TITLE:   GCN CIRCULAR
NUMBER:  41837
SUBJECT: LIGO/Virgo/KAGRA S250818k: HCT optical follow-up
DATE:    25/09/16 08:43:46 GMT
FROM:    V. Swain at IIT Bombay <vishwajeet.s@iitb.ac.in>

V. Swain (IITB), G. C. Anupama (IIA), D.K. Sahu (IIA), S. Barway (IIA),  H. Das (IIA), J. Basu (IIA), S. Chamoli (IIA), S. P. Singh (IAO), A. Tiwari (IAO), T. Mohan (IITB), A.P. Saikia (IITB), V. Bhalerao (IITB):

We observed the optical transient AT2025ulz (Stein et. al, GCN #41414) in the localization region of a candidate binary neutron star merger S250818k (LVK, GCN #41437) using the 2.0m Himalayan Chandra Telescope (HCT) of the Indian Astronomical Observatory (IAO). We obtained single exposure in the SDSS r' filter starting at 2025-09-11T16:04:33.16 UT (~24.61 days post trigger). Image subtraction was performed using a reference frame from CFHT/MegaCam, and the transient was detected in the resulting difference image. The derived photometry is reported below:

| JD (mid)       | Filter | Exposure (s) | Mag (AB)     | Limiting Mag (AB) |
| -------------- | ------ | ------------ | ------------ | ----------------- |
| 2460930.169828 | r'     | 600          | 20.82 ± 0.11 | 21.9              |

The magnitudes are calibrated against PanSTARRS DR1 (Chambers et al., 2016) and not corrected for Galactic extinction.

Our result is consistent with the photometry followed by (Busmann et. al, GCN #41421; Hall et. al, GCN #41433; Karambelkar et. al, GCN #41436; Nicholl et. al, GCN #41439; Srivastav et. al, GCN #41451; O’Connor et. al, GCN #41452; Gillanders et. al, GCN #41454; Mo et. al, GCN #41456; Liu et. al, GCN #41461; Ackley et. al, GCN #41468; Klose et. al, GCN #41474; Banerjee et. al, GCN #41476; Perley et. al, GCN #41480; D'Avanzo et. al, GCN #41489; Malesani et. al, GCN #41492; Smartt et. al, GCN #41493; Santos et. al, GCN #41501; Becerra et. al, GCN #41502; An et. al, GCN #41503; Passaleva et. al, GCN #41504; Taguchi et. al, GCN #41505; Troja et. al, GCN #41506; Freeburn et. al, GCN #41507; Angulo et. al, GCN #41518; Antier et. al, GCN #41519; Banerjee et. al, GCN #41532; Lipunov et. al, GCN #41534; Busmann et. al, GCN #41535; Kasliwal et. al, GCN #41538; Gillanders et. al, GCN #41540; Becerra et. al, GCN #41544; Pankov et. al, GCN #41548).

Further observations are planned.

These observations were carried out under the ToO program HCT-2025-C3-P43. We thank the HCT staff for their support during the observations. The Indian Astronomical Observatory is operated by the Indian Institute of Astrophysics, Bengaluru, India.