GCN Circular 45857
Subject
EP261006a: Swift/XRT detection of the fading X-ray counterpart
Event
Date
2026-10-08T11:31:24Z (15 hours ago)
From
Nicolas Monsalves <n.m.monsalvesgonzalez@gmail.com>
Via
Web form
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.