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Register a new userNine years of multi-frequency monitoring of the blazar PKS 0048-097: spectral and temporal variability
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Alicja Wierzcholska
Publication date
2016
fields
Physics
and research's language is
English
Authors
Alicja Wierzcholska
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Blazars are highly variable, radio-loud active galactic nuclei with jets oriented at a small angle to the line of sight. The observed emission of these sources covers the whole electromagnetic spectrum from radio frequencies up to the high or even very high energy gamma-ray range. To understand the complex physics of these objects, multi-wavelength observations and studies on the variability and correlations between different wavelengths are therefore essential. The long-term multi-frequency observations of PKS 0048-097 are analysed here to investigate its spectral and temporal features. The studies includes nine years of observations of the blazar, which is well studied in the optical and radio domain, but not in the other frequencies. Multi-wavelength data collected with OVRO, KAIT, Catalina, Swift/UVOT, Swift/XRT and Fermi/LAT were studied. The performed analysis revealed strong variability in all wavelengths that is most clearly manifested in the X-ray range. The correlation studies do not exhibit any relation between different wavelengths, except for the very strong positive correlation between the optical emission in V and R bands.
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A study of the gravitationally lensed blazar PKS 1830-211 was carried out using multi waveband data collected by Fermi-LAT, Swift-XRT and Swift-UVOT telescopes between MJD 58400 to MJD 58800 (9 Oct 2018 to 13 Nov 2019). Flaring states were identified by analysing the gamma-ray light curve. Simultaneous multi-waveband SED were obtained for those flaring periods. A cross-correlation analysis of the multi-waveband data was carried out, which suggested a common origin of the gamma-ray and X-ray emission. The broadband emission mechanism was studied by modelling the SED using a leptonic model. Physical parameters of the blazar were estimated from the broadband SED modelling. The blazar PKS 1830-211 is gravitationally lensed by at least two galaxies and has been extensively studied in the literature because of this property. The self-correlation of the gamma-ray light curve was studied to identify the signature of lensing, but no conclusive evidence of correlation was found at the expected time delay of 26 days.
Observations of very high energy gamma-rays from blazars provide information about acceleration mechanisms occurring in their innermost regions. Studies of variability in these objects allow a better understanding of the mechanisms at play. To investigate the spectral and temporal variability of VHE (>100 GeV) gamma-rays of the well-known high-frequency-peaked BL Lac object PKS 2155-304 with the H.E.S.S. imaging atmospheric Cherenkov telescopes over a wide range of flux states. Data collected from 2005 to 2007 are analyzed. Spectra are derived on time scales ranging from 3 years to 4 minutes. Light curve variability is studied through doubling timescales and structure functions, and is compared with red noise process simulations. The source is found to be in a low state from 2005 to 2007, except for a set of exceptional flares which occurred in July 2006. The quiescent state of the source is characterized by an associated mean flux level of 4.32 +/-0.09 x 10^-11 cm^-2 s^-1 above 200 GeV, or approximately 15% of the Crab Nebula, and a power law photon index of 3.53 +/-0.06. During the flares of July 2006, doubling timescales of ~2 min are found. The spectral index variation is examined over two orders of magnitude in flux, yielding different behaviour at low and high fluxes,which is a new phenomenon in VHE gamma-ray emitting blazars. The variability amplitude characterized by the fractional r.m.s. is strongly energy-dependent and is proportional to E^(0.19 +/- 0.01). The light curve r.m.s. correlates with the flux. This is the signature of a multiplicative process which can be accounted for as a red noise with a Fourier index of ~2. This unique data set shows evidence for a low level gamma-ray emission state from PKS 2155-304, which possibly has a different origin than the outbursts. The discovery of the light curve lognormal behaviour might be an indicator ..
Time variability of the photon flux is a known feature of active galactic nuclei (AGN) and in particular of blazars. The high frequency peaked BL Lac (HBL) object PKS 2155-304 is one of the brightest sources in the TeV band and has been monitored regularly with different instruments and in particular with the H.E.S.S. experiment above 200 GeV for more than 11 years. These data together with the observations of other instruments and monitoring programs like SMARTS (optical), Swift-XRT/RXTE/XMM-Newton (X-ray) and Fermi-LAT (100 MeV < E < 300 GeV) are used to characterize the variability of this object in the quiescent state over a wide energy range. Variability studies are made by looking at the lognormality of the light curves and at the fractional root mean square (rms) variability Fvar in several energy bands. Lognormality is found in every energy range and the evolution of Fvar with the energy shows a similar increase both in X-rays and in TeV bands.
We present a multi-wavelength temporal analysis of the blazar 3C 454.3 during the high $gamma$-ray active period from May-December, 2014. Except for X-rays, the period is well sampled at near-infrared (NIR)-optical by the emph{SMARTS} facility and the source is detected continuously on daily timescale in the emph{Fermi}-LAT $gamma$-ray band. The source exhibits diverse levels of variability with many flaring/active states in the continuously sampled $gamma$-ray light curve which are also reflected in the NIR-optical light curves and the sparsely sampled X-ray light curve by the emph{Swift}-XRT. Multi-band correlation analysis of this continuous segment during different activity periods shows a change of state from no lags between IR and $gamma$-ray, optical and $gamma$-ray, and IR and optical to a state where $gamma$-ray lags the IR/optical by $sim$3 days. The results are consistent with the previous studies of the same during various $gamma$-ray flaring and active episodes of the source. This consistency, in turn, suggests an extended localized emission region with almost similar conditions during various $gamma$-ray activity states. On the other hand, the delay of $gamma$-ray with respect to IR/optical and a trend similar to IR/optical in X-rays along with strong broadband correlations favor magnetic field related origin with X-ray and $gamma$-ray being inverse Comptonized of IR/optical photons and external radiation field, respectively.
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