GCN Circular 45857
EP261006a: Swift/XRT detection of the fading X-ray counterpart
From: Nicolas Monsalves
Issued: 2026-10-08 11:31 UTC
Type: X-ray Transient
Nicolas Monsalves, Muskan Yadav, Hira Waseem, and Eleonora Troja (U Rome) reports on behalf of the ERC BHianca team:
We observed the field of the fast X-ray transient EP261006a (Lao et al. GCN 45826) with the Neil Gehrels Swift Observatory. Observations (PI: Troja) started on 2026 Oct 7.74 UT (~24 hr after the trigger) for a total exposure of 1.8 ks in Photon Counting mode.
Within the EP/FXT localization (Lao et al. GCN 45826), a bright source is detected by the UVOT in all filters with uvw2~18.4 AB mag, not corrected for Galactic extinction. Its position is consistent with the nucleus of the galaxy Gaia DR3 2587044864659595392 and the candidate optical counterpart (Eyles-Ferris et al. GCN 45831; Wu et al. GCN 45852; Bochenek & Perley GCN 45853; Chen et al. GCN 45854). Archival GALEX images show only a weak source at this position with NUV ~ 23.5 AB mag, suggesting significant variability between the two epochs.
Swift/XRT observations detect an X-ray source at >99.99% significance at RA, Dec (J2000) = 01:27:09.74, +12:53:21.8 with an uncertainty of 6.7 arcsec (90% C.L.), 6 arcsec from the galaxy’s nucleus and the candidate optical counterpart. The limited number of counts does not allow for an enhanced XRT position.
The XRT count rate of 0.010 (+0.004, -0.003) ct/s corresponds to an unabsorbed 0.3-10 keV flux of ~4e-13 erg cm^-2 s^-1 (photon index 1.64 and Galactic N_H as in Lao et al. GCN 45826), suggesting substantial fading with respect to the earlier EP/FXT observations. Using a simple power-law to describe the decay rate as t^alpha, we derive a temporal index alpha ~- 1. The X-ray fading does not match the behavior of the candidate optical counterpart, still blue and brightening at a similar epoch (r = 18.7 at ~28 hr, Bochenek & Perley GCN 45853). Based on the marginal positional coincidence and different temporal behavior, the association between the X-ray afterglow and candidate optical and UV counterpart cannot be confirmed at present.
We thank the Swift team for rapidly scheduling these observations.
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Relayed automatically from NASA's General Coordinates Network.
Transient Alerts » in reply to EP261006a — X-ray Transient
In plain words Notable
On 6 October 2026 the Einstein Probe, a space X-ray telescope, spotted a sudden flash of X-rays at the position of a galaxy, with a measured redshift of 0.1853 (very roughly 2-3 billion light-years away). Optical telescopes then found a bright, blue, new light source right at the galaxy's centre that was not there two days earlier and has brightened to about 18th magnitude over several days. Its blue colour and central location fit a tidal disruption event, where a black hole tears apart a passing star, but the spectra show no clear features to confirm this, and a fast-fading exploding star remains possible. The X-rays faded by about a factor of 300 within a day, while the visible light kept rising, so the two may not come from the same process. Radio telescopes (VLA and MeerKAT) detected nothing.
How unusual is it? A measured distance, an optical counterpart at the galaxy's centre and a rising blue light curve make this a well-characterised fast X-ray transient, with a likely tidal disruption event, which is more than the typical alert. It is not exceptional, since such events are being found regularly, and the classification is still unconfirmed; clear emission lines or radio emission would settle the nature, and a different type of explosion would change the picture.
Written by an AI model (claude-sonnet-5-5) from the official reports in this thread, and rewritten as new reports arrived (version 7); last updated . It may contain mistakes: the reports themselves are the authority.
EP261006a: Swift/XRT detection of the fading X-ray counterpart
The whole thread (20 posts)
- EP261006a — X-ray Transient
- EP261006a / AT 2026aetp: Optical follow-up with the 1.3-m DFOT
- EP261006a : MeerKAT radio observations.
- EP261006a: 10 GHz VLA observations
- EP261006a / AT 2026aetp: Luminance-band optical follow-up with small-aperture telescopes
- EP261006a: Kilonova-Catcher optical follow-up and detection
- Updated explainer: what the latest reports add (AI-written)
- EP261006a / AT 2026aetp: Multiband optical follow-up with the PKU 60cm Telescope
- EP261006a: Liverpool Telescope optical follow-up
- Explainer: what this is and why it matters (AI-written)
- EP261006a: Xinglong 2.16m/BFOSC optical follow-up observations
- EP261006a: Optical follow-up observations with MATCH
- EP261006a / AT 2026aetp: VLT/X-shooter spectroscopic redshift z = 0.1853
- EP261006a: Swift/XRT detection of the fading X-ray counterpart
- EP261006a: Optical follow-up observations with Kinder
- EP261006a: Liverpool Telescope optical observations
- EP261006a: Early Optical Observations by WFST
- EP261006a: NOT spectroscopy of a blue continuum
- EP261006a: LT and NOT discovery of an optical counterpart
- EP261006a: SVOM/VT optical upper limits