Fermi-LAT observation of nonblazar AGNs Sahakyan, N.; Baghmanyan, V.; Zargaryan, D.
Astronomy and astrophysics (Berlin),
06/2018, Letnik:
614
Journal Article
Recenzirano
Odprti dostop
Context. Fermi Large Area Telescope (Fermi-LAT) has recently detected γ-ray emission from active galactic nuclei (AGN) that do not show clear evidence for optical blazar characteristics or have jets ...pointing away from the observer (nonblazar AGNs). These are interesting γ-ray emitters providing an alternative approach to studying high energy emission processes. Aims. This paper investigates the spectral and temporal properties of γ-ray emission from nonblazar AGNs using the recent Fermi-LAT observational data. Methods. The data collected by Fermi-LAT during 2008–2015, from the observations of 26 nonblazar AGNs, including 11 Fanaroff–Riley Type I (FRI) and ten FRII radio galaxies and steep spectrum radio quasars (SSRQs) and five narrow line seyfert 1s (NLSy1s) are analysed using the new PASS 8 event selection and instrument response function. Possible spectral changes above GeV energies are investigated with a detailed spectral analysis. Light curves generated with normal and adaptive time bins are used to study the γ-ray flux variability. Results. Non-blazar AGNs have a γ-ray photon index in the range of 1.84–2.86 and a flux varying from a few times 10−9 photon cm−2 s−1 to 10−7 photon cm−2 s−1. Over long time periods, the power law provides an adequate description of the γ-ray spectra of almost all sources. Significant curvature is observed in the γ-ray spectra of NGC 1275, NGC 6251, SBS 0846 + 513, and PMN J0948 + 0022 and their spectra are better described by log parabola or by the power law with exponential cut-off models. The γ-ray spectra of PKS 0625-25 and 3C 380 show a possible deviation from a simple power-law shape, indicating a spectral cut-off around the observed photon energy of Ecut = 131.2 ± 88.04 GeV and Ecut = 55.57 ± 50.74 GeV, respectively. Our analysis confirms the previous finding of an unusual spectral turnover in the γ-ray spectrum of Cen A: the photon index changes from Γ = 2.75 ± 0.02 to 2.31 ± 0.1 at 2.35 ± 0.08 GeV. In the Γ−Lγ plane, the luminosity of nonblazar AGNs is spread in the range of (1041–1047) erg s−1, where those with the lowest luminosity are FRI radio galaxies (but typically appear with a harder photon index) and those with the highest luminosity have are SSRQs/NLSY1s (with softer photon indexes). We confirm the previously reported short-timescale flux variability of NGC 1275 and 3C 120. The γ-ray emission from NLSY1s, 1H 0323 + 342, SBS 0846 + 513, and PMN J0948 + 0022 is variable, showing flares in short scales sometimes accompanied by a moderate hardening of their spectra (e.g., for MJD 56146.8 the γ-ray photon index of SBS 0846 + 513 was Γ = 1.73 ± 0.14). Non-blazar AGNs 3C 111, Cen A core, 3C 207, 3C 275.1, 3C 380, 4C + 39.23B, PKS 1502 + 036, and PKS 2004-447 show a long-timescale flux variability in the γ-ray band.
Rapid Gamma-Ray Variability of NGC 1275 Baghmanyan, V.; Gasparyan, S.; Sahakyan, N.
The Astrophysical journal,
10/2017, Letnik:
848, Številka:
2
Journal Article
Recenzirano
Odprti dostop
We report on a detailed analysis of the γ-ray light curve of NGC 1275 using the Fermi Large Area Telescope data accumulated during 2008-2017. Major γ-ray flares were observed in 2015 October and 2016 ...December/2017 January when the source reached a daily peak flux of , achieving a flux of within 3 hr, which corresponds to an apparent isotropic γ-ray luminosity of . The most rapid flare had an e-folding time as short as hr, which had never been previously observed for any radio galaxy in γ-ray band. Also, γ-ray spectral changes were observed during these flares: in the flux versus photon index plane, the spectral evolution follows correspondingly a counterclockwise and a clockwise loop inferred from the light curve generated by an adaptive binning method. On 2016 December 30 and 2017 January 1 the X-ray photon index softened ( ) and the flux increased nearly ∼3 times as compared with the quiet state. The observed hour-scale variability suggests a very compact emission region ( ), implying that the observed emission is most likely produced in the subparsec-scale jet if the entire jet width is responsible for the emission. During the active periods, the γ-ray photon index hardened, shifting the peak of the high-energy spectral component to , making it difficult to explain the observed X-ray and γ-ray data in the standard one-zone synchrotron self-Compton model.
We report on broadband observations of CTA 102 (z = 1.037) during the active states in 2016-2017. In the γ-ray band, Fermi-LAT observed several prominent flares that followed a harder-when-brighter ...behavior: the hardest photon index Γ = 1.61 0.10 being unusual for flat-spectrum radio quasars. The peak γ-ray flux above 100 MeV, observed on MJD 57,738.47 within 4.31 minutes, corresponds to an isotropic γ-ray luminosity of , comparable to the highest values observed from blazars so far. The analyses of the Swift UVOT/XRT data show an increase in the UV/optical and X-ray bands that is contemporaneous with the bright γ-ray periods. The X-ray spectrum observed by Swift XRT and NuSTAR during the γ-ray flaring period is characterized by a hard photon index of ∼1.30. The shortest e-folding time was 4.08 1.44 hr, suggesting a very compact emission region cm. We modeled the spectral energy distribution of CTA 102 in several periods (having different properties in UV/optical, X-ray, and γ-ray bands) assuming a compact blob inside and outside the BLR. We found that the high-energy data are better described when the infrared thermal radiation of the dusty torus is considered. In the flaring periods when the correlation between the γ-ray and UV/optical/X-ray bands is lacking, the γ-ray emission can be produced from the interaction of fresh electrons in a different blob, which does not make a dominant contribution at lower energies.
Context. Important information on the evolution of a jet can be obtained by comparing the physical state of the plasma at its propagation through the broad-line region (where the jet is most likely ...formed) into the intergalactic medium, where it starts to decelerate significantly. Aims. We compare the constraints on the physical parameters in the innermost (≤pc) and outer (≥kpc) regions of the 3C 120 jet by means of a detailed multiwavelength analysis and theoretical modeling of their broadband spectra. Methods. The data collected by Fermi LAT (γ-ray band), Swift (X-ray and ultraviolet bands), and Chandra (X-ray band) are analyzed together and the spectral energy distributions are modeled using a leptonic synchrotron and inverse Compton model, taking into account the seed photons originating inside and outside the jet. The model parameters are estimated using the Markov chain Monte Carlo method. Results. The γ-ray flux from the inner jet of 3C 120 was characterized by rapid variation from MJD 56 900 to MJD 57 300. Two strong flares were observed on April 24, 2015, when within 19.0 min and 3.15 h the flux was as high as (7.46 ± 1.56) × 10-6 photon cm-2 s-1 and (4.71 ± 0.92) × 10-6 photon cm-2 s-1, respectively, with ≥10σ. During these flares the apparent isotropic γ-ray luminosity was Lγ ≃ (1.20−1.66) × 1046 erg s-1 which is not common for radio galaxies. The broadband emission in the quiet and flaring states can be described as synchrotron self-Compton emission, while inverse Compton scattering of dusty torus photons cannot be excluded for the flaring states. The X-ray emission from the knots can be reproduced by inverse Compton scattering of cosmic microwave background photons only if the jet is highly relativistic (even when δ = 10,Ue/UB is still ≥80). These extreme requirements can be somewhat softened assuming the X-rays are from the synchrotron emission of a second population of very high energy electrons. Conclusions. We found that the jet power estimated at two scales is consistent, suggesting that the jet does not suffer severe dissipation, it simply becomes radiatively inefficient.
On the gamma-ray emission from 3C 120 Sahakyan, N.; Zargaryan, D.; Baghmanyan, V.
Astronomy and astrophysics (Berlin),
02/2015, Letnik:
574
Journal Article
Recenzirano
Odprti dostop
We report the analysis of Fermi Large Area Telescope data from five years of observations of the broad line radio galaxy 3C 120. The accumulation of a larger data set results in the detection of high ...energy γ-rays up to 10 GeV, with a detection significance of about 8.7σ. A power law spectrum with a photon index of 2.72 ± 0.1 and an integrated flux of Fγ = (2.35 ± 0.5) × 10-8photoncm-2 s-1 above 100 MeV well describe the data averaged over five years of observations. The variability analysis of the light curve with 180, and 365 day bins reveals flux increase (nearly twice its average level) during the last year of observation. This variability on month timescales indicates the compactness of the emitting region. The γ-ray spectrum can be described as synchrotron self-Compton emission from the electron population producing the radio-to-X-ray emission in the jet. The required electron energy density exceeds the magnetic field energy density only by a factor of 2, meaning there is no significant deviation from equipartition.
We report on anomalously long duration (2 ms) count rate bursts following the impact of cosmic ray showers near a 7.62 cm x⊘7.62 cm LaBr3 scintillation detector at the High Altitude Water Cherenkov ...array in Mexico, previously described by Stenkin et al. (2001), and termed “neutron bursts.” The largest burst produced 198 counts within 2 ms in our LaBr3 detector. We simulate the neutron burst albedo flux (that is, secondary emissions from an extensive air shower core impacting the ground), and show that (1) the characteristic spectra and count rates are well explained by neutron absorption in the ground and (2) any cosmic ray secondary that produces neutrons, either through hadron inelastic collisions, or photoneutron production by gamma‐rays, produces the same characteristic spectra. This implies that other natural phenomena that produce downward beams of gamma‐rays, like Terrestrial gamma ray flashes, should produce a similar “neutron burst” signature from the photoneutron reactions occurring in the soil.
Plain Language Summary
When very large cosmic ray showers (CRS) impact the ground, neutrons are produced in the soil that will rattle around until they become captured by soil particles and release energetic gamma‐rays. This produces a slow explosion of particles emanating from the ground following a CRS impact, and is termed a 'neutron burst'. We present recent observations of neutron bursts from a hand held sized gamma‐ray detector at the High Altitude Water Cherenkov (HAWC) array in Mexico, that exhibit interesting spectral features (the presence of positron annihilation), and an interesting time structure (hundreds of counts within a few ms). Our simulations indicate that Terrestrial gamma‐ray flashes (TGFs, bursts of gamma‐rays associated with lightning) should also produce these neutron bursts. An implication of this work is that existing deployments of ground based TGF instruments, comprised of small gamma‐ray detectors, can additionally be used to observe signatures of large cosmic ray showers on clear days.
Key Points
We report on fairweather count rate bursts with 2 ms duration following the impact of a large cosmic ray shower near a small scintillation detector at HAWC
Simulations show that the spectra and decay time can be produced by either hadronic interactions, or photoneutron reactions from gamma‐rays
These results imply that downward TGFs could produce a similar delayed neutron signature in the soil near ground based detectors
We present the γ -ray observations of the radio galaxy PKS 0625-35, using the Fermi Large Area Telescope data accumulated during 2008-2017. γ -rays up to 100 GeV have been detected with a detection ...significance of about 32.3σ . A power law spectrum with a photon index of 1.88±0.04 and an integrated flux of F
γ
= (1.02 ± 0.10)×10
-8
photon cm-2 s-1 above 100 MeV well describes the data averaged over 9 years of observations. There is a hint of deviation from a simple power-law shape around tens of GeV energies; however, the low statistics does not allow one to reject power law modeling. The spectral energy distributions during high and low X-ray states are modeled using one-zone leptonic models that include the synchrotron, synchrotron self Compton processes; the model parameters are estimated using the Markov Chain Monte Carlo method. The modeling shows that in the jet of PKS 0625-35 the particles (electrons) are accelerated to energies higher than 50 TeV.
We present the gamma-ray observations of the radio galaxy PKS 0625-35, using the Fermi Large Area Telescope data accumulated during 2008-2017. gamma-rays up to 100 GeV have been detected with a ...detection significance of about 32.3sigma. A power law spectrum with a photon index of 1.88 + or - 0.04 and an integrated flux of F.sub.gamma = (1.02 + or - 0.10) x 10.sup.-8 photon cm.sup.-2 s.sup.-1 above 100 MeV well describes the data averaged over 9 years of observations. There is a hint of deviation from a simple power- law shape around tens of GeV energies; however, the low statistics does not allow one to reject power law modeling. The spectral energy distributions during high and low X-ray states are modeled using one-zone leptonic models that include the synchrotron, synchrotron self Compton processes; the model parameters are estimated using the Markov Chain Monte Carlo method. The modeling shows that in the jet of PKS 062535 the particles (electrons) are accelerated to energies higher than 50 TeV. Keywords: PKS 0625-35: gamma-rays: radio galaxy
We report on a detailed investigation of the \(\gamma\)-ray emission from 26 non-blazar AGNs based on the Fermi LAT data accumulated for 7 years. The photon index of non-blazar AGNs changes in the ...range of 1.84-2.86 and the flux varies from a few times \(10^{-9} photon\: cm^{-2} s^{-1}\) to \(10^{-7} photon\: cm^{-2}s^{-1}\). Over long time periods, power-law provides an adequate description of the \(\gamma\)-ray spectra of almost all sources. Significant curvature is observed in the \(\gamma\)-ray spectra of NGC 1275, NGC 6251, SBS 0846+513 and PMN J0948+0022 and their spectra are better described by log-parabola or power-law with exponential cut-off models. The \(\gamma\)-ray spectra of PKS 0625-25 and 3C 380 show a possible deviation from a simple power-law shape, indicating a spectral cutoff around the observed photon energy of \(E_{cut}=131.2\pm88.04\) GeV and \(E_{cut}=55.57\pm50.74\) GeV, respectively. Our analysis confirms the previous finding of an unusual spectral turnover in the \(\gamma\)-ray spectrum of Cen A: the photon index changes from \(2.75\pm0.02\) to \(2.31\pm0.1\) at \(2.35\pm0.08\) GeV. In the \(\Gamma-L_{\gamma}\) plane, the luminosity of non-blazar AGNs is spread in the range of \(10^{41}-10^{47}\: erg\: s^{-1}\), where the lowest luminosity have FRI radio galaxies (but typically appear with a harder photon index) and the highest- SSRQs/NLSY1s (with softer photon indexes). We confirm the previously reported short-timescale flux variability of NGC 1275 and 3C 120. The \(\gamma\)-ray emission from NLSY1s, 1H 0323+342, SBS 0846+513 and PMN J0948+0022, is variable, showing flares in short scales sometimes accompanied by a moderate hardening of their spectra (e.g., on MJD 56146.8 the \(\gamma\)-ray photon index of SBS 0846+513 was \(1.73\pm0.14\)). 3C 111, Cen A core, 3C 207, 3C 275.1, 3C 380, 4C+39.23B, PKS 1502+036 and PKS 2004-447 show a long-timescale flux variability in the \(\gamma\)-ray band.
We report on a detailed analysis of the \(\gamma\)-ray light curve of NGC 1275 using the Fermi large area telescope data accumulated in 2008-2017. Major \(\gamma\)-ray flares were observed in October ...2015 and December 2016/January 2017 when the source reached a daily peak flux of \((2.21\pm0.26)\times10^{-6}\:{\rm photon\:cm^{-2}\:s^{-1}}\), achieving a flux of \((3.48\pm0.87)\times10^{-6}\:{\rm photon\:cm^{-2}\:s^{-1}}\) within 3 hours, which corresponds to an apparent isotropic \(\gamma\)-ray luminosity of \(\simeq3.84\times10^{45}\:{\rm erg\:s^{-1}}\). The most rapid flare had e-folding time as short as \(1.21\pm0.22\) hours which had never been previously observed for any radio galaxy in \(\gamma\)-ray band. Also \(\gamma\)-ray spectral changes were observed during these flares: in the flux versus photon index plane the spectral evolution follows correspondingly a counter clockwise and a clockwise loop inferred from the light curve generated by an adaptive binning method. On December 30, 2016 and January 01, 2017 the X-ray photon index softened (\(\Gamma_{\rm X}\simeq 1.75-1.77\)) and the flux increased nearly \(\sim3\) times as compared with the quiet state. The observed hour-scale variability suggests a very compact emission region (\(R_\gamma\leq5.22\times10^{14}\:(\delta/4)\:{\rm cm}\)) implying that the observed emission is most likely produced in the subparsec-scale jet if the entire jet width is responsible for the emission. During the active periods the \(\gamma\)-ray photon index hardened, shifting the peak of the high energy spectral component to \(>{\rm GeV}\), making it difficult to explain the observed X-ray and \(\gamma\)-ray data in the standard one-zone synchrotron self-Compton model.
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