[Users] Residual eccentricity in BNS

[Samuel Tootle]

Hi Lorenzo,

For closer separations the eccentricity can grow as the approximations of time symmetry and quasi-equilibrium become increasingly less accurate. The differences due to component masses would be negligible in comparison.

Indeed eccentricity reduction is possible and we have implemented this in the FUKA codes, however, I am not aware of this capability currently existing in Lorene. Be default FUKA solutions utilize 3.5PN estimates to reduce residual eccentricity prior to an initial evolution. In most cases this is sufficient, but further reduction is also possible by fixing certain parameters (omega and adot, see https://arxiv.org/abs/2103.09911 for details). Though, for equal mass such methods are usually not necessary.

Hopefully someone with more experience with Lorene can chime in, but I hope the above answers some of your inquiries.

Warm regards,
Samuel Tootle
Postdoctoral Researcher
Physics Department
University of Idaho

Feb 28, 2024 08:56:42 Lorenzo Cipriani <lorenzo.cipriani at graduate.univaq.it>:

> Hi everyone,
> 
> I hope this message finds you well.
> 
> I am running an equal mass BNS simulation and I find that the Newtonian distance from the origin of the center of mass of one star has strong oscillations that seem to be due to a residual eccentricity in the initial data (computed with Lorene). A modulation is also present in the GW strain and doesn't seem to disappear changing the omega_0 parameter in the fixed frequency integration to compute h (I use Kuibit by the way).
> 
> I have a few questions:
>       1) Does the eccentricity in the ID grow only if the two stars are closer or are there other relevant parameters?
>       2) Given an initial eccentricity, do lower masses simply show the modulation more (since the modulus of the strain is smaller)?
>       3) Looking online I have found references to a routine to reduce eccentricity for BBH simulations. Is there anything similar for BNS?
> 
> I know some residual eccentricity is to be expected, but this seems excessive. For reference I have uploaded the parameter file as well.
> 
> Thank you very much,
> Lorenzo Cipriani
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