arXiv subjects

Giacomo Principe

Publications and source records attributed to Giacomo Principe.

At least 19 recordsLinked to original sources

arXiv2026

Ionization Structure and Metal Enrichment of the Galactic Center Minispiral Observed with JWST

Sagittarius A* (Sgr A*) is the nearest quiescent supermassive black hole, and its proximity offers a unique opportunity to study its surrounding fuel supply. We leverage extensive spatial and spectroscopic information provided by the JWST/MIRI MRS instrument to disentangle mid-infrared ionized gas structures in the central 0.1 parsec of the Galaxy. The Galactic Minispiral's Bar and Northern Arm are revealed by their distinct morphological and kinematic signatures. Several compact (<1 arcsec) gas structures including X7 also appear within ~0.05 parsec of Sgr A* in the plane of the sky, moving with blue-shifted radial velocities >~600 km/s. Fine-structure-line measurements spanning ionization potentials ~7-55 eV are used to constrain the incident radiation field, metal abundances (neon, argon, sulfur, nickel, and iron), and dust depletion/destruction for each identified gas structure. Overall, the Minispiral gas shows mildly super-solar abundances (~1.8*solar), a Wolf--Rayet star-driven radiation field consistent with ionization parameters spanning -1.6<~log U<~-0.6, and significant nickel and iron dust destruction. In particular, the enhanced ionization state and strong dust processing in the high-velocity compact gas structures suggest that these features may experience localized fast shocks.

astro-ph.GA↗

arXiv2026

A Search for GeV Emission from Magnetar Giant Flare Candidates with Fermi-LAT

The discovery of delayed GeV emission from the extragalactic magnetar giant flare (MGF) GRB 200415A, located in the nearby Sculptor galaxy, revealed for the first time that these rare transients can launch relativistic outflows that power high-energy afterglows. Motivated by the recent identification of additional nearby MGF candidates in the archival data of the Fermi Gamma-ray Burst Monitor, we conduct a search for GeV counterparts with the Fermi Large Area Telescope (LAT). We analysed post-trigger time intervals taken in the range $10^{2}$--$10^{4}$\,s using a maximum-likelihood approach and performed a stacking analysis of all candidates with LAT coverage. In addition, we searched for photon triplets through \chng{a waiting time} analysis to identify events potentially associated with MGFs. We recover the known delayed signal from GRB\,200415A but find no GeV emission from the remaining six candidates. For three events, the earliest emission is unconstrained because the $10^{2}$\,s interval contains zero exposure after standard selections. For the events with LAT coverage, we obtain upper limits at 95\% confidence level of order $F_E \sim 10^{-9}\,\mathrm{erg\,cm^{-2}\,s^{-1}}$ for the individual events, and a stacked population-averaged limit of $\approx2\times10^{-10}\,\mathrm{erg\,cm^{-2}\,s^{-1}}$. We interpret these upper limits within the relativistic fireball framework, where the prompt spectral peaks favor a baryonic-poor regime ($η> η_*$). For the candidates with hard prompt spectrum and early LAT coverage, the baryonic-poor condition restricts the mass of relativistic ejecta to $M_b \lesssim 1\text{--}4 \times 10^{22}$\,g; the LAT upper limits confirm that the predicted GeV afterglow from such clean outflows falls below current instrumental sensitivity.

astro-ph.HE↗

arXiv2026

Infrared Line Diagnostics Fail to Constrain Sgr A*'s UV Output

Sgr A*, the 4 x $10^6 M_{sun}$ supermassive black hole at the Galactic Center, exhibits frequent flaring with X-ray luminosities of $L_X \sim 10^{35}-10^{36}$ erg s$^{-1}$, while its ultraviolet (UV) emission remains unconstrained due to extreme extinction ($A_V \sim 30$ mag). We use JWST/MIRI time-resolved spectroscopy of the central 0.3" region to search for mid-infrared emission line variability driven by Sgr A* flares, comparing the results to CLOUDY photoionization models spanning flare luminosities of $L_{UV} = 10^{32}$-$10^{39}$ erg s$^{-1}$. We detect no continuum-correlated variability in any mid-infrared line, including [Fe II] (5.34 $μ$m), [Ne II] (12.813 $μ$m), [Fe II] (17.936 $μ$m), and [S III] (18.713 $μ$m) over lags of $\sim1$-8~hr. Despite expectations of a flare-driven response, we show that the lack of variability is consistent with the physical conditions in the spatially extended line-emitting gas, where light-crossing timescales of $\sim0.1$-$10$ days and recombination and cooling timescales much longer than the variability timescales suppress any observable response to individual flares. We further find that the modeled infrared spectra are dominated by continuum emission rather than isolated line emission. The brightest predicted lines are intrinsically weak (lower than 10$^{-5}$ mJy), and their detectability is further reduced by the large kinematic broadening expected at the CLOUDY-predicted emitting radii, reducing their contrast against the continuum. Extending the analysis to higher-ionization mid-infrared and near-infrared lines does not improve sensitivity. These results demonstrate that infrared emission lines trace a steady-state radiation field rather than individual flaring events, and therefore infrared line diagnostics cannot be used to constrain the instantaneous UV flux of Sgr A*.

astro-ph.HE↗

arXiv2026

AGILE detection of transient γ-ray emission from the region of the supergiant fast X-ray transient source IGR J17354-3255

Context. On April 14, 2009, the AGILE satellite detected a γ-ray flare from an unknown transient source. Subsequent X-ray follow-up observations with Swift and INTEGRAL identified the supergiant fast X-ray transient (SFXT) IGR J17354-3255 as the best candidate counterpart, based on positional coincidence and a similar temporal behaviour. Aside from this hint, no SFXT has been firmly detected at high energies to date. Overall, SFXTs comprise a subclass of high-mass X-ray binaries (HMXBs) that host a massive OB supergiant star as a companion donor. They tend to display the most extreme X-ray variability among HMXBs. These systems might be able to emit photons at MeV-TeV energies in the form of fast flares lasting from hours to a few days, with a low-duty cycle. Aims. In this work, we analyse archival AGILE data to search for γ-ray flares consistent with IGR J17354-3255 and evaluate their possible physical origin. Methods. We identified a transient source, AGL J1736-3250, which emitted 19 γ-ray flares and was seen to be positionally consistent with IGR J17354-3255. Most flares, detected on a 1 d timescale, concentrate most of their emission on two, four, and six hour timescales, resembling those observed in the X-ray band from IGR J17354-3255. Results. An orbital phase analysis revealed that approximately half of the γ-ray activity occurs around the apastron passage of the compact object hosted in the binary system. We also incorporated archival Swift and INTEGRAL observations to provide phase-folded light curves at lower energies. Our collected results strongly support a physical association between IGR J17354-3255 and AGL J1736-3250, offering evidence that SFXTs could constitute a new class of Galactic high-energy transient emitters.

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arXiv2026

Towards a foundation model for astrophysical source detection: An End-to-End Gamma-Ray Data Analysis Pipeline Using Deep Learning

The increasing volume of gamma-ray data demands new analysis approaches that can handle large-scale datasets while providing robustness for source detection. We present a Deep Learning (DL) based pipeline for detection, localization, and characterization of gamma-ray sources. We extend our AutoSourceID (ASID) method, initially tested with \textit{Fermi}-LAT simulated data and optical data (MeerLICHT), to Cherenkov Telescope Array Observatory (CTAO) simulated data. This end-to-end pipeline demonstrates a versatile framework for future application to other surveys and potentially serves as a building block for a foundational model for astrophysical source detection.

astro-ph.IM↗

arXiv2025

TXS 0506+056-like blazar sources and their role as possible neutrino emitters

The interest in blazars as candidate neutrino emitters grew after the 3$σ$ evidence for a contemporaneous joint photon and neutrino emission from the flaring blazar TXS 0506+056 in 2017. Blazars, a class of extragalactic sources with relativistic jets pointing toward Earth, present a broadband emission interpretable via leptonic and hadronic processes, the latter relevant for proton acceleration and neutrino production. Several emission models have been developed to explain this multi-messenger observation, but the details of the neutrino production and the nature of TXS 0506+056 are not yet fully understood. In this work we investigate the properties of sources similar to TXS 0506+056. We select a sample of blazars from the Fermi 4LAC-DR2 catalog by constraining a number of key parameters in ranges centered on TXS 0506+056 values. We estimate their disk accretion efficiency and model their spectral energy distribution (SED) in terms of lepto-hadronic emission, gaining information respectively on the potential similarity of their environment with that of TXS 0506+056 and on their neutrino flux and detectability prospects at TeV energies. Our study shows the candidates' high energy emission to be dominated by leptonic processes. Part of them also show a high accretion rate, characteristic of FSRQs. For these sources, the very high energy (VHE) and neutrino fluxes appear undetectable by current and future instruments in an average emission state.

astro-ph.HE↗

arXiv2025

Optimizing searches for gravitational wave bursts using coherent WaveBurst 2G

The most general searches for gravitational wave transients (GWTs) rely on data analysis methods that do not assume prior knowledge of the signal waveform, direction, or arrival time on Earth. These searches provide data-driven signal reconstructions that are crucial both for testing available emission models and for discovering yet-to-be-uncovered sources. Here, we discuss progress in the detection performance of the coherent WaveBurst second-generation pipeline (cWB-2G), which is highly adaptable to both minimally modeled and model-informed searches for GWTs. Several search configurations for GWTs are examined using approximately 14.8 days of observation time from the third observing run by LIGO-Virgo-KAGRA (LVK). Recent enhancements include a ranking statistic fully based on multivariate classification with eXtreme Gradient Boosting, a thorough validation of the statistical significance accuracy of GWT candidates, and a measurement of the correlations of false alarms and simulated detections between different concurrent searches. For the first time, we provide a comprehensive comparison of cWB-2G performance on data from networks made of two and three detectors, and we demonstrate the advantage of combining concurrent searches for GWTs of generic morphology in a global observatory. This work offers essential insights for assessing our data analysis strategies in ongoing and future LVK searches for generic GWTs.

gr-qc↗

arXiv2025

The time-domain gamma-ray sky seen by the Fermi-LAT

The Fermi Gamma-ray Space Telescope is currently celebrating its 15th anniversary of operation. Since its launch, the Fermi-Large Area Telescope (LAT), the main instrument onboard the Fermi satellite, has remarkably unveiled the sky at GeV energies providing outstanding results in time-domain gamma-ray astrophysics. In particular, LAT has observed some of the most powerful transient phenomena in the Universe (such as gamma-ray bursts, blazar flares, magnetar flares, ...) enabling the possibility to test our current understanding of the laws of physics in extreme conditions. In this paper I will review some of the main recent results with a focus on the transient phenomena seen by LAT with a multi-wavelength and multi-messenger connection.

astro-ph.HE↗

arXiv2024

Extended gamma-ray emission from particle escape in pulsar wind nebulae -- Application to HESS J1809-193 and HESS J1825-137

There is growing evidence from gamma-ray observations at high and very high energies that particle escape is a key aspect shaping the morphological properties of pulsar wind nebulae (PWNe) at various evolutionary stages. We aim to provide a simple model for the gamma-ray emission from these objects including the transport of particles across the different components of the system. We applied it to sources HESS J1809-193 and HESS J1825-137. We developed a multi-zone framework applicable to dynamically young PWNe, taking into account the diffusive escape of relativistic electron-positron pairs out of the nebula into the parent supernova remnant (SNR) and their confinement downstream of the magnetic barrier of the forward shock until an eventual release into the surrounding interstellar medium (ISM). For a wide range of turbulence properties in the nebula, the GeV-TeV inverse-Compton radiation from pairs that escaped into the remnant can be a significant if not dominant contribution to the emission from the system. It may dominate the pion-decay radiation from cosmic rays accelerated at the forward shock and advected downstream of it. In the TeV-PeV range, the contribution from particles escaped into the ISM can exceed by far that of the SNR+PWN components. Applied to HESS J1809-193 and HESS J1825-137, we found that spatially extended GeV-TeV emission components can be accounted for mostly from particles escaped into the ISM, while morphologically more compact components above 50-100TeV are ascribed to the PWNe. In these two cases, the model suggests high turbulence in the nebula and a forward shock accelerating cosmic rays up to ~100TeV at most. The model provides the temporal and spectral properties of the flux of particles originally energized by the pulsar wind and ultimately released in the ISM. (Abridged).

astro-ph.HE↗

arXiv2024

Constraining possible $γ$-ray burst emission from GW230529 using Swift-BAT and Fermi-GBM

GW230529 is the first compact binary coalescence detected by the LIGO-Virgo-KAGRA collaboration with at least one component mass confidently in the lower mass-gap, corresponding to the range 3-5$M_{\odot}$. If interpreted as a neutron star-black hole merger, this event has the most symmetric mass ratio detected so far and therefore has a relatively high probability of producing electromagnetic (EM) emission. However, no EM counterpart has been reported. At the merger time $t_0$, Swift-BAT and Fermi-GBM together covered 100$\%$ of the sky. Performing a targeted search in a time window $[t_0-20 \text{s},t_0+20 \text{s}]$, we report no detection by the Swift-BAT and the Fermi-GBM instruments. Combining the position-dependent $γ-$ray flux upper limits and the gravitational-wave posterior distribution of luminosity distance, sky localization and inclination angle of the binary, we derive constraints on the characteristic luminosity and structure of the jet possibly launched during the merger. Assuming a top-hat jet structure, we exclude at 90$\%$ credibility the presence of a jet which has at the same time an on-axis isotropic luminosity $\gtrsim 10^{48}$ erg s$^{-1}$, in the bolometric band 1 keV-10 MeV, and a jet opening angle $\gtrsim 15$ deg. Similar constraints are derived testing other assumptions about the jet structure profile. Excluding GRB 170817A, the luminosity upper limits derived here are below the luminosity of any GRB observed so far.

astro-ph.HE↗

arXiv2024

On the Origin of the X-ray Emission in Heavily Obscured Compact Radio Sources

X-ray continuum emission of active galactic nuclei (AGNs) may be reflected by circumnuclear dusty tori, producing prominent fluorescence iron lines at X-ray frequencies. Here we discuss the broad-band emission of three radio-loud AGN belonging to the class of compact symmetric objects (CSOs), with detected narrow Fe\,K$α$ lines. CSOs have newly-born radio jets, forming compact radio lobes with projected linear sizes of the order of a few to hundreds of parsecs. We model the radio--to--$γ$-ray spectra of compact lobes in {J1407+2827}, J1511+0518, and {J2022+6137}, which are among the nearest and the youngest CSOs known to date, and are characterized by an intrinsic X-ray absorbing column density of $N_{\rm H} > 10^{23}$\,cm$^{-2}$. In addition to the archival data, we analyze the newly acquired \chandra\ X-ray Observatory and Sub-Millimeter Array (SMA) observations, and also refine the $γ$-ray upper limits from the \fermi\ Large Area Telescope (LAT) monitoring. The new \chandra\ data exclude the presence of the extended X-ray emission components on scales larger than $1.5^{\prime \prime}$. The SMA data unveil a correlation of the spectral index of the electron distribution in the lobes and $N_{\rm H}$, which can explain the $γ$-ray quietness of heavily obscured CSOs. Based on our modeling, we argue that the inverse-Compton emission of compact radio lobes may account for the intrinsic X-ray continuum in all these sources. Furthermore, we propose that the observed iron lines may be produced by a reflection of the lobes' continuum from the surrounding cold dust.

astro-ph.HE↗

arXiv2024

Search for hyperbolic encounters of compact objects in the third LIGO-Virgo-KAGRA observing run

Gravitational-wave (GW) observations provide unique information about compact objects. As detectors sensitivity increases, new astrophysical sources of GW could emerge. Close hyperbolic encounters are one such source class: scattering of stellar mass compact objects is expected to manifest as GW burst signals in the frequency band of current detectors. We present the search for GW from hyperbolic encounters in the second half of the third Advanced LIGO-Virgo observing run (O3b). We perform a model-informed search with machine-learning enhanced Coherent WaveBurst algorithm. No significant event has been identified in addition to known detections of compact binary coalescences. We inject in the O3b data non-spinning third Post-Newtonian order accurate hyperbolic encounter model with component masses between [2, 100] $M_{\odot}$, impact parameter in [60, 100] ${GM}/{c^2}$ and eccentricity in [1.05, 1.6]. We further discuss the properties of the simulation recovered. For the first time, we report the sensitivity volume achieved for such sources, which for O3b data reaches up to 3.9$\pm 1.4 \times 10^5$ Mpc$^3$year for compact objects with masses between [20, 40] $M_{\odot}$, corresponding to a rate density upper limit of 0.589$\pm$0.094 $\times10^{-5}$Mpc$^{-3}$year$^{-1}$. Finally, we present projected sensitive volume for the next observing runs of current detectors, namely O4 and O5.

gr-qc↗

arXiv2023

Hunting the gamma-ray emission from Fast Radio Burst with Fermi-LAT

Fast radio bursts (FRBs) are one of the most exciting new mysteries of astrophysics. Their origin is still unknown, but recent observations seem to link them to soft gamma repeaters and, in particular, to magnetar giant flares (MGFs). The recent detection of a MGF at GeV energies by the Fermi Large Area Telescope (LAT) motivated the search for GeV counterparts to the >100 currently known FRBs. To date, none of these has a known gamma-ray counterpart. Taking advantage of more than 12 years of Fermi-LAT data, we perform a search for gamma-ray emission from almost all the reported repeating and non-repeating FRBs. We analyze on different time scales the Fermi-LAT data for each individual source separately and perform a cumulative analysis on the repeating ones. In addition, we perform the first stacking analysis at GeV energies of this class of sources in order to constrain the gamma-ray properties of the FRBs. The stacking analysis is a powerful method that allows for a possible detection from below-threshold FRBs providing important information on these objects. In this proceeding we present preliminary results of our study and we discuss their implications for the predictions of gamma-ray emission from this class of sources.

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

The Fermi Gamma Ray Sky: summary of recent Observations

The Fermi Gamma-ray Space Telescope was launched more than 13 years ago and since then it has dramatically changed our knowledge of the gamma-ray sky. With more than three billions photons from the whole sky, collected in the energy range between 20 MeV and more than 300 GeV, and beyond 6,000 detected sources, LAT observations have been crucial to improving our understanding of particle acceleration and gamma-ray production in astrophysical sources. In this proceeding, I will review recent science highlights from the LAT. I will focus on the recent source catalog release, as well as on the main transient phenomena seen with the LAT with multi-wavelength and multi-messenger connection.

astro-ph.HE↗

arXiv2022

Advancing the Landscape of Multimessenger Science in the Next Decade

The last decade has brought about a profound transformation in multimessenger science. Ten years ago, facilities had been built or were under construction that would eventually discover the nature of objects in our universe could be detected through multiple messengers. Nonetheless, multimessenger science was hardly more than a dream. The rewards for our foresight were finally realized through IceCube's discovery of the diffuse astrophysical neutrino flux, the first observation of gravitational waves by LIGO, and the first joint detections in gravitational waves and photons and in neutrinos and photons. Today we live in the dawn of the multimessenger era. The successes of the multimessenger campaigns of the last decade have pushed multimessenger science to the forefront of priority science areas in both the particle physics and the astrophysics communities. Multimessenger science provides new methods of testing fundamental theories about the nature of matter and energy, particularly in conditions that are not reproducible on Earth. This white paper will present the science and facilities that will provide opportunities for the particle physics community renew its commitment and maintain its leadership in multimessenger science.

astro-ph.HE↗

arXiv2022

The Future of Gamma-Ray Experiments in the MeV-EeV Range

Gamma-rays, the most energetic photons, carry information from the far reaches of extragalactic space with minimal interaction or loss of information. They bring messages about particle acceleration in environments so extreme they cannot be reproduced on earth for a closer look. Gamma-ray astrophysics is so complementary with collider work that particle physicists and astroparticle physicists are often one in the same. Gamma-ray instruments, especially the Fermi Gamma-ray Space Telescope, have been pivotal in major multi-messenger discoveries over the past decade. There is presently a great deal of interest and scientific expertise available to push forward new technologies, to plan and build space- and ground-based gamma-ray facilities, and to build multi-messenger networks with gamma rays at their core. It is therefore concerning that before the community comes together for planning exercises again, much of that infrastructure could be lost to a lack of long-term planning for support of gamma-ray astrophysics. Gamma-rays with energies from the MeV to the EeV band are therefore central to multiwavelength and multi-messenger studies to everything from astroparticle physics with compact objects, to dark matter studies with diffuse large scale structure. These goals and new discoveries have generated a wave of new gamma-ray facility proposals and programs. This paper highlights new and proposed gamma-ray technologies and facilities that have each been designed to address specific needs in the measurement of extreme astrophysical sources that probe some of the most pressing questions in fundamental physics for the next decade. The proposed instrumentation would also address the priorities laid out in the recent Astro2020 Decadal Survey, a complementary study by the astrophysics community that provides opportunities also relevant to Snowmass.

astro-ph.HE↗

arXiv2021

Multiwavelength View of the M87 Black Hole Captured by the Event Horizon Telescope

In 2017, the first image of the center of the M87 galaxy was captured by the Event Horizon Telescope (EHT). It revealed a ring morphology and a size consistent with theoretical expectations for the light pattern around a weakly accreting supermassive black hole of ~6.5 billion solar masses. In parallel to the EHT measurements, an extensive multi-wavelength (MWL) campaign with ground- and space-based facilities from radio all the way up to the TeV energy range was organized. In this note we will give an overview of the results from this campaign. M87 was found to be in a historically low state. At X-ray energies the emission from the core dominates over HST-1. We present the most complete simultaneous, MWL spectrum of the active nucleus to date.

astro-ph.HE↗