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Register a new userA decade of joint MOJAVE$-$Fermi AGN monitoring: localisation of the gamma-ray emission region
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Within the MOJAVE VLBA program (Monitoring of Jets in AGN with VLBA Experiments), we have accumulated observational data at 15 GHz for hundreds of jets in $gamma$-ray bright active galactic nuclei since the beginning of the Fermi scientific observations in August 2008. We investigated a time delay between the flux density of AGN parsec-scale radio emission at 15 GHz and 0.1$-$300 GeV Fermi LAT photon flux, taken from constructed light curves using weekly and adaptive binning. The correlation analysis shows that radio is lagging $gamma$-ray radiation by up to 8 months in the observers frame, while in the source frame, the typical delay is about 2-3 months. If the jet radio emission, excluding the opaque core, is considered, significant correlation is found at greater time lags. We supplement these results with VLBI kinematics and core shift data to conclude that the dominant high-energy production zone is typically located within the 15 GHz VLBA core at a distance of a few parsecs from the central nucleus.
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Westerlund 1 (Wd 1) is the most massive stellar cluster in the Galaxy and associated with an extended region of TeV emission. Here we report the results of a search for GeV gamma-ray emission in this region. The analysis is based on ~4.5 years of Fermi-LAT data and reveals significantly extended emission which we model as a Gaussian, resulting in a best-fit sigma of sigma_S = (0.475 +/- 0.05) deg and an offset from Wd 1 of ~1 deg. A partial overlap of the GeV emission with the TeV signal as reported by H.E.S.S. is found. We investigate the spectral and morphological characteristics of the gamma-ray emission and discuss its origin in the context of two distinct scenarios. Acceleration of electrons in a Pulsar Wind Nebula provides a reasonably natural interpretation of the GeV emission, but leaves the TeV emission unexplained. A scenario in which protons are accelerated in or near Wd 1 in supernova explosion(s) and are diffusing away and interacting with molecular material, seems consistent with the observed GeV and TeV emission, but requires a very high energy input in protons, ~10^51 erg, and rather slow diffusion. Observations of Wd 1 with a future gamma-ray detector such as CTA provide a very promising route to fully resolve the origin of the TeV and GeV emission in Wd 1 and provide a deeper understanding of the high-energy (HE) astrophysics of massive stellar clusters.
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.
We present the fourth in a series of catalogs of gamma-ray bursts (GRBs) observed with Fermis Gamma-Ray Burst Monitor (Fermi-GBM). It extends the six year catalog by four more years, now covering the ten year time period from trigger enabling on 2008 July 12 to 2018 July 11. During this time period GBM triggered almost twice a day on transient events of which we identifyied 2356 as cosmic GRBs. Additional trigger events were due to solar are events, magnetar burst activities, and terrestrial gamma-ray flashes. The intention of the GBM GRB catalog series is to provide updated information to the community on the most important observables of the GBM-detected GRBs. For each GRB the location and main characteristics of the prompt emission, the duration, peak flux, and fluence are derived. The latter two quantities are calculated for the 50-300 keV energy band, where the maximum energy release of GRBs in the instrument reference system is observed and also for a broader energy band from 10-1000 keV, exploiting the full energy range of GBMs low-energy detectors. Furthermore, information is given on the settings of the triggering criteria and exceptional operational conditions during years 7 to 10 in the mission. This fourth catalog is an official product of the Fermi-GBM science team, and the data files containing the complete results are available from the High-Energy Astrophysics Science Archive Research Center (HEASARC).
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.
In May - July 2014, the flat spectrum radio quasar 3C 454.3 exhibited strong flaring behaviour. Observations with the Large Area Telescope detector on-board the Fermi Gamma-ray Space Telescope captured the $gamma$-ray flux at energies 0.1 $leq E_{gamma}leq$ 300 GeV increasing fivefold during this period, with two distinct peaks in emission. The $gamma$-ray emission is analysed in detail, in order to study the emission characteristics and put constraints on the location of the emission region. We explore variability in the spectral shape of 3C 454.3, search for evidence of a spectral cutoff, quantify the significance of VHE emission and investigate whether or not an energy-dependence of the emitting electron cooling exists. $gamma$-ray intrinsic doubling timescales as small as $tau_{int} = 0.68$ $pm$ 0.01 h at a significance of > 5$sigma$ are found, providing evidence of a compact emission region. Significant $E_{gamma, emitted}geq$ 35 GeV and $E_{gamma, emitted}geq$ 50 GeV emission is also observed. The location of the emission region can be constrained to $rgeq1.3$ $times$ $R_{BLR}^{out}$, a location outside the broad-line region. The spectral variation of 3C 454.3 also suggests that these flares may be originating further downstream of the supermassive black hole than the emission before and after the flares.
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