TITLE:   GCN CIRCULAR
NUMBER:  40779
SUBJECT: sb25061218/SVOM: OHP/T120 and OHP/T193 optical imaging and spectroscopic observations
DATE:    25/06/18 14:55:00 GMT
FROM:    Christophe Adami at LAM <christophe.adami@lam.fr>

C. Adami (LAM/Pytheas/AMU), B. Schneider (LAM), S. Basa (LAM/OHP/Pytheas/AMU), E. Le Floc'h (CEA/Irfu), M. Dennefeld (IAP/Sorbonne U.), J. L. Atteia (IRAP), J. Rodriguez (CEA), F. Cangemi (APC), A. Coleiro (APC), Y. Degot-Longui (Pytheas/OHP), J. Balcaen (Pytheas/OHP), report on behalf of a larger collaboration:

We observed the field of the SVOM sb25061218 X-ray transient (Rodriguez et al., GCN 40712) using the T120cm and the T193cm equipped with the MISTRAL spectro-imager at Observatoire de Haute-Provence (France). 

Our T120 observations consisted of multiple epochs from 2025-06-12T22:07 to 2025-06-13T00:46 UT (from 1.52 to 4.17 hr after the trigger) in r-band and covered both uncatalogued X-ray sources seen with Swift/XRT (Evans et al. GCN 40721). We examined the sources visible at the XRT #1 and #2 positions and noted a rebrightening of ~0.3 mag of the SIMBAD object [VV2006] J131446.6+544804 compared to SDSS (r = 18.66 +/- 0.02) and PanStarrs (r = 18.65 +/- 0.01) surveys, possibly associated with the XRT source #1.

The magnitudes derived for that source (measured with STDWeb/STDPipe tools (Karpov 2025)) are:
date-obs (UT) | exposure time (min) | band, mag (AB) | mag err
2025-06-12T22:07 | 12min | r | 18.33 | 0.02
2025-06-12T22:36 | 12min | r | 18.29 | 0.04
2025-06-12T23:54 | 4min | r | 18.35 | 0.03
2025-06-13T00:46 | 8min | r | 18.36 | 0.03

On 2025-06-13 starting at 20:27 UT (23.85 hr after the trigger), we re-observed the object [VV2006] J131446.6+544804 with the spectro-imager MISTRAL. We obtained 1 minute of observation in r-band and measured a magnitude of r = 18.37 +/- 0.07, consistent with the T120 measurements on 2025-06-12. We also obtained 1 hour of spectroscopic observations using the MISTRAL blue mode with a spectral resolution of ~700 under moderately good conditions. When compared to the SDSS public spectrum, a preliminary analysis of the spectrum shows a flux increase of the Fe II complex between ~6600 AA and ~7000 AA. This increase is roughly consistent with the r-band magnitude variation observed by the T120 and MISTRAL.

Our observations suggest that the Swift/XRT source #1 is the X-ray counterpart to [VV2006] J131446.6+544804 source, a known QSO at z~ 0.49. The X-ray detection of this source is the first reported to our knowledge and may indicate that the SVOM trigger is also associated with this event. This object being radio-quiet (2.9 mJy at 1.4GHz, NVSS) without typical lines of TDEs, and the observed r-band magnitudes being nearly constant after the initial increase suggest a likely association with an accretion event onto the central black hole of this QSO and would disfavor a GRB nature of this event. 

We encourage further observations of [VV2006] J131446.6+544804.

We acknowledge the excellent support from Observatoire de Haute-Provence and in particular the students and professors from the summer camp OHP 2025 (Institut Origines) and the SOPHIE observer, Xavier Delfosse.