GCN Circular 41469

S. Dichiara (PSU), S. R. Oates (Lancaster U.), J.A. Kennea (PSU),
K.L. Page (U. Leicester) report on behalf of the Swift-XRT and Swift-UVOT team:

Swift-XRT has performed follow-up observations of the Einstein Probe/WXT-detected
source EP250821a (Hu et al., GCN 41459
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 ), collecting 1.6 ks of Photon Counting (PC)
mode data between T0+4.9 ks and T0+10 ks after the trigger. A likely counterpart has
been found. The details of this source are:

  Source 1 (SWIFT J191932.6-432250):
  ==================================
    RA (J2000.0):   289.8861  =  19 19 32.66
    Dec (J2000.0):  -43.3807  =  -43 22 50.5
    Error:	    3.6 (arcsec, radius, 90% confidence).
    Detect flag:    GOOD
    Distance:	    2.2 arcmin from the Einstein Probe/WXT position.
    Mean rate:	    0.263 +/- 0.015 ct s^-1
    Mean flux:	    (9.44 +/- 0.53)e-12 erg cm^-2 s^-1
    Peak rate:	    0.440 +/- 0.099 ct s^-1
    Peak flux:	    (1.58 +/- 0.35)e-11 erg cm^-2 s^-1
    ECF:	    3.58e-11 erg cm^-2 ct^-1
		      assuming NH=1.38e+21 cm^-2, gamma=1.93
		      determined from a spectral fit.
    XMM UL:	    5.5e-12 erg cm^-2 s^-1, (0.3-10 keV)
		      so the source is 2.9-sigma above this 3-sigma upper limit.
    The source may be fading, at the 1.1-sigma level.


All fluxes are 0.3-10 keV, observed. For all flux conversions and comparisons with
catalogues and upper limits  from other missions, we assumed a power-law spectrum
with NH=3x10^20 cm^-2 and photon index (Gamma)=1.7 unless otherwise stated.

The results of the XRT-team automatic analysis of the XRT observations, including a
position-specific upper limit calculator, are available at
https://www.swift.ac.uk/EP/EP_FIELD00059.

Source 1 was also detected in the UVOT u-band image. The preliminary UVOT position
is:
    RA (J2000.0):   289.883  =	19 19 31.86
    Dec (J2000.0):  -43.3816  =   -43 22 53.8


Preliminary detection u-band magnitude in 1897 s exposure is 18.88 +/- 0.08, using
the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373)

We note that the source is also listed in the Guide Star Catalog  (Lasker et al,
2008) and
it has clearly brightened from its catalogued brightness (Bmag = 22.3).

This circular is an official product of the Swift XRT and UVOT team.