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The Astronomer's Telegram                   http://www.astronomerstelegram.org

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ATEL #16578							    ATEL #16578

Title:	Detection of the X-ray source associated with SDSS J133519.91+072807.4
		by Einstein Probe
Author:	Y. L. Wang, H. Sun, Z. X. Ling, Y. Liu, C. Zhang, C. C. Jin,
		H. Q. Cheng, W. Chen, C. Z. Cui, D. W. Fan, H. B. Hu, J. W. Hu, M.
		H. Huang, D. Y. Li, T. Y. Lian, H. Y. Liu, M. J. Liu, Z. Z. Lv, X.
		Mao, H. W. Pan, X. Pan, W. X. Wang, Q. Y. Wu, X. P. Xu, Y. F. Xu,
		H. N. Yang, W. Yuan, M. Zhang, W. D. Zhang, W. J. Zhang, Z. Zhang,
		D. H. Zhao (NAOC, CAS), E. Kuulkers, A. Santovincenzo (ESA), P. O'Brien
		(Univ. of Leicester), K. Nandra, A. Rau (MPE), B. Cordier (CEA), on
		behalf of the Einstein Probe team
Queries:	ep_ta@nao.cas.cn
Posted:	8 Apr 2024; 08:09 UT
Subjects:X-ray, AGN, Transient, Tidal Disruption Event

We report on the independent detection of an X-ray source EP240327a by
the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission.
The observation began at 2024-03-27T09:51:02 (UTC) with a total exposure
time of 24011 seconds. The position of the source is R.A. = 203.853 deg,
DEC = 7.488 deg (J2000) with an uncertainty of 3 arcmin (radius, 90% C.L.
statistical and systematic). The averaged spectrum can be fitted by an
absorbed blackbody model (tbabs*bbody) with NH fixed to the Galactic value
of 2.7 x 10^20 cm^-2. The blackbody temperature is estimated to be 79(-15/+18)
eV. The derived average unabsorbed flux in 0.5 - 4.0 keV is 3.5(-0.9/+0.9)
x 10^-12 erg/s/cm^2. 

The position of EP240327a is 1.7 arcmin away from the X-ray source reported
by Hernandez-Garcia et al. (ATel #16576), suggesting EP240327a being consistent
with this source within its positional error, which is found to be associated
with the nucleus in the early-type galaxy SDSS J133519.91+072807.4 at a
redshift of 0.0024.

The source was not detected, however, in a few of other WXT long-term observations.
One of the observations started one day earlier, at 2024-03-26T20:51:07
(UTC) with a total exposure time of 23297 seconds, giving an upper limit
of 2.4 x 10^-12 erg/s/cm^2 in 0.5 - 4.0 keV (3 sigma limit). The other
observation with a relatively long exposure time of 42267 seconds started
at 2024-03-30T06:52:37 (UTC), giving an upper limit of 1.8 x 10^-12 erg/s/cm^2
in 0.5 - 4.0 keV (3 sigma limit).

The above observation was made with the WXT instrument during the commissioning
phase of EP. 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). EP is a mission
of the Chinese Academy of Sciences in collaboration with ESA, MPE and CNES.

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