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explaining Jet Brightness / Power relation ?

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#1 Widdekind



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Posted 17 December 2012 - 05:10 AM

There is an observed relation, between the brightness & power of relativistic jets, common to Quasars & GRBs:




Could the following calculations help explain the same?


P = \frac{e^2}{6 \pi \epsilon_0 c} \left( \gamma^3 \dot{\beta} \right)^2


 = \frac{d}{dt} \left( \gamma m_e c^2 \right) = m_e c^2 \left( \gamma^3 \beta \dot{\beta} \right)




\gamma^3 \dot{\beta} = \frac{3 c}{2} \frac{m_e c^2}{\left( \frac{e^2}{4 \pi \epsilon_0} \right) } = \frac{3 c}{2 r_e} \beta


\frac{\gamma^3}{\beta} d\beta = \frac{3 c}{2 r_e} dt


\frac{\gamma^3}{\beta} \frac{d\gamma}{\frac{d\gamma}{d\beta}} = \frac{3 c}{2 r_e} dt


\frac{\gamma^3}{\beta} \frac{d\gamma}{\gamma^3 \beta} = \frac{3 c}{2 r_e} dt


\frac{\gamma^2}{\gamma^2-1} d\gamma = \frac{3 c}{2 r_e} dt


By partial fractions:


\frac{\gamma^2}{\gamma^2-1} = 1 + \frac{1/2}{\gamma-1} - \frac{1/2}{\gamma+1}


So the integration yields:


\Delta \left( \gamma + ln \left( \sqrt{\frac{\gamma - 1}{\gamma + 1}} \right) \right) = \frac{3 c}{2 r_e} \Delta t


For relativistic jets \gamma \gg 1, and most of the power is radiated early on, at high \gamma.  So:


\Delta \gamma \approx \frac{3 c}{2 r_e} \Delta t


\boxed{ t_{cool} \approx \frac{2 r_e}{3 c} \gamma_0 }


But the initial electron energy was also proportional to \gamma_0.  So, the average power of emissions is (calculated to be) constant at all energy scales:


\bar{P} \equiv \frac{E_0}{t_{cool}} \approx \frac{\gamma_0 m_e c^2}{ \frac{2 r_e}{3 c} \gamma_0 } = \frac{3 c}{2 r_e} m_e c^2 \approx 10^{10} W


Perhaps some similar sort of scale invariance, whereby the power emitted by decelerating electrons is quasi-constant, could account, for the observed Jet brightness / power relation.

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