TITLE: GCN CIRCULAR NUMBER: 41410 SUBJECT: LIGO/Virgo/KAGRA S250818k: Coverage and upper limits from MAXI/GSC observations DATE: 25/08/18 09:38:35 GMT FROM: Satoshi Sugita at Aoyama Gakuin U. sugita@phys.aoyama.ac.jp

M. Nakajima, H. Negoro, K. Takagi (Nihon U.), N. Kawai, T. Mihara, (RIKEN), S. Sugita, M. Serino, Y. Kawakubo, H. Hiramatsu, Y. Kondo (AGU) report on behalf of the MAXI team:

We examined MAXI/GSC all-sky X-ray images (2-20 keV) after compact binary merger candidate S250818k at 2025-08-18 01:20:06.030 UTC.

At the trigger time of S250818k, the high-voltage of MAXI/GSC was on. The instantaneous field of view of GSC at the GW trigger time covered 1% of the 90% credible region of the bayestar sky map, in which we found no significant new X-ray source. The first one-orbit (92 min) scan observation with GSC after the event covered 78% of the 90% credible region of the bayestar skymap from 01:20:06 to 02:51:15 UTC (T0+0 to T0+5469 sec).

No significant new source was found in the region in the one-orbit scan observation. A typical 1-sigma averaged upper limit obtained in one scan observation is 20 mCrab at 2-20 keV.

If you require information about X-ray flux by MAXI/GSC at specific coordinates, please contact the submitter of this circular by email.

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