EP J171159.4-333253/EP250623a: Optical upper limits with Kinder and RIFT observations
ATel #17280; A. K. H. Kong (NTHU), A. Aryan (NCU), K.-L. Li (NCKU), T.-W. Chen, W.-J. Hou (both NCU), J. H. Gillanders (Oxford), S. J. Smartt (Oxford/QUB), Y.-H. Lee, Y. J. Yang, A. Sankar. K, Y.-C. Pan, C.-C. Ngeow, M.-H. Lee, H.-C. Lin, C.-H. Lai, H.-Y. Hsiao, C.-S. Lin, J.-K. Guo (all NCU), S. Yang, Z. N. Wang, L. L. Fan, G. H. Sun (all HNAS), H.-W. Lin (UMich), H. F. Stevance, S. Srivastav, L. Rhodes (all Oxford), M. Nicholl, M. Fulton, T. Moore, K. W. Smith, C. Angus, A. Aamer (all QUB), A. Schultz and M. Huber (both IfA, Hawaii)
on 9 Jul 2025; 06:28 UT
Credential Certification: Albert Kong (akong@phys.nthu.edu.tw)
Subjects: Optical, Binary, Neutron Star, Transient
We observed the eclipsing, bursting neutron star X-ray binary EP J171159.4-333253/EP250623a (Ni et al. ATel #17247; Wu et al. ATel #17255; Marino et al. ATel #17271) using the 40 cm SLT at Lulin Observatory in Taiwan as part of the Kinder collaboration (Chen & Yang et al., 2025, ApJ, 983, 86). The first SLT epoch of observations started at 14:52 UTC on the 27th of June 2025 (MJD = 60853.619), 3.84d after the EP WXT trigger.
We utilized the astroalign (Beroiz et al. 2020, A&C, 32, 100384) and astropy (Astropy Collaboration et al. 2022, ApJ, 935, 167) packages to align and stack the individual frames. We did not find any evidence of a new and uncataloged source in the stacked images at the position of the proposed optical (ATel #17272) and radio (ATel #17258) counterparts.
On 3 July 2025, we did another set of follow-up observations with the 0.5-m RIFT at Lulin from 15:19 to 15:40 UTC. Six 200-s unfiltered images were taken, and no new source was detected at the reported location.
Moreover, we utilized the Python-based package AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform the PSF photometry on our stacked frames. The details of the observations and measured 3-sigma upper limit (in the AB system) are as follows:
Telescope | Filter | MJD (start) | Exposure (s) | Magnitude | avg. Seeing | med. Airmass
SLT | rp | 60853.619 | 300*12 | >18.8 | 2".26 | 1.87
RIFT | white | 60859.638 | 200*6 | >19.0 | 3".81 | 1.90
The presented magnitudes are calibrated using the field stars from the SkyMapper catalog (r-band) and are not corrected for the expected Galactic foreground extinction corresponding to a reddening of A_r = 2.5 mag in the direction of the transient (Schlafly & Finkbeiner 2011).
Based on our observations, the source may brighten on 28 June, 2025 (ATel #17272) and it has become fainter since 3 July, 2025. We encourage further monitoring of this source in all wavelengths.