The synchrotron maser emission from relativistic magnetized shocks: Dependence on the pre-shock temperature. (arXiv:2006.03081v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Babul_A/0/1/0/all/0/1">Aliya-Nur Babul</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Sironi_L/0/1/0/all/0/1">Lorenzo Sironi</a>
Electromagnetic precursor waves generated by the synchrotron maser
instability at relativistic magnetized shocks have been recently invoked to
explain the coherent radio emission of Fast Radio Bursts. By means of
two-dimensional particle-in-cell simulations, we explore the properties of the
precursor waves in relativistic electron-positron perpendicular shocks as a
function of the pre-shock magnetization $sigmagtrsim 1$ (i.e., the ratio of
incoming Poynting flux to particle energy flux) and thermal spread $Delta
gamma equiv kT/mc^2=10^{-5}-10^{-1}$. We measure the fraction $f_xi$ of
total incoming energy that is converted into precursor waves, as computed in
the post-shock frame. At fixed magnetization, we find that $f_{xi}$ is nearly
independent of temperature as long as $Delta gamma lesssim 10^{-1.5}$ (with
only a modest decrease of a factor of three from $Deltagamma =10^{-5}$ to
$Deltagamma =10^{-1.5}$), but it drops by nearly two orders of magnitude for
$Delta gamma gtrsim 10^{-1}$. At fixed temperature, the scaling with
magnetization $f_xisim 10^{-3},sigma^{-1}$ is consistent with our earlier
one-dimensional results. For our reference $sigma=1$, the power spectrum of
precursor waves is relatively broad (fractional width $sim 1-3$) for cold
temperatures, whereas it shows pronounced line-like features with fractional
width $sim 0.2$ for $10^{-3} lesssim Delta gamma lesssim 10^{-1.5} $. For
$sigmagtrsim 1$, the precursor waves are beamed within an angle $simeq
sigma^{-1/2}$ from the shock normal (as measured in the post-shock frame), as
required so they can outrun the shock. Our results can provide
physically-grounded inputs for FRB emission models based on maser emission from
relativistic shocks.
Electromagnetic precursor waves generated by the synchrotron maser
instability at relativistic magnetized shocks have been recently invoked to
explain the coherent radio emission of Fast Radio Bursts. By means of
two-dimensional particle-in-cell simulations, we explore the properties of the
precursor waves in relativistic electron-positron perpendicular shocks as a
function of the pre-shock magnetization $sigmagtrsim 1$ (i.e., the ratio of
incoming Poynting flux to particle energy flux) and thermal spread $Delta
gamma equiv kT/mc^2=10^{-5}-10^{-1}$. We measure the fraction $f_xi$ of
total incoming energy that is converted into precursor waves, as computed in
the post-shock frame. At fixed magnetization, we find that $f_{xi}$ is nearly
independent of temperature as long as $Delta gamma lesssim 10^{-1.5}$ (with
only a modest decrease of a factor of three from $Deltagamma =10^{-5}$ to
$Deltagamma =10^{-1.5}$), but it drops by nearly two orders of magnitude for
$Delta gamma gtrsim 10^{-1}$. At fixed temperature, the scaling with
magnetization $f_xisim 10^{-3},sigma^{-1}$ is consistent with our earlier
one-dimensional results. For our reference $sigma=1$, the power spectrum of
precursor waves is relatively broad (fractional width $sim 1-3$) for cold
temperatures, whereas it shows pronounced line-like features with fractional
width $sim 0.2$ for $10^{-3} lesssim Delta gamma lesssim 10^{-1.5} $. For
$sigmagtrsim 1$, the precursor waves are beamed within an angle $simeq
sigma^{-1/2}$ from the shock normal (as measured in the post-shock frame), as
required so they can outrun the shock. Our results can provide
physically-grounded inputs for FRB emission models based on maser emission from
relativistic shocks.
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