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From: atel@astronomerstelegram.org
Subject: ATel  17491
Message-Id: <20251113070623.377CA288092A@server.astronomerstelegram.org>
Date: Thu, 13 Nov 2025 02:06:23 -0500 (EST)

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Posted: Within the last 24 hours
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ATEL #17491							     ATEL #17491

Title:	EP251020a: Einstein Probe detection of an X-ray transient associated
		with the galaxy nucleus of LEDA 962438
Author:	Y.-H. I. Yin (HKU), S. Q. Jiang, H. Y. Ren, G. J. Yang, K.-J.
		Zhang, D. Zhu, Y. Liu, D. Y. Li, C. Jin, W. Yuan (NAOC), L. Dai, C.
		K. Kan, Z. Zhang (HKU) on behalf of the Einstein Probe (EP) team
Queries:	ep_ta@bao.ac.cn
Posted:	13 Nov 2025; 03:47 UT
Subjects:X-ray, Transient, Tidal Disruption Event

We report on the detection of an X-ray transient by the Wide-field X-ray
Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP251020a.
The source was first detected in an observation starting at 2025-10-20T05:39:14
(UTC), and was continuously detected in subsequent observations spanning
from 2025-11-03T12:51:08 to 2025-11-11T09:34:52. The WXT position of the
source is R.A. = 74.292 deg, DEC = -11.746 deg (J2000) with an uncertainty
of 3.1 arcmin in radius (90% C.L. statistical and systematic). The WXT
position is consistent with a galaxy LEDA 962438.

Stacking of the WXT data taken prior to the WXT detection indicated that
the source has already emerged on late September, 2025. The peak WXT 0.5-4
keV spectrum can be fitted with an absorbed power law with a fixed Galactic
hydrogen column density of 8.00 x 10^20 cm^-2 and a photon index of 3.75
(-0.61/+0.61). The derived peak unabsorbed 0.5-4 keV flux is 1.37(-0.20/+0.24)
x 10^(-11) erg/s/cm^2, corresponding to an X-ray luminosity of around 2.3e43
erg/s.

We performed a target-of-opportunity follow-up observation with the Follow-up
X-ray Telescope (FXT) at 2025-11-05T09:35:40 (UTC). Within the WXT error
circle, the X-ray source was detected at R.A. = 74.2769 deg, DEC = -11.7286
deg (J2000) with an uncertainty of 10 arcsec in radius (90% C.L. statistical
and systematic). The FXT position is around 2 arcsec away from the galaxy
LEDA 962438. The average FXT 0.5-10 keV spectrum can be fitted with an
absorbed power law and an absorbed blackbody, with a fixed Galactic hydrogen
column density of 8.00 x 10^20 cm^-2, a photon index of 3.13 (-0.37/+0.37),
and a temperature of around 0.12 keV. The derived average unabsorbed 0.5-10
keV flux is 1.35 (-0.17/+0.16) x 10^(-11) erg/s/cm^2.

We also requested a Swift/XRT ToO observation which started at 2025-11-07T11:50:57
(UTC). The observation, with an exposure time of 1.6 ks, unveiled the X-ray
source located at R.A. = 74.27657, -11.72783 deg (J2000) with an uncertainty
of 2.3 arcsec in radius (90% C.L. statistical and systematic). The XRT
position is around 1.03 arcsec away from the center of LEDA 962438. The
derived average unabsorbed 0.3-10 keV flux is around 2.10 (-0.30/+0.37)
x 10^(-11) erg/s/cm^2.

The UVOT detection reveals a source at the nucleus of LEDA 962438 on the
images of 6 filters, which is brightened compared to the GALEX history
detection.

Preliminary detections of UVOT in AB system are:

Filter    Exp(s)    Mag

B         132       17.02 +/- 0.06

U         132       18.37 +/- 0.09

V         132       16.29 +/- 0.06

UVW1      264       19.29 +/- 0.13

UVM2      373       19.80 +/- 0.14

UVW2      528       19.74 +/- 0.10

The magnitudes in the table are not corrected for the Galactic extinction.

Based on the current multi-wavelength results, this source is likely a
new tidal disruption event (TDE) in the galaxy LEDA 962438. Further multi-wavelength
monitoring has been planned to track its long-term evolution.

Launched on January 9, 2024, EP is a space X-ray observatory to monitor
the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook
of X-ray and Gamma-ray Astrophysics).

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Password Certification: Yuan Liu (liuyuan@bao.ac.cn)
https://www.astronomerstelegram.org/?read=17491
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