GCN Circular 45998
EP261015a: Follow-up with the 1.3-m DFOT
From: unknown
Issued: 2026-10-20 15:08 UTC
Type: X-ray Transient
Pankaj Pawar, Sarvesh Yadav, Anshika Gupta, Debalina Kar, Dhruv Jain, Yash Sharma, Kuntal Misra and Shashi B. Pandey (ARIES, Nainital, India) report:
We observed the field of the Einstein Probe transient EP261015a
(Li et al., GCN 45956) with the 1.3-m Devasthal Fast Optical Telescope
(DFOT), India, on 2026 October 17. Observations started at 18:29:19 UT,
approximately 53.23 hours after the event onset. We obtained six 300-s
exposures in the R filter.
We detect a source at the position of the candidate optical counterpart
reported by Corcoran et al. (GCN 45955). Our preliminary PSF photometry
of the stacked image, obtained using AutoPhOT (Brennan & Fraser 2022,
A&A, 667, A62), is:
Mid-time (UT) Tmid-T0 (d) Filter Exposure (s) Mag (Vega)
2026-10-17 18:44:31 2.22842 R 6 x 300 22.18 +/- 0.19
T0 is 2026-10-15 13:15:36 UT, the start of the EP-WXT event
(Li et al., GCN 45956). Photometry was calibrated against 26 nearby
Gaia DR3 XP stars using synthetic Johnson-Cousins R magnitudes on the
Vega system. The magnitude is not corrected for Galactic extinction.
Our photometry was performed on unsubtracted images and is contaminated
by the underlying galaxy noted by Ma et al. (GCN 45963). The reported
measurement includes galaxy light and is not a galaxy-subtracted
transient magnitude.
Our observation predates the optical rebrightening reported by Fortin
et al. (GCN 45991). The galaxy contribution and different passbands prevent
a direct comparison with their template-subtracted r-band photometry.
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.
Relayed automatically from NASA's General Coordinates Network.
Transient Alerts » in reply to EP261015a — X-ray Transient, Optical Transient
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: Follow-up with the 1.3-m DFOT
The whole thread (16 posts)
- EP261015a — X-ray Transient, Optical Transient
- Updated explainer: what the latest reports add (AI-written)
- EP261015a: GSP multiband optical follow-up observations
- EP261015a: Further optical observations with the 1.3m DFOT
- EP261015a: AZT-20 Assy-Turgen optical observations
- Explainer: what this is and why it matters (AI-written)
- EP261015a: Wendelstein 43 cm optical observations
- EP261015a: Follow-up with the 1.3-m DFOT
- EP261015a: Terskol Zeiss-2000 optical upper limit
- EP261015a: COLIBRÍ optical observations of a rebrightening
- EP261015a: Tentative fading of the candidate optical counterpart from the NOT
- EP261015a: refined analysis of EP-FXT observations
- EP261015a: J-band upper limit with WINTER
- EP261015a: SVOM/VT optical observation
- EP261015a: COLIBRÍ optical observations
- EP261015a: Einstein Probe detection of a fast X-ray transient