GCN Circular 36972

A. Reguitti (INAF-OAB / INAF-OAPd), L. Tartaglia (INAF-OAAb), L. Tomasella (INAF-OAPd), E. Cappellaro (INAF-OAPd), F. Onori (INAF-OAAb), F. De Luise (INAF-OAAb), E. Bigongiari (Univ. Pisa), R. Brivio (INAF-OAB), M. Ferro (INAF-OAB), Y.-D Hu (INAF-OAB), B. Patricelli (Univ. Pisa), P. D'Avanzo (INAF-OAB), E. Brocato (INAF-OAAb) report on behalf of the GRAWITA collaboration:


We carried out optical follow-up observations of the well-localised LIGO/Virgo/KAGRA GW trigger S240615dg (LVK Collaboration, GCN Circ. 36669
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) with the Schmidt telescopes sited at the INAF Campo Imperatore and Asiago observatories (Italy). Observations from Campo Imperatore started on 2024-06-15 at 23:30 UT (~12 hours after the GW trigger) with the Sloan-r filter.

We covered 3 square degrees within the 90% localisation region of the S240615dg GW event with 3 pointings of 1 square degree each, centred at J2000 celestial coordinates: (6.886, 45.598); (8.112, 45.597) and (8.117, 46.445). These observations covered ~ 60% of the final 90% credible region (LVK Collaboration, GCN Circ. #36704
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). The 3sigma limiting AB magnitude is r ~ 19 mag.

Observations from Asiago started on 2024-06-17 at 00:00 UT (~ 1.52 days after the GW trigger) with the Sloan-r filter. A second epoch has been carried out starting on 2024-06-18 at 00:10 UT (~ 2.53 days after the GW trigger), also with the Sloan-r filter. We covered 4 square degrees within the 90% localisation region of the S240615dg GW event with 3 pointings of 1 square degree each, centred at J2000 celestial coordinates: (6.892, 45.564); (6.875, 46.418); (8.116, 46.419) and (8.123, 45.556). These observations covered ~ 80% of the final 90% credible region (LVK Collaboration, GCN Circ. #36704
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). The typical 3sigma limiting AB magnitudes are r ~ 21.5 mag and r ~ 21.6 mag for the first and second epoch, respectively.

Preliminary analysis, which includes image subtraction with the template images from the PanSTARRS all-sky survey does not show evidence for promising candidate counterparts. 

We note that within the localisation region of the Swift/XRT source N.32 from GCN Circ. #36698
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 (Page et al. 2024), we detect a source (WISEA J002455.22+452006.9) with variability at a 2.5sigma level. However, the source is reported in the GAIA EDR3 catalog with a parallax of 2.8+-0.17 mas and a proper motion of 21 mas/yr which, together with a red r-z colour of 1.9 mag from the PS1 catalog, is consistent with a late-type dwarf Galactic star experiencing a high-energy flare.