The late-time radio behavior of GRB afterglows: testing the standard model. (arXiv:1911.01938v2 [astro-ph.HE] UPDATED)
<a href="http://arxiv.org/find/astro-ph/1/au:+Kangas_T/0/1/0/all/0/1">Tuomas Kangas</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Fruchter_A/0/1/0/all/0/1">Andrew Fruchter</a>
We examine a sample of 21 gamma-ray burst (GRB) afterglow light curves at
radio frequencies, and compare them to the X-ray and/or optical properties of
the afterglows and to the predictions of the standard jet/fireball model. Our
sample includes every textit{Swift} GRB with an X-ray light curve indicating a
jet break and with a published radio light curve, as well as several other
targets with observed X-ray or and/optical jet breaks. We examine the late-time
decline of each burst, and attempt to fit an analytical model based on the
standard GRB afterglow equations to each data set. We show that most of the
events in our textit{Swift} GRB sample are incompatible with the radio light
curve behavior predicted by conventional afterglow theory. Many exhibit a
late-time radio decline incompatible with the post-break X-ray or optical
afterglow. Only one radio afterglow in this sample, at any time, shows the
eventually expected decline of $sim t^{-2}$, although two others show it in
their mm light curve. Several others remain consistent with the standard model
if such a decline began after the observations. The radio behavior alone does
not, however, indicate whether a GRB can be fit by our modeling code. Indeed,
several of the well-fit GRBs may only appear so due to a lack of
multi-wavelength data. While a second source of emission can account for some
of the anomalous radio behavior, our tests indicate this is often not the case
unless the main jet component is simultaneously suppressed.
We examine a sample of 21 gamma-ray burst (GRB) afterglow light curves at
radio frequencies, and compare them to the X-ray and/or optical properties of
the afterglows and to the predictions of the standard jet/fireball model. Our
sample includes every textit{Swift} GRB with an X-ray light curve indicating a
jet break and with a published radio light curve, as well as several other
targets with observed X-ray or and/optical jet breaks. We examine the late-time
decline of each burst, and attempt to fit an analytical model based on the
standard GRB afterglow equations to each data set. We show that most of the
events in our textit{Swift} GRB sample are incompatible with the radio light
curve behavior predicted by conventional afterglow theory. Many exhibit a
late-time radio decline incompatible with the post-break X-ray or optical
afterglow. Only one radio afterglow in this sample, at any time, shows the
eventually expected decline of $sim t^{-2}$, although two others show it in
their mm light curve. Several others remain consistent with the standard model
if such a decline began after the observations. The radio behavior alone does
not, however, indicate whether a GRB can be fit by our modeling code. Indeed,
several of the well-fit GRBs may only appear so due to a lack of
multi-wavelength data. While a second source of emission can account for some
of the anomalous radio behavior, our tests indicate this is often not the case
unless the main jet component is simultaneously suppressed.
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