Determining the viewing angle of neutron star merger jets with VLBI radio images. (arXiv:2101.05138v2 [astro-ph.HE] UPDATED)
<a href="http://arxiv.org/find/astro-ph/1/au:+Fernandez_J/0/1/0/all/0/1">Joseph John Fernández</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kobayashi_S/0/1/0/all/0/1">Shiho Kobayashi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Lamb_G/0/1/0/all/0/1">Gavin P. Lamb</a>
Very long base interferometry (VLBI) radio images recently proved to be
essential in breaking the degeneracy in the ejecta model for the neutron star
merger event GW170817. We discuss the properties of synthetic radio images of
merger jet afterglow by using semi-analytic models of laterally spreading or
non-spreading jets. The image centroid initially moves away from the explosion
point in the sky with an apparent superlumianal velocity. After reaching a
maximum displacement its motion is reversed. This behavior is in line with that
found in full hydrodynamics simulations. Since the evolution of the centroid
shift and jet image size are significantly different in the two jet models,
observations of these characteristics for very bright events might be able to
confirm or constrain the lateral expansion law of merger jets. We explicitly
demonstrate how $theta_{rm obs}$ is obtained by the centroid shift of radio
images or its apparent velocity provided the ratio of the jet core size
$theta_{c}$ and the viewing angle $theta_{rm obs}$ is determined by
afterglow light curves. We show that a simple method based on a point-source
approximation provides reasonable angular estimates ($10-20%$ errors at most).
By taking a sample of structured Gaussian jet results, we find that the model
with $theta_{rm obs} sim 0.32$ rad can explain the main features of the
GW170817 afterglow light curves and the radio images.
Very long base interferometry (VLBI) radio images recently proved to be
essential in breaking the degeneracy in the ejecta model for the neutron star
merger event GW170817. We discuss the properties of synthetic radio images of
merger jet afterglow by using semi-analytic models of laterally spreading or
non-spreading jets. The image centroid initially moves away from the explosion
point in the sky with an apparent superlumianal velocity. After reaching a
maximum displacement its motion is reversed. This behavior is in line with that
found in full hydrodynamics simulations. Since the evolution of the centroid
shift and jet image size are significantly different in the two jet models,
observations of these characteristics for very bright events might be able to
confirm or constrain the lateral expansion law of merger jets. We explicitly
demonstrate how $theta_{rm obs}$ is obtained by the centroid shift of radio
images or its apparent velocity provided the ratio of the jet core size
$theta_{c}$ and the viewing angle $theta_{rm obs}$ is determined by
afterglow light curves. We show that a simple method based on a point-source
approximation provides reasonable angular estimates ($10-20%$ errors at most).
By taking a sample of structured Gaussian jet results, we find that the model
with $theta_{rm obs} sim 0.32$ rad can explain the main features of the
GW170817 afterglow light curves and the radio images.
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