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GCN Circular 34508

Subject
GRB 230812B : GRANDMA/Kilonova-Catcher optical afterglow detection and upper limits
Date
2023-08-21T13:15:22Z (6 months ago)
From
Damien Turpin at CEA-Saclay <dturpin-astro@hotmail.com>
Via
Web form
D. Turpin (CEA), M. Serrau (KNC), S. Leonini (KNC), M. Freeberg (KNC), 
F. D. Romanov (KNC), S. Karpov (FZU), S. Antier (OCA/Artemis) report 
on behalf of the GRANDMA/Kilonova-Catcher collaboration:

The Kilonova-Catcher telescope network responded to the alert of 
GRB 230812B (Fermi GBM detection: Roberts et al., GCN 34391;
Fermi LAT detection: Scotton et al., GCN 34392). 

The KNC observations were taken by M. Serrau at the Chante-Perdrix 
Observatory (France), S. Leonini at the Montarrenti Observatory (Italy),
M. Freeberg at the Hidden Valley Observatory (USA) and F.~D. Romanov at the
iTelescope.Net in Sierra Remote Observatory (USA).
The afterglow is first detected in 50x30s R-band coadded images at
about 1 day (midtime of the exposure) after the Fermi/GBM trigger time.

Below, we report a subset of our photometric measurements.
---------------------------------------------------------------------------------
T-T0 (midtime,day)  |Exposure| Filter | Mag +/- err |Mag.Lim. (5sig AB) |Observer
---------------------------------------------------------------------------------
1.024 | 50 x 30s | Rc | 20.35 +/- 0.16 | -- | Leonini
1.164 | 15 x 180s | V | 20.74 +/- 0.08 | -- | Serrau
1.182 | 30 x 180s | Rc | 20.87 +/- 0.15 | -- | Serrau
1.425 | 6 x 300s | Ic | -- | 18.6 | Romanov
2.334 | 12 x 300s | r | -- | 20.6 | Freeberg
2.397 | 12 x 300s | i | -- | 19.3 | Freeberg
2.624 | 30 x 180s | Rc | 21.29 +/- 0.11 | -- | Serrau
4.080 | 40 x 180s | V | 22.20 +/- 0.21 | -- | Serrau

Our detections are consistent with the fading behavior previously reported
by GRANDMA Mao et al., GCN 34404; Pyshna et al., GCN 34425. We note that
the underlying host galaxy may contribute to our late photometric 
measurements. The reported magnitudes are not corrected for the galactic
dust extinction in the line of sight of the burst. 

The GRANDMA/Kilonova-Cacther images have been calibrated using field
stars from the PanSTARRS-DR1 catalog using the STDpipe pipeline
(Karpov 2022) and the ps1/r to Rc mag conversion from (Pancino et al. 2022).

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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