arXiv subjects

A. Adelfio

Publications and source records attributed to A. Adelfio.

4 recordsLinked to original sources

arXiv2026

Exploring the nature of Galactic unassociated sources detected by the Fermi-LAT

We investigate the nature of the unassociated sources detected by the Fermi-LAT close (|b|<10{\deg}) to the Galactic plane, representing 16% of all sources in the 4FGL-DR4 catalog. The bulk of these sources (referred to as soft Galactic unassociated sources, SGUs) exhibit properties not found in known classes of gamma-ray emitters, as confirmed by a machine-learning classification approach. In particular, these properties include a steep, curved spectrum peaking below 1 GeV and a specific Galactic-latitude distribution with both a narrow and a broad component (dubbed the spike and the shoulder, respectively). Some source clusters are highlighted. New plausible source classes are explored, but only star-forming regions are found to account for a significant fraction (at most 10%) of the unassociated population. A thorough search for counterparts to the 175 brightest sources brings out a number of plausible counterparts but does not reveal clues about the nature of the whole population. We investigate the possibility that SGUs originate from mismodeled clumps of diffuse emission. Using Monte Carlo simulations, the SGU spectra can be reproduced in this scenario under an ad hoc condition concerning the clump spatial extension. The possible connection between the SGUs and gas not accounted for by the 12CO tracer is explored using the 13CO MOPRA data but leads to inconclusive results. The origin of SGUs being related to diffuse emission remains plausible. However, a scenario whereby SGUs represent a new class of gamma-ray emitters cannot be fully excluded.

astro-ph.HE↗

arXiv2026

Gamma-ray signature of superluminous supernovae: Fermi-LAT GeV detection of SN 2017egm and evidence of a central engine

Superluminous supernovae (SLSNe) are a rare class of transients with peak luminosities 10-100 times greater than those of standard core-collapse supernovae (SNe). The mechanisms powering their extreme brightness remain debated, with circumstellar medium (CSM) interaction, or energy injection from a central engine like a magnetar wind nebula being the most plausible scenarios. To further constrain the underlying mechanism, we carried out a systematic search for GeV gamma-ray emission using the Fermi-LAT telescope from a sample of nearby hydrogen-poor (Type I) and hydrogen-rich (Type II) SLSNe over the past 16 years. Among the sample, only SN 2017egm shows significant gamma-ray emission, with likelihood test statistic (TS) values of 26-33 (i.e., >5$\sigma$) depending on the adopted time window. The signal arises between 50 and 160 days after explosion and is well described by a power-law spectrum with index $\Gamma=2.17 \pm 0.23$. The emission is consistent both in terms of its light curve and its spectrum, with predictions from magnetar models requiring either low nebular magnetization or faster spin-down than dipole losses. The CSM shell interaction scenario can reproduce the observed flux level but not the observed timing of the gamma-ray signal. In addition, the observed ratio, $L_{\gamma}/L_{opt} \sim 1$, is inconsistent with theoretical expectations and not in line with ratio measurements in other interacting CSM-dominated objects (e.g., novae or SNe) where this ratio is less than $10^{-2}$. Our study strongly suggests that a central engine like a magnetar plays a key role in this SLSN and could explain the bulk of the optical and gamma-ray light curves properties. Finally, simulations of 50 hours of CTAO observations indicate that a SN 2017egm-like event would be detectable up to 140 Mpc in the magnetar model but not in the CSM model due to strong gamma-gamma absorption.

astro-ph.HE↗

arXiv2025

Puzzling Variation of Gamma Rays from the Sun over the Solar Cycle Revealed with Fermi-LAT

The steady-state gamma-ray emission from the Sun is thought to consist of two emission components due to interactions with Galactic cosmic rays: (1) a hadronic disk component, and (2) a leptonic extended component peaking at the solar edge and extending into the heliosphere. The flux of these components is expected to vary with the 11-year solar cycle, being highest during solar minimum and lowest during solar maximum, as it varies with the cosmic-ray flux. No study has yet analyzed the flux variation of each component over solar cycles. In this work, we measure the temporal variations of the flux of each component over 15 years of Fermi Large Area Telescope observations and compare them with the sunspot number and Galactic cosmic-ray flux from AMS-02 near Earth. We find that the flux variation of the disk anticorrelates with the sunspot number and correlates with cosmic-ray protons, as expected, confirming its emission mechanism. In contrast, the extended component exhibits a more complex variation: despite an initial anticorrelation with the sunspot number, we find neither anticorrelation with the sunspot number nor correlation with cosmic-ray electrons over the full 15-year period. This most likely suggests that cosmic-ray transport and modulation in the inner heliosphere are unexpectedly complex and may differ for electrons and protons or, alternatively, that there is an additional, unknown component of gamma rays or cosmic rays. These findings impact space weather research and emphasize the need for close monitoring of Cycle 25 and the ongoing polarity reversal.

astro-ph.HE↗

arXiv2024

Search for Extended GeV Sources in the Inner Galactic Plane

The recent detection of extended $\gamma$-ray emission around middle-aged pulsars is interpreted as inverse-Compton scattering of ambient photons by electron-positron pairs escaping the pulsar wind nebula, which are confined near the system by unclear mechanisms. This emerging population of $\gamma$-ray sources was first discovered at TeV energies and remains underexplored in the GeV range. To address this, we conducted a systematic search for extended sources along the Galactic plane using 14 years of Fermi-LAT data above 10 GeV, aiming to identify a number of pulsar halo candidates and extend our view to lower energies. The search covered the inner Galactic plane ($\lvert l\rvert\leq$ 100$^{\circ}$, $\lvert b\rvert\leq$ 1$^{\circ}$) and the positions of known TeV sources and bright pulsars, yielding broader astrophysical interest. We found 40 such sources, forming the Second Fermi Galactic Extended Sources Catalog (2FGES), most with 68% containment radii smaller than 1.0$^{\circ}$ and relatively hard spectra with photon indices below 2.5. We assessed detection robustness using field-specific alternative interstellar emission models and by inspecting significance maps. Noting 13 sources previously known as extended in the 4FGL-DR3 catalog and five dubious sources from complex regions, we report 22 newly detected extended sources above 10 GeV. Of these, 13 coincide with H.E.S.S., HAWC, or LHAASO sources; six coincide with bright pulsars (including four also coincident with TeV sources); six are associated with 4FGL point sources only; and one has no association in the scanned catalogs. Notably, six to eight sources may be related to pulsars as classical pulsar wind nebulae or pulsar halos.

astro-ph.HE↗