The Evolution of Gamma-ray Burst Jet Opening Angle through Cosmic Time. (arXiv:1912.00057v1 [astro-ph.HE])

The Evolution of Gamma-ray Burst Jet Opening Angle through Cosmic Time. (arXiv:1912.00057v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Lloyd_Ronning_N/0/1/0/all/0/1">Nicole Lloyd-Ronning</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hurtado_V/0/1/0/all/0/1">Valeria U. Hurtado</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Aykutalp_A/0/1/0/all/0/1">Aycin Aykutalp</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Johnson_J/0/1/0/all/0/1">Jarrett Johnson</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ceccobello_C/0/1/0/all/0/1">Chiara Ceccobello</a>

Jet opening angles of long gamma-ray bursts (lGRBs) appear to evolve in
cosmic time, with lGRBs at higher redshifts being on average more narrowly
beamed than those at lower redshifts. We examine the nature of this
anti-correlation in the context of collimation by the progenitor stellar
envelope. First, we show that the data indicate a strong correlation between
gamma-ray luminosity and jet opening angle, and suggest this is a natural
selection effect – only the most luminous GRBs are able to successfully launch
jets with large opening angles. Then, by considering progenitor properties
expected to evolve through cosmic time, we show that denser stars lead to more
collimated jets, and argue that the observed anti-correlation between opening
angle and redshift can be accounted for if lGRB massive star progenitors at
high redshifts have higher average density compared to those at lower
redshifts. This may be viable for an evolving IMF, and under the assumption
that average density scales directly with mass, this relationship is consistent
with the form of the IMF characteristic mass evolution suggested in the
literature. The jet angle-redshift anti-correlation may also be explained if
the lGRB progenitor population is dominated by massive stars at high redshift,
while lower redshift lGRBs allow for a greater diversity of progenitor systems.
Overall, however, we find both the jet angle-redshift anti-correlation and jet
angle-luminosity correlation are consistent with the conditions of jet launch
through, and collimation by, the envelope of a massive star progenitor.

Jet opening angles of long gamma-ray bursts (lGRBs) appear to evolve in
cosmic time, with lGRBs at higher redshifts being on average more narrowly
beamed than those at lower redshifts. We examine the nature of this
anti-correlation in the context of collimation by the progenitor stellar
envelope. First, we show that the data indicate a strong correlation between
gamma-ray luminosity and jet opening angle, and suggest this is a natural
selection effect – only the most luminous GRBs are able to successfully launch
jets with large opening angles. Then, by considering progenitor properties
expected to evolve through cosmic time, we show that denser stars lead to more
collimated jets, and argue that the observed anti-correlation between opening
angle and redshift can be accounted for if lGRB massive star progenitors at
high redshifts have higher average density compared to those at lower
redshifts. This may be viable for an evolving IMF, and under the assumption
that average density scales directly with mass, this relationship is consistent
with the form of the IMF characteristic mass evolution suggested in the
literature. The jet angle-redshift anti-correlation may also be explained if
the lGRB progenitor population is dominated by massive stars at high redshift,
while lower redshift lGRBs allow for a greater diversity of progenitor systems.
Overall, however, we find both the jet angle-redshift anti-correlation and jet
angle-luminosity correlation are consistent with the conditions of jet launch
through, and collimation by, the envelope of a massive star progenitor.

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