Swift observations of the very intense flaring activity of Mrk 421 during 2006. I. Phenomenological picture of electron acceleration and predictions for MeV/GeV emission

By A. Giommi, M. Perri, F. Verrecchia, G. Torino, Italy., CA SLAC-, Usa, A. S. D. C. -. I. D. D. F. Perugia, I. Perugia. 2009

202 citations recorded by the source corpus.

Aims. We present the results of a deep spectral analysis of all Swift observations of Mrk 421 between April 2006 and July 2006, when it reached its highest X-ray flux recorded until the end of 2006. The peak flux was about 85 milli-Crab in the 2.0–10.0 keV band, and the peak energy (Ep) of the spectral energy distribution (SED) was often at energies higher than 10 keV. We study trends between the spectral parameters, and the physical insights the parameters provide into the underlying acceleration and emission mechanisms. Methods. We performed a spectral analysis of Swift observations to investigate trends between the spectral parameters. We searched for acceleration and energetic features phenomenologically linked to the SSC model parameters, by predicting their effects in the γ-ray band, and in particular, the spectral shape expected in the Fermi Gamma-ray Space Telescope-LAT band. Results. We confirm that the X-ray spectrum is described well by a log-parabolic distribution close to Ep, that the peak flux of the SED (S p) is correlated with Ep ,a nd thatEp is anti-correlated with the curvature parameter (b). The spectral evolution in the Hardness-ratio-flux plane shows both clockwise and counter-clockwise patterns. During the most energetic flares, the UV-to-soft-X-ray spectral shape requires an electron distribution spectral index of s � 2.3. Conclusions. We demonstrate that the UV-to-X-ray emission from Mrk 421 is probably generated by a population of electrons that is actually curved, and has a low energy power-law tail. The observed spectral curvature is consistent with both stochastic acceleration or energy-dependent acceleration probability mechanisms, whereas the power-law slope of XRT-UVOT data is close to that inferred from the GRBs X-ray afterglow and in agreement with the universal first-order relativistic shock acceleration models. This scenario implies that magnetic turbulence may play a twofold role: spatial diffusion relevant to the first order process and momentum diffusion relevant to the second order process.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/964c2521116b9ba862850642b55f111c1616b7d3

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In Astronomy and Astrophysics

1. Bing Zhang
2. F. Tavecchio
3. Y. Kovalev
4. E. Orlando
5. C. Dermer
6. M. Lister
7. A. Pushkarev
8. L. Maraschi

In Nuclear and High Energy Physics

1. F. Tavecchio
2. C. Dermer
3. E. Orlando
4. Y. Kovalev
5. M. Lister
6. L. Maraschi
7. A. Pushkarev
8. P. Padovani

Citations in the corpus

• THE SPECTRAL ENERGY DISTRIBUTIONS OF FERMI BLAZARS (2016, methodology): “Our statistical results confirm the anti-correlation (Massaro et al. 2004, Tramacere et al. 2007, 2009), and this kind of anti-correlation is a strong signature of a stochastic component in the acceleration (Tramacere et al. 2011).”
• STOCHASTIC ACCELERATION AND THE EVOLUTION OF SPECTRAL DISTRIBUTIONS IN SYNCHRO-SELF-COMPTON SOURCES: A SELF-CONSISTENT MODELING OF BLAZARS’ FLARES (2011, methodology): “…of a large collection of X-ray observations of Mkn 421, Tramacere et al. (2007); Tramacere (2007); Electronic address: andrea.tramacere@unige.ch Tramacere et al. (2009) pointed out that the observed anticorrelation between the peak energy and the curvature measured in the synchrotron Spectral…”
• A simplified view of blazars: clearing the fog around long‐standing selection effects (2011, methodology): “(iii) Non-thermal component To represent the non-thermal/jet component, we assume a simple homogeneous synchrotron self-Compton model (SSC, see, e.g. Tramacere et al. 2009, and references therein) with relativistic electrons distributed as a power law at low energies and as a log-parabola at high…”
• INSIGHTS INTO THE HIGH-ENERGY γ-RAY EMISSION OF MARKARIAN 501 FROM EXTENSIVE MULTIFREQUENCY OBSERVATIONS IN THE FERMI ERA (2011, background): “However, some smoothly curved spectral shape might perhaps be an alternative representation of the electron spectrum (e.g., Stawarz & Petrosian 2008; Tramacere et al. 2009).”
• Simultaneous Planck, Swift, and Fermi observations of X-ray and γ-ray selected blazars (2011, methodology): “When deviations from a single power-law model were found, we adopted a log-parabolic law of the form F(E) = KE(a+b log E) (Massaro et al. 2004), which has been shown to fit the X-ray spectrum of blazars of the HSP type well (e.g. Giommi et al. 2005; Tramacere et al. 2009).”