D simulations. Building on these new findings, we propose here a revised scenario for EHBL, in which the emission is produced by electrons accelerated at the recollimation shock and subsequently further energized through stochastic acceleration in the turbulent downstream flow. We apply a simple version of this scenario to the prototypical EHBL 1ES 0229-200, showing that the SED can be satisfactorily reproduced with standard values of the main physical parameters.

astro-ph.HE↗

arXiv2022

Polarization constraints on the X-ray corona in Seyfert Galaxies: MCG-05-23-16

We report on the first observation of a radio-quiet Active Galactic Nucleus (AGN) using polarized X-rays: the Seyfert 1.9 galaxy MCG-05-23-16. This source was pointed with the Imaging X-ray Polarimetry Explorer (IXPE) starting on May 14, 2022 for a net observing time of 486 ks, simultaneously with XMM-Newton (58 ks) and NuSTAR (83 ks). A polarization degree smaller than $Π<4.7\%$ (at the 99% c.l.) is derived in the 2-8 keV energy range, where emission is dominated by the primary component ascribed to the hot corona. The broad-band spectrum, inferred from a simultaneous fit to the IXPE, NuSTAR, and XMM-Newton data, is well reproduced by a power law with photon index $Γ=1.85\pm0.01$ and a high-energy cutoff $E_{\rm C}=120\pm15$ keV. A comparison with Monte Carlo simulations shows that a lamp-post and a conical geometry of the corona are consistent with the observed upper limit, a slab geometry is allowed only if the inclination angle of the system is less than 50$^{\circ}$.

astro-ph.HE↗

arXiv2022

Testing particle acceleration models for BL LAC jets with the Imaging X-ray Polarimetry Explorer

To determine if the recently launched Imaging X-ray Polarimetry Explorer (IXPE) can follow the polarization variations induced by different particle acceleration mechanisms in blazars jets we simulate observations of a high synchrotron peak (HSP) blazar variable in polarization degree and angle according to the theoretical predictions of Tavecchio et al., 2020 (shock acceleration model), Bodo et al., 2021 (magnetic reconnection model) and Marscher \& Jorstad, 2021 (Turbulent Extreme Multi-Zone Model). We use the Monte Carlo tool ixpeobssim to create realistic IXPE data products. We create simulated light-curves of polarization degree and angle by time-slicing the simulated data into arbitrary short time bins. We use a $χ^2$ test to assess the performance of the observations in detecting the time variability of the polarization properties. A series of 10 ks long observations permits IXPE to follow the time variability of the polarization degree in the case of the shock acceleration model. In the case of the magnetic reconnection model, the nominal injected model provides the best fit of the simulated IXPE data for time bins of 5-10 ks, depending on the tested flux level. For the TEMZ model, shorter time slices of 0.5 ks are needed for obtaining a formally good fit of the simulated IXPE data with the injected model. On the other hand, we find that a fit with a constant model provides a $χ^2$ lower than the fit with the nominal injected model when using time slices of 20 ks, 60/70 ks, and 5 ks for the case of the shock acceleration, magnetic reconnection and TEMZ model, respectively. We conclude that, provided that the statistics of the observation allows slicing of the data in adequately short time bins, IXPE observations of an HSP blazar at a typical flux level can detect the time variability predicted by popular models for particle acceleration in jets.

astro-ph.HE↗

arXiv2022

Direct observation of an extended X-ray jet at $z$=6.1

We report on the direct observation of an extended X-ray jet in the $z$=6.1 radio-loud Active Galactic Nucleus PSO J030947.49+271757.31 from a deep Chandra X-ray observation (128 ksec). This detection represents the most distant kpc off-nuclear emission resolved in the X-rays to date. The angular distance of the emission is $\sim$4" (corresponding to $\sim$20 kpc at $z$=6.1), along the same direction of the jet observed at parsec scales in previous VLBA high-resolution radio observations. Moreover, the 0.5-7.0 keV isophotes coincide with the extended radio emission as imaged by the VLA Sky Survey at 3 GHz. The rest-frame 2-10 keV luminosity of the extended component is L$_{2-10keV}$=5.9$\times$10$^{44}$ erg s$^{-1}$, about 8% of the core: this makes it one of the most luminous jets resolved in the X-rays so far. Through Spectral Energy Distribution modelling we find that this emission can be explained by the Inverse Compton interaction with the photons of the Cosmic Microwave Background assuming that the jet's physical parameters are similar to those in the local Universe. At the same time, we find that the radiation produced by a putative population of high-energetic electrons through the synchrotron process observed at low redshift is quenched at high redshift, hence becoming negligible.

astro-ph.HE↗

arXiv2021

Probing Dark Matter and Fundamental Physics with the Cherenkov Telescope Array

Astrophysical observations provide strong evidence that more than 80% of all matter in the Universe is in the form of dark matter (DM). Two leading candidates of particles beyond the Standard Model that could constitute all or a fraction of the DM content are the so-called Weakly Interacting Massive Particles (WIMPs) and Axion-Like Particles (ALPs). The upcoming Cherenkov Telescope Array, which will observe gamma rays between 20 GeV and 300 TeV with unprecedented sensitivity, will have unique capabilities to search for these DM candidates. A particularly promising target for WIMP searches is the Galactic Center. WIMPs with annihilation cross sections correctly producing the DM relic density will be detectable with CTA, assuming an Einasto-like density profile and WIMP masses between 200 GeV and 10 TeV. Regarding new physics beyond DM, CTA observations will also enable tests of fundamental symmetries of nature such as Lorentz invariance.

astro-ph.HE↗

arXiv2021

Probing extreme environments with the Cherenkov Telescope Array

The physics of the non-thermal Universe provides information on the acceleration mechanisms in extreme environments, such as black holes and relativistic jets, neutron stars, supernovae or clusters of galaxies. In the presence of magnetic fields, particles can be accelerated towards relativistic energies. As a consequence, radiation along the entire electromagnetic spectrum can be observed, and extreme environments are also the most likely sources of multi-messenger emission. The most energetic part of the electromagnetic spectrum corresponds to the very-high-energy (VHE, E>100 GeV) gamma-ray regime, which can be extensively studied with ground based Imaging Atmospheric Cherenkov Telescopes (IACTs). The results obtained by the current generation of IACTs, such as H.E.S.S., MAGIC, and VERITAS, demonstrate the crucial importance of the VHE band in understanding the non-thermal emission of extreme environments in our Universe. In some objects, the energy output in gamma rays can even outshine the rest of the broadband spectrum. The Cherenkov Telescope Array (CTA) is the next generation of IACTs, which, with cutting edge technology and a strategic configuration of ~100 telescopes distributed in two observing sites, in the northern and southern hemispheres, will reach better sensitivity, angular and energy resolution, and broader energy coverage than currently operational IACTs. With CTA we can probe the most extreme environments and considerably boost our knowledge of the non-thermal Universe.

astro-ph.IM↗

arXiv2021

Time Domain Astronomy with the THESEUS Satellite

THESEUS is a medium size space mission of the European Space Agency, currently under evaluation for a possible launch in 2032. Its main objectives are to investigate the early Universe through the observation of gamma-ray bursts and to study the gravitational waves electromagnetic counterparts and neutrino events. On the other hand, its instruments, which include a wide field of view X-ray (0.3-5 keV) telescope based on lobster-eye focusing optics and a gamma-ray spectrometer with imaging capabilities in the 2-150 keV range, are also ideal for carrying out unprecedented studies in time domain astrophysics. In addition, the presence onboard of a 70 cm near infrared telescope will allow simultaneous multi-wavelegth studies. Here we present the THESEUS capabilities for studying the time variability of different classes of sources in parallel to, and without affecting, the gamma-ray bursts hunt.

astro-ph.IM↗

arXiv2020

Constraining the shear acceleration model for the X-ray emission of large-scale extragalactic jets

The nature of the intense X-ray emission from powerful extragalactic jets at large ($>1$ kpc) scale is still debated. The scenario that invokes the inverse Compton scattering of the CMB by electrons is challenged by the lack of gamma-ray emission in the GeV band. An alternative assumes synchrotron emission by a distinct population of ultra-high energy electrons. Here we present a concrete attempt to apply this scenario, exploring the specific model in which the ultra-high energy electrons are accelerated in a shear layer surrounding the jet. We limit the study to non-relativistic flows and particle acceleration is treated by a Fokker-Planck equation. The observed relation between low energy (radio, optical) and X-ray emission prompts us to assume that the required population of pre-accelerated particles is provided by a shock responsible for the acceleration of the electrons emitting at low frequencies. We apply the model to the emission of the principal knots of the jets of PKS 0637-752 and PKS 1136-135, two of the best studied objects. For the set of fiducial parameters adopted, the condition that the jet power does not exceeds a limiting value of $10^{48}$ erg s$^{-1}$ constrains the magnetic field above $10$ $μ$G and indicates moderate beaming ($δ\simeq 2$) for PKS 0637-752. For both sources, the requirement that acceleration of the electrons proceeds faster than radiative cooling can be met if the magnetic turbulence in the shear layer follows a Kolmogorov spectrum, $I(k)\propto k^{-q}$ with $q=5/3$, but cannot satisfied in the Bohm-like case ($q=1$).

astro-ph.HE↗

arXiv2020

Probing shock acceleration in BL Lac jets through X-ray polarimetry: the time-dependent view

Polarimetric measurements, especially if extended at high energy, are expected to provide important insights into the mechanisms underlying the acceleration of relativistic particles in jets. In a previous work we have shown that the polarization of the synchrotron X-ray emission produced by highly energetic electrons accelerated by a mildly relativistic shock carries essential imprints of the geometry and the structure of the magnetic fields in the downstream region. Here we present the extension of our analysis to the non-stationary case, especially suitable to model the highly variable emission of high-energy emitting BL Lacs. We anticipate a large ($Π\approx 40\%$), almost time-independent degree of polarization in the hard/medium X-ray band, a prediction soon testable with the upcoming mission {\it IXPE}. The situation in other bands, in particular in the optical, is more complex. A monotonic decrease of the optical degree of polarization is observed during the development of a flare. At later stages $Π$ reaches zero and then it starts to increase, recovering large values at late times. The instant at which $Π=0$ is marked by a rotation of the polarization angle by $90$ degrees. However, at optical frequencies it is likely that more than one region contributes to the observed emission, potentially making it difficult to detect the predicted behavior.

astro-ph.HE↗

arXiv2020

On the distribution of fluxes of gamma-ray blazars: hints for a stochastic process?

We examine a model for the observed temporal variability of powerful blazars in the $γ$-ray band in which the dynamics is described in terms of a stochastic differential equation, including the contribution of a deterministic drift and a stochastic term. The form of the equation is motivated by the current astrophysical framework, accepting that jets are powered through the extraction of the rotational energy of the central supermassive black hole mediated by magnetic fields supported by a so-called \emph{magnetically arrested} accretion disk. We apply the model to the $γ$-ray light curves of several bright blazars and we infer the parameters suitable to describe them. In particular, we examine the differential distribution of fluxes ($dN/dF_γ$) and we show that the predicted probability density function for the assumed stochastic equation naturally reproduces the observed power law shape at large fluxes $dN/dF_γ \propto F_γ^{-α}$ with $α>2$.

astro-ph.HE↗

arXiv2020

EeV Astrophysical neutrinos from FSRQs?

Flat Spectrum Radio Quasars (FSRQ) are the most powerful blazars in the gamma-ray band. Although they are supposed to be good candidates in producing high energy neutrinos, no secure detection of FSRQs has been obtained up to now, except for a possible case of PKS B1424-418. In this work, we compute the expected flux of high energy neutrinos from FSRQs using standard assumptions for the properties of the radiation fields filling the regions surrounding the central supermassive black hole. We obtain as a result that high energy neutrinos are naturally expected from FSRQs in the sub-EeV-EeV energy range and not at PeV energies. This justifies the non-observation of neutrinos from FSRQs with the present technology, since only neutrinos below 10 PeV have been observed. We found that for a non-negligible range of the parameters the cumulative flux from FSRQs is comparable to or even exceeds the expected cosmogenic neutrino flux. This result is intriguing and highlights the importance to disentangle these point-source emissions from the diffuse cosmogenic background.

astro-ph.HE↗

arXiv2019

Anisotropic electron populations in BL Lac jets: consequences for the observed emission

We investigate the impact on the properties of high-energy emitting BL Lac objects of a population of electrons with an anisotropic momentum distribution. We adopt a simple phenomenological description of the anisotropy, in which the most energetic electrons have a small pitch angle and the least energetic electrons are isotropic, as proposed by Sobacchi \& Lyubarsky (2019). We explore (i) a simple model that assumes a phenomenological shape for the electron energy distribution, and (ii) a self-consistent scheme in which the electrons follow a distribution which is the result of the balance between injection and radiative losses (we include the effects of the anisotropy on the synchrotron cooling rate). Considering the BL Lac object Mkn 421 as representative of the entire class, we show that in both cases the emission can be satisfactorily reproduced under equipartition between the magnetic field and the relativistic electrons. This is in better agreement with the idea that jets are launched as Poynting dominated flows with respect to the standard isotropic scenario, which requires both a low magnetization and a low radiative efficiency to reproduce the observed emission. The hard spectrum predicted for the inverse Compton continuum at TeV energies could be used as a potential test of the anisotropic model.

astro-ph.HE↗