arXiv subjects

G. La Mura

Publications and source records attributed to G. La Mura.

At least 19 recordsLinked to original sources

arXiv2026

Extinction by non-spherical particles with the Nanoparticle Transition Matrix Code: perspectives for interstellar medium and exoplanetary atmospheres

The Transition Matrix formalism (T-matrix) offers an optimal solution to the problem of radiation scattering and absorption by particles characterized by a high degree of chemical and structural complexity. However, its application to particle models composed by large numbers of constituents has been classically limited by the substantial computational burden of the method. As a consequence, the investigation of light scattering and absorption processes, in cases where particle asymmetry and non-homogeneity play an important role, typically need to rely on simplifying assumptions which affect the correct interpretation of transmission spectra and extinction curves. This problem has a huge impact on many fields of scientific application. In particular, it has a critical role in astrophysics, because the ability to model highly non homogeneous particles is a fundamental requirement to understand the role of extinction in different phases of the interstellar medium (ISM), as well as to probe the physics of light transmission in planetary atmospheres within the Solar System and beyond. In this work we describe the application of a new implementation of the T-matrix formalism, based on the framework originally developed by Borghese, Denti & Saija and named Nano-particle Transition Matrix code, which leverages the power of modern hardware architectures to reduce the computational costs related to the solution of relevant particle models for the investigation of dust in the ISM and of hot-Jupyter class planets atmospheric aerosols. This new implementation of the T-matrix formalism can handle calculations of increasing complexity with a direct impact in the interpretation of the transmission spectra of transiting exoplanets produced by the James Webb Space Telescope (JWST) and of those expected from the Atmospheric Remote-sensing Infrared Exoplanet Large survey (ARIEL).

astro-ph.IM↗

arXiv2026

Nano-particle Transition Matrix code implementation

Electromagnetic scattering and absorption by material particles is a fundamental physical problem with a broad range of applications, going from laboratory experiments, biology and material sciences, all the way up to environmental studies and astrophysical investigations. In spite of its primary importance, an exact theoretical treatment is only possible for a limited range of ideal cases, while realistic situations require the development of numerical solutions. In the course of the years, several techniques were developed to model the effects of scattering and absorption in more general cases, using approaches such as the Discrete Dipole Approximation (DDA), the Finite Difference Time Domain (FDTD) method, the Transition Matrix formalism (T-matrix) or the Mean Field Theory (MFT). Among these possibilities, the T-matrix approach grants the highest degree of flexibility in modeling aggregates of spherically symmetric particles with arbitrary overall morphology and composition, but its application has been limited by the challenging computational requirements of the method in realistic cases. This paper describes the Nano-Particle Transition Matrix Code project (NP_TMcode), a new implementation of the T-matrix formalism that, taking advantage of high performance parallel hardware architectures, allows the solution of increasingly complex models, while substantially reducing the computing time. The paper describes the code structure, with a particular focus on the algorithm optimization, and it presents the results of the performance analysis for a set of development applications.

astro-ph.IM↗

arXiv2026

SN 2016iog: A fast declining Type II-L supernova with an ultra-faint tail, persistently interacting with circumstellar material

We present optical photometric and spectroscopic observations of the rapidly declining Type IIL supernova (SN) 2016iog. SN 2016iog reached its peak $\sim$ 14 days after explosion, with an absolute magnitude in the $V$ band of $-18.64 \pm 0.15$ mag, followed by a steep decline of $8.85 \pm 0.15$~mag~(100\,d)$^{-1}$ post-peak. Such a high decline rate makes SN~2016iog one of the fastest declining Type~IIL SNe observed to date. The rapid rise in the light curve, combined with the nearly featureless continuum observed in the spectrum at +9.3 days, suggests the presence of interaction. In the recombination phase, we observed broad H$α$ lines that persist at all epochs. In addition, the prominent double-peaked H$α$ feature observed in the late-time spectrum (+190.8 days) is likely attributable either to significant dust formation within a cool dense shell or to asymmetric circumstellar material. These features suggest the presence of sustained interaction around SN~2016iog. We propose that the observed characteristics of SN~2016iog can be qualitatively explained by assuming a low-mass H-rich envelope surrounding a red supergiant progenitor star with low-density circumstellar material.

astro-ph.SR↗

arXiv2025

Interstellar dust as a dynamic environment

In spite of accounting for only a small fraction of the mass of the Interstellar Medium (ISM), dust plays a primary role in many physical and chemical processes in the Universe. It is the main driver of extinction of radiation in the UV/optical wavelength range and a primary source of thermal IR emission. Dust grains contain most of the refractory elements of the ISM and they host chemical processes that involve complex molecular compounds. However, observational evidence suggests that grain structure is highly non-trivial and that dust particles are characterized by granularity, asymmetry and stratification, which significantly affect their interaction with radiation fields. Accurate modeling of such interaction is fundamental to properly explain observational results, but it is a computationally demanding task. Here we present the possibility to investigate the effects of radiation/particle interactions in non-spherically symmetric conditions using a novel implementation of the Transition Matrix formalism, designed to run on scalable parallel hardware facilities.

astro-ph.GA↗

arXiv2025

Identification of low energy neutral and charged cosmic ray events in large wide field observatories

The lower energy thresholds of large wide-field gamma-ray observatories are often determined by their capability to deal with the very low-energy cosmic ray background. In fact, in observatories with areas of tens or hundreds of thousands of square meters, the number of background events generated by the superposition of random, very low energy cosmic rays is huge and may exceed by far the possible signal events. In this article, we argue that a trigger strategy based on pattern recognition of the shower front can significantly reject the background, keeping a good efficiency and a good angular accuracy (few square degrees) for gamma rays with energies as low as tens of GeV. In this way, alerts can be followed or emitted within time lapses of the order of the second, enabling wide-field gamma-ray observatories to better contribute to global multi-messenger networks of astrophysical observatories.

astro-ph.HE↗

arXiv2023

The relation between optical and $γ$-ray emission in BL Lac sources

The relativistic jets produced by some Active Galactic Nuclei (AGNs) are among the most efficient persistent sources of non-thermal radiation and represent an ideal laboratory for studying high-energy interactions. In particular, when the relativistic jet propagates along the observer's line of sight, the beaming effect produces dominant signatures in the observed spectral energy distribution (SED), from the radio domain up to the highest energies, with the further possibility of resulting in radiation-particle multi-messenger associations. In this work, we investigate the relationships between the emission of $γ$ rays and the optical spectra of a sample of AGN, selected from BL Lac sources detected by the $Fermi$ Large Area Telescope ($Fermi$-LAT). We find that there is a close relationship between the optical and $γ$-ray spectral indices. Despite all the limitations due to the non-simultaneity of the data, this observation strongly supports a substantial role of Synchrotron-Self Compton (SSC) radiation in a single zone leptonic scenario for most sources. This result simplifies the application of theoretical models to explore the physical parameters of the jets in this type of sources.

astro-ph.HE↗

arXiv2022

The Mercedes water Cherenkov detector

The concept of a small, single-layer water Cherenkov detector, with three photomultiplier tubes (PMTs), placed at its bottom in a $120^{\circ}$ star configuration (\emph{Mercedes} Water Cherenkov Detector) is presented. The PMTs are placed near the lateral walls of the stations with an adjustable inclination and may be installed inside or outside the water volume. To illustrate the technical viability of this concept and obtain a first-order estimation of its cost, an engineering design was elaborated. The sensitivity of these stations to low energy Extensive Air Shower (EAS) electrons, photons and muons is discussed, both in compact and sparse array configurations. It is shown that the analysis of the intensity and time patterns of the PMT signals, using machine learning techniques, enables the tagging of muons, achieving an excellent gamma/hadron discrimination for TeV showers. This concept minimises the station production and maintenance costs, allowing for a highly flexible and fast installation. Mercedes Water Cherenkov Detectors (WCDs) are thus well-suited for use in high-altitude large gamma-ray observatories covering an extended energy range from the low energies, closing the gap between satellite and ground-based measurements, to very high energy regions, beyond the PeV scale.

physics.ins-det↗

arXiv2022

Hunting extreme BL Lacertae blazars with Fermi-LAT

The emission of very-high-energy photons (VHE, E>100 GeV$) in blazars is closely connected to the production of ultra-relativistic particles and the role of these gamma-ray sources as cosmic particle accelerators. This work focuses on a selection of 22 gamma-ray objects from the 2BIGB catalog of high-synchrotron-peaked sources, which are classified as blazar candidates of uncertain type in the 4FGL-DR2 catalog. We study these sources by means of a re-analysis of the first 10 years of gamma-ray data taken with the Fermi Large Area Telescope, including the attenuation by the extragalactic background light. Their broadband spectral energy distributions are also evaluated, using multi-wavelength archival data in the radio, optical, and X-ray bands, in terms of one-zone synchrotron-self-Compton models, adding an external Compton component when needed. Out of this analysis, we identify 17 new extreme high-synchrotron-peaked (EHSP) candidates and compare their physical parameters with those of prototypical EHSP blazars. Finally, the resulting models are used to assess their detectability by the present and future generation of ground-based imaging atmospheric Cherenkov telescopes. We find two VHE candidates within the reach of the current and next generation of Cherenkov telescopes: J0847.0-2336 and J1714.0-2029.

astro-ph.HE↗

arXiv2021

A model-driven search for extreme BL Lacs among LAT BCU

The emission of very-high-energy photons (VHE, E$>$100 GeV) in active galactic nuclei (AGN) is closely connected with the production of ultra-relativistic particles. Among AGN, the subclass of extreme BL Lacertae are of particular interest because they challenge state-of-art models on how these cosmic particle accelerators operate. By cross-matching two gamma-ray catalogs (this is, 4FGL-DR2 and 2BIGB), we identified 23 high-synchrotron-peaked (HSP) blazar candidates with photometric or spectroscopic redshifts, good multi-wavelength coverage, that are possibly detectable by VHE instruments. We performed a new analysis of Fermi Large Area Telescope data including the effects of attenuation from the extragalactic background light and complemented these results by collecting multiwavelength data from optical, radio and X-ray archival observations. Their broadband spectral energy distributions were interpreted in terms of synchrotron-self-Compton models with external-Compton components and compared with the properties of prototypical extreme HSP blazars. Finally, we test their detectability with imaging atmospheric Cherenkov telescopes (IACTs) and propose a new method for selecting these extreme targets for these ground-based telescopes.

astro-ph.HE↗

arXiv2021

Gamma-Ray Burst detection prospects for next generation ground-based VHE facilities

Gamma-ray Bursts (GRB) were discovered by satellite-based detectors as powerful sources of transient $γ$-ray emission. The Fermi satellite detected an increasing number of these events with its dedicated Gamma-ray Burst Monitor (GBM), some of which were associated with high energy photons $(E > 10\, \mathrm{GeV})$, by the Large Area Telescope (LAT). More recently, follow-up observations by Cherenkov telescopes detected very high energy emission $(E > 100\, \mathrm{GeV})$ from GRBs, opening up a new observational window with implications on the interpretation of their central engines and on the propagation of very energetic photons across the Universe. Here, we use the data published in the 2nd Fermi-LAT Gamma Ray Burst Catalogue to characterise the duration, luminosity, redshift and light curve of the high energy GRB emission. We extrapolate these properties to the very high energy domain, comparing the results with available observations and with the potential of future instruments. We use observed and simulated GRB populations to estimate the chances of detection with wide-field ground-based $γ$-ray instruments. Our analysis aims to evaluate the opportunities of the Southern Wide-field-of-view Gamma-ray Observatory (SWGO), to be installed in the Southern Hemisphere, to complement CTA. We show that a low-energy observing threshold $(E_{low} < 200\, \mathrm{GeV})$, with good point source sensitivity $(F_{lim} \approx 10^{-11}\, \mathrm{erg\, cm^{-2}\, s^{-1}}$ in $1\, \mathrm{yr})$, are optimal requirements to work as a GRB trigger facility and to probe the burst spectral properties down to time scales as short as $10\, \mathrm{s}$, accessing a time domain that will not be available to IACT instruments.

astro-ph.IM↗

arXiv2021

Detection of VHE gamma-ray transients with monitoring facilities

The observation of Very High Energy gamma rays (VHE, E > 100 GeV) led us to the identification of extremely energetic processes and particle acceleration sites both within our Galaxy and beyond. We expect that VHE facilities, like CTA, will explore these sources with an unprecedented level of detail. However, the transient and unpredictable nature of many important processes requires the development of proper monitoring strategies, to observe them. With this study, we estimate the properties of VHE transients that can be effectively detected by monitoring facilities. We use data collected by the Fermi-LAT instrument, during its monitoring campaign, to select events that are likely associated with VHE emission. We use this sample to estimate the frequency, the luminosity and the time-scales of different transients, focusing on blazar flares and Gamma Ray Bursts (GRBs). We discuss how the balance between Field of View, sensitivity and duty cycle of an observatory affects the likelihood to detect transients that occur at the inferred rates and we conclude describing the contribution that current and near-future monitoring facilities can bring to the identification and study of VHE transient emission.

astro-ph.IM↗

arXiv2021

Monitoring Gamma-Ray Burst VHE emission with the Southern Wide-field-of-view Gamma-ray Observatory

It has been established that Gamma-Ray Bursts (GRB) can produce Very High Energy radiation (E > 100 GeV), opening a new window on the investigation of particle acceleration and radiation properties in the most energetic domain. We expect that next-generation instruments, such as the Cherenkov Telescope Array (CTA), will mark a huge improvement in their observation. However, constraints on the target visibility and the limited duty cycle of Imaging Atmospheric Cherenkov Telescopes (IACT) reduce their ability to react promptly to transient events and to characterise their general properties. Here we show that an instrument based on the Extensive Air Shower (EAS) array concept, proposed by the Southern Wide Field-of-view Gamma-ray Observatory (SWGO) Collaboration, has promising possibilities to detect and track VHE emission from GRBs. Observations made by the Fermi Large Area Telescope (Fermi-LAT) identified some events with a distinct spectral component, extending above $1\,$GeV or even $10\,$GeV, which can represent a substantial fraction of the emitted energy and also arise in early stages of the process. Using models based on these properties, we estimate the possibilities that a wide field of view and large effective area ground-based monitoring facility has to probe VHE emission from GRBs. We show that the ability to monitor VHE transients with a nearly continuous scanning of the sky grants an opportunity to access simultaneous electromagnetic counterparts to Multi-Messenger triggers up to cosmological scales, in a way that is not available to IACTs.

astro-ph.IM↗

arXiv2020

The Radio Structure of the Narrow-Line Seyfert 1 Mrk 783 with VLBA and e-MERLIN

In this paper, we present the analysis of new radio and optical observations of the narrow-line Seyfert 1 galaxy Mrk 783. 1.6 GHz observations performed with the e-MERLIN interferometer confirm the presence of the diffuse emission previously observed. The Very Long Baseline Array (VLBA) also detects the nuclear source both at 1.6 GHz (L-Band) and 5 GHz (C-band). While the L-band image shows only an unresolved core, the C-band image shows the presence of a partially resolved structure, at a position angle of 60°. The brightness temperature of the emission in both bands ($>10^6$ K) suggests that it is a pc-scale jet produced by the AGN. The relatively steep VLBA spectral index ($α_{VLBA} = 0.63\pm0.03$) is consistent with the presence of optically thin emission on milliarcsecond scales. Finally, we investigated two possible scenarios that can result in the misalignment between the kpc and pc-scale radio structure detected in the galaxy. We also analysed the optical morphology of the galaxy, which suggests that Mrk 783 underwent a merging in relatively recent times.

astro-ph.GA↗

arXiv2020

Radio morphology of southern narrow-line Seyfert 1 galaxies with Very Large Array observations

We present the results of new radio observations carried out with the Karl G. Jansky Very Large Array C-configuration at 5.5 GHz for a sample of southern narrow-line Seyfert 1 galaxies (NLS1s). This work increases the number of known radio-detected NLS1s in the southern hemisphere, and confirms that the radio emission of NLS1s is mainly concentrated in a central region at kpc-scale and only a few sources show diffuse emission. In radio-quiet NLS1s, the radio luminosity tends to be higher in steep-spectrum sources and be lower in flat-spectrum sources, which is opposite to radio-loud NLS1s. This may be because the radio emission of steep NLS1s is dominated by misaligned jets, AGN-driven outflows, or star formation superposing on a compact core. Instead the radio emission of flat NLS1s may be produced by a central core which has not yet developed radio jets and outflows. We discover new NLS1s harboring kpc-scale radio jets and confirm that a powerful jet does not require a large-mass black hole to be generated. We also find sources dominated by star formation. These NLS1s could be new candidates in investigating the radio emission of different mechanisms.

astro-ph.GA↗

arXiv2020

Searching for misaligned active galactic nuclei among blazar candidates in the Fourth Fermi-LAT catalog

Radio-loud sources with blazar-like properties, but having a jet that does not directly point in the direction of the observer are among the most interesting classes of gamma-ray emitters. These sources are known as Misaligned Active Galactic Nuclei (MAGN). Understanding MAGN properties is useful to improve the knowledge of blazar energetics. We searched for new MAGN candidates among the remaining blazars of uncertain type detected by the Fermi Large Area Telescope (LAT) using a methodology based on characterizing their radio morphology. We identified seven new candidates associated with gamma-ray sources. Their features are consistent with a source with a misaligned relativistic jet consistent with the definition of MAGN.

astro-ph.GA↗

arXiv2019

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

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

astro-ph.HE↗

arXiv2019

A correlation between [O III] line property and X-ray spectral complexity in narrow-line Seyfert 1 galaxies?

We present a detailed study of 11 narrow-line Seyfert 1 galaxies (NLS1s) from the Six-degree Field Galaxy Survey (6dFGS) that both have optical and X-ray spectroscopic observations. There are five complex NLS1s (C-NLS1s) and six simple NLS1s (S-NLS1s). We propose a possible correlation between [O III] line asymmetry and X-ray complexity. The outflow or wind from the inner accretion disk is commonly present in NLS1s and mostly directed along the system axis. In C-NLS1s only weak wind effects are measured, the X-ray spectral complexity might be caused by the presence of ionized material in the wind. On the contrary, the wind in S-NLS1s is fast, the ionized material could be swept by such a strong wind, thus the complex feature is missing which results in a simple X-ray spectrum. Furthermore, this outflow scenario seems to be an inclination effect. Since the speed of the wind is higher in a small inclination while lower in a large inclination, S-NLS1s might be sources viewed at small angles while C-NLS1s might be sources viewed at large angles.

astro-ph.GA↗

arXiv2019

Line shapes in narrow-line Seyfert 1 galaxies: a tracer of physical properties?

Line profiles can provide fundamental information on the physics of active galactic nuclei (AGN). In the case of narrow-line Seyfert 1 galaxies (NLS1s) this is of particular importance since past studies revealed how their permitted line profiles are well reproduced by a Lorentzian function instead of a Gaussian. This has been explained with different properties of the broad-line region (BLR), which may present a more pronounced turbulent motions in NLS1s with respect to other AGN. We investigated the line profiles in a recent large NLS1 sample classified using SDSS, and we divided the sources into two subsamples according to their line shapes, Gaussian or Lorentzian. The line profiles clearly separate all the properties of NLS1s. Black hole mass, Eddington ratio, [O III], and Fe II strength are all very different in the Lorentzian and Gaussian samples. We interpret this in terms of evolution within the class of NLS1s. The Lorentzian sources may be the youngest objects, while Gaussian profiles may be typically associated to more evolved objects. Further detailed spectroscopic studies are needed to fully confirm our hypothesis.

astro-ph.GA↗