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Register a new userSynchrotron-Self Compton Spectral Evolution of PKS 2155-304
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The high frequency peaked blazar PKS 2155-304 is one of the brightest and most intensively studied prototype of BL Lac objects. Gamma-rays from PKS 2155-304 have been detected from the MeV to TeV ranges. We computed a synchrotron self-Compton (SSC) model, based on the temporal behavior of the particles distribution, responsible for the high-energy emission. Using the available simultaneous multiwavelength data, we simulated the overall spectral energy distribution (SED) and the spectral variability of this source.
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We present the results of BeppoSAX observations of PKS 2155-304 during an intense gamma-ray flare. The source was in a high X-ray state. A temporal analysis of the data reveals a tendency of the amplitude of variations to increase with energy, and the presence of a soft lag with a timescale of the order 10^3 s. A curved continuum spectrum, with no evidence of spectral features, extends up to ~50 keV, while there is indication of a flatter component emerging at higher energies, consistent with the interpretation of the broad band spectral energy distribution (SED) as due to synchrotron self-Compton (SSC) emission from a single region. Notably, the fitting of the SED with such a model is consistent with an interpretation of the detected soft lag as due to radiative cooling, supporting the idea that radiation losses play an important role in variability. The observed shifts of the SED peaks between the lowest and highest flux levels can be accounted for by an increase of the break energy in the relativistic particle spectrum. The model predicts emission at TeV energies in good agreement with the recently reported detection.
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 ..
The X-ray selected BL Lac PKS 2155-304 has been observed using the University of Durham Mark 6 very high energy gamma ray telescope during 1998. We find no evidence for TeV emission during these recent observations when the X-ray flux was observed to be low. We have reconsidered our measurements made in 1997 November when PKS 2155-304 was in a bright X-ray state and extended X-ray and GeV gamma ray observations were made as part of a multiwavelength campaign. Comparisons are made of the VHE emission during this time with the available data from other wavelengths.
We present theoretical modelling for the very rapid TeV variability of PKS 2155--304 observed recently by the H.E.S.S. experiment. To explain the light-curve, where at least five flaring events were well observed, we assume five independent components of a jet that are characterized by slightly different physical parameters. An additional, significantly larger component is used to explain the emission of the source at long time scales. This component dominates the emission in the X-ray range, whereas the other components are dominant in the TeV range. The model used for our simulation describes precisely the evolution of the particle energy spectrum inside each component and takes into account light travel time effects. We show that a relatively simple synchrotron self-Compton scenario may explain this very rapid variability. Moreover, we find that absorption of the TeV emission inside the components due to the pair creation process is negligible.
We have performed an optical observation campaign on PKS 2155-304, whose aim was to determine the variability properties of this object on very short time scales in several photometric bands. We detected variability on time scales as short as 15 min. The Fourier properties of the light curves have been investigated using structure function analysis. The power spectra are well described by a power-law with an index -2.4. It is compatible with the index found in the X-ray domain. The value of this index shows that the light curves cannot be generated by a sum of exponential pulses. Using historical data, we find that the longest time scale of variability in the optical domain lies between 10 and 40 days. We find a strong correlation between flux and spectral index, which we interpret as the signature of an underlying constant component. As a result we do not find evidence of spectral variation for the active nucleus in the optical domain. A lag has been found between the light curves in different optical bands. The short-wavelength light curves lead the long-wavelength ones. The amplitude of the lag is about 40 min for a factor 2 in wavelength. Our results are compared with predictions from different models. None of them can explain naturally the set of results obtained with this campaign, but we bring out some clues for the origin of the variability.
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