TITLE:   GCN CIRCULAR
NUMBER:  36326
SUBJECT: LIGO/Virgo/KAGRA S240422ed: GRANDMA Optical Upper Limits for EP240426a
DATE:    24/04/26 21:27:56 GMT
FROM:    Damien Dornic <ddornic@km3net.de>

E. G. Elhooseiny (NRIAG), D. Dornic (CPPM), M. Pillas, S. Antier (OCA), J. Dibasso (Berkeley) A. Klotz (IRAP), P. Thierry (AGORA association),  S. Karpov (FZU), C. Andrade (UMN), M. Coughlin (UMN), P.A. Duverne (APC), P. Hello (IJCLAB), I. Tosta e Melo (UniCT-DFA), T. Pradier (Unistra/IPHC), D. Turpin (CEA), M. Freeberg (KNC), H-B Eggenstein (KNC), J.-G. Ducoin (CPPM), N. Leroy (IJCLAB), X. Wang (Tsinghua Univ)
Report on behalf of GRANDMA collaboration.

We responded to the detection of a candidate counterpart EP240426a by EP/FXT (H. Sun et al, GCN 36313).

We investigated in ATLAS forced photometry any possible previous detection from 2024-04-26T00:41:14 to 2024-04-23T01:27:14. No detection at mag [12.2 - 20.0] mag in the orange filter. We then performed observations with GRANDMA (Kilonova-catcher telescopes in Australia and LesMakes-T60 at la Reunion) of EP240426a from 2024-04-26T11:18:00 to 2024-04-26T18:03:15 UTC, under moderate weather conditions.

We used T0 as the LIGO/Virgo/Kagra S240422ed GW candidate event (LIGO-Virgo-Kagra, GCN 36236).

In the following table we report the preliminary photometry of our
observations. Upper limits are reported at the 5-sigma limit,
in the AB system.

T-T0 (day)|    MJD   | Observatory | Exp. | Filter   | Upp.Mag.  | Comments
------------------------------------------------------------------------------------------------------------
3.58    |60426.477894 | KNC       |10x180s| Johnson V |  16.7     | calibrated in R, Gaia
3.63    |60426.528530 | KNC       |7x180s | Rc        |  16.6     | calibrated in sdssg, PS1, low elevation and moon
3.76    |60426.652754 | LesMakes-T60|5x120s | Clear    |  19.5     | calibrated in sdssr, PS1
3.78    |60426.668757 | LesMakes-T60|15x120s| Clear    |  20.9     | calibrated in sdssr, PS1
3.80    |60426.696710 | LesMakes-T60|40x120s| sdssr   |  21.2     | calibrated in sdssr, PS1


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

In our low latency analysis (Antier & Karpov, in prep), comparing to PS1, we did not find any clear new source candidate in our frames from LesMakes-T60 within 15 arcmin radius centered around ra=121.8567 dec=-29.4609, in addition to the near-by existing galaxy.

Our results are consistent with the non-detection reported by GMG (GCN 36315), DECam-r (GCN 36317) and YAHPT (GCN 36323).

We can provide any image from this campaign by request.

We thank the EP collaboration 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 [2]. Kilonova-Catcher (KNC) is the citizen science program of GRANDMA (kilonovacatcher.in2p3.fr).