TITLE: GCN CIRCULAR NUMBER: 38596 SUBJECT: EP241217a: Swift-XRT Detection DATE: 24/12/17 18:53:37 GMT FROM: Maia Williams at PSU mjw6837@psu.edu
M. A. Williams (PSU), M. H. Siegel (PSU), J.A. Kennea (PSU), K.L. Page (U. Leicester), S. Dichiara (PSU), and P.A. Evans (U. Leicester) report on behalf of the Swift team:
Swift-XRT has performed follow-up observations of the Einstein Probe/WXT-detected X-ray transient EP241217a. We searched for X-ray sources in 1.6 ks of Photon Counting (PC) mode data. The total exposure at the position of the transient (see below) is 1.537 ks, obtained between T0+24.2 ks and T0+25.7 ks.
An uncatalogued X-ray source is detected within the estimated 90% EP/FXT error region (20 arcsec) and is believed to be associated with the EP source (GCN Circ. 38586). Using 1537 s of PC mode data and 1 UVOT image, we find an enhanced XRT position (using the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue): RA, Dec = 46.94156, +30.92977 which is equivalent to:
RA (J2000): 03h 07m 45.98s Dec(J2000): +30d 55' 47.2"
with an uncertainty of 4.1 arcsec (radius, 90% confidence). This position is 0.091 arcmin from the EP/FXT position and 3.2 arcsec from the optical counterpart reported by Levan et al. (GCN Circ. 38587). The peak count rate is 0.039 (+0.007, -0.006) ct s-1 (0.3 — 10 keV), which suggests fading from the EP/WXT detection. The observed 0.3-10 keV flux is 1.5 (+0.7, -0.3) × 10-12 erg cm-2 s-1.
A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.7 (+0.5, -0.4). The best-fitting absorption column is 1.9 x 10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.89 × 10^-11 (4.99 × 10^-11) erg cm^-2 count^-1.
A summary of the PC-mode spectrum is thus: Galactic column: 1.9 x 10^21 cm^-2 Photon index: 1.7 (+0.5, -0.4)
The Swift/UVOT began settled observations of the field of EP 241217a 24.3 ks after the EP trigger. A 1639s image taken in the u filter shows no optical transient at the XRT position down to a limit of u=20.83. We show no source consistent with that reported by Levan et al. (GCN Circ. 38587), Izzo et al. (GCN Circ. 38588), and Fan et al. (GCN Circ 38592). The lack of UVOT detection would be consistent with the redshift of z=4.6 reported by Levan et al. (GCN Circ. 38593).
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