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Register a new userX-ray Emission of Mkn 421: New Clues From Its Spectral Evolution. I. Temporal Analysis
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Authors
G. Fossati
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Mkn 421 was repeatedly observed with BeppoSAX in 1997-1998. This is the first of two papers where we present the results of a thorough temporal and spectral analysis of all the data available to us, focusing in particular on the flare of April 1998, which was simultaneously observed also at TeV energies. Here we focus on the time analysis, while the spectral analysis and physical interpretation are presented in the companion paper. The detailed study of the flare in different energy bands reveals very important new results: i) hard photons lag the soft ones by 2-3 ks -a behavior opposite to what is normally found in high energy peak BL Lacs X-ray spectra; ii) the flare light curve is symmetric in the softest X-ray band, while it becomes increasingly asymmetric at higher energies, with the decay being progressively slower than the rise; iii) the flux decay of the flare can be intrinsically achromatic if a stationary underlying emission component is present. The temporal and spectral information obtained challenge the simplest models currently adopted for the (synchrotron) emission and most importantly provide clues on the particle acceleration process.
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Mkn 421 was repeatedly observed with BeppoSAX in 1997-1998. The source showed a very rich phenomenology, with remarkable spectral variability. This is the second of two papers presenting the results of a thorough temporal and spectral analysis of all the data available to us, focusing in particular on the flare of April 1998, which was simultaneously observed also at TeV energies. The spectral analysis and correlations are presented in this paper, while the data reduction and timing analysis are the content of the companion paper. The spectral evolution during the flare has been followed over few ks intervals, allowing us to detect for the first time the peak of the synchrotron component shifting to higher energies during the rising phase, and then receding. This spectral analysis nicely confirms the delay of the flare at the higher energies, which in Paper I we quantified as a hard lag of a few ks. Furthermore, at the highest energies, evidence is found of variations of the inverse Compton component. The spectral and temporal information obtained challenge the simplest models currently adopted for the (synchrotron) emission and most importantly provide clues on the particle acceleration process. A scenario accounting for all the observational constraints is discussed, where electrons are injected at progressively higher energies during the development of the flare, and the achromatic decay is ascribed to the source light crossing time exceeding the particle cooling timescales.
The X-ray observations of Mkn 421 show significant spectral curvature that can be reproduced by a log-parabola function. The spectra can also be fitted by an analytical model considering synchrotron emission from an electron distribution that is accelerated at a shock front with an energy-dependent diffusion(EDD model). The spectral fit of NuSTAR and Swift-XRT observations using EDD model during different flux states reveal the model parameters are strongly correlated. We perform a detailed investigation of this correlation to decipher the information hidden underneath. The model predicts the synchrotron peak energy to be correlated with the peak spectral curvature which is consistent with the case of Mkn 421. Expressing the energy dependence of the diffusion in terms of the magnetohydrodynamic turbulence energy index, it appears the turbulence shifts from Kolmogorov/Kraichnan type to Bohm limit during high flux states. Further, the correlation between the best-fit parameters of EDD model lets us derive an expression for the product of source magnetic field(B) and jet Doppler factor($delta$) in terms of synchrotron and Compton peak energies. The synchrotron peak energy is obtained using the simultaneous Swift-XRT and NuSTAR observations; whereas, the Compton peak energy is estimated by performing a linear regression analysis of the archival spectral peaks. The deduced $delta$B varies over a wide range; however, it satisfies reasonably well with the values estimated solely from the spectral peak energies independent of the EDD model. This highlights the plausible connection between the microscopic description of the electron diffusion with the macroscopic quantities deciding the broadband spectrum of Mkn 421.
Mkn 421 was repeatedly observed with BeppoSAX in 1997-1998. We present highlights of the results of the thorough temporal and spectral analysis discussed by Fossati et al. (1999) and Maraschi et al. (1999), focusing on the flare of April 1998, which was simultaneously observed also at TeV energies. The detailed study of the flare in different energy bands reveals a few very important new results: (a) hard photons lag the soft ones by 2-3 ks *a behavior opposite to what is normally found in High energy peak BL Lacs X-ray spectra*; (b) the flux decay of the flare can be intrinsically achromatic if a stationary underlying emission component is present. Moreover the spectral evolution during the flare has been followed by extracting X-ray spectra on few ks intervals, allowing to detect for the first time the peak of the synchrotron component shifting to higher energies during the rising phase, and then receding. The spectral analysis confirms the delay in the flare at the higher energies, as above a few keV the spectrum changes only after the peak of the outburst has occurred. The spectral and temporal information obtained challenge the simplest models currently adopted for the (synchrotron) emission and most importantly provide clues on the particle acceleration process. A theoretical picture accounting for all the observational constraints is discussed, where electrons are injected at low energies and then progressively accelerated during the development of the flare.
The X-ray spectral curvature of blazars is traditionally explained by an empirical log-parabola function characterized by three parameters, namely the flux, curvature and spectral index at a given energy. Since their exact relationship with the underlying physical quantities is unclear, interpreting the physical scenario of the source through these parameters is difficult. To attain an insight on the X-ray spectral shape, we perform a detailed study of the X-ray spectra of the blazar MKN 421, using an analytical model where the electron diffusion from the particle acceleration site is energy-dependent. The resultant synchrotron spectrum is again determined by three parameters, namely, the energy index of the escape time scale, the quantity connecting the electron energy to the observed photon energy and the normalization. The X-ray observations of MKN 421, during July 2012 - April 2013 by NuSTAR and Swift-XRT are investigated using this model and we find a significant correlation between model parameters and the observational quantities. Additionally, a strong anti-correlation is found between the fit parameters defining the spectral shape, which was not evident from earlier studies using empirical models. This indicates the flux variations in MKN 421 and possibly other blazars, may arise from a definite physical process that needs to be further investigated.
We report the results of a new analysis of 13 wide band BeppoSAX observations of the BL Lac object Mkn 421. The data from LECS, MECS and PDS, covering an energy interval from 0.1 to over 100 keV, have been used to study the spectral variability of this source. We show that the energy distributions in different luminosity states can be fitted very well by a log-parabolic law F(E)=K*(E/E_1)^(-(a+b*Log(E/E_1))), which provides good estimates of the energy and flux of the synchrotron peak in the SED. In the first four short observations of 1997 Mkn 421 was characterized by a very stable spectral shape, with average values a=2.25 and b=0.45. In the observations of 1998 smaller values for both parameters, a~2.07 and b~0.34, were found and the peak energy in the SED was in the range 0.5-0.8 keV. Also in the observations of May 1999 and April-May 2000 the log-parabolic model gave a satisfactory description of the overall SED of Mkn 421. In particular, in the observations of spring 2000 the source was brighter than the other observations and showed a large change of the spectral curvature. Spectral parameters estimates gave a~1.8 and b~0.19 and the energy of the maximum in the SED moved to the range 1-5.5 keV. We give a possible interpretation of the log-parabolic spectral model in terms of particle acceleration mechanisms. An energy distribution of emitting particles with curvature close to the one observed can be explained by a simple model for statistical acceleration with the assumption that the probability for a particle to increase its energy is a decreasing function of the energy itself. A consequence of this mechanism is the existence of a linear relation between the spectral parameters a and b, well confirmed by the estimated values of these two parameters for Mkn421.(Abridged)
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