Justin D. Finke

Context. Gamma-ray bursts (GRBs) have long been considered candidate sources of ultrahigh-energy cosmic rays (UHECRs) due to their large energy release and relativistic outflows. The detection of multi-TeV γ rays from GRB 221009A and its rarity have renewed interest in this connection and motivate considering an additional narrow-jet long GRB population in the local Universe. Aims. We investigate whether such a local narrow-jet population can contribute to the observed diffuse UHECR energy spectrum. We also examine the associated cosmogenic neutrino and cascade γ -ray emissions to assess the multimessenger viability of this scenario. Methods. We fit the observed UHECR spectrum and mass composition data using three source-evolution models: a uniform comoving source emissivity, a standard-jet GRB population tracing the star formation rate (SFR), and a standard + narrow jet GRB population tracing SFR. We propagate a mixed-composition UHECR injection and calculate the cosmogenic neutrino and cascade γ -ray fluxes. Results. The standard + narrow jet model fits the observed UHECR spectrum and composition, with the highest-energy flux dominated by the narrow-jet population confined to z ≤ z _max, * ≃ 0.36. This low-redshift dominance lowers the cosmogenic neutrino flux compared to the standard-jet GRB population. For the narrow-jet GRB population, the fit implies a baryon loading factor ξ _CR ^nj ≃ 10. Conclusions. Such a locally enhanced long-GRB population can therefore explain the highest-energy UHECR flux without violating current multimessenger constraints. Future UHE searches can further probe this scenario through the associated cosmogenic fluxes.

The shock fronts of supernova remnants (SNRs) are believed to be significant sites of acceleration of cosmic-ray particles. Previous researchers have shown that a particle distribution similar to a log-parabola can be generated when particles have an energy-dependent escape. We explore the acceleration of electrons at SNR shock fronts and show that modeling this energy-dependent particle escape model can produce spectral energy distributions consistent with observations of two lepton-radiation-dominated SNRs: RX J1713.7−3946 and SN 1006. The model includes the evolution of both the electron distribution and photon spectra as a result of the combined effects of the SNR evolution and electron energy loss. The electron-escape energy dependence is quite weak, but the electron distribution and photon spectra turn out to be very sensitive to changes in the electron escape. We also explore how sensitive the spectra and electron distributions are to the parameters used in the log-parabola model.

The extragalactic background light (EBL), the cumulative radiation from all extragalactic sources, traces galaxy formation and cosmic evolution. High-energy γ -rays attenuated via pair production with EBL photons are a powerful probe of the EBL. In this work, we use very-high-energy (VHE; E _γ > 100 GeV) γ -rays to measure the local EBL intensity and test its consistency with galaxy counts and direct measurements. Our analysis employs a sample of 286 spectra from 45 sources observed with Imaging Atmospheric Cherenkov telescopes. A model-dependent study shows seven EBL templates require only ≤10% rescaling to fit the observed γ -ray attenuation. The galaxy-count-anchored model gives the closest match. We then derive template-marginalized TeV optical depths from a representative model subset. We combine them with Fermi-LAT GeV measurements to reconstruct the EBL at z = 0 using empirical and physically motivated models. The two reconstructions agree and follow the integrated galaxy light to within 2–3 nW m ⁻² sr ⁻¹ (typically μ m. Both are consistent with low-zodiacal-light observations, including outer solar system and dark cloud measurements. In contrast, the near-IR excess reported by the Infrared Telescope in Space and CIBER exceeds our reconstructed intensity by 3 σ –5 σ , implying an additional ≳5–10 nW m ⁻² sr ⁻¹ incompatible with the γ -ray optical depths. Combined with GeV constraints on EBL evolution to z ≃ 4, these TeV optical depths provide a VHE-anchored determination of the local EBL intensity. The agreement with galaxy counts and deep-space measurements indicates that known galaxy populations account for most of the optical and near-IR background, leaving limited room for an additional diffuse component.

The extragalactic background light (EBL) from ultraviolet to infrared comprises the emission from all stars, galaxies, and actively accreting black holes in the observable Universe. A precise measurement of the EBL is critically important to probe models of star formation and galaxy evolution. The EBL can be measured via the absorption imprint left on the spectra of γ -ray blazars. In this work, we rely on 15 yr of Fermi-LAT data and 1576 blazars to measure the EBL optical depth in the 0 < z < 4.3 range. We detect the EBL attenuation with ≅23 σ significance and measure the optical depth in 19 redshift bins, extending the coverage and improving on our previous results. This allows us to reconstruct the evolution of the EBL and find general consistency with recent EBL models. These results represent the most precise determination of the EBL with GeV γ rays to date.

We revisit observations of Fe absorption lines along lines of sight to background stars and quasars through the interior of SN 1006. Previous such studies of Fe II absorption lines have revealed a lower-than-expected mass of Fe for a Type Ia supernova remnant. We give a more detailed treatment of the photoionization, including the effect of UV–X-ray synchrotron radiation emitted by cosmic-ray electrons at the forward shock, and find that most of the Fe ejecta is photoionized to higher charge states, leading to higher Fe masses inferred from the Fe II absorption lines than in previous work. While the problem of lower-than-expected Fe mass is resolved, a precise determination of the Fe mass in order to understand the progenitor and the nature of the explosion depends sensitively on the time dependence of the synchrotron luminosity and, when inferred from existing X-ray observations, the degree of ejecta clumping. Plausible assumptions suggest a Chandrasekhar mass white dwarf progenitor, but other scenarios cannot so far be ruled out.

This paper presents an analysis of the predicted optical-to-X-ray spectral index (α_ox) within the context of ultra-luminous X-ray sources (ULXs) associated with stellar-mass black holes (BHs) and neutron stars (NSs). We used the population synthesis code COSMIC to simulate the evolution of binary systems and investigate the relationship between ultraviolet (UV) and X-ray emission during the ULX phase, namely the α_ox relation. Furthermore, we investigated the impact of metallicity on α_ox values. Notably, it predicts a significant anti-correlation between α_ox and UV luminosity (L_UV), consistent with observations. The slope of this relationship varies with metallicity for black hole ULXs (BH-ULXs). The neutron star ULX (NS-ULX) population shows a relatively consistent slope around −0.33 across metallicities, with minor variations. The number of ULXs decreases with increasing metallicity, consistent with observational data. The X-ray luminosity function (XLF) shows a slight variation in its slope with metallicity, exhibiting a relative excess of high-luminosity ULXs at lower metallicities. The inclusion of the beaming effect in the analysis shows a significant impact on the XLF and α_ox, particularly at high accretion rates, where the emission is focused into narrower cones. We found that UV emission in ULXs is predominantly disc-dominated, which is the likely origin of the α_ox relation, with the percentage of disc-dominated ULXs increasing as metallicity rises.

Significant improvements in our understanding of nuclear γ-ray line production and instrument performance allow us to better characterize the continuum emission from electrons at energies ≳300 keV during solar flares. We represent this emission by the sum of a power-law (PL) extension of hard X-rays and a power law times an exponential function (PLexp). We fit the γ-ray spectra in 25 large flares observed over 40 yr with this continuum and the calculated spectra of all known nuclear components. The PLexp is separated spectroscopically from the other components, and its presence is required with >99% confidence in 18 of the flares. Its distinct origin is suggested by significant differences between its time histories and those of the PL and nuclear components in 18 of the flares. RHESSI imaging/spectroscopy of the 2005 January 20 flare reveals that the PL and nuclear components come from the footpoints, while the PLexp component comes from the corona. While the index and flux of the anisotropic PL component are dependent on the flare’s heliocentric angle, the PLexp parameters do not show comparable dependences with 99.5% confidence. The PLexp spectrum is flat at low energies and rolls over at a few megaelectronvolts (MeV). Such a shape can be produced by thin-target bremsstrahlung from electrons with a spectrum that peaks between 3 and 5 MeV and by inverse Compton scattering of soft X-rays by 10–20 MeV electrons, or by a combination of the two. These electrons can produce radiation detectable at other wavelengths.

SIRI-2 is a collection of strontium iodide gamma-ray detectors sensitive at approximately 400 keV to 10 MeV, launched on the Department of Defense's STPSat-6 to geosynchronous orbit. SIRI-2 detected the gamma-ray burst (GRB) 221009A and, unlike most GRB detectors, was not saturated and did not require any pulse pileup corrections. The energetics of this burst as measured by SIRI-2 are consistent with those found by other instruments, and the Band function fits to the spectra are consistent with those from the unsaturated Insight and GECAM instruments and similar to corrected spectra from the Fermi Gamma-ray Burst Monitor, but softer than those found by Konus-Wind when that instrument was saturated. The total fluence measured with SIRI-2 was measured to be 0.140 ± 0.002 erg cm⁻², lower than other instruments, likely due to the increasing background of SIRI-2 forcing the calculation to use a smaller time interval. An extrapolation of the distributions of fluences from GRBs to the fluence of 221009A measured with SIRI-2 indicates bursts brighter than this one should occur about once every 4000 yr.

SIRI-2 is a collection of Strontium Iodide gamma-ray detectors sensitive at approximately 400 keV to 10 MeV, launched on the Department of Defense's STPSat-6 to geosynchronous orbit. SIRI-2 detected the gamma-ray burst (GRB) 221009A and, unlike most GRB detectors, was not saturated and did not require any pulse pile-up corrections. The energetics of this burst as measured by SIRI-2 are consistent with those found by other instruments, and the Band function fits to the spectra are consistent with that from the unsaturated Insight and GECAM instruments, and similar to corrected spectra from the Fermi Gamma-ray Burst Monitor, but softer than those found by Konus-Wind when that instrument was saturated. The total fluence measured with SIRI-2 was measured to be 0.140 +/- 0.002 erg cm-2, lower than other instruments, likely due to the increasing background of SIRI-2 forcing the calculation to use a smaller time interval. An extrapolation of the distributions of fluences from GRBs to the fluence of 221009A measured with SIRI-2 indicates bursts brighter than this one should occur about once every 4,000 years.