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Register a new userThe WIMP-induced Gamma Ray Spectrum of Active Galactic Nuclei
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As direct and indirect dark matter detection experiments continue to place stringent constraints on WIMP masses and couplings, it becomes imperative to expand the scope of the search for particle dark matter by looking in new and exotic places. One such place may be the core of active galactic nuclei where the density of dark matter is expected to be extremely high. Recently, several groups have explored the possibility of observing signals of dark matter from its interactions with the high-energy jets emanating from these galaxies. In this work, we build upon these analyses by including the other components of the WIMP-induced gamma ray spectrum of active galactic nuclei; namely, (1) the continuum from WIMP annihilation into light standard model states which subsequently radiate and/or decay into photons and (2) the direct (loop-induced) decay into photons. We work in the context of models of universal extra dimensions (in particular, a model with two extra dimensions) and compute all three components of the gamma ray spectrum and compare with current data. We find that the model with two extra dimensions exhibits several interesting features which may be observable with the Fermi gamma ray telescope. We also show that, in conjunction with other measurements, the gamma ray spectrum from AGN can be an invaluable tool for restricting WIMP parameter space.
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We report the detection of a non-zero time delay between radio emission measured by the VLBA at 15.4 GHz and gamma-ray radiation (gamma-ray leads radio) registered by the Large Area Telescope (LAT) on board the Fermi Gamma-Ray Space Telescope for a sample of 183 radio and gamma-ray bright active galactic nuclei (AGNs). For the correlation analysis we used 100 MeV - 100 GeV gamma-ray photon fluxes, taken from monthly binned measurements from the first Fermi LAT catalog, and 15.4 GHz radio flux densities from the MOJAVE VLBA program. The correlation is most pronounced if the core flux density is used, strongly indicating that the gamma-ray emission is generated within the compact region of the 15 GHz VLBA core. Determining the Pearsons r and Kendalls tau correlation coefficients for different time lags, we find that for the majority of sources the radio/gamma-ray delay ranges from 1 to 8 months in the observers frame and peaks at about 1.2 months in the sources frame. We interpret the primary source of the time delay to be synchrotron opacity in the nuclear region.
Active Galactic Nuclei can be copious extragalactic emitters of MeV-GeV-TeV gamma rays, a phenomenon linked to the presence of relativistic jets powered by a super-massive black hole in the center of the host galaxy. Most of gamma-ray emitting active galactic nuclei, with more than 1500 known at GeV energies, and more than 60 at TeV energies, are called blazars. The standard blazar paradigm features a jet of relativistic magnetized plasma ejected from the neighborhood of a spinning and accreting super-massive black hole, close to the observer direction. Two classes of blazars are distinguished from observations: the flat-spectrum radio-quasar class (FSRQ) is characterized by strong external radiation fields, emission of broad optical lines, and dust tori. The BL Lac class (from the name of one of its members, BL Lacertae) corresponds to weaker advection-dominated flows with gamma-ray spectra dominated by the inverse Compton effect on synchrotron photons. This paradigm has been very successful for modeling the broadband spectral energy distributions of blazars. However, many fundamental issues remain, including the role of hadronic processes and the rapid variability of those BL Lac objects whose synchrotron spectrum peaks at UV or X-ray frequencies. A class of gamma-ray--emitting radio galaxies, which are thought to be the misaligned counterparts of blazars, has emerged from the results of the Fermi-Large Area Telescope and of ground-based Cherenkov telescopes. Blazars and their misaligned ounterparts make up most of the >100 MeV extragalactic gamma ray background (EGB), and are uspected of being the sources of ultra-high energy cosmic rays. The future Cherenkov Telescope Array, in synergy with the Fermi-Large Area Telescope and a wide range of telescopes in space and on he ground, will write the next chapter of blazar physics.
Active galactic nuclei (AGN) with jets seen at small viewing angles are the most luminous and abundant objects in the $gamma$-ray sky. AGN with jets misaligned along the line-of-sight appear fainter in the sky, but are more numerous than the brighter blazars. We calculate the diffuse $gamma$-ray emission due to the population of misaligned AGN (MAGN) unresolved by the Large Area Telescope (LAT) on the {it Fermi} Gamma-ray Space Telescope ({it Fermi}). A correlation between the $gamma$-ray luminosity and the radio-core luminosity is established and demonstrated to be physical by statistical tests, as well as compatible with upper limits based on {it Fermi}-LAT data for a large sample of radio-loud MAGN. We constrain the derived $gamma$-ray luminosity function by means of the source count distribution of the radio galaxies (RGs) detected by the {it Fermi}-LAT. We finally calculate the diffuse $gamma$-ray flux due to the whole MAGN population. Our results demonstrate that the MAGN can contribute from 10% up to nearly the entire measured Isotropic Gamma-Ray Background (IGRB). We evaluate a theoretical uncertainty on the flux of almost an order of magnitude.
The majority of the activity around nearby (z ~ 0) supermassive black holes is found in low-luminosity active galactic nuclei (LLAGN), the most of them being classified as low ionization nuclear emission regions. Although these sources are well studied from radio up to X-rays, they are poorly understood in gamma-rays. In this work we take advantage of the all sky-surveying capabilities of the Large Area Telescope on board Fermi Gamma ray Space Telescope to study the whole Palomar sample of LLAGN in gamma-rays. Precisely, the four radio-brightest LLAGN in the sample are identified as significant gamma-ray emitters, all of which are recognized as powerful Fanaroff-Riley I galaxies. These results suggest that the presence of powerful radio jets is of substantial importance for observing a significant gamma-ray counterpart even if these jets are misaligned with respect to the line of sight. We also find that most of the X-ray-brightest LLAGN do not have a significant gamma-ray and strong radio emission, suggesting that the X-rays come mainly from the accretion flow in these cases. A detailed analysis of the spectral energy distributions (SEDs) of NGC 315 and NGC 4261, both detected in gamma-rays, is provided where we make a detailed comparison between the predicted hadronic gamma-ray emission from a radiatively inefficient accretion flow (RIAF) and the gamma-ray emission from a leptonic jet-dominated synchrotron self-Compton (SSC) model. Both SEDs are better described by the SSC model while the RIAF fails to explain the gamma-ray observations.
We present a detailed statistical analysis of the correlation between radio and gamma-ray emission of the Active Galactic Nuclei (AGN) detected by Fermi during its first year of operation, with the largest datasets ever used for this purpose. We use both archival interferometric 8.4 GHz data (from the VLA and ATCA, for the full sample of 599 sources) and concurrent single-dish 15 GHz measurements from the Owens Valley Radio Observatory (OVRO, for a sub sample of 199 objects). Our unprecedentedly large sample permits us to assess with high accuracy the statistical significance of the correlation, using a surrogate-data method designed to simultaneously account for common-distance bias and the effect of a limited dynamical range in the observed quantities. We find that the statistical significance of a positive correlation between the cm radio and the broad band (E>100 MeV) gamma-ray energy flux is very high for the whole AGN sample, with a probability <1e-7 for the correlation appearing by chance. Using the OVRO data, we find that concurrent data improve the significance of the correlation from 1.6e-6 to 9.0e-8. Our large sample size allows us to study the dependence of correlation strength and significance on specific source types and gamma-ray energy band. We find that the correlation is very significant (chance probability <1e-7) for both FSRQs and BL Lacs separately; a dependence of the correlation strength on the considered gamma-ray energy band is also present, but additional data will be necessary to constrain its significance.
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