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ATEL #16514							    ATEL #16514

Title:	Swift follow-up observation of the EP X-ray transient EP240305a
		(EPW20240305aa)
Author:	M. J. Liu (NAOC, CAS), D. Y. Li (NAOC, CAS), Y. Liu (NAOC,
		CAS),  H. Q. Cheng (NAOC, CAS), W. D. Zhang (NAOC, CAS), C. Zhang
		(NAOC, CAS), Z. X. Ling (NAOC, CAS),  C. C. Jin (NAOC, CAS), W. Chen
		(NAOC, CAS), C. Z. Cui (NAOC, CAS), D. W. Fan (NAOC, CAS), H. B. Hu
		(NAOC, CAS), J. W. Hu (NAOC, CAS), M. H. Huang (NAOC, CAS),  T. Y.
		Lian (NAOC, CAS), H. Y. Liu (NAOC, CAS), Z. Z. Lv (NAOC, CAS), X.
		Mao (NAOC, CAS), H. W. Pan (NAOC, CAS), X. Pan (NAOC, CAS), H. Sun
		(NAOC, CAS), W. X. Wang (NAOC, CAS), Y. L. Wang (NAOC, CAS), Q. Y.
		Wu (NAOC, CAS), X. P. Xu (NAOC, CAS), Y. F. Xu (NAOC, CAS), H. N.
		Yang (NAOC, CAS), M. Zhang (NAOC, CAS), W. J. Zhang (NAOC, CAS), 
		S. N. Zhang (IHEP, CAS), Z. Zhang (NAOC, CAS), D. H. Zhao (NAOC, CAS),
		E. Kuulkers (ESA), P. O'Brien (Univ. of Leicester), and W. Yuan
		(NAOC, CAS), on behalf of the Einstein Probe team
Queries:	liuyuan@bao.ac.cn
Posted:	8 Mar 2024; 03:54 UT
Subjects:X-ray, Transient

Following up on the new X-ray flare EPW20240305aa (renamed EP240305a hereafter)
detected by Einstein Probe (EP) WXT on March 5 2024 (ATel #16509), we performed
a target of opportunity observation with Swift. The Swift XRT observation
began at 2024-03-07T15:44:51(UTC) with an exposure time of 2652 seconds
in the Photon Counting mode. An X-ray source is detected within the positional
error circle (3 arcmin radius) of the EP source with a signal-to-noise
(S/N) ratio of 4.5. The XRT position is RA (J2000) = 122.921, Dec(J2000)
=-54.652, with an uncertainty of 5.3 arcsec (radius, 90% C.L. statistical
and systematic). The source brightness is one order of magnitude above
the flux limit previously set by the eROSITA survey (Tubin-Arenas, D.,
et al. 2024). We suggest that this XRT source is most likely associated
with EP240305a. The XRT position is about 2 arcmin away from the faint
eROSITA source (Merloni, A., et al 2024) mentioned in ATel #16509 , thus
excluding the association of the XRT source with the eROSITA source. 

The XRT spectrum can be fitted by an absorbed power law model with a photon
index of 1.3 (-0.8/+0.7). The absorption value is fixed based on the modelling
of the EP data (ATel #16509). The corresponding unabsorbed flux in 0.5-4.0
keV is 3.9e-13 erg/s/cm2, which has dropped by three orders of magnitude
in about 49 hours since the EP WXT's observation. The UVOT image shows
a faint optical source within the error circle of the XRT position, which
is also found in the GAIA DR3 source catalog, with a parallax measurement
of 0.134+/-0.111 mas. Therefore, we tend to suggest that the source detected
by EP and Swift is possibly a new Galactic X-ray transient. Further follow-up
observations are encouraged to identify the nature of this X-ray transient.

We thank the Neil Gehrels Swift Observatory team for making the ToO observation
possible. 

Reference:

Merloni, A., et al., The SRG/eROSITA all-sky survey - First data release
and X-ray catalog of the western Galactic hemisphere, A&A, in press

Tubin-Arenas, D., et al., The eROSITA Upper Limits: Description and access
to the data, A&A, in press

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