Time-domain approximant with Bilby (proposal of example)
Dear Bilby developers,
together with Tom Colin (intern student), we have put together a Jupyter notebook based on Tuto 3.2 of the ODW 2021 [https://github.com/gw-odw/odw-2021/blob/master/Tutorials/Day_3/Tuto_3.2_Parameter_estimation_for_compact_object_mergers.ipynb] that does a simplified analysis of GW150914 using a time-domain approximant using the same model (SEOBNRv4). Our objective is to get consistent posterior distributions. Here is our notebook:
We align the template in time so that it matches well that of the frequency domain approximant. The time alignment code is:
max_index = np.argmax(h_plus**2 + h_cross**2)
delta_index = len(h_plus) - max_index
h_plus = np.roll(h_plus, delta_index)
h_cross = np.roll(h_cross, delta_index)
which leaves a small time shift with the time-domain waveform obtained from the frequency-domain generator (using the exact same model). See sanity check in the notebook.
Questions:
- We obtain consistent posterior for the intrinsic parameters (chirp mass and mass ratio) but we see large discrepancies for geocent_time. See this corner plot
We conclude that the time alignment procedure is wrong. We struggle to understand how to align. Could you please give us a hint?
- We believe this type of exercise could be useful to other users. We could come up with a standalone script that could be added to the example/ folder of the Bilby tree. Is this something you would value?
Notes:
For the time-domain approximants, we coded the two following functions (see cell [19]):
def lalsim_SimInspiralTD(...): def _base_lal_cbc_td_waveform(...): def lal_binary_black_hole_td(...):
which are the exact symmetric of the following frequency-domain functions already included Bilby:
def lalsim_SimInspiralFD(...): https://git.ligo.org/lscsoft/bilby/-/blob/master/bilby/gw/utils.py#L735
def _base_lal_cbc_fd_waveform(...): https://git.ligo.org/lscsoft/bilby/-/blob/master/bilby/gw/source.py#L247
def lal_binary_black_hole(...): https://git.ligo.org/lscsoft/bilby/-/blob/master/bilby/gw/source.py#L14
Those could be added to the Bilby code as well.
Best, Eric and Tom