Tsunefumi Mizuno

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.

We present the largest sample of gamma-ray transients observed by any CubeSat mission so far. Observations were acquired by a 1U CubeSat GRBAlpha, the smallest astrophysical space observatory, and a 3U CubeSat VZLUSAT-2. Both missions were technological pathfinders and carried a novel CsI scintillator-based detector read out by silicon photomultipliers. They operated on Sun-synchronous low Earth orbits below 550 km for about 4 yr; GRBAlpha between 2021 March and 2025 June and VZLUSAT-2 between 2022 January and 2025 November. Despite being technological experiments, they observed over 300 gamma-ray transients including gamma-ray bursts (GRBs), solar flares, soft gamma repeaters, and one outburst from an X-ray binary. Among these are the two brightest GRBs ever observed, GRB 221009A and GRB 230307A, without saturation and GRBs at redshifts up to z = 4.2. GRBAlpha also contributed to the InterPlanetary Network. Regular monitoring of transients was demonstrated by a detection rate of two transients or one GRB a week and the shortest time between two subsequent detections of only 42 minutes. We show that a constellation of nanosatellites around the Earth would observe at least 60% of Fermi/GBM GRBs with 5 σ significance and over 90% at the 3 σ level. GRBAlpha and VZLUSAT-2 prove that routine monitoring of the gamma-ray sky can also be done by low-cost and quickly developed nanosatellite missions.

In this work, we report the successful application of silicon photomultipliers (SiPMs) in gamma-ray burst (GRB) detectors used in CubeSats operating in the low Earth orbit (LEO) radiation environment. It is known that SiPMs are susceptible to radiation damage, leading to an increase in the dark count rate. This results in an increase in the low-energy threshold in detectors combining SiPMs and scintillators. Despite this drawback, they became popular in gamma-ray detectors on CubeSats due to their low operating voltage, small size and fast response. Therefore, it is important to characterise their long-term performance in the space environment. Here, we describe the changes in the dark count rate and low-energy threshold of S13360-3050PE multi-pixel photon counters (MPPCs) by Hamamatsu Photonics K.K., using measurements from the GRBAlpha, GRBBeta, and VZLUSAT-2 CubeSats. In the case of GRBAlpha, the measurement of SiPM performance in space lasted over 4 years. GRBAlpha was a 1U CubeSat launched on 2021/03/22 to a 550 km altitude polar orbit carrying a CsI(Tl) scintillator GRB detector employing eight MPPCs and sensitive in the range of 30-900 keV. GRBAlpha de-orbited on 2025/06/09. VZLUSAT-2 was a 3U CubeSat launched on 2022/01/13 to a 535 km altitude polar orbit and de-orbited on 2025/11/30. GRBBeta was launched on 2024/07/09 to a 580 km altitude, 62° inclination orbit. Both VZLUSAT-2 and GRBBeta carry detectors similar to the one on GRBAlpha. We have flight-proven the Hamamatsu MPPCs S13360-3050 PE and demonstrated that SiPMs, shielded by 2.5 mm of PbSb alloy, can be used in a LEO environment on a scientific mission lasting beyond 4 years. This shows the potential for SiPMs to be employed in future satellites.

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.

We present the first X-ray polarization measurements of the PSR/PWN complex within SNR Kes 75. Two ∼500 ks IXPE observations were conducted in 2024 October/November and 2025 April. The second observation yields a significant phase-average 2–8 keV polarization degree of PD = 9.9% ± 2.5% at PA = 36 . ° 8 ± 7 . ° 3, implying a toroidal field aligned with the pulsar wind nebula (PWN) symmetry axis. The first epoch, however, has only a polarization upper limit. During this epoch, an additional pulsed component is visible at Δ ϕ ≈ 0.5, detected at ∼3.7 σ . An unbinned phase-resolved analysis reveals a high-polarization degree rotating vector model position angle sweep at the ∼99.5% confidence level, with angles fixed at those inferred from the PWN morphology; this can explain the loss of phase-average polarization. Additional observations are needed to pin down the nature of the anomalous pulse.

Large High Altitude Air Shower Observatory (LHAASO) has reported 43 sub-PeV γ-ray sources, which are promising candidates for cosmic-ray (CR) accelerators above the PeV energy, commonly called PeVatrons. Multiwavelength observations are crucial for identifying the underlying particle species and estimating the CR energy content of these sources. In this work we investigate the region around LHAASO J1912+1014u (and HESS J1912+101) using Fermi-LAT γ-ray data and FUGIN CO data. We analyzed 15 yr of Fermi-LAT data in the 0.4–409.6 GeV energy range. By improving the standard Fermi-LAT diffuse emission model, we significantly reduced the large residuals around the source in the 1.6–12.8 GeV band. We detected a statistically significant excess above the diffuse background, which likely represents ≥10 GeV emission associated with the LHAASO/H.E.S.S. source. The GeV excess exhibits a hard spectrum (photon index of about 2.1) and is well reproduced by interstellar gas templates with systemic velocities of about 25 km s⁻¹ or 60 km s⁻¹. We performed a comprehensive fit to the GeV–TeV spectral energy distribution. Although a leptonic scenario can reproduce the observed spectrum, a hadronic scenario is favored once electron cooling is considered. The inferred CR proton spectrum has an index of ∼2.2, and the total CR proton energy above 1 GeV is (1–5) × 10⁴⁹ erg, depending on the assumed velocity range of the associated interstellar gas. A stringent upper limit on diffuse X-ray emission further supports the proton PeVatron scenario.

Flat spectrum radio quasars (FSRQs) often remain undetected at very-high-energy (VHE, E > 100 GeV) γ rays due to their typically soft spectra and large distance. In December 2023, the Large-Sized Telescope prototype (LST-1) detected for the first time VHE γ-ray emission from the FSRQ OP 313 during an extraordinarily bright flare, becoming the furthest blazar ever observed at VHE with z=0.997. We aim to characterize the γ-ray emission of OP 313 during this flare, comparing it with its average emission state in order to understand the processes leading to this detection. Its remarkable distance also enables studies on the extragalactic gackground light (EBL), with the goal of evaluating the attenuation of VHE γ-ray photons. Thanks to an intensive multiwavelength campaign, we also study in detail the broadband emission of the source. We characterize the γ-ray emission during the flare in December 2023 and the low emission state observed in January 2024 thanks to the LST-1 and MAGIC data and quasi-simultaneous -LAT observations. Fermi This dataset also enables us to evaluate the EBL attenuation by systematically exploring the EBL intensity over the γ-ray spectrum. . Finally, we study the multiwavelength emission and interpret the broadband spectral energy distribution (SED) within blazar radiative models including the thermal contributions from the accretion disk, dusty torus and broad line region. We measured significant VHE γ-ray emission during the December 2023 flare at a level of 0.3 Crab units above 100 GeV . We also characterize the flare brightness in the high-energy (HE, E>100 MeV) γ-ray band, which was found to be a factor of 50 above the average emission seen by -LAT. The HE and VHE observations allow us to set constrains on the EBL density, resulting in an upper limit at 0.6 μm of łambda I_ Fermi łambda <8.74 nW m 2 $ sr^-1 with LST-1 data, and łambda I_ łambda <14.7 nW m^2 sr^-1 with MAGIC data, including systematic uncertainties. Finally, thanks to the extensive multiwavelength campaign organized, we are able to construct and model the broadband SED of OP 313 within the framework of a two-zone leptonic model where the γ-ray emission is produced via inverse-Compton scattering of the broad line region, accretion disk, and dusty torus photon fields. However, the dominant external photon field remains unknown, as several combinations are able to successfully explain the $γ-ray emission observed .

We report on the average and orbital phase-resolved polarization of Cyg X-3 in the hard state (HS) during the 2023 Imaging X-ray Polarimetry Explorer (IXPE) observational campaign. We find the polarization degree of 21.4% ± 0.4% and polarization angle of 92 . ° 2 ± 0 . ° 5 , compatible within 3 σ with the first HS IXPE observation in 2022. As the observed polarization depends on both the accretion geometry and the X-ray emission mechanism, and assuming it arises from reflection from the optically thick envelope surrounding the central source, our result indicates that both are similar in the two HS epochs on year-long timescales despite the source transitioning through an intermediate state in between. We discuss time- and energy-dependent polarization properties and their implications for the geometry and stability of the accretion funnel, finding significant orbital modulation of the polarization properties.