GCN Circular 36712

D. Akl (AUS), M. Masek (FZU), C. Andrade (UMN), E. G. Elhosseiny (NRIAG), H. Peng (THU), L. Wang (XAO), S. Antier (OCA), S. Karpov (FZU), M. Coughlin (UMN), I. Tosta e Melo (UniCT-DFA), P. Hello (IJCLAB), P-A Duverne (APC), D. Turpin (CEA-Saclay/Irfu), N. Guessoum (AUS), T. Pradier (Unistra/IPHC), J.-G. Ducoin (CPPM), M. Prouza (FZU), A. Klotz (CNRS-OMP-IRAP), M. Boer, A. S. Gervasoni, C. Limonta (OCA), Y.Rajabov, O.Burkhonov (UBAI), Y.Tillayev, S.Ehgamberdiev (UBAI, NUUz), T.Sadibekova (UBAI-CEA), M. Freeberg (KNC), M. Serrau (KNC), M. Odeh (AKO), C. Galdies (KNC), A. LeCalloch (Berkeley), N. Leroy (IJCLAB) on behalf of the GRANDMA collaboration:

We performed observations with GRANDMA and its citizen science project Kilonova-catcher (KNC) on the X-ray transient EP240618a detected by Einstein Probe (EP) WXT on T0 = 2024-06-18T05:43:43 (GCN 36690
Not found
 
 
). We monitored the source with 8 telescopes, including TAROT-TCH, TAROT-TCA, FRAM-CTA-N, and UBAI-NT60, from 2024-06-18T16:10:05 to 2024-06-19T09-57-01 UTC, starting 0.43 days after the EP trigger.

We did not find any new optical counterpart in any of our frames within 30 arcsec radius centered around Ra/Dec: 281.648/ 23.833 deg, consistent with the reports from (Tinyanont et al., GCN 36710
Not found
 
 
; Adami et al., GCN 36702
Not found
 
 
; Wu et al., GCN 36701
Not found
 
 
).

All our observations and upper limits, which correspond to the depth of the full image (given in the AB system), can be found here:
https://skyportal-icare.ijclab.in2p3.fr/public/sources/EP240618a/version/8f2a92bbe70faf40c582c7454103e505

Below is the version that can be downloaded from the link above:
+--------------+----------+----------------+-------------------+
|          MJD | Filter   |        Lim.Mag |       Instrument  |
+==============+==========+================+===================+
| 60479.687743 | I        |          18.96 | UBAI/NT-60        |
+--------------+----------+----------------+-------------------+
| 60479.942361 | sdssg    |          17.82 | TAROT/TCA         |
+--------------+----------+----------------+-------------------+
| 60479.942361 | sdssg    |          17.83 | TAROT/TCH         |
+--------------+----------+----------------+-------------------+
| 60479.912306 | R        |          17.97 | FRAM-CTA-N        |
+--------------+----------+----------------+-------------------+
| 60480.406678 | R        |          19.29 | KNC               |
+--------------+----------+----------------+-------------------+
| 60479.993831 | V        |          20.26 | KNC               |
+--------------+----------+----------------+-------------------+
| 60479.759005 | I        |          19.54 | KNC               |
+--------------+----------+----------------+-------------------+
| 60479.683044 | R        |          19.54 | KNC               |
+--------------+----------+----------------+-------------------+
| 60480.027174 | sdssg    |          20.15 | KNC               | 
+--------------+----------+----------------+-------------------+
| 60479.983422 | sdssr    |          19.93 | KNC               |
+--------------+----------+----------------+-------------------+
| 60479.664595 | V        |          19.65 | KNC               |
+--------------+----------+----------------+-------------------+
| 60479.949853 | G        |          20.40 | KNC               |
+--------------+----------+----------------+-------------------+

We note that in the Skyportal page above, bessellv, bessellr, and besselli filters correspond to V, R, and I Johnson Cousin filters, respectively.
Generic telescopes in the Skyportal page correspond to KNC telescopes.

All the data have been reduced by a single data processing pipeline STDPIPE (Karpov et al., 2022). 

Images obtained in Johnson Cousin filters were calibrated using the Gaia DR3 Synphot catalog while images taken with Sloan filters were calibrated using the Pan-STARRS DR1 Catalog.
We note that some amateur telescopes use green filters, which were calibrated with the Gaia catalog.

We use the Skyportal application (skyportal.io) to monitor our observational campaign.

We thank the Einstein Probe team for useful communication.

GRANDMA is a worldwide telescope network (https://grandma.lal.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/).