GCN Circular 33878

R. Jayaraman, M.M. Fausnaugh, G. Mo, E. Katsavounidis, R. Vanderspek, G.R. Ricker (MIT) report:

We report observations by the Transiting Exoplanet Survey Satellite (TESS) of the Bilby sky localization region from the LVK NSBH merger event S230518h (GCN 33813
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, 33816
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; LVK collaboration), the Swope candidates presented in GCN 33829
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 (Coulter et al.), and the MeerLICHT candidates presented in Astronote 2023-121 (Johnston et al.). TESS has a field-of-view of 2304 deg^2, a plate scale of 21 arcseconds per pixel, and a passband between 600 and 1000 nm. TESS images its entire field of view every 200 seconds.

We used the techniques described in Mo et al. (ApJL 948 L3, 2023) to calculate an overlap between the TESS field of view and the gravitational-wave (GW) localization sky map. 15.3% of the Bilby sky localization probability fell within TESS cameras 3 and 4. TESS was observing its pre-determined fields of view for Sector 65 (https://tess.mit.edu/tess-year-5-observations/) beginning ~6.8 days prior to the LVK trigger. Observations continued until 11 hours after the time of the trigger, at which time a scheduled TESS data downlink via the Deep Space Network commenced. TESS full frame images (FFIs) were then calibrated using the TICA software package (Fausnaugh et al. RNAAS 4 251, 2020) and subsequently became public on MAST ~36 hours after downlink. Shortly after the TICA data became public at MAST, we commenced analysis of the TESS FFIs. 

The Earth rose above TESS’s sun shade limits at 2023-05-10 04:23 UTC, and set below the sun shade limits at 2023-05-18 13:30 UTC, which was 32 minutes after the GW trigger. Earthshine during these times introduces strong scattered light signals in the TESS images, which manifests as a time-variable background that complicates our analysis.

Using reference images from before the GW trigger, we performed difference imaging on the TESS images. We extracted differential light curves for 10,343 galaxies at distances between 200 and 400 Mpc from the GLADE+ catalog (Dálya et al. MNRAS 514 1 1403–1411, 2022) that lie within the GW sky localization. Comparing the flux in each light curve during the 4 days prior to the GW trigger with the flux during the 11 hours after the trigger, we find 40 light curves with a nominal 7-sigma difference. Visual inspection shows that these deviations are mainly caused by crowding of bright stars in the TESS photometric apertures, implying that these deviations are related to residual systematic errors in the photometry. Smaller deviations might still be candidates, but interpretation of these light curves is difficult due to the variable backgrounds from earthshine.

The three Swope candidates (GCN 33829
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, Coulter et al.) and two MeerLICHT candidates (Astronote 2023-121, Johnston et al.) lie within the TESS field of view. We extracted differential light curves for these candidates as well. The TESS light curves on 4-hour timescales are consistent with zero flux, implying the following 5-sigma upper limits on any transients from 6.8 days prior to 11 hours after the GW trigger:

Swope Candidate   | TESS magnitude limits
SSS23a                         | >18.98
SSS23b                         | >18.51
SSS23c                         | >18.03

MeerLICHT Candidate | TESS magnitude limits
AT 2023iyb                        | >18.93
AT 2023ixg                         | >17.38

For AT2023iyb, the TESS limit is consistent with the MeerLICHT measurement if the transient is relatively blue.