arXiv subjects

Elena Orlando

Publications and source records attributed to Elena Orlando.

At least 19 recordsLinked to original sources

arXiv2026

The large-scale ordered magnetic field in the Galactic halo and the Local Bubble

All-sky synchrotron polarization observations are the primary probe of the Galactic large-scale ordered magnetic field, which includes both coherent and ordered random components. However, magnetic-field strength estimates from synchrotron observations depend on the assumed cosmic-ray electron model, which itself depends on the magnetic-field configuration and strength throughout the Galaxy. We aim to constrain the strength of the large-scale ordered magnetic field in the Galactic halo through a self-consistent treatment of cosmic-ray electrons with the latest coherent magnetic-field configuration inferred by Xu & Han from rotation measures corrected for local contamination. We compare the Planck 30 GHz synchrotron polarization maps with our large-scale synchrotron models that jointly treat cosmic rays and magnetic fields and are constrained by direct cosmic-ray measurements and multifrequency data. Since the inferred field strength depends on the halo size, we explore different cosmic-ray propagation halo sizes. We observe a significant large-scale synchrotron excess relative to model predictions, which requires an ordered toroidal magnetic-field component in the halo with a peak strength of 3.2-4.3 microG, depending on the assumed halo size. We also find that the cosmic-ray propagation model with a 4 kpc halo size is preferred by the Planck synchrotron polarization maps. This result shows that the ordered magnetic field is 4-6 times stronger than the coherent field inferred from rotation measures by Xu & Han, establishing the presence of an ordered random field in the Galactic halo that is not traced by rotation measures. Attributing the synchrotron excess entirely to the Local Bubble would require non-standard Local Bubble properties, disfavoring this interpretation.

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arXiv2025

Cosmic-ray and Interstellar Gas Properties in the Solar Neighborhood Revealed by Diffuse Gamma Rays

To investigate the interstellar medium (ISM) and Galactic cosmic rays (CRs) in the solar neighborhood, we analyzed $γ$-ray data by Fermi Large Area Telescope (LAT) for five nearby molecular cloud regions. Our data includes the MBM/Pegasus region (MBM~53, 54, 55 clouds and Pegasus loop), R CrA region (R Coronae Australis clouds), Chamaeleon region (Chamaeleon clouds), Cep/Pol region (Cepheus and Polaris flare), and Orion region (Orion clouds). The ISM templates are constructed by a component decomposition of the 21~cm {\HI} line, the Planck dust emission model, and the carbon monoxide (CO) 2.6~mm line. Through $γ$-ray data analysis the ISM gas is successfully decomposed into non-local {\HI}, narrow-line and optically thick {\HI}, broad-line and optically thin {\HI}, CO-bright {\Htwo}, and CO-dark {\Htwo} for all five regions. CR intensities evaluated by the $γ$-ray emissivity of broad {\HI} agree well with a model based on directly-measured CR spectra at the Earth, with a gradient giving a higher CR intensity toward the inner Galaxy at the 10\% level in ${\sim}$ 500~pc. The ratio of CO-dark {\Htwo} to CO-bright {\Htwo} anti-correlates with the {\Htwo} mass traced by the CO 2.6~mm line, and reaches 5--10 for small systems of ${\sim}$1000 solar mass.

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arXiv2024

Yet Another Sunshine Mystery: Unexpected Asymmetry in GeV Emission from the Solar Disk

The Sun is one of the most luminous $γ$-ray sources in the sky and continues to challenge our understanding of its high-energy emission mechanisms. This study provides an in-depth investigation of the solar disk $γ$-ray emission, using data from the Fermi Large Area Telescope spanning 2008 August to 2022 January. We focus on $γ$-ray events with energies exceeding 5 GeV, originating from 0.5$^\circ$ angular aperture centered on the Sun, and implement stringent time cuts to minimize potential sample contaminants. We use a helioprojection method to resolve the $γ$-ray events relative to the solar rotation axes and combine statistical tests to investigate the distribution of events over the solar disk. We found that integrating observations over large time windows may overlook relevant asymmetrical features, which we reveal in this work through a refined time-dependent morphological analysis. We describe significant anisotropic trends and confirm compelling evidence of energy-dependent asymmetry in the solar disk $γ$-ray emission. Intriguingly, the asymmetric signature coincides with the Sun's polar field flip during the cycle 24 solar maximum, around 2014 June. Our findings suggest that the Sun's magnetic configuration plays a significant role in shaping the resulting $γ$-ray signature, highlighting a potential link between the observed anisotropies, solar cycle, and the solar magnetic fields. These insights pose substantial challenges to established emission models, prompting fresh perspectives on high-energy solar astrophysics.

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arXiv2024

Testing Cosmic-Ray Propagation Scenarios with AMS-02 and Voyager Data

AMS-02 on board the ISS provides precise measurements of Cosmic Rays (CR) near Earth, while Voyager measures CR in the local interstellar medium, beyond the effects of solar modulation. Based on these data, we test and revise various CR propagation scenarios under standard assumptions: pure diffusion, diffusion with convection, diffusion with reacceleration, and diffusion with reacceleration and convection. We report on the scenarios' performance against CR measurements, aiming to limit the number of model parameters as much as possible. For each scenario we find parameters that are able to reproduce Voyager and AMS-02 data for the entire energy band for all the CR species tested. Above several GV we observe a similar injection spectral index for He and C, with He harder than H. Some scenarios previously disfavored are now reconsidered. For example, contrary to usual assumptions, we find that the pure diffusion scenario does not need an upturn in the diffusion coefficient at low energy, while it needs the same number of low-energy breaks in the injection spectrum as diffusive-reacceleration scenarios. We show that scenarios differ in modeled spectra of one order of magnitude for positrons at around 1 GeV and of a factor of 2 for antiprotons at several GV. The force-field approximation describes well the AMS-02 and Voyager spectra analyzed, except antiprotons. We confirm the excess around 10 GeV in the antiproton spectrum for all scenarios. Also, for all scenarios, the resulting solar modulation should be stronger for positrons than for nuclei, with reacceleration models requiring much larger modulation.

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arXiv2022

Transport of Cosmic ray electrons from 1 AU to the Sun

Gamma rays are produced by cosmic ray (CR) protons interacting with the particles at solar photosphere and by cosmic ray electrons and positrons (CRes) via inverse Compton scattering of solar photons. The former come from the solar disk while the latter extend beyond the disk. Evaluation of these emissions requires the flux and spectrum of CRs in the vicinity of the Sun, while most observations provide flux and spectra near the Earth, at around 1 AU from the Sun. Past estimates of the quiet Sun gamma-ray emission use phenomenological modulation procedures to estimate spectra near the Sun (see review by Orlando and Strong 2021 and references therein). We show that CRe transport in the inner heliosphere requires a kinetic approach and use a novel approximation to determine the variation of CRe flux and spectrum from 1 AU to the Sun including effects of (1) the structure of large scale magnetic field, (2) small scale turbulence in the solar wind from several in situ measurements, in particular, those by Parker Solar Probe that extend this information to 0.1 AU, and (3) most importantly, energy losses due to synchrotron and inverse Compton processes. We present results on the flux and spectrum variation of CRes from 1 AU to the Sun for several transport models. In forthcoming papers we will use these results for a more accurate estimate of quiet Sun inverse Compton gamma-ray spectra, and, for the first time, the spectrum of extreme ultraviolet to hard X-ray photons produced by synchrotron emission. These can be compared with the quiet Sun gamma-ray observation by Fermi (see, e.g.~Fermi-LAT Collaboration, 2011) and X-ray upper limits set by RHESSI (Hannah et al., 2010).

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arXiv2022

A New Component from the Quiet Sun: Synchrotron Radiation from Galactic Cosmic-Ray Electrons

The quiet Sun, i.e. in its non-flaring state or non-flaring regions, emits thermal radiation from radio to ultraviolet. The quiet Sun produces also non-thermal radiation observed in gamma rays due to interactions of Galactic Cosmic Rays (GCR) with the solar gas and photons. We report on a new component: the synchrotron emission by GCR electrons in the solar magnetic field. To the best of our knowledge this is the first time this emission has been theoretically claimed and modeled. We find that the measured GCR electrons with energies from tens of GeV to a few TeV produce synchrotron emission in X-rays, which is a few orders of magnitude lower than current upper limits of the quiet Sun set by RHESSI and FOXSI. For a radially decreasing solar magnetic field we find the expected synchrotron intensity to be almost constant in the solar disk, to peak in the close proximity of the Sun, and to quickly drop away from the Sun. We also estimate the synchrotron emission from radio to gamma rays and we compare it with current observations, especially with LOFAR. While it is negligible from radio to UV compared to the solar thermal radiation, this emission can potentially be observed at high energies with NuSTAR and more promising future FOXSI observations. This could potentially allow for constraining CR densities and magnetic-field intensities at the Sun. This study provides a more complete description and a possible new way for understanding the quite Sun and its environment.

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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.

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arXiv2022

Using TeV Cosmic Rays to probe the Heliosphere's Boundary with the Local Interstellar Medium

The heliosphere is the magnetic structure formed by the Sun's atmosphere extending into the local interstellar medium (ISM). The boundary separating the heliosphere from the ISM is a still largely unexplored region of space. Even though both Voyager spacecraft entered the local ISM and are delivering data, they are two points piercing a vast region of space at specific times. The heliospheric boundary regulates the penetration of MeV- GeV galactic cosmic rays (CR) in the inner heliosphere. Interstellar keV neutral atoms are crucial to the outer heliosphere since they can penetrate unperturbed and transfer energy to the solar wind. Missions such as NASA's IBEX and Cassini are designed to detect neutral atoms and monitor charge exchange processes at the heliospheric boundary. The heliosphere does not modulate the TeV CR intensity, but it does influence their arrival direction distribution. Ground-based CR observatories have provided accurate maps of CR anisotropy as a function of energy in the last couple of decades. Combining observations to produce all-sky coverage makes it possible to investigate the heliosphere's impact on TeV CR particles. We can numerically calculate the pristine TeV CR distribution in the local ISM with state-of-the-art heliosphere models. Only by subtracting the heliospheric influence is it possible to use TeV CR observations to infer propagation properties and the characteristics of magnetic turbulence in the ISM. Numerical calculations of CR particle trajectories through heliospheric models provide a complementary tool to probe the boundary region properties. A program boosting heliospheric modeling with emphasis on the boundary region and promoting combined CR experimental data analyses from multiple experiments benefits CR astrophysics and provides additional data and tools to explore the interaction between the heliosphere and the local ISM.

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arXiv2022

Exploring the MeV Sky with a Combined Coded Mask and Compton Telescope: The Galactic Explorer with a Coded Aperture Mask Compton Telescope (GECCO)

The sky at MeV energies is currently poorly explored. Here we present an innovative mission concept that builds on and improves past and currently proposed missions at such energies. We outline the motivations for combining a coded mask and a Compton telescope and we define the scientific goals of such a mission. The Galactic Explorer with a Coded Aperture Mask Compton Telescope (GECCO) is a novel concept for a next-generation telescope covering hard X-ray and soft gamma-ray energies. The potential and importance of this approach that bridges the observational gap in the MeV energy range are presented. With the unprecedented angular resolution of the coded mask telescope combined with the sensitive Compton telescope, a mission such as GECCO can disentangle the discrete sources from the truly diffuse emission. Individual Galactic and extragalactic sources are detected. This also allows to understand the gamma-ray Galactic center excess and the Fermi Bubbles, and to trace the low-energy cosmic rays, and their propagation in the Galaxy. Nuclear and annihilation lines are spatially and spectrally resolved from the continuum emission and from sources, addressing the role of low-energy cosmic rays in star formation and galaxy evolution, the origin of the 511 keV positron line, fundamental physics, and the chemical enrichment in the Galaxy. Such an instrument also detects explosive transient gamma-ray sources, which enable identifying and studying the astrophysical objects that produce gravitational waves and neutrinos in a multi-messenger context. By looking at a poorly explored energy band it also allows discoveries of new astrophysical phenomena.

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arXiv2021

A study of super-luminous stars with the Fermi Large Area Telescope

The $γ$-ray emission from stars is induced by the interaction of cosmic rays with stellar atmospheres and photon fields. This emission is expected to come in two components: a stellar disk emission, where $γ$-rays are mainly produced in atmospheric showers generated by hadronic cosmic rays, and an extended halo emission, where the high density of soft photons in the surroundings of stars create a suitable environment for $γ$-ray production via inverse Compton (IC) scattering by cosmic-ray electrons. Besides the Sun, no other disk or halo from single stars has ever been detected in $γ$-rays. However, by assuming a cosmic-ray spectrum similar to that observed on Earth, the predicted $γ$-ray emission of super-luminous stars, like e.g. Betelgeuse and Rigel, could be high enough to be detected by the Fermi Large Area Telescope (LAT) after its first decade of operations. In this work, we use 12 years of Fermi-LAT observations along with IC models to study 9 super-luminous nearby stars, both individually and via stacking analysis. Our results show no significant $γ$-ray emission, but allow us to restrict the stellar $γ$-ray fluxes to be on average $<3.3 \times 10^{-11}$ ph cm$^{-2}$ s$^{-1}$ at a 3$σ$ confidence level, which translates to an average local density of electrons in the surroundings of our targets to be less than twice of that observed for the Solar System.

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arXiv2021

StellarICS: Inverse Compton Emission from the Quiet Sun and Stars from keV to TeV

The study of the quiet Sun in gamma rays started over a decade ago, and rapidly gained a wide interest. Gamma rays from the quiet Sun are produced by Cosmic Rays (CRs) interacting with its surface (disk component) and with its photon field (spatially extended inverse-Compton component, IC). The latter component is maximum close to the Sun and it is above the background even at large angular distances, extending over the whole sky. First detected with EGRET, it is studied now with Fermi-LAT with high statistical significance. Observations of the IC component allow us to obtain information on CR electrons and positrons close to the Sun and in the heliosphere for the various periods of solar activity and polarity. They allow to learn about CR interactions and propagation close to stars, in the heliosphere and on the solar surface, and to understand the Sun itself, its environment, and its activity. Analyses of solar observations are usually model-driven. Hence advances in model calculations and constraints from precise CR measurements are timely and needed. Here we present our StellarICS code to compute the gamma-ray IC emission from the Sun and also from single stars. The code is publicly available and it is extensively used by the scientific community to analyze Fermi-LAT data. It has been used by the Fermi-LAT collaboration to produce the solar models released with the FSSC Fermi Tools. Our modeling provides the basis for analyzing and interpreting high-energy data of the Sun and of stars. After presenting examples of updated solar IC models in the Fermi-LAT energy range that account for the various CR measurements, we extend the models to keV, MeV, and TeV energies for predictions for future possible telescopes such as AMEGO, GECCO, e-ASTROGAM, HAWC, LHAASO, SWGO, and present X-ray telescopes. We also present predictions for some of the closest and most luminous stars.

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arXiv2019

The origin of Galactic cosmic rays: challenges to the standard paradigm

A critical review of the standard paradigm for the origin of Galactic cosmic rays is presented. Recent measurements of local and far-away cosmic rays reveal unexpected behaviours, which challenge the commonly accepted scenario. These recent findings are discussed, together with long-standing open issues. Despite the progress made thanks to ever-improving observational techniques and theoretical investigations, at present our understanding of the origin and of the behaviour of cosmic rays remains incomplete. We believe it is still unclear whether a modification of the standard paradigm, or rather a radical change of the paradigm itself is needed in order to interpret all the available data on cosmic rays within a self-consistent scenario.

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arXiv2019

Studying the magnetized ISM with all-sky polarimetric radio maps

Synchrotron radiation from the interstellar medium (ISM) of our galaxy dominates the sky brightness at low radio frequencies, and carries information about relativistic and thermal electron distributions across a range of astrophysical environments. The polarization of the radiation, as modified by Faraday rotation effects in the ISM, also contains extensive information about the magnetic field. Comprehensive all-sky broadband mapping of this radiation, when combined with high frequency radio data, gamma ray data, cosmic ray (CR) measurements and sophisticated modeling, can revolutionize our understanding of the ISM and the processes that influence its evolution. Current widefield imagery of the galactic synchrotron emission is heterogeneous in frequency coverage, sky coverage, angular resolution and calibration accuracy, limiting utility for ISM studies. A new generation of all-digital low frequency array technologies is opening a path to matched resolution, high fidelity polarimetric imaging across a fully sampled swath of radio frequencies from a few tens to many hundreds of MHz, generating a transformational dataset for a broad range of scientific applications.

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arXiv2019

Supermassive black holes at high redshifts

MeV blazars are the most luminous persistent sources in the Universe and emit most of their energy in the MeV band. These objects display very large jet powers and accretion luminosities and are known to host black holes with a mass often exceeding $10^9 M_{\odot}$. An MeV survey, performed by a new generation MeV telescope which will bridge the entire energy and sensitivity gap between the current generation of hard X-ray and gamma-ray instruments, will detect $>$1000 MeV blazars up to a redshift of $z=5-6$. Here we show that this would allow us: 1) to probe the formation and growth mechanisms of supermassive black holes at high redshifts, 2) to pinpoint the location of the emission region in powerful blazars, 3) to determine how accretion and black hole spin interplay to power the jet.

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arXiv2019

Positron Annihilation in the Galaxy

The 511 keV line from positron annihilation in the Galaxy was the first $γ$-ray line detected to originate from outside our solar system. Going into the fifth decade since the discovery, the source of positrons is still unconfirmed and remains one of the enduring mysteries in $γ$-ray astronomy. With a large flux of $\sim$10$^{-3}$ $γ$/cm$^{2}$/s, after 15 years in operation INTEGRAL/SPI has detected the 511 keV line at $>50σ$ and has performed high-resolution spectral studies which conclude that Galactic positrons predominantly annihilate at low energies in warm phases of the interstellar medium. The results from imaging are less certain, but show a spatial distribution with a strong concentration in the center of the Galaxy. The observed emission from the Galactic disk has low surface brightness and the scale height is poorly constrained, therefore, the shear number of annihilating positrons in our Galaxy is still not well know. Positrons produced in $β^+$-decay of nucleosynthesis products, such as $^{26}$Al, can account for some of the annihilation emission in the disk, but the observed spatial distribution, in particular the excess in the Galactic bulge, remains difficult to explain. Additionally, one of the largest uncertainties in these studies is the unknown distance that positrons propagate before annihilation. In this paper, we will summarize the current knowledge base of Galactic positrons, and discuss how next-generation instruments could finally provide the answers.

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arXiv2019

Cosmic Rays and Interstellar Medium with Gamma-Ray Observations at MeV Energies

Latest precise cosmic-ray (CR) measurements and present gamma-ray observations have started challenging our understanding of CR transport and interaction in the Galaxy. Moreover, because the density of CRs is similar to the density of the magnetic field, gas, and starlight in the interstellar medium (ISM), CRs are expected to affect the ISM dynamics, including the physical and chemical processes that determine transport and star formation. In this context, observations of gamma-ray emission at MeV energies produced by the low-energy CRs are very important and urgent. A telescope covering the energy range between ~0.1 MeV and a few GeV with a sensitivity more than an order of magnitude better than previous instruments would allow for the first time to study in detail the low-energy CRs, providing information on their sources, their spectra throughout the Galaxy, their abundances, transport properties, and their role on the evolution of the Galaxy and star formation. Here we discuss the scientific prospects for studies of CRs, ISM (gas, interstellar photons, and magnetic fields) and associated gamma-ray emissions with such an instrument.

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arXiv2017

Solar gamma rays and modulation of cosmic rays in the inner heliosphere

The first evidence of the gamma-ray emission from the quiescent Sun was found in the archival EGRET data that was later confirmed by Fermi-LAT observations with high significance. This emission is produced by Galactic cosmic rays (CRs) penetrating the inner heliosphere and inter- acting with the solar atmosphere and optical photons. The solar emission is characterized by two spatially and spectrally distinct components: (i) disk emission due to the CR cascades in the solar atmosphere, and (ii) spatially extended inverse Compton (IC) emission due to the CR electrons scattering off of solar photons. The intensity of both components associated with Galactic CRs anti-correlate with the level of the solar activity being the brightest during solar minimum. In this paper we discuss updates of the models of the IC component of the emission based on CR measurements made at different levels of solar activity, and we make predictions for e- ASTROGAM and AMEGO, proposed low-energy gamma-ray missions.

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arXiv2017

GALPROP cosmic-ray propagation code: recent results and updates

Information on cosmic-ray (CR) composition comes from direct CR measurements while their distribution in the Galaxy is evaluated from observations of their associated diffuse emission in the range from radio to gamma rays. Even though the main interaction processes are identified, more and more precise observations provide an opportunity to study more subtle effects and pose a challenge to the propagation models. GALPROP is a sophisticated CR propagation code that is being developed for about 20 years. It provides a unified framework for interpretations of data from many different types of experiments. It is used for a description of direct CR measurements and associated interstellar emissions (radio to gamma rays), thereby providing important information about CR injection and propagation in the interstellar medium. By accounting for all relevant observables at a time, the GALPROP code brings together theoretical predictions, interpretation of the most recent observations, and helps to reveal the signatures of new phenomena. In this paper we review latest applications of GALPROP and address ongoing and near future improvements. We are discussing effects of different propagation models, and of the transition from cylindrically symmetrical models to a proper 3D description of the components of the interstellar medium and the source distribution.

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