Pre-discovery detection of AT 2023hrq, a supernova candidate in the field of the Coma galaxy cluster

ATel #16037; A. Horti-David, K. Sarneczky, J. Vinko (Konkoly Observatory)
on 14 May 2023; 20:56 UT
Credential Certification: Jozsef Vinko (vinko@astro.as.utexas.edu)

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We report pre-discovery detection of AT 2023hrq on CCD frames taken with the 0.6/0.9m Schmidt telescope at Piszkesteto Station of Konkoly Observatory, Hungary. The transient was discovered by Pan-STARRS at RA=13:00:06.993, DEC=+27:52:37.70 (2000) on 2023-05-08.399 UT.

At Konkoly Observatory we are regularly monitoring the area of the Coma galaxy cluster. After examining the frames taken before the discovery epoch of AT 2023hrq, we found multiple pre-discovery detections.

The unfiltered frames taken on the same night (with exposure time of 12x120 sec) were co-added to increase the signal-to-noise, then a deep reference image was subtracted from the co-added frames.

AT 2023hrq was first detected at the reported position on the subtracted frame taken on 2023-03-22.051 UT. Forced aperture photometry (tied to PS1 w-band magnitudes) resulted in a white-band brightness of 22.0 +/- 0.2 mag, close to the detection limit of 22.4 mag. The last non-detection observed with the same setup during similar conditions occurred on 2023-02-22.136 UT with a limiting magnitude of 22.3.

We also report follow-up photometry of AT 2023hrq with the same setup:

UT Date Magnitude (error)
2023-03-22.051 22.01 (0.19)
2023-03-29.048 21.24 (0.11)
2023-04-20.897 20.03 (0.05)
2023-04-22.922 20.12 (0.05)

After supplementing the data above by detections from Pan-STARRS and ATLAS reported on the TNS website and converting the magnitudes to fluxes, we find that the light curve is consistent with the t^2 power-law (the fireball model) with a rise time longer than 50 days, because the transient is still rising. Such a long rise time suggests an unusual transient, maybe a superluminous supernova in a background galaxy. Indeed, the likely host galaxy, SDSS J130006.91+275236.8, has a photometric redshift of z ~ 0.32, which would imply M ~ -21.3 absolute magnitude for AT 2023hrq.

A spectroscopic classification and further photometric monitoring is encouraged.