Observational characteristics of radiation-mediated shocks in photospheric gamma-ray burst emission. (arXiv:2206.11701v1 [astro-ph.HE])

Observational characteristics of radiation-mediated shocks in photospheric gamma-ray burst emission. (arXiv:2206.11701v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Samuelsson_F/0/1/0/all/0/1">Filip Samuelsson</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ryde_F/0/1/0/all/0/1">Felix Ryde</a>

Emission from the photosphere in gamma-ray burst (GRB) jets can be
substantially affected by subphotospheric energy dissipation, which is
typically caused by radiation-mediated shocks (RMSs). We study the
observational characteristics of such emission, in particular the spectral
signatures. Invoking an internal collision framework to estimate relevant shock
initial conditions, we use an RMS model to generate synthetic photospheric
spectra. The spectra are then fitted with a standard cutoff power-law (CPL)
function to compare with corresponding GRB catalogues. We find mean values and
standard deviations for the low-energy index and the peak energy as
$langlealpha_{rm cpl}rangle = -0.76pm 0.227$ and $langlelog(E_{rm
peak}/{rm keV})rangle = 2.42pm 0.408$, respectively. The range of
$alpha_{rm cpl}$ values obtained cover a substantial fraction of the
catalogued values, $langlealpha_{rm cpl}^{rm cat}rangle = -0.80 pm
0.311$. This requires that the free fireball acceleration starts at $r_0 sim
10^{10}~$cm, which is in agreement with hydrodynamical simulations. The CPL
function generally provides a good fit, even though the synthetic spectra
typically exhibit an additional break at lower energies. We also identify a
non-negligible parameter region for what we call “optically shallow shocks”:
shocks that do not accumulate enough scatterings to reach a steady-state
spectrum before decoupling. These occur for optical depths $tau lesssim 55 ,
u_u^{-2}$, where $u_u = gamma_ubeta_usim 2$ is the dimensionless specific
momentum of the upstream as measured in the shock rest frame. We conclude that
photospheric emission, which has passed through an RMS, is spectrally
consistent with the dominant fraction of observed bursts.

Emission from the photosphere in gamma-ray burst (GRB) jets can be
substantially affected by subphotospheric energy dissipation, which is
typically caused by radiation-mediated shocks (RMSs). We study the
observational characteristics of such emission, in particular the spectral
signatures. Invoking an internal collision framework to estimate relevant shock
initial conditions, we use an RMS model to generate synthetic photospheric
spectra. The spectra are then fitted with a standard cutoff power-law (CPL)
function to compare with corresponding GRB catalogues. We find mean values and
standard deviations for the low-energy index and the peak energy as
$langlealpha_{rm cpl}rangle = -0.76pm 0.227$ and $langlelog(E_{rm
peak}/{rm keV})rangle = 2.42pm 0.408$, respectively. The range of
$alpha_{rm cpl}$ values obtained cover a substantial fraction of the
catalogued values, $langlealpha_{rm cpl}^{rm cat}rangle = -0.80 pm
0.311$. This requires that the free fireball acceleration starts at $r_0 sim
10^{10}~$cm, which is in agreement with hydrodynamical simulations. The CPL
function generally provides a good fit, even though the synthetic spectra
typically exhibit an additional break at lower energies. We also identify a
non-negligible parameter region for what we call “optically shallow shocks”:
shocks that do not accumulate enough scatterings to reach a steady-state
spectrum before decoupling. These occur for optical depths $tau lesssim 55 ,
u_u^{-2}$, where $u_u = gamma_ubeta_usim 2$ is the dimensionless specific
momentum of the upstream as measured in the shock rest frame. We conclude that
photospheric emission, which has passed through an RMS, is spectrally
consistent with the dominant fraction of observed bursts.

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