Fermi-LAT observation of nonblazar AGNs Sahakyan, N.; Baghmanyan, V.; Zargaryan, D.
Astronomy and astrophysics (Berlin),
06/2018, Volume:
614
Journal Article
Peer reviewed
Open access
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.
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, Volume:
574
Journal Article
Peer reviewed
Open access
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.
Room-temperature absorption and luminescence spectra of the
R
1
and
R
2
bands in a single crystal of Ruby Al
2
O
3
:Cr
3+
(0.05%) are analyzed. The luminescence in these bands was induced both by ...crystal excitation by radiation of a halogen lamp and by selective excitation of two upper levels of the
2
T
1
state by radiation of a tunable laser diode (656–662 nm). Four different shifts of the
R
1
and
R
2
bands and four different distances between these bands the change in which is a multiple of ~0.52 cm
–1
are discovered in spectra of se-lectively excited luminescence bands. Detailed comparative analysis of the obtained spectral bands of luminescence and corresponding absorption spectra allowed determining combined structure of each of the
R
1
and
R
2
bands formed by satellite doublets of these bands associated with all four stable isotopes
50
Cr
3+
,
52
Cr
3+
,
53
Cr
3+
, and
54
Cr
3–
of chromium ions. Splitting of these doublets varies from 7.04 to 9.14 сm
–1
depending on the isotope mass.
Capabilities of YAG ceramics doped with 10% Yb ions for operation as a radiation balanced laser are evaluated. Parameters of the optimal cooling and radiation balanced lasing for YAG-Yb
3+
ceramics ...are determined. It is shown, that YAG-Yb ceramics have better characteristics for optical cooling and lasing generation compared to ZBLANP-Yb
3+
and Rb
2
NaYF
6
-Yb
3+
.
The central region of the Milky Way is one of the foremost locations to look for dark matter (DM) signatures. We report the first results on a search for DM particle annihilation signals using new ...observations from an unprecedented gamma-ray survey of the Galactic Center (GC) region, i.e., the Inner Galaxy Survey, at very high energies (& GSIM;100 GeV) performed with the H.E.S.S. array of five ground-based Cherenkov telescopes. No significant gamma-ray excess is found in the search region of the 2014-2020 dataset and a profile likelihood ratio analysis is carried out to set exclusion limits on the annihilation cross section (sigma v). Assuming Einasto and Navarro-Frenk-White (NFW) DM density profiles at the GC, these constraints are the strongest obtained so far in the TeV DM mass range. For the Einasto profile, the constraints reach (sigma v) values of 3.7 x 10-26 cm3 s-1 for 1.5 TeV DM mass in the W+W- annihilation channel, and 1.2 x 10-26 cm3 s-1 for 0.7 TeV DM mass in the tau+tau- annihilation channel. With the H.E.S.S. Inner Galaxy Survey, ground-based gamma-ray observations thus probe (sigma v) values expected from thermal-relic annihilating TeV DM particles.
Context.
In recent years, winds from massive stars have been considered promising sites for investigating relativistic particle acceleration. In particular, the resulting bow-shaped shocks from the ...interaction of the supersonic winds of runaway stars with interstellar matter have been intensively observed at many different wavelengths, from radio to
γ
-rays.
Aims.
In this study we investigate the O4If star, BD+43° 3654, the bow shock of which is, so far, the only one proven to radiate both thermal and non-thermal emission at radio frequencies. In addition, we consider NGC 7635, the Bubble Nebula, as a bow shock candidate and examine its apex for indications of thermal and non-thermal radio emission.
Methods.
We observed both bow shocks in radio frequencies with the Very Large Array (VLA) in the C and X bands (4–8 GHz and 8–12 GHz) and with the Effelsberg telescope at 4–8 GHz. We analysed single-dish and interferometric results individually, in addition to their combined emission, obtained spectral index maps for each source, and calculated their spectral energy distributions.
Results.
We find that both sources emit non-thermal emission in the radio regime, with the clearest evidence for NGC 7635, whose radio emission has a strongly negative spectral index along the northern rim of the bubble. We present the first high-resolution maps of radio emission from NGC 7635, finding that the morphology closely follows the optical nebular emission. Our results are less conclusive for the bow shock of BD+43° 3654, as its emission becomes weaker and faint at higher frequencies in VLA data. Effelsberg data show a much larger emitting region (albeit a region of thermal emission) than is detected with the VLA for this source.
Conclusions.
Our results extend the previous radio results from the BD+43° 3654 bow shock to higher frequencies, and with our NGC 7635 results we double the number of bow shocks around O stars with detected non-thermal emission, from one to two. Modelling of the multi-wavelength data for both sources shows that accelerated electrons at the wind termination shock are a plausible source for the non-thermal radio emission, but energetics arguments suggest that any non-thermal X-ray and
γ
-ray emission could be significantly below existing upper limits. Enhanced synchrotron emission from compressed galactic cosmic rays in the radiative bow shock could also explain the radio emission from the BD+43° 3654 bow shock, but not from NGC 7635. The non-detection of point-like radio emission from BD+43° 3654 puts an upper limit on the mass-loss rate of the star that is lower than values quoted in the literature.
Context.
Young massive stellar clusters are extreme environments and potentially provide the means for efficient particle acceleration. Indeed, they are increasingly considered as being responsible ...for a significant fraction of cosmic rays (CRs) that are accelerated within the Milky Way. Westerlund 1, the most massive known young stellar cluster in our Galaxy, is a prime candidate for studying this hypothesis. While the very-high-energy
γ
-ray source HESS J1646−458 has been detected in the vicinity of Westerlund 1 in the past, its association could not be firmly identified.
Aims.
We aim to identify the physical processes responsible for the
γ
-ray emission around Westerlund 1 and thus to understand the role of massive stellar clusters in the acceleration of Galactic CRs better.
Methods.
Using 164 h of data recorded with the High Energy Stereoscopic System (H.E.S.S.), we carried out a deep spectromorphological study of the
γ
-ray emission of HESS J1646−458. We furthermore employed H I and CO observations of the region to infer the presence of gas that could serve as target material for interactions of accelerated CRs.
Results.
We detected large-scale (∼2° diameter)
γ
-ray emission with a complex morphology, exhibiting a shell-like structure and showing no significant variation with
γ
-ray energy. The combined energy spectrum of the emission extends to several tens of TeV, and it is uniform across the entire source region. We did not find a clear correlation of the
γ
-ray emission with gas clouds as identified through H I and CO observations.
Conclusions.
We conclude that, of the known objects within the region, only Westerlund 1 can explain the majority of the
γ
-ray emission. Several CR acceleration sites and mechanisms are conceivable and discussed in detail. While it seems clear that Westerlund 1 acts as a powerful particle accelerator, no firm conclusions on the contribution of massive stellar clusters to the flux of Galactic CRs in general can be drawn at this point.
PDF
