TITLE:   GCN CIRCULAR
NUMBER:  36771
SUBJECT: EP240625a: GRANDMA Optical Upper Limits
DATE:    24/06/27 09:44:18 GMT
FROM:    Sarah Antier at OCA <sarah.antier@oca.eu>


D. Akl (AUS), A. Le Calloch (Berkeley), Y.Rajabov (UBAI), M. Molham (KAO), 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), D. Turpin (CEA-Saclay/Irfu), T. Pradier (Unistra/IPHC), N. Guessoum (AUS), M. Masek (FZU), M. Prouza (FZU), A. Kaeouch (OUCA), M. Freeberg (KNC), 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 EP240625a detected by Einstein Probe (EP) WXT on T0 = 2024-06-25T01:48:23 (Pan et al., GCN 36757). We monitored the source with three telescopes, including FRAM-CTA-N, KNC-T32, KNC-ASO, from 2024-06-26T00:33:03.632 to 2024-06-26T16:15:48 UTC, starting 0.95 days after the EP trigger. 

We did not find any new optical counterpart in any of our frames within 10 arcsec radius centered around Ra/Dec: 310.7308, -15.9760.

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/EP240625a/version/fbcf272073852fb0c34826deb0914978

Below is the version that can be downloaded from the link above:
+--------------+----------+----------------+-------------------+
|          MJD | Filter   |        Lim.Mag |       Instrument  |
+==============+==========+================+===================+
| 60487.008465 | L        |          20.04 | KNC-ASO           |
+--------------+----------+----------------+-------------------+
| 60487.061847 | G        |          19.84 | KNC-ASO           |
+--------------+----------+----------------+-------------------+
| 60487.034262 | R        |          19.46 | KNC-ASO           |
+--------------+----------+----------------+-------------------+
| 60487.091242 | B        |          20.13 | KNC-ASO           |
+--------------+----------+----------------+-------------------+
| 60487.182141 | R        |          16.95 | FRAM-CTA-N        |
+--------------+----------+----------------+-------------------+
| 60487.626447 | V        |          19.88 | KNC-T32           |
+--------------+----------+----------------+-------------------+

We note that bessellv and bessellr filters correspond to V and R Johnson Cousin filters, respectively.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 also note that some amateur telescopes use green filters, which were calibrated with the Gaia eDR3 catalog.Generic telescopes correspond to KNC telescopes.

We performed force photometry on our images at Ra, Dec: 20:42:55.47, -15:58:24.00 (S.Y. Fu et al., GCN 36761). We did not detect a source at that position at 20.7 upper limit in the V-band (Vega, 5 sigma).

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.

We thank the Einstein Probe team for useful communication.

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/).