Swift/XRT confirms new outburst from XTE J1739-285

Following the INTEGRAL announcement of a new outburst from the X-ray transient source XTE J1739-285 (ATel #13474), which was suggested to host the fastest rotating millisecond X-ray pulsar (Kaaret et al., 2007, ApJL 657, 2, L97-L100), a Swift/XRT ToO observation was triggered to measure the source position down to the arcsec accuracy and obtain a description of the soft X-ray spectrum.
Swift/XRT observed the source on 2020 February 13th at 23.48 UTC for about 1 ks. A single source has been detected within the XRT field of view. The best determined position is at RA=264.9744 and DEC=-28.4963 (J2000), with an associated accuracy of 2.2 arcsec (at 90% c.l.; as provided by the XRT data analysis on-line tool; Evans et al. 2009, MNRAS, 397, 1177). This is compatible (to within an offset of 1.4 arcsec) with the best known position of XTE J1739-285 determined by Chandra (ATel #777). The source spectrum could be well described by an absorbed power-law with an absorption column density of (3.3+/-0.8)E22 cm^(-2) and a photon index of 1.5+/-0.3. The estimated 0.5-10 keV flux is 1.7E-10 erg/cm^2/s (not corrected for absorption). No type-I bursts have been recorded during the observation.
We also performed a combined fit between the current XRT data and the INTEGRAL IBIS/ISGRI data reported in ATel #13474. The combined XRT and IBIS/ISGRI spectrum is well described by an absorbed powerlaw with a high-energy cutoff (reduced chi squared 1.11 for 47 d.o.f.). The best-fit parameters are nH=2.6±0.8 Gamma;=1.0±0.4, and Ecut=17¹⁵-7 keV. The derived flux in the 0.5-100 keV is of 3.5e-10 erg/s/cm².
We thank the Swift team for the very rapid scheduling of the ToO observation.