• Energy Research

Centaurus A (Cen A) is the nearest radio galaxy discovered as a very-high-energy (VHE; 100 GeV–100 TeV) γ-ray source by the High Energy Stereoscopic System (H.E.S.S.). It is a faint VHE γ-ray emitter, though its VHE flux exceeds both the extrapolation from early Fermi-LAT observations as well as expectations from a (misaligned) single-zone synchrotron-self Compton (SSC) description. The latter satisfactorily reproduces the emission from Cen A at lower energies up to a few GeV. New observations with H.E.S.S., comparable in exposure time to those previously reported, were performed and eight years of Fermi-LAT data were accumulated to clarify the spectral characteristics of the γ-ray emission from the core of Cen A. The results allow us for the first time to achieve the goal of constructing a representative, contemporaneous γ-ray core spectrum of Cen A over almost five orders of magnitude in energy. Advanced analysis methods, including the template fitting method, allow detection in the VHE range of the core with a statistical significance of 12σ on the basis of 213 hours of total exposure time. The spectrum in the energy range of 250 GeV–6 TeV is compatible with a power-law function with a photon index Γ = 2.52 ± 0.13stat ± 0.20sys. An updated Fermi-LAT analysis provides evidence for spectral hardening by ΔΓ ≃ 0.4 ± 0.1 at γ-ray energies above 2.8+1.0−0.6 GeV at a level of 4.0σ. The fact that the spectrum hardens at GeV energies and extends into the VHE regime disfavour a single-zone SSC interpretation for the overall spectral energy distribution (SED) of the core and is suggestive of a new γ-ray emitting component connecting the high-energy emission above the break energy to the one observed at VHE energies. The absence of significant variability at both GeV and TeV energies does not yet allow disentanglement of the physical nature of this component, though a jet-related origin is possible and a simple two-zone SED model fit is provided to this end.

Abstract We investigated the multiband emission from the pulsar binaries XSS J12270−4859, PSR J2039−5617, and PSR J2339−0533, which exhibit orbital modulation in the X-ray and gamma-ray bands. We constructed the sources’ broadband spectral energy distributions and multiband orbital light curves by supplementing our X-ray measurements with published gamma-ray results, and we modeled the data using intrabinary shock (IBS) scenarios. While the X-ray data were well explained by synchrotron emission from electrons/positrons in the IBS, the gamma-ray data were difficult to explain with the IBS components alone. Therefore, we explored other scenarios that had been suggested for gamma-ray emission from pulsar binaries: (1) inverse-Compton emission in the upstream unshocked wind zone and (2) synchrotron radiation from electrons/positrons interacting with the kilogauss magnetic field of the companion. Scenario (1) requires that the bulk motion of the wind substantially decelerates to ∼1000 km s−1 before reaching the IBS for increased residence time, in which case the formation of a strong shock is untenable, inconsistent with the X-ray phenomenology. Scenario (2) can explain the data if we assume the presence of electrons/positrons with a Lorentz factor ∼ 108 (∼0.1 PeV) that pass through the IBS and tap a substantial portion of the pulsar voltage drop. These findings raise the possibility that the orbitally modulating gamma-ray signals from pulsar binaries can provide insights into the flow structure and energy conversion within pulsar winds and particle acceleration nearing PeV energies in pulsars. These signals may also yield greater understanding of kilogauss magnetic fields potentially hosted by the low-mass stars in these systems.

The unidentified very-high-energy (VHE;E> 0.1 TeV)γ-ray source, HESS J1826−130, was discovered with the High Energy Stereoscopic System (HESS) in the Galactic plane. The analysis of 215 h of HESS data has revealed a steadyγ-ray flux from HESS J1826−130, which appears extended with a half-width of 0.21° ± 0.02stat° ± 0.05sys°. The source spectrum is best fit with either a power-law function with a spectral index Γ = 1.78 ± 0.10stat± 0.20sysand an exponential cut-off at 15.2−3.2+5.5TeV, or a broken power-law with Γ1= 1.96 ± 0.06stat± 0.20sys, Γ2= 3.59 ± 0.69stat± 0.20sysfor energies below and aboveEbr= 11.2 ± 2.7 TeV, respectively. The VHE flux from HESS J1826−130 is contaminated by the extended emission of the bright, nearby pulsar wind nebula, HESS J1825−137, particularly at the low end of the energy spectrum. Leptonic scenarios for the origin of HESS J1826−130 VHE emission related to PSR J1826−1256 are confronted by our spectral and morphological analysis. In a hadronic framework, taking into account the properties of dense gas regions surrounding HESS J1826−130, the source spectrum would imply an astrophysical object capable of accelerating the parent particle population up to ≳200 TeV. Our results are also discussed in a multiwavelength context, accounting for both the presence of nearby supernova remnants, molecular clouds, and counterparts detected in radio, X-rays, and TeV energies.

Abstract We present 294 pulsars found in GeV data from the Large Area Telescope (LAT) on the Fermi Gamma-ray Space Telescope. Another 33 millisecond pulsars (MSPs) discovered in deep radio searches of LAT sources will likely reveal pulsations once phase-connected rotation ephemerides are achieved. A further dozen optical and/or X-ray binary systems colocated with LAT sources also likely harbor gamma-ray MSPs. This catalog thus reports roughly 340 gamma-ray pulsars and candidates, 10% of all known pulsars, compared to ≤11 known before Fermi. Half of the gamma-ray pulsars are young. Of these, the half that are undetected in radio have a broader Galactic latitude distribution than the young radio-loud pulsars. The others are MSPs, with six undetected in radio. Overall, ≥236 are bright enough above 50 MeV to fit the pulse profile, the energy spectrum, or both. For the common two-peaked profiles, the gamma-ray peak closest to the magnetic pole crossing generally has a softer spectrum. The spectral energy distributions tend to narrow as the spindown power E ̇ decreases to its observed minimum near 1033 erg s−1, approaching the shape for synchrotron radiation from monoenergetic electrons. We calculate gamma-ray luminosities when distances are available. Our all-sky gamma-ray sensitivity map is useful for population syntheses. The electronic catalog version provides gamma-ray pulsar ephemerides, properties, and fit results to guide and be compared with modeling results.