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Register a new userStudy of microwave/gamma-ray properties for Fermi-LAT bright AGNs
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Blazars are a small fraction of all extragalactic sources but, unlike other objects, they are strong emitters across the entire electromagnetic spectrum. Recent data in the microwave region of the electromagnetic spectrum have become available to allow for systematic studies of blazars over large cosmological volumes. This frequency band is indeed particularly suited for the selection of blazars since at these frequencies the contamination from radio extended components with steep spectra is no longer present and the emission from the accretion process is negligible. During the first 3 months of scientific operations Fermi-LAT detected 106 bright, high-galactic latitude (| b |> 10 deg) AGNs with high significance. In this study we investigate the possible relations between the microwave and the gamma-ray emissions for Fermi-LAT detected AGNs belonging to WMAP 5th year bright source catalog.
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The X-ray, Ultraviolet, Optical emission from radio-quiet AGNs, black hole binaries, and other compact sources, in general, follow a lognormal flux distribution, a linear rms-flux relation, and a (broken) power-law power spectral densities (PSDs). These characteristics are normally attributed to the multiplicative combination of fluctuations in the accretion disk. Similar features have been inferred for some well-observed blazars in different energy bands, but a systematic study over a long duration is still missing. Using a continuous gamma-ray light curves over 3-days cadence from August 2008 - October 2015, we present the first systematic study of these features in four sources: the FR I radio galaxy NGC 1275 and three blazars- Mrk 421, B2 1520+31 and PKS 1510-089. For all, except Mrk 421, the flux spans $gtrsim$ 2 orders of magnitude. For blazars, a log-normal profile describes the flux histograms better compared to a Gaussian, while none is favored for NGC 1275, the only non-blazar source, suggesting a complex distribution. Regardless of flux histogram profile, the rms-flux relation is linear for all with PSDs being consistent with a power-law shot noise spectrum despite hints of breaks. The inferred results are consistent with the properties of unresolved magnetic reconnection sites, as inferred in the X-ray emission from the whole Solar disk and the statistical characteristics of magnetic reconnection based minijets-in-a-jet model. The results, thus, suggest a strong jet-accretion-disk coupling with energy input from the central source being distributed over a wide range in time and energy by the reconnection process depending on the geometry and local physical conditions.
A list of 205 gamma-ray strong objects was reported recently as a result of a 3-month integration with the Large Area Telescope on board the Fermi Gamma-Ray Space Telescope. We attempted identification of these objects, cross-correlating the gamma-ray positions with VLBI positions of a large all-sky sample of extragalactic radio sources selected on the basis of their parsec-scale flux density. The original associations reported by the Fermi team are confirmed and six new identifications are suggested. A Monte-Carlo analysis shows that the fraction of chance associations in our analysis is less than 5 per cent, and confirms that the vast majority of gamma-ray bright extragalactic sources are radio loud blazars with strong parsec-scale jets. A correlation between the parsec-scale radio and gamma-ray flux is supported by our analysis of a complete VLBI flux-density-limited sample of extragalactic jets. The effectiveness of using a VLBI catalog to find associations between gamma-ray detections and compact extragalactic radio sources, especially near the Galactic plane, is demonstrated. It is suggested that VLBI catalogs should be used for future identification of Fermi LAT objects.
We present the results of simultaneous multi-frequency imaging observations at 22, 43, 86, and 129,GHz of OJ,287. We used the Korean VLBI Network as part of the Interferometric MOnitoring of GAmma-ray Bright active galactic nuclei (iMOGABA). The iMOGABA observations were performed during 31 epochs from 2013 January 16 to 2016 December 28. We also used 15,GHz OVRO and 225,GHz SMA flux density data. We analyzed four flux enhancements in the light curves. The estimated time scales of three flux enhancements were similar with time scales of $sim$50 days at two frequencies. A fourth flux enhancement had a variability timescale approximately twice as long. We found that 225,GHz enhancements led the 15,GHz enhancements by a range of 7 to 30 days in the time delay analysis. We found the fractional variability did not change with frequency between 43 and 86,GHz. We could reliably measure the turnover frequency, $ u_{rm c}$, of the core of the source in three epochs. This was measured to be in a range from 27 to 50,GHz and a flux density at the turnover frequency, $S_{rm m}$, ranging from 3-6,Jy. The derived SSA magnetic fields, $B_{rm SSA}$, are in a range from $0.157pm0.104$ to $0.255pm0.146$ mG. We estimated the equipartition magnetic field strengths to be in a range from $0.95pm0.15$ to $1.93pm0.30$ mG. The equipartition magnetic field strengths are up to a factor of 10 higher than the values of $B_{rm SSA}$. We conclude that the downstream jet may be more particle energy dominated.
We present a statistical characterization of the $gamma$-ray emission from the four emph{Fermi}-LAT sources: FR I radio galaxy NGC 1275, BL Lac Mrk 421, FSRQs B2 1520+31 and PKS 1510-089 detected almost continuously over a time integration of 3-days between August 2008 - October 2015. The observed flux variation is large, spanning $gtrsim 2$ orders of magnitude between the extremes except for Mrk~421. We compute the flux distributions and compare with Gaussian and lognormal ones. We find that the 3 blazars have distribution consistent with a lognormal, suggesting that the variability is of a non-linear, multiplicative nature. This is further supported by the computation of the flux-rms relation, which is observed to be linear for the 3 blazars. However, for NGC 1275, the distribution does not seem to be represented either by a lognormal or a Gaussian, while its flux-rms relation is still found to be linear. We also compute the power spectra, which suggest the presence of a break, but are consistent with typical scale-free power-law shot noise. The results are broadly consistent with the statistical properties of the magnetic reconnection powered minijets-in-a-jet model. We discuss other possible scenarios and implications of these observations on jet processes and connections with the central engine.
Aims. We use a sample of 83 core-dominated active galactic nuclei (AGN) selected from the MOJAVE (Monitoring of Jets in AGN with VLBA Experiments) radio-flux-limited sample and detected with the Fermi Large Area Telescope (LAT) to study the relations between non-simultaneous radio, optical, and gamma-ray measurements. Methods. We perform a multi-band statistical analysis to investigate the relations between the emissions in different bands and reproduce these relations by modeling of the spectral energy distributions of blazars. Results. There is a significant correlation between the gamma-ray luminosity and the optical nuclear and radio (15 GHz) luminosities of blazars. We report a well defined positive correlation between the gamma-ray luminosity and the radio-optical loudness for quasars and BL Lacertae type objects (BL Lacs). A strong positive correlation is found between the radio luminosity and the gamma-ray-optical loudness for quasars, while a negative correlation between the optical luminosity and the gamma-ray-radio loudness is present for BL Lacs. Modeling of these correlations with a simple leptonic jet model for blazars indicates that variations of the accretion disk luminosity (and hence the jet power) is able to reproduce the trends observed in most of the correlations. To reproduce all observed correlations, variations of several parameters, such as the accretion power, jet viewing angle, Lorentz factor, and magnetic field of the jet, are required.
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