Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userBroad Line Radio Galaxies Observed with Fermi-LAT: The Origin of the GeV Gamma-Ray Emission
168
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
We report on a detailed investigation of the gamma-ray emission from 18 broad line radio galaxies (BLRGs) based on two years of Fermi Large Area Telescope (LAT) data. We confirm the previously reported detections of 3C 120 and 3C 111 in the GeV photon energy range; a detailed look at the temporal characteristics of the observed gamma-ray emission reveals in addition possible flux variability in both sources. No statistically significant gamma-ray detection of the other BLRGs was however found in the considered dataset. Though the sample size studied is small, what appears to differentiate 3C 111 and 3C 120 from the BLRGs not yet detected in gamma-rays is the particularly strong nuclear radio flux. This finding, together with the indications of the gamma-ray flux variability and a number of other arguments presented, indicate that the GeV emission of BLRGs is most likely dominated by the beamed radiation of relativistic jets observed at intermediate viewing angles. In this paper we also analyzed a comparison sample of high accretion-rate Seyfert 1 galaxies, which can be considered radio-quiet counterparts of BLRGs, and found none were detected in gamma-rays. A simple phenomenological hybrid model applied for the broad-band emission of the discussed radio-loud and radio-quiet type 1 active galaxies suggests that the relative contribution of the nuclear jets to the accreting matter is > 1 percent on average for BLRGs, whilst <0.1 percent for Seyfert 1 galaxies.
rate research
Read More
Fast radio bursts (FRBs) are one of the most exciting new mysteries of astrophysics. Their origin is still unknown, but recent observations seems to link them to Soft Gamma Repeaters and, in particular, to magnetar giant flares (MGFs). The recent detection of a MGF at GeV energies by the textit{Fermi} Large Area Telescope (LAT) motivated the search for GeV counterparts to the >100 currently known FRBs. Taking advantage of more than 12 years of textit{Fermi}-LAT data, we perform a search for gamma-ray emission from all the reported repeating and non-repeating FRBs. We analyse on different-time scales the textit{Fermi}-LAT data of each individual source separately, including a cumulative analysis on the repeating ones. In addition, we perform the first stacking analysis at GeV energies of this class of sources in order to constrain the gamma-ray properties of the FRBs that are undetected at high energies. The stacking analysis is a powerful method that allow a possible detection from below-threshold FRBs providing important information on these objects. In this talk we present the preliminary results of our study and we discuss their implications for the predictions of gamma-ray emission from this class of sources
We performed a systematic X-ray study of eight nearby $gamma$-ray bright radio galaxies with {em Suzaku} for understanding the origin of their X-ray emissions. The {em Suzaku} spectra for five of those have been presented previously, while the remaining three (M,87, PKS,0625$-$354, and 3C,78) are presented here for the first time. Based on the Fe-K line strength, X-ray variability, and X-ray power-law photon indices, and using additional information on the [O III] line emission, we argue for a jet origin of the observed X-ray emission in these three sources. We also analyzed five years of {em Fermi} Large Area Telescope (LAT) GeV gamma-ray data on PKS,0625$-$354 and 3C,78 to understand these sources within the blazar picture. We found significant $gamma$-ray variability in the former object. Overall, we note that the {em Suzaku} spectra for both PKS,0625$-$354 and 3C,78 are rather soft, while the LAT spectra are unusually hard when compared with other $gamma$-ray detected low-power (FR,I) radio galaxies. We demonstrate that the constructed broad-band spectral energy distributions of PKS,0625$-$354 and 3C,78 are well described by a one-zone synchrotron/synchrotron self-Compton model. The results of the modeling indicate lower bulk Lorentz factors compared to those typically found in other BL Lac objects, but consistent with the values inferred from modeling other LAT-detected FR,I radio galaxies. Interestingly, the modeling also implies very high peak ($sim 10^{16}$,Hz) synchrotron frequencies in the two analyzed sources, contrary to previously-suggested scenarios for FR I/BL Lac unification. We discuss the implications of our findings in the context of the FR,I/BL Lac unification schemes.
The middle-aged supernova remnant (SNR) CTB 37A is known to interact with several dense molecular clouds through the detection of shocked ${rm H_{2}}$ and OH 1720 MHz maser emission. In the present work, we use eight years of $textit Fermi$-LAT Pass 8 data, with an improved point-spread function and an increased acceptance, to perform detailed morphological and spectral studies of the $gamma$-ray emission toward CTB 37A from 200 MeV to 200 GeV. The best fit of the source extension is obtained for a very compact Gaussian model with a significance of 5.75$sigma$ and a 68% containment radius of $0.116^{circ}$ $pm$ $0.014^{circ}_{rm stat}$ $pm$ $0.017^{circ}_{rm sys}$ above 1 GeV, which is larger than the TeV emission size. The energy spectrum is modeled as a LogParabola, resulting in a spectral index $alpha$ = 1.92 $pm$ 0.19 at 1 GeV and a curvature $beta$ = 0.18 $pm$ 0.05, which becomes softer than the TeV spectrum above 10 GeV. The SNR properties, including a dynamical age of 6000 yr, are derived assuming the Sedov phase. From the multiwavelength modeling of emission toward the remnant, we conclude that the nonthermal radio and GeV emission is mostly due to the reacceleration of preexisting cosmic rays (CRs) by radiative shocks in the adjacent clouds. Furthermore, the observational data allow us to constrain the total kinetic energy transferred to the trapped CRs in the clouds. Based on these facts, we infer a composite nature for CTB 37A to explain the broadband spectrum and to elucidate the nature of the observed $gamma$-ray emission.
Almost 10 yr of $gamma$-ray observations with the Fermi Large Area Telescope (LAT) have revealed extreme $gamma$-ray outbursts from flat spectrum radio quasars (FSRQs), temporarily making these objects the brightest $gamma$-ray emitters in the sky. Yet, the location and mechanisms of the $gamma$-ray emission remain elusive. We characterize long-term $gamma$-ray variability and the brightest $gamma$-ray flares of six FSRQs. Consecutively zooming in on the brightest flares, which we identify in an objective way through Bayesian blocks and a hill-climbing algorithm, we find variability on subhour time scales and as short as minutes for two sources in our sample (3C279, CTA102) and weak evidence for variability at time scales less than the Fermi satellites orbit of 95 minutes for PKS1510-089 and 3C454.3. This suggests extremely compact emission regions in the jet. We do not find any signs for $gamma$-ray absorption in the broad-line region (BLR), which indicates that $gamma$-rays are produced at distances greater than hundreds of gravitational radii from the central black hole. This is further supported by a cross-correlation analysis between $gamma$-ray and radio/millimeter light curves, which is consistent with $gamma$-ray production at the same location as the millimeter core for 3C273, CTA102, and 3C454.3. The inferred locations of the $gamma$-ray production zones are still consistent with the observed decay times of the brightest flares if the decay is caused by external Compton scattering with BLR photons. However, the minute-scale variability is challenging to explain in such scenarios.
We report the detection of GeV gamma-ray emission from the molecular cloud complex that surrounds the supernova remnant (SNR) W44 using the Large Area Telescope (LAT) onboard Fermi. While the previously reported gamma-ray emission from SNR W44 is likely to arise from the dense radio-emitting filaments within the remnant, the gamma-ray emission that appears to come from the surrounding molecular cloud complex can be ascribed to the cosmic rays (CRs) that have escaped from W44. The non-detection of synchrotron radio emission associated with the molecular cloud complex suggests the decay of neutral pi mesons produced in hadronic collisions as the gamma-ray emission mechanism. The total kinetic energy channeled into the escaping CRs is estimated to be (0.3--3)x10^{50} erg, in broad agreement with the conjecture that SNRs are the main sources of Galactic CRs.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
