Picky Partners: The Pairing of Component Masses in Binary Black Hole Mergers. (arXiv:1905.12669v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Fishbach_M/0/1/0/all/0/1">Maya Fishbach</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Holz_D/0/1/0/all/0/1">Daniel E. Holz</a>
We examine the relationship between individual black hole (BH) masses in
merging binary black hole (BBH) systems. Analyzing the ten BBH detections from
LIGO/Virgo’s first two observing runs, we find that the masses of the black
holes comprising each binary are unlikely to be randomly drawn from the same
underlying distribution. Instead, we find that the two BHs prefer to be of
comparable mass. We show that it is $sim 7$ times more likely that the
component BHs in a given binary are always equal (to within 5%) than that they
are randomly paired. If we insist that component BHs are randomly drawn from
the same underlying power-law distribution with slope $gamma$, we find $gamma
= -0.8^{+1.0}_{-1.0}$ (median and 90% credible interval). However, it is more
likely that the probability of a merger between two BHs depends on their mass
ratio, $q leq 1$. If we assume a scaling of $q^beta$, so that $beta=0$
corresponds to random pairings, we find $beta>0$ is favored at credibility
$0.994$. If we additionally introduce a minimum mass ratio threshold,
$q_mathrm{min} < q < 1$, we find $beta = 5.0^{+6.2}_{-7.5}$, $q_mathrm{min}
= 0.6^{+0.3}_{-0.4}$, and $gamma = -1.4^{+0.9}_{-0.8}$. This implies that only
1% of merging binaries have mass ratios less than $q_{1%} =
0.66^{+0.24}_{-0.27}$, compared to $q_{1%} = 0.17^{+0.07}_{-0.06}$ if the
pairing is done at random. We conclude that merging black holes do not form
random pairings; instead they are selective about their partners, preferring to
mate with black holes of a similar mass. The details of these selective
pairings provide insight into the underlying formation channels of merging
binary black holes.
We examine the relationship between individual black hole (BH) masses in
merging binary black hole (BBH) systems. Analyzing the ten BBH detections from
LIGO/Virgo’s first two observing runs, we find that the masses of the black
holes comprising each binary are unlikely to be randomly drawn from the same
underlying distribution. Instead, we find that the two BHs prefer to be of
comparable mass. We show that it is $sim 7$ times more likely that the
component BHs in a given binary are always equal (to within 5%) than that they
are randomly paired. If we insist that component BHs are randomly drawn from
the same underlying power-law distribution with slope $gamma$, we find $gamma
= -0.8^{+1.0}_{-1.0}$ (median and 90% credible interval). However, it is more
likely that the probability of a merger between two BHs depends on their mass
ratio, $q leq 1$. If we assume a scaling of $q^beta$, so that $beta=0$
corresponds to random pairings, we find $beta>0$ is favored at credibility
$0.994$. If we additionally introduce a minimum mass ratio threshold,
$q_mathrm{min} < q < 1$, we find $beta = 5.0^{+6.2}_{-7.5}$, $q_mathrm{min}
= 0.6^{+0.3}_{-0.4}$, and $gamma = -1.4^{+0.9}_{-0.8}$. This implies that only
1% of merging binaries have mass ratios less than $q_{1%} =
0.66^{+0.24}_{-0.27}$, compared to $q_{1%} = 0.17^{+0.07}_{-0.06}$ if the
pairing is done at random. We conclude that merging black holes do not form
random pairings; instead they are selective about their partners, preferring to
mate with black holes of a similar mass. The details of these selective
pairings provide insight into the underlying formation channels of merging
binary black holes.
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