Gabrijela Zaharijas

We present a catalog of sources detected above 50 GeV by the Fermi Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope using 16 yr of observations. This is the Fourth Catalog of Hard Fermi-LAT Sources (4FHL), containing 673 objects detected in the 50 GeV–2 TeV energy range. The sensitivity and localization are improved by factors of ∼2 relative to the previous LAT catalog at the same energies (i.e. the 2FHL catalog). The increased photon statistics, nearly doubling the number of detections relative to 2FHL, enables more detailed population studies, including the identification of fainter sources detected above 50 GeV. The majority of the sources (84.7%) are associated with extragalactic counterparts, primarily BL Lacertae objects, including 36 objects at redshift z > 1. Galactic sources account for 10% of the sample, mainly pulsar wind nebulae and supernova remnants, while ∼5% remain unassociated or of uncertain classification. Twenty sources are reported with no counterpart in previous LAT catalogs. The 4FHL catalog provides the deepest all-sky survey above 50 GeV to date. By focusing on this high-energy regime, it provides a fundamental link between space-based gamma-ray observations and the TeV range probed by current and future ground-based observatories. It serves as a useful reference for guiding future observations, including new potential targets for follow-up with Cherenkov telescopes.

We used 14 yr of Fermi-Large Area Telescope (LAT) data to derive a catalog of sources detected between 20 MeV and 200 MeV. For each source, we provide localization, spectral, and association information. The Second Catalog of Low-Energy Fermi-LAT sources (2FLE) contains 450 sources detected at TS > 25, with median energy flux of F _{20−200 MeV} = 1.9 × 10 ⁻¹¹ erg cm ⁻² s ⁻¹ . Using a Bayesian and likelihood-ratio method, we associate 377 2FLE sources with a known counterpart. Of the 325 objects associated with extragalactic sources, the dominant source class represented in the 2FLE is flat-spectrum radio quasars (FSRQs). Additionally, 52 sources are associated with Galactic sources. Three associated sources have not been previously detected in any γ -ray catalog and represent new γ -ray sources of known origin. We find that 387 2FLE sources have a positional cross-match with the 4FGL-DR4 sources. For the sources with a 4FGL counterpart, we compare the spectrum of the 2FLE source to the 4FGL spectral energy distributions. Of the 73 2FLE sources without multiwavelength associations, 13 have a 4FGL counterpart that lacks classification, while 60 are newly discovered γ -ray sources. Among these 60 sources, 51 are found at high latitudes (∣ b ∣ > 10°), and 38 are detected at >10 σ . Additionally, we find that out of the 450 2FLE sources, 45 are TeV emitters, and six are spatially coincident with a 1LHAASO source. The 2FLE helps us pave the way for reducing the “MeV gap,” and provides the most sensitive catalog between 20 and 200 MeV to date.

High-energy astrophysical neutrinos serve as crucial messengers for understanding hadronic acceleration processes and identifying the origins of cosmic rays, with blazars among the most promising neutrino sources. The KM3NeT experiment reported the detection of an ultrahigh-energy neutrino with an energy estimate of ∼220 PeV, the most energetic yet observed. The neutrino arrival direction has a 99% confidence region of 3° radius centered at R.A. 94 . ° 3, decl. −7 . ° 8 (J2000). In this work, 17 candidate blazars located within this region are identified. Comprehensive new observations and archival data analysis for these sources are presented. The study provides a complete multiwavelength coverage across radio, optical, X-ray, and gamma-ray bands, including proprietary data and dedicated follow-up observations. This study highlights flaring behavior in several candidate counterparts. One object exhibits a radio flare coinciding with the neutrino arrival time, with a pre-trial chance probability of 0.26%. Only for this object, the chance probability could be quantified in a robust way. Another candidate displays a rising trend in X-ray flux in a 1 yr window around the neutrino arrival time, while a third undergoes a gamma-ray flare during the same period. Based on the observational findings presented here, while none of these candidates can conclusively be linked to the neutrino, the implications of a possible blazar origin for the KM3NeT event are discussed.

We investigate the nature of the unassociated sources detected by the Fermi-LAT close (∣ b ∣ < 10°) 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 ¹² CO tracer is explored using the ¹³ CO 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.

The origin of diffuse neutrinos and γ rays is unknown, and galaxy clusters hosting active galactic nuclei and starburst galaxies are the most probable sources of these cosmic messengers. In this work, we investigate the diffuse γ -ray and neutrino emission from the Virgo, Perseus, and Coma clusters using a detailed numerical method, combining MHD simulations with Monte Carlo methods. The MHD simulation provides the distributions of the temperature, gas, and magnetic field in the clusters. The Monte Carlo simulations are used to investigate the cosmic-ray (CR) propagation in the intracluster medium and subsequently the secondaries stemming from CRs. Our primary assumption is that CR injection follows the gas density of the clusters, providing a physically motivated approximation. High-density regions in clusters are associated with strong turbulence and prominent shock structures, making them the natural sites for efficient CR acceleration. Our predicted γ -ray flux from the individual clusters lies well below the present LHAASO upper limits. The MAGIC observations of the central source NGC 1275 of the Perseus cluster are significantly higher than our results. Further, we estimate the cumulative γ -ray and neutrino fluxes from clusters with masses ≳5 × 10 ¹³ M _⊙ in the local Universe (within 500 Mpc). The diffuse γ -ray flux reported by the Fermi-LAT collaboration is significantly higher than our results. Our predictions are consistent with IceCube’s existing upper limits on the unresolved neutrino flux from galaxy clusters ( M > 10 ¹⁴ M _⊙ ) up to z = 2.

Dwarf spheroidal galaxies (dSphs) are excellent targets for indirect dark matter (DM) searches using gamma-ray telescopes because they are thought to have high DM content and a low astrophysical background. The sensitivity of these searches is improved by combining the observations of dSphs made by different gamma-ray telescopes. We present the results of a combined search by the most sensitive currently operating gamma-ray telescopes, namely: the satellite-borne Fermi -LAT telescope; the ground-based imaging atmospheric Cherenkov telescope arrays H.E.S.S., MAGIC, and VERITAS; and the HAWC water Cherenkov detector. Individual datasets were analyzed using a common statistical approach. Results were subsequently combined via a global joint likelihood analysis. We obtain constraints on the velocity-weighted cross section 〈 σv 〉 for DM self-annihilation as a function of the DM particle mass. This five-instrument combination allows the derivation of up to 2-3 times more constraining upper limits on 〈 σv 〉 than the individual results over a wide mass range spanning from 5 GeV to 100 TeV. Depending on the DM content modeling, the 95% confidence level observed limits reach 1.5×10 ^-24 cm ³ s ^-1 and 3.2×10 ^-25 cm ³ s ^-1 , respectively, in the τ ⁺ τ ^- annihilation channel for a DM mass of 2 TeV.

Numerous observations confirm the existence of dark matter (DM) at astrophysical and cosmological scales, yet the fundamental nature of this elusive component of our Universe remains unknown. Theory and simulations of Galaxy formation predict that DM should cluster on small scales in bound structures called subhalos or DM clumps. While the most massive DM subhalos host baryonic matter and are observed as dwarf galaxies of the Milky Way (MW), less massive, unpopulated subhalos could be abundant in the Galaxy as well and yield high-energy gamma rays as final products of DM annihilation. Recently, it has been highlighted that the brightest halos should also have a sizeable extension in the sky. In this study, we examine the prospects offered by the Cherenkov Telescope Array Observatory (CTAO), a next-generation gamma-ray instrument, for detecting and characterizing such objects. Previous studies have primarily focused on high-latitude observations; here, we assess the potential impact of the CTAO’s Galactic Plane Survey, which will provide unprecedentedly deep survey data for the inner five degrees of the Galactic plane. Our modeling accounts for tidal effects on the subhalo population, examining the conditions under which DM subhalos can be detected and distinguished from conventional astrophysical sources. We find that regions a few degrees above or below the Galactic plane offer the highest likelihood for DM subhalo detection. For an individual subhalo—the brightest from among various realizations of the MW subhalo population—we find that detection at the 5 σ level is achievable for an annihilation cross section of ⟨ σ v ⟩ ∼ 3 × 10 − 25 cm 3 / s for TeV-scale DM annihilating into b b ¯ . For a full population study, depending on the distribution and luminosity model of Galactic subhalos, still unconstrained cross sections in the range ⟨ σ v ⟩ ∼ 10 − 23 – 10 − 22 cm 3 / s for TeV DM candidates are necessary for the brightest subhalos to be detected.

An increasing number of pulsar wind nebulae (PWNe) are being identified in the TeV band by ground-based Imaging Air Cherenkov Telescopes such that they constitute the dominant source class of Galactic TeV emitters. However, MeV–GeV PWN counterparts are still largely lacking. To date, only a dozen PWNe are identified by the Fermi–Large Area Telescope (LAT) in the MeV–GeV band. Most PWNe are located along the Galactic plane embedded within the prominent, diffuse Galactic γ-ray emission, which makes these sources difficult to disentangle from the bright diffuse background. We present a systematic search for γ-ray counterparts to known PWNe in the 300 MeV–2 TeV energy band using the Fermi–LAT. We target the locations of previously identified PWNe that lack detected Fermi–LAT pulsars to minimize associated pulsar contamination. The sample includes six previously identified Fermi PWNe and eight Fermi–LAT sources associated with PWNe. We report the analysis of 58 regions of interest and classify Fermi–LAT detected sources as either a likely PWN or a candidate PWN counterpart based on their morphological and spectral characteristics across the broadband spectrum. There are nine unidentified Fermi–LAT sources that we consider as likely PWN counterparts, which, if confirmed to be PWNe, would greatly increase the PWN population detected by the Fermi–LAT from 12 to 21. The remaining Fermi–LAT detected sources are considered weaker PWN candidates. A second approach in the systematic search for γ-ray emitting PWNe will involve studying the off-pulse phases of Fermi–LAT detected pulsars for the presence of an obscured PWN and will be reported in a subsequent paper.