GCN Circular 45983
Subject
EP261015b: NOT Optical Observations
Event
Date
2026-10-18T11:55:44Z (18 hours ago)
From
Gregory Corcoran at University College Dublin <gregory.corcoran@ucdconnect.ie>
Via
Web form
G. Corcoran (UCD), D. B. Malesani (DAWN/NBI and Radboud), A. Martin-Carrillo (UCD), D. Xu (NAOC), A. L. Bouquin (NOT), A. Caballero Almagro (NOT), E. Georgiou (NOT and Turku Univ.) report on behalf of a larger collaboration,
We observed the field of the Einstein Probe fast X-ray transient EP 261015b (Li et al., GCN 45958) using the Nordic Optical Telescope (NOT) equipped with the ALFOSC imager. A total of 3x300 s were secured in the SDSS r band, with an observation mid time on 2026-10-17 at 00:00:59 UT (1.42 days after the EP trigger). We performed image subtraction against a Legacy Survey stacked image as well as a stack of archival CFHT images.
Li et al. (GCN 45971) present two X-ray sources detected by EP/FXT, for which we report the following information. All our photometry is calibrated using nearby stars in the Pan-STARRS catalog, given in the AB system and not corrected for Galactic extinction.
EPF_J000211.4+305730: The high proper-motion star UCAC4 605-000101 does not subtract cleanly due to its positional offset between our image and the template, as well as its bright flux. We measure an integrated magnitude of r = 16.06 +/- 0.02. This measurement is consistent with the Pan-STARRS DR2 catalogued value of r = 16.0673 +/- 0.0006. Within the FXT uncertainty region, we do not identify any other source in our difference images.
EPF_J000231.0+305716: In addition to the QSO SDSS J000230.78+305717.6 (already noted by Li et al., GCN 45971), located just to the west of the error circle, there exists an additional spectroscopically confirmed QSO within it (https://www.legacysurvey.org/viewer/desi-spectrum/dr1/targetid39628499002066108). The SDSS classified QSO produces a negative residual in our subtraction while the DESI QSO produces a positive residual (integrated mag r = 22.29 +/- 0.06, compared to r = 22.60 +/- 0.07 measured from the Legacy Survey image). Just outside of the uncertainty region we detect a third variable source consistent with the Pan-STARRS object PSO J000.6355+30.9555 (LS DR 11 ID 48424592024274146). This object appears point-like and while it does not have a spectroscopic QSO classification, there is tentative evidence for long-term variability in ZTF and WISE W1 data. We therefore believe that this is also likely a QSO or some form of variable star. No other sources are identified in or near this uncertainty region in our difference images.
We also examined the difference images outside of the EP-FXT regions but within the WXT localisation and after manual vetting of the residuals we did not identify any candidate counterparts down to a 3 sigma upper limit of r > 24.5.
Given the relatively late epoch of our follow-up, the lack of measured variability for UCAC4 605-000101 does not rule out a stellar flare. A stellar flare would also be in agreement with the non-detection of any candidate counterparts but does not confirm it either. The presence of multiple QSOs near or within the FXT localisation of EPF_J000231.0+305716 would point towards it being an unrelated X-ray source with X-ray emission coming from one or multiple of the mentioned QSOs. Overall, while a stellar flare origin is consistent with our data, the nature of this event is not fully confirmed, and we encourage further X-ray observations in order to test the X-ray sources for variability.