TITLE: GCN CIRCULAR NUMBER: 37015 SUBJECT: EP240801a: GRANDMA and Kilonova-Catcher optical observations DATE: 24/08/02 14:53:29 GMT FROM: Damien Turpin at CEA-Saclay dturpin-astro@hotmail.com

D. Turpin (CEA-Saclay/Irfu), S. Aghayeva (Shamakhy Obs.), Z. Vidadi (Shamakhy Obs.), N. Kochiashvili (Abastumani Obs.), S. Antier (OCA), S. Karpov (FZU), M. Coughlin (UMN), C. Andrade (UMN), I. Tosta e Melo (UniCT-DFA), P. Hello, N. Leroy (IJCLAB), P-A Duverne (APC), T. Pradier (Unistra/IPHC), N. Guessoum (AUS), M. Masek (FZU), M. Prouza (FZU), M. Freeberg (KNC), S. Leonini (KNC, Montarrenti Obs.), Y. Rajabov (UBAI), M. Serrau (KNC) on behalf of the GRANDMA and Kilonova-Catcher collaborations:

We performed observations with GRANDMA and its citizen science project Kilonova-catcher (KNC) on the X-ray transient EP240801a detected by Einstein Probe (EP) WXT on T0 = 2024-08-01T09:06:03 (Zhou et al., GCN 36997). Our observations were made from 2024-08-01T20:32:01 to 2024-08-02T08:49:33 UTC, starting 11.4 hours after the EP trigger.

In most of our images, we did not detect the optical counterpart identified by several other follow-up observations (Fu et al., GCN 36998; Li et al. GCN 36999; Zheng et al. 36999; Aryan et al., GCN 37002; An et al., GCN 37004; Pérez-Fournon et al. GCN 37007; Malesani et al., GCN 37008, Zhu et al. GCN 37010, Moskvitin et al. GCN 37012, Quirola-Vásquez et al. GCN 37013; Moretti et al., GCN 37014). Our 5 sigma upper limits are consistent with the reported detections. In addition, we note a marginal detection in one of our stacked frame taken at a midtime ~15.7h after the EP/WXT trigger time with the following magnitude G = 22.1 +/- 0.5.

All our observations and upper limits (5 sigma C.L.) can be found here: +--------------+------------+-------+-----------+---------------------+ | MJD | T-T0 (hrs) |Filter | Mag (AB) | Instrument | +==============+============+===================+=====================+ | 60523.855567 | 11.43 | sdssr |20.7 (U.L) | AbAO-T70 | +--------------+------------+-------+-----------+---------------------+ | 60523.951979 | 13.74 | R |20.2 (U.L.)| UBAI-NT60 | +--------------+------------+-------+-----------+---------------------+ | 60523.953785 | 13.79 | R |19.3 (U.L.)| KNC-Montarrenti Obs.| +--------------+------------+-------+-----------+---------------------+ | 60523.997921 | 14.85 | R |18.4 (U.L.)| FRAM-CTA-N | +--------------+------------+-------+-----------+---------------------+ | 60524.032112 | 15.67 | Gaia G|22.1+/-0.5 | KNC-Dauban Meade 12"| +--------------+------------+-------+-----------+---------------------+ | 60524.055914 | 16.24 | sdssr |21.0 (U.L.)| KNC-CDK17 | +--------------+------------+-------+-----------+---------------------+ | 60524.279711 | 21.61 | sdssr |21.2 (U.L.)| KNC-T160FL | +--------------+------------+-------+-----------+---------------------+

Images obtained in Johnson Cousin filters were calibrated using the Gaia DR3 Synphot catalog, images taken with Sloan filters were calibrated using the Pan-STARRS DR1 Catalog while images taken with Gaia G filters were calibrated using the Gaia eDR3 catalog.

All the data have been reduced by a single data processing pipeline STDPIPE (Karpov et al., 2022). We use the Skyportal application (skyportal.io) to monitor our observational campaign.

GRANDMA is a worldwide telescope network (grandma.ijclab.in2p3.fr) devoted to the observation of transients in the context of multi-messenger astrophysics (Antier et al. 2020 MNRAS 497, 5518). Kilonova-Catcher (KNC) is the citizen science program of GRANDMA (http://kilonovacatcher.in2p3.fr/).

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