A sample of GRB radio afterglows inconsistent with the standard jet model. (arXiv:1911.01938v1 [astro-ph.HE])
<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 present a sample of 15 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 has been chosen so that each afterglow exhibits a jet break at some
frequency, usually X-ray. We examine the late-time decline of each burst in the
radio and in the X-ray, and attempt to fit an analytical model based on the
conventional GRB afterglow equations to each data set. We show that the GRBs in
our sample are mostly incompatible with the light curve behavior predicted by
conventional GRB afterglow theory. In particular, we observe a lack of visible
jet breaks in the radio light curve, even when one is seen in the X-ray. No
radio afterglow in this sample, at any time, shows the expected post-break
decline of $sim t^{-2}$, although a few remain consistent with the standard
model if such a decline began soon after the observations. The observed decline
in the radio is often described at least as well by a single power law as by
the standard model, in some cases being consistent with the expected pre-break
decline (assuming $nu_{mathrm{radio}} > nu_m$) until late times. Notably,
signs of a jet break are visible in the millimeter-wave afterglow of GRB
161219B and GRB 111215A, perhaps suggesting that only lower radio-frequency
afterglows behave anomalously. Nonetheless, the observed behavior conflicts
with our current theoretical understanding of radio afterglows.
We present a sample of 15 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 has been chosen so that each afterglow exhibits a jet break at some
frequency, usually X-ray. We examine the late-time decline of each burst in the
radio and in the X-ray, and attempt to fit an analytical model based on the
conventional GRB afterglow equations to each data set. We show that the GRBs in
our sample are mostly incompatible with the light curve behavior predicted by
conventional GRB afterglow theory. In particular, we observe a lack of visible
jet breaks in the radio light curve, even when one is seen in the X-ray. No
radio afterglow in this sample, at any time, shows the expected post-break
decline of $sim t^{-2}$, although a few remain consistent with the standard
model if such a decline began soon after the observations. The observed decline
in the radio is often described at least as well by a single power law as by
the standard model, in some cases being consistent with the expected pre-break
decline (assuming $nu_{mathrm{radio}} > nu_m$) until late times. Notably,
signs of a jet break are visible in the millimeter-wave afterglow of GRB
161219B and GRB 111215A, perhaps suggesting that only lower radio-frequency
afterglows behave anomalously. Nonetheless, the observed behavior conflicts
with our current theoretical understanding of radio afterglows.
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