Optical detection of the X-ray transient EP240809a from ATLAS and host galaxy search using archival DECaPS data

ATel #16767; Jiazheng Zhu (USTC), Ning Jiang (USTC), Tinggui Wang (USTC), Yibo Wang (USTC), W. X. Li (NAOC)
on 12 Aug 2024; 17:11 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Jiazheng Zhu (jiazheng@mail.ustc.edu.cn)

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We report an optical counterpart of the X-ray transient, EP240809a, which was initially discovered by the Einstein Probe (EP) mission and confirmed by Swift at the location of R.A. = 268.26769 deg, DEC = -45.86380 deg with an uncertainty of 0.48 arcsec (Liu et al., ATel#16765). The optical flare was first detected by ATLAS o-band on 2024-07-17T03:00:00 (UTC) with a magnitude of 19.31 +- 0.22, three weeks before the WXT detection. The closest non-detection epoch was at 2024-07-13T04:00:00 (UTC) and the latest magnitudes on 2024-08-11 is 18.78 +-0.10 in o-band. All the photometry was obtained using the ATLAS Forced Photometry service. We obtained a primary SED near the peak using the photometry from Swift and ATLAS. We assumed that the absorption column density of soft X-ray is entirely contributed by the interstellar medium, therefore EP240809a has an estimated E(B-V) ~ 0.26. The UV-optical SED is generally blue, with a temperature of (18,000 +- 860)K when fitted by a blackbody. Both the blackbody temperature and the soft X-ray flare are consistent with a TDE scenario. To test the possible TDE origin of EP240809a, we collect the i,z,Y images of the DECam Plane Survey (DECaPS) taken in 2019 to search for the host galaxy signal. The DECam images show non-detection at the location within the circle of 90% confidence level of the UVOT position. Unfortunately, there are four stars within the 3 arcsec region. The closest star is ~19.6 mag in i-band with an (i-Y) color ~ 0.3. Follow-up spectroscopic observations are highly encouraged to identify the nature of this transient.

EP-WXT detection of an X-ray transient EP240809a and Swift follow-up observation