GCN Circular 36149

L. Rhodes (Oxford), S. J. Smartt (Oxford/QUB), J. H. Gillanders, S. Srivastav, A. Cooper (Oxford), M. Fulton, K. W. Smith (QUB) and T.-W. Chen (NCU) report on behalf of a larger collaboration:

C. R. Bom et al (GCN 36146
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) report a candidate optical counterpart associated with a known AGN for the binary black hole merger S240413p, as reported by LVK (GCN 36075
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).

Here we note the proposed AGN counterpart (WISEAJ105115.40-2B054824.6; Veron-Cetty & Veron, 2006, A&A 455, 733) for S240313p has been exhibiting continued AGN flaring activity since ~January 2021, as detected by the survey telescopes ATLAS and ZTF. The publicly available Lasair lightcurve illustrates this long-lived behaviour in the ZTF data (https://lasair-ztf.lsst.ac.uk/objects/ZTF19aaflhnr).

ATLAS data are available through the forced photometry server (https://fallingstar.com) and also show similar elevated activity over the last ~150 days. 

Based on the most recent observations, we see no evidence for additional flaring activity, after the merger time of S240313p, which is above its previously observed behaviour across the last ~3 years. Thus, it seems unlikely that S240313p has produced a detectable flare within this AGN. The excess flux Bom et al (GCN 36146
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) observe is certainly real, but the apparent flaring can be attributed to the large time difference between their recent observation and the template image from Pan-STARRS (Pan-STARRS1 3Pi survey: Chambers et al. 2016arXiv161205560C) which is made from a stack of images taken from 2009-2014.