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Register a new userAnalysis of Fermi-LAT observations, UHECRs and neutrinos from the radio galaxy Centaurus B
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Centaurus B (Cen B) is one of the closest and brightest radio-loud galaxy in the southern sky. This radio galaxy, proposed as a plausible candidate for accelerating ultra-high-energy cosmic rays (UHECRs), is near the highest-energy neutrino event reported (IC35) in the High-Energy Starting Events catalog. Pierre Auger observatory reported the highest energy comic rays during 10 years of collecting data with some of them around this source. In this paper, the analysis of the gamma-ray spectrum and the light curve above 200 MeV is presented with nine years of cumulative Fermi-LAT data around Cen B. Taking into consideration the multi-wavelength observations carried out about this radio galaxy, leptonic and hadronic scenarios are introduced in order to fit the spectral energy distribution, assuming that the gamma-ray flux is produced in a region close to the core and in the extended lobes. Using the best-fit values found, several physics properties of this radio galaxy are derived. Furthermore, a statistical analysis of the cosmic ray distribution around Cen B is performed, finding that this distribution is not different from the background at a level of significance of 5%. Considering the UHECR event associated to this source by Moskalenko et al. and extrapolating its luminosity to low energies, we do not find enough evidence to associate the highest-energy neutrino event (IC35) with this radio galaxy.
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Centaurus B is a nearby radio galaxy positioned in the Southern hemisphere close to the Galactic plane. Here we present a detailed analysis of about 43 months of accumulated Fermi-LAT data of the gamma-ray counterpart of the source initially reported in the 2nd Fermi-LAT catalog, and of newly acquired Suzaku X-ray data. We confirm its detection at GeV photon energies, and analyze the extension and variability of the gamma-ray source in the LAT dataset, in which it appears as a steady gamma-ray emitter. The X-ray core of Centaurus B is detected as a bright source of a continuum radiation. We do not detect however any diffuse X-ray emission from the known radio lobes, with the provided upper limit only marginally consistent with the previously claimed ASCA flux. Two scenarios that connect the X-ray and gamma-ray properties are considered. In the first one, we assume that the diffuse non-thermal X-ray emission component is not significantly below the derived Suzaku upper limit. In this case, modeling the inverse-Compton emission shows that the observed gamma-ray flux of the source may in principle be produced within the lobes. This association would imply that efficient in-situ acceleration of the radiating electrons is occurring and that the lobes are dominated by the pressure from the relativistic particles. In the second scenario, with the diffuse X-ray emission well below the Suzaku upper limits, the lobes in the system are instead dominated by the magnetic pressure. In this case, the observed gamma-ray flux is not likely to be produced within the lobes, but instead within the nuclear parts of the jet. By means of synchrotron self-Compton modeling we show that this possibility could be consistent with the broad-band data collected for the unresolved core of Centaurus B, including the newly derived Suzaku spectrum.
Ultrahigh energy cosmic ray (UHECR) protons interacting with the cosmic microwave background (CMB) produce UHE electrons and gamma-rays that in turn initiate electromagnetic cascades on CMB and infrared photons. As a result, a background of diffuse isotropic gamma radiation is accumulated in the energy range $Elsim 100$ GeV. The Fermi-LAT collaboration has recently reported a measurement of the extragalactic diffuse background finding it less intense and softer than previously measured by EGRET. We show that this new result constrains UHECR models and the flux of cosmogenic neutrinos. In particular, it excludes models with cosmogenic neutrino fluxes detectable by existing neutrino experiments, while next-generation detectors as e.g. JEM-EUSO can observe neutrinos only for extreme parameters.
We review the high energy properties of Misaligned AGNs associated with gamma-ray sources detected by Fermi in 24 months of survey. Most of them are nearby emission low power radio galaxies (i.e FRIs) which probably have structured jets. On the contrary, high power radio sources (i.e FRIIs) with GeV emission are rare. The small number of FRIIs does not seem to be related to their higher redshifts. Assuming proportionality between the radio core flux and the gamma-ray flux, several of them are expected to be bright enough to be detected above 100 MeV in spite of their distance. We suggest that beaming/jet structural differences are responsible for the detection rate discrepancy observed between FRIs and FRIIs.
Radio and $gamma$-ray measurements of large lobes of several radio galaxies provide adequate basis for determining whether emission in these widely separated spectral regions is largely by energetic electrons. This is very much of interest as there is of yet no unequivocal evidence for a significant energetic proton component to account for $gamma$-ray emission by neutral pion decay. A quantitative assessment of the proton spectral distribution necessitates full accounting of the local and background radiation fields in the lobes; indeed, doing so in our recent analysis of the spectral energy distribution of the Fornax A lobes considerably weakened previous conclusions on the hadronic origin of the emission measured by the Fermi satellite. We present the results of similar analyses of the measured radio, X-ray and $gamma$-ray emission from the lobes of Centaurus A, Centaurus B, and NGC 6251. The results indicate that the measured $gamma$-ray emission from these lobes can be accounted for by Compton scattering of the radio-emitting electrons off the superposed radiation fields in the lobes; consequently, we set upper bounds on the energetic proton contents of the lobes.
3C 207 is a lobe-dominant radio galaxy with one sided jet and the bright knots in kpc-Mpc scale were resolved in the radio, optical and X-ray bands. It was confirmed as a gamma-ray emitter with Fermi/LAT, but it is uncertain whether the gamma-ray emission region is the core or knots due to the low spatial resolution of Fermi/LAT. We present an analysis of its Fermi/LAT data in the past 9 years. Different from the radio and optical emission from the core, it is found that the gamma-ray emission is steady without detection of flux variation over 2 sigma confidence level. This likely implies that the gamma-ray emission is from its knots. We collect the radio, optical, and X-ray data of knot-A, the closest knot from the core at 1 arcsec, and compile its spectral energy distribution (SED). Although the single-zone synchrotron+SSC+IC/CMB model by assuming knot-A at rest can reproduce the SED in the radio-optical-X-ray band, the predicted gamma-ray flux is lower than the LAT observations and the derived magnetic field strength deviates the equipartition condition with 3 orders of magnitude. Assuming that knot-A is relativistically moving, its SED from radio to gamma-ray bands would be well represented with the single-zone synchrotron+SSC+IC/CMB model under the equipartition condition. These results likely suggest that the gamma-ray emission may be from knot-A via the IC/CMB process and the knot should have relativistical motion. The jet power derived from our model parameters is also roughly consistent with the kinetic power estimated with the radio data.
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