GRB 250430A: detection of the optical afterglow – 30 Apr. 2025

We detected the optical afterglow of GRB 250430A, about 4.75 hours after the burst.

GRB 250430A: 30 Apr. 2025.

On 30 April 2025 at 17:21:31 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 250430A (trigger=1308754). This detection was reported via the Nasa’s GCN 40292 Circular. An afterglow was detected.

At 22:19 UTC, about 4.75 hours after the burst, we remotely slewed the Celestron C14+Paramount ME+SBIG ST-10XME robotic unit  to the transient position, collecting 7, 300-second unfiltered exposures, then we averaged them. 

We detected a faint object at the following position (J2000.0):

RA: 15 33 32.94s
Decl.: -18 07 06.5
R:  20.8 (assuming R-mags from Gaia DR2 for the reference stars).

This position is consistent with Odeh et al., GCN 40295.

At the position above, there is a very faint source on Panstarrs DR1 images, as well as on DESI Legacy DR10 images (the latter, mentioned by Garnichey et al., GCN 40301, is extremely faint, r = 23.22), likely the host galaxy of GRB 250430A. Assuming this hypothesis and the redshift for the GRB (z=0.77; the photometric redshift of the galaxy is z=0.89, consistent with the former), the light travel  time of the source was 6.7 billion years!

We published a GCN circular 40316.

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