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Transient Alerts » in reply to EP261015a — X-ray Transient, Optical Transient

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In plain words Notable

On 15 October 2026 the Einstein Probe, a space telescope that watches for X-ray flashes, saw a brief flare lasting about two minutes in the southern sky. Telescopes then found a faint, slowly changing point of visible light in the same area, sitting on top of a faint galaxy. It faded a little in the first day and a half, then brightened again around day 4 and stayed roughly steady through day 5. A separate, fainter X-ray source in the same region was found to belong to an unrelated, known quasar (a distant galaxy with a very bright core). The leading idea is that the visible light is a supernova (an exploding star) linked to the X-ray flash, but this is not confirmed, and no distance has been measured yet.

How unusual is it? A visible-light counterpart was found for an X-ray flash, and its rebrightening hints at an accompanying supernova, which is not typical of most such flashes. It is not exceptional because the link is unconfirmed, the source could be an unrelated transient, and no distance is known; a spectrum confirming a supernova or giving a distance would settle the verdict.

Written by an AI model (claude-sonnet-5-5) from the official reports in this thread, and rewritten as new reports arrived (version 4); last updated . It may contain mistakes: the reports themselves are the authority.

EP261015a: Further optical observations with the 1.3m DFOT
GCN Circulars —

GCN Circular 46006
EP261015a: Further optical observations with the 1.3m DFOT
From: unknown
Issued: 2026-10-21 13:36 UTC
Type: X-ray Transient, Optical Transient

Pankaj Pawar, Yash Sharma, Anshika Gupta, Debalina Kar, Dhruv Jain, Kuntal Misra, Amit K Ror and Shashi B. Pandey (ARIES, Nainital, India) report:

We observed the field of EP261015a (Li et al., GCN 45956) and its candidate optical counterpart (Corcoran et al., GCN 45955) with the 1.3m Devasthal Fast Optical Telescope (DFOT), India, on 2026 October 20. Observations started at 19:07:22 UT. We obtained six 300-s exposures in the R filter.

We reduced and stacked the images using standard procedures. After archival Legacy Survey template subtraction, we find a tentative positive residual at the counterpart position. Our preliminary PSF photometry, using an empirical PSF constructed with AutoPhOT (Brennan & Fraser 2022, A&A, 667, A62), is:

Mid-time (UT) Tmid-T0 (d) Filter Exposure (s) Mag (AB)
2026-10-20 19:22:34 5.25484 R 6 x 300 22.30 +/- 0.24

T0 is 2026-10-15 13:15:36 UT (Li et al., GCN 45956). Photometry was calibrated against nearby stars using standardised synthetic Johnson–Cousins R magnitudes derived from Gaia DR3 XP spectra. Magnitudes are reported on the AB system using the zero-point conversion of Bessell & Murphy (2012, PASP, 124, 140), and are not corrected for Galactic extinction.

The uncertainty includes an allowance for tested subtraction systematics; additional uncertainty from residual galaxy contamination and the different science/template passbands may remain. We regard the measurement as tentative.

Our observation follows the optical rebrightening reported by Fortin et al. (GCN 45991). Further observations are encouraged to establish the subsequent evolution of the source.

Filter: only the top 10% of the past year's X-ray transients are posted, ranked by the number of GCN Circulars written about them (now ≥ 11). Thresholds are recalculated monthly. Only events first reported within the past 7 days can start a thread.
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