Afterglow constraints on the viewing angle of binary neutron star mergers and determination of the Hubble constant. (arXiv:2005.01754v2 [astro-ph.HE] UPDATED)

<a href="http://arxiv.org/find/astro-ph/1/au:+Nakar_E/0/1/0/all/0/1">Ehud Nakar</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Piran_T/0/1/0/all/0/1">Tsvi Piran</a>

One of the key properties of any binary is its viewing angle (i.e.,

inclination), $theta_{rm obs}$. In binary neutron star (BNS) mergers it is of

special importance due to the role that it plays in the measurement of the

Hubble constant, $H_0$. The opening angle of the jet that these mergers launch,

$theta_j$, is also of special interest. Following the detection of the first

BNS merger, GW170817, there were numerous attempts to estimate these angles

using the afterglow light curve, finding a wide range of values. Here we

provide a simple formula for the ratio $theta_{rm obs}/theta_j$ based on the

afterglow light curve and show that this is the only quantity that can be

determined from the light curve alone. Namely, it is impossible to determine

each of the angles separately without additional information. Our result

explains the inconsistency of the values found by the various studies of

GW170817 that were largely driven by the different priors taken in each study.

Among the additional information that can be used to estimate $theta_{rm

obs}$ and $theta_j$, the most useful is a VLBI measurement of the afterglow

image superluminal motion. An alternative is an identification of the afterglow

transition to the sub-relativistic phase. These observations are possible only

for mergers observed at small viewing angles, whose afterglow is significantly

brighter than the detector’s threshold. We discuss the implications of these

results to measurements of $H_0$ using GW observations. We show that while the

viewing angle will be measured only in a small fraction of future BNS mergers,

it can significantly reduce the uncertainty in $H_0$ in each one of these

events, possibly to a level of 4-5%. A minority of the mergers with high

precision measurements of this kind may dominate in the future the precision in

which $H_0$ will be measured using this method.

One of the key properties of any binary is its viewing angle (i.e.,

inclination), $theta_{rm obs}$. In binary neutron star (BNS) mergers it is of

special importance due to the role that it plays in the measurement of the

Hubble constant, $H_0$. The opening angle of the jet that these mergers launch,

$theta_j$, is also of special interest. Following the detection of the first

BNS merger, GW170817, there were numerous attempts to estimate these angles

using the afterglow light curve, finding a wide range of values. Here we

provide a simple formula for the ratio $theta_{rm obs}/theta_j$ based on the

afterglow light curve and show that this is the only quantity that can be

determined from the light curve alone. Namely, it is impossible to determine

each of the angles separately without additional information. Our result

explains the inconsistency of the values found by the various studies of

GW170817 that were largely driven by the different priors taken in each study.

Among the additional information that can be used to estimate $theta_{rm

obs}$ and $theta_j$, the most useful is a VLBI measurement of the afterglow

image superluminal motion. An alternative is an identification of the afterglow

transition to the sub-relativistic phase. These observations are possible only

for mergers observed at small viewing angles, whose afterglow is significantly

brighter than the detector’s threshold. We discuss the implications of these

results to measurements of $H_0$ using GW observations. We show that while the

viewing angle will be measured only in a small fraction of future BNS mergers,

it can significantly reduce the uncertainty in $H_0$ in each one of these

events, possibly to a level of 4-5%. A minority of the mergers with high

precision measurements of this kind may dominate in the future the precision in

which $H_0$ will be measured using this method.

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