arXiv subjects

Massimo Persic

Publications and source records attributed to Massimo Persic.

At least 19 recordsLinked to original sources

arXiv2024

Non-thermal emission in M31 and M33

Spiral galaxies M31 and M33 are Fermi/LAT-detected gamma-ray sources. We model the broadband non-thermal (NT) emission of the central region of M31 (R < 5.5 kpc) and of the disk of M33 (R ~ 9 kpc). For either galaxy, we self-consistently model the broadband SED of the diffuse NT emission based on published radio and gamma-ray data. All relevant radiative processes involving relativistic and thermal electrons (synchrotron, Compton scattering, bremsstrahlung, and free-free emission and absorption), along with relativistic protons (neutral-pion decay following interaction with thermal protons), are considered, using exact emissivity formulae. We also use the Fermi/LAT validated gamma-ray emissivities for pulsars. We find that, in both sources, the radio emission is composed of primary and secondary electron synchrotron and thermal bremsstrahlung. The M33 gamma-ray emission appears to be mainly hadronic, similar to the Magellanic Clouds (Persic & Rephaeli 2022). In contrast, we find suggestions of a more complex situation in the central region of M31, whose emission could be a mix of pulsar emission and hadronic emission, with the latter possibly originating from both the disk and the vicinity of the nuclear black hole. The alternative modelling of the spectra of M31 and M33 is motivated by the different hydrogen distribution in the two galaxies: the hydrogen deficiency in the central region of M31 partially unveils emissions from the nuclear BH and the pulsar population in the bulge and inner disk. If this were to be the case in M33 as well, these emissions would be outshined by diffuse pionic emission originating within the flat central-peak gas distribution in M33.

astro-ph.HE↗

arXiv2023

Prospects of additional contribution at Optical-NIR band of EBL in the light of VHE spectra

The Extragalactic Background Light (EBL) that spans the UV-IR band originates from direct and dust-reprocessed starlight integrated over the history of the Universe. EBL measurements are very challenging due to foreground emission like the zodiacal light and interplanetary dust emission. Indeed, some optical/NIR direct measurements overpredict EBL models based on galaxy counts. On the other hand, there is some debate on possible additional components of the Optical-NIR photon density: e.g., population-III stars, axion-photon decay, direct collapse of black holes, intra-halo light etc. Owing to the absorption of Very High Energy (VHE) $\gamma$ rays by interaction with EBL photons, we study the prospects of accommodating an additional population of EBL sources in the Optical-NIR band on top of the standard galaxy-count--based component. To this aim we use 105 VHE spectra of 37 blazars with known redshifts, $0.03 $1.5, we estimate, at different wavelengths, upper limits to the additional low energy photon fields which would contribute to the absorption of $\gamma$-rays. Considering these limits we suggest that there is room for photons from Pop III stars and axion-like particle (ALP) annihilation. However, these additional hypothetical photon fields are bound to fall significantly below direct published EBL measurements by several instruments, and therefore our limits are either in tension or even inconsistent with such measurements.

astro-ph.HE↗

arXiv2022

The Fourth Catalog of Active Galactic Nuclei Detected by the Fermi Large Area Telescope -- Data Release 3

An incremental version of the fourth catalog of active galactic nuclei (AGNs) detected by the Fermi-Large Area Telescope is presented. This version (4LAC-DR3) derives from the third data release of the 4FGL catalog based on 12 years of E>50 MeV gamma-ray data, where the spectral parameters, spectral energy distributions (SEDs), yearly light curves, and associations have been updated for all sources. The new reported AGNs include 587 blazar candidates and four radio galaxies. We describe the properties of the new sample and outline changes affecting the previously published one. We also introduce two new parameters in this release, namely the peak energy of the SED high-energy component and the corresponding flux. These parameters allow an assessment of the Compton dominance, the ratio of the Inverse-Compton to the synchrotron peak luminosities, without relying on X-ray data.

astro-ph.HE↗

arXiv2022

Diffuse non-thermal emission in the disks of the Magellanic Clouds

The Magellanic Clouds, two dwarf galaxy companions to the Milky Way, are among the Fermi Large Area Telescope (LAT) brightest gamma-ray sources. Aiming at a comprehensive modeling of the non-thermal electromagnetic and neutrino emission in both Clouds, we self-consistently model the radio and gamma-ray spectral energy distribution from their disks based on recently published Murchison Widefield Array and Fermi/LAT data. All relevant radiative processes involving relativistic and thermal electrons (synchrotron, Compton scattering, and bremsstrahlung) and relativistic protons (neutral-pion decay following interaction with thermal protons) are considered, using exact emission formulae. Our joint spectral analyses indicate that radio emission in the Clouds has both primary and secondary electron synchrotron and thermal bremsstrahlung origin, whereas gamma rays originate mostly from neutral-pion decay with some contributions from relativistic bremsstrahlung and Compton scattering off starlight. The proton spectra in both galaxies are modeled as power laws in energy with similar spectral indices, ~2.4, and energy densities, ~1 eV/cm3. The predicted 0.1-10 GeV neutrino flux is too low for detection by current and upcoming experiments. Our analyses confirm earlier suggestions of a largely hadronic origin of the gamma-ray emission in both Magellanic Clouds.

astro-ph.HE↗

arXiv2019

Non-thermal emission in lobes of radio galaxies: III. 3C 98, Pictor A, DA 240, Cygnus A, 3C 326, and 3C 236

Recent analyses of the broad spectral energy distributions (SED) of extensive lobes of local radio-galaxies have confirmed the leptonic origin of their Fermi/LAT gamma-ray emission, significantly constraining the level of hadronic contribution. SED of distant (D > 125 Mpc) radio-galaxy lobes are currently limited to the radio and X-ray bands, hence give no information on the presence of non-thermal (NT) protons but are adequate to describe the properties of NT electrons. Modeling lobe radio and X-ray emission in 3C 98, Pictor A, DA 240, Cygnus A, 3C 326, and 3C 236, we fully determine the properties of intra-lobe NT electrons and estimate the level of the related gamma-ray emission from Compton scattering of the electrons off the superposed Cosmic Microwave Background, Extragalactic Background Light, and source-specific radiation fields.

astro-ph.HE↗

arXiv2019

Non-thermal emission in radio galaxy lobes: II. Centaurus A, Centaurus B, and NGC 6251

Radio and $\gamma$-ray measurements of large lobes of several radio galaxies provide adequate basis for determining whether emission in these widely separated spectral regions is largely by energetic electrons. This is very much of interest as there is of yet no unequivocal evidence for a significant energetic proton component to account for $\gamma$-ray emission by neutral pion decay. A quantitative assessment of the proton spectral distribution necessitates full accounting of the local and background radiation fields in the lobes; indeed, doing so in our recent analysis of the spectral energy distribution of the Fornax A lobes considerably weakened previous conclusions on the hadronic origin of the emission measured by the Fermi satellite. We present the results of similar analyses of the measured radio, X-ray and $\gamma$-ray emission from the lobes of Centaurus A, Centaurus B, and NGC 6251. The results indicate that the measured $\gamma$-ray emission from these lobes can be accounted for by Compton scattering of the radio-emitting electrons off the superposed radiation fields in the lobes; consequently, we set upper bounds on the energetic proton contents of the lobes.

astro-ph.HE↗

arXiv2019

Nonthermal emission in the lobes of Fornax A

Current measurements of the spectral energy distribution in radio, X-and-gamma-ray provide a sufficiently wide basis for determining basic properties of energetic electrons and protons in the extended lobes of the radio galaxy Fornax A. Of particular interest is establishing observationally, for the first time, the level of contribution of energetic protons to the extended emission observed by the Fermi satellite. Two recent studies concluded that the observed gamma-ray emission is unlikely to result from Compton scattering of energetic electrons off the optical radiation field in the lobes, and therefore that the emission originates from decays of neutral pions produced in interactions of energetic protons with protons in the lobe plasma, implying an uncomfortably high proton energy density. However, our exact calculation of the emission by energetic electrons in the magnetized lobe plasma leads to the conclusion that all the observed emission can, in fact, be accounted for by energetic electrons scattering off the ambient optical radiation field, whose energy density (which, based on recent observations, is dominated by emission from the central galaxy NGC 1316) we calculate to be higher than previously estimated.

astro-ph.HE↗

arXiv2015

Synchrotron and Compton Spectra from a Steady-State Electron Distribution

Energy densities of relativistic electrons and protons in extended galactic and intracluster regions are commonly determined from spectral radio and (rarely) $γ$-ray measurements. The time-independent particle spectral density distributions are commonly assumed to have a power-law (PL) form over the relevant energy range. A theoretical relation between energy densities of electrons and protons is usually adopted, and energy equipartition is invoked to determine the mean magnetic field strength in the emitting region. We show that for typical conditions, in both star-forming and starburst galaxies, these estimates need to be scaled down substantially due to significant energy losses that (effectively) flatten the electron spectral density distribution, resulting in a much lower energy density than deduced when the distribution is assumed to have a PL form. The steady-state electron distribution in the nuclear regions of starburst galaxies is calculated by accounting for Coulomb, bremsstrahlung, Compton, and synchrotron losses; the corresponding emission spectra of the latter two processes are calculated and compared to the respective PL spectra. We also determine the proton steady-state distribution by taking into account Coulomb and pion production losses, and briefly discuss implications of our steady-state particle spectra for estimates of proton energy densities and magnetic fields.

astro-ph.HE↗

arXiv2014

An extreme gravitationally redshifted iron line at 4.8 keV in Mrk 876

X-ray spectral lines at unforeseen energies are important because they can shed light on the extreme physical conditions of the environment around the supermassive black holes of active galactic nuclei (AGN). Mrk 876 displays such a line at 4.8 keV rest-frame energy. A possible interpretation of its origin can be found in the hotspot scenario. In this scenario the primary radiation from a flare in the hot corona of an AGN illuminates a limited portion of the accretion disk that emits by fluorescence. In this context the line can represent an extreme gravitationally redshifted Fe line originating on the accretion disk below 6 gravitational radii from a rotating supermassive black hole. The correct estimate of the line significance requires a dedicated approach. Based on an existing rigorous approach, we have performed extensive Monte Carlo simulations. We determine that the line is a real feature at ~99 confidence level.

astro-ph.HE↗

arXiv2014

Estimates of relativistic electron and proton energy densities in starburst galactic nuclei from radio measurements

The energy density of energetic protons, U_p, in several nearby starburst nuclei (SBNs) has been directly deduced from gamma-ray measurements of the radiative decay of neutral pions produced in interactions with ambient protons. Lack of sufficient sensitivity and spatial resolution makes this direct deduction unrealistic in the foreseeable future for even moderately distant SBNs. A more viable indirect method for determining U_p in star-forming galaxies is to use its theoretically based scaling to the energy density of energetic electrons, U_e, which can be directly deduced from radio synchrotron and possibly also nonthermal hard X-ray emission. In order to improve the quantitative basis and diagnostic power of this leptonic method we reformulate and clarify its main aspects. Doing so we obtain a basic expression for the ratio U_p/U_e in terms of the proton and electron masses and the power-law indices that characterize the particle spectral distributions in regions where the total particle energy density is at equipartition with that of the mean magnetic field. We also express the field strength and the particle energy density in the equipartition region in terms of the region's size, mean gas density, IR and radio fluxes, and distance from the observer, and determine values of U_p in a sample of nine nearby and local SBNs.

astro-ph.HE↗

arXiv2013

Nonthermal Emission from Star-Forming Galaxies

The detections of high-energy gamma-ray emission from the nearby starburst galaxies M82 & NGC253, and other local group galaxies, broaden our knowledge of star-driven nonthermal processes and phenomena in non-AGN star-forming galaxies. We review basic aspects of the related processes and their modeling in starburst galaxies. Since these processes involve both energetic electrons and protons accelerated by SN shocks, their respective radiative yields can be used to explore the SN-particle-radiation connection. Specifically, the relation between SN activity, energetic particles, and their radiative yields, is assessed through respective measures of the particle energy density in several star-forming galaxies. The deduced energy densities range from O(0.1) eV/cm^3 in very quiet environments to O(100) eV/cm^3 in regions with very high star-formation rates.

astro-ph.HE↗

arXiv2013

CTA: the future of ground-based gamma-ray astrophysics

Very high energy (VHE; E > 100 GeV) gamma-rays provide a unique probe into the non-thermal processes in the universe. The ground-based Imaging Air Cherenkov telescopes (IACTs) for detecting VHE gamma-rays have been perfected, so a relatively fast and inexpensive assembly of IACTs is now possible. Next generation instruments will have a sensitivity about 10 times better than current facilities, and will extend the accessible gamma-ray bandwidth at both energy ends (down to 30 GeV and up to 300 TeV) with improved angular and energy resolutions. Some key physics drivers, that are discussed here, suit specific features of the upcoming IACT facility, the Cherenkov Telescope Array (CTA). The resulting technical solutions chosen for CTA, and the current status of the project, are also outlined.

astro-ph.HE↗

arXiv2012

Emitting electrons and source activity in Markarian 501

We study the variation of the broad-band spectral energy distribution (SED) of the BL Lac object Mrk 501 as a function of source activity, from quiescent to flaring. Through chi-square-minimization we model eight simultaneous SED datasets with a one-zone Synchrotron-Self-Compton (SSC) model, and examine how model parameters vary with source activity. The emerging variability pattern of Mrk 501 is complex, with the Compton component arising from gamma-electron scatterings that sometimes are (mostly) Thomson and sometimes (mostly) extreme Klein-Nishina. This can be seen from the variation of the Compton to synchrotron peak distance according to source state. The underlying electron spectra are faint/soft in quiescent states and bright/hard in flaring states. A comparison with Mrk 421 suggests that the typical values of the SSC parameters are different in the two sources: however, in both jets the energy density is particle dominated in all states.

astro-ph.HE↗

arXiv2011

BL Lac Objects: Laboratories to study the environment and properties of emitting particles in relativistic jets

We report the variation of the spectral energy distribution (SED) of blazars as a function of source activity, based on available, simultaneous multi-wavelength (MWL) observations of BL Lac objects. We use a fully automatized \c{hi}2 minimization procedure, instead of the commonly used eye-ball fit, to model the data sets with a one-zone Synchrotron-Self-Compton (SSC) model. The obtained SSC parameters are then analyzed as a function of source luminosity, and the correlation between parameters is shown. Possibilities of improving the present observational and modeling status of BL Lac objects are also discussed.

astro-ph.CO↗

arXiv2011

Emission models and EBL as a tool to measure the redshift of BL Lac objects

We introduce a new method to determine the redshift of unknown-redshift BL Lac Objects. The method relies on simultaneous multi-wavelength (MWL) observations of BL Lac objects in optical, X-ray, HE (E>100 MeV) gamma-rays and VHE (E>100 GeV)gamma-rays. It involves best-fitting spectral energy distribution (SED) from optical through HE gamma-rays with a Synchrotron-Self-Compton (SSC) model. We extrapolate such best fitting model into VHE regime, and assume that it represents the intrinsic emission of the object. We then compare the observed VHE flux which has been affected by the interaction with Extragalactic Background Light (EBL). Constraining the measured vs intrinsic emission leads to the determination of gamma-gamma opacity. Comparing the obtained opacity with the predicted opacity based on EBL model, we obtain the redshift of the photon source.

astro-ph.CO↗

arXiv2011

Environment and properties of emitting electrons in blazar jets: Mrk 421 as a laboratory

Here we report our recent study on the spectral energy distribution (SED) of the high frequency BL Lac object Mrk 421 in different luminosity states. We used a full-fledged chi2-minimization procedure instead of more commonly used "eyeball" fit to model the observed flux of the source (from optical to very high energy), with a Synchrotron-Self-Compton (SSC) emission mechanism. Our study shows that the synchrotron power and peak frequency remain constant with varying source activity, and the magnetic field decreases with the source activity while the break energy of electron spectrum and the Doppler factor increase. Since a lower magnetic field and higher density of electrons result in increased electron-photon scattering efficiency, the Compton power increases, so does the total emission.

astro-ph.HE↗

arXiv2011

The environment and distribution of emitting electrons as a function of source activity in Markarian 421

For the high-frequency peaked BL Lac object Mrk 421 we study the variation of the spectral energy distribution (SED) as a function of source activity, from quiescent to active. We use a fully automatized chi-squared minimization procedure, instead of the "eyeball" procedure more commonly used in the literature, to model nine SED datasets with a one-zone Synchrotron-Self-Compton (SSC) model and examine how the model parameters vary with source activity. The latter issue can finally be addressed now, because simultaneous broad-band SEDs (spanning from optical to VHE photon energies) have finally become available. Our results suggest that in Mrk 421 the magnetic field decreases with source activity, whereas the electron spectrum's break energy and the Doppler factor increase -- the other SSC parameters turn out to be uncorrelated with source activity. In the SSC framework these results are interpreted in a picture where the synchrotron power and peak frequency remain constant with varying source activity, through a combination of decreasing magnetic field and increasing number density of electrons below the break energy: since this leads to an increased electron-photon scattering efficiency, the resulting Compton power increases, and so does the total (= synchrotron plus Compton) emission.

astro-ph.HE↗

arXiv2011

Evidence for an axion-like particle from blazar spectra?

Observations with the Imaging Atmospheric Cherenkov Telescopes H.E.S.S., MAGIC, CANGAROO III and VERITAS have shown that the Universe is more transparent than expected to gamma rays above 100GeV. As a natural explanation, the DARMA scenario has previously been proposed, wherein photons can oscillate into a new very light axionlike particle and vice-versa in the presence of cosmic magnetic fields. Here we demonstrate that the most recent observations further support the DARMA scenario, thereby making the existence of a very light axion-like particle more likely.

astro-ph.HE↗