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Asaf Pe'er

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Emission models from X-ray binaries: jet contribution

  Observations of the low/hard state of many X-ray binaries reveal a rich broad band spectrum, extending from the radio to the hard X-rays. The radio spectrum observed is typically flat (F_ν ∝ ν^0). At higher frequencies (typically from optical to the soft X-ray), the spectrum is thermal, dominated by the accretion disk and the companion star, while at higher energies (X- and γ-rays), a power law is typically observed.
  Traditionally, the flat radio spectra is explained as due to synchrotron emission from electrons that propagate along the jet. The key idea is that sycnhrotron emission from power-law distributed electrons have a characteristic break frequency (synchrotron self absorption), at the transition from the optically thin (at high frequencies) to optically thick (at low frequencies) emission. As the magnetic field decays along the jet(due to Poynting flux conservation), this break frequency shifts to lower and lower frequencies. For unresolved jets, the integrated spectra from different jet regions is flat. This idea was first suggested by Blandford & Kongil (1979), and is in wide use until today.
  This model, however, suffers several drawbacks: for example, it does not consider radiative and adiabatic energy losses (or assumed that they are replenished). This had motivated us to investigate a different model, in which electrons are assumed to be accelerated once, at the base of the jet, and lose their energy while propagating along the jet. Furthermore, we have assumed that a power law exists only above a low energy Maxwellian. Using these assumptions, we were able to show that a flat radio spectrum is a natural outcome - but due to the peak frequency of emission from the Maxwellian, rather than the self absorption frequency. Thus, a power law is not even needed !. This work is done in collaboration with P. Casella.
  Furthermore, we showed that an X-ray spectrum F_ν ∝ ν^{-1/2} is a natural outcome resulting from fast cooling in strong magnetic field close to the jet base; this was further extended (in collaboration with S. Markoff) to show that at least part of the X-ray spectra may also have jet origin. In addition, our model provides a natural explanation to the outliers in the radio-X correlation observed in these objects.
Selected Publications
  • Pe'er, A., & Casella, P. (2009), "A Model for Emission from Jets in X-Ray Binaries: Consequences of a Single Acceleration Episode" Ap.J., 699, 1919
  • Casella, P., & Pe'er, A. (2009), "On the Role of the Magnetic Field on Jet Emission in X-Ray Binaries" Ap.J., 703, L63
  • Pe'er, A., & Markoff, S. (2012), "X-Ray Emission from Transient Jet Model in Black Hole Binaries" Ap.J., 753, 177
  • Pe'er, A. (2013), "Energetic and Broad Band Spectral Distribution of Emission from Astronomical Jets" Space Science Review, in press

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