NICER observations of the ongoing X-ray outburst from SAX J1747.0-2853

Following the announcement of a possible new transient discovered by MAXI (ATel #12576) and the follow-up communication that this transient was instead the previously known neutron star LMXB SAX J1747.0-2853 (ATel #12578), we observed this source with NICER starting from 2019 March 14 17:37 UT, for a total exposure of approximately 1.5 ksec. The average source count rate measured by NICER was 115 cts/sec (0.2-12 keV). The 1-10 keV power spectrum does not present significant periodic or quasi-periodic signals in the frequency range 1E-3Hz - 1E3 Hz. The continuum X-ray spectrum extracted from the NICER data in the 0.6-10 keV energy range is well described (red. chi^2 1.16 for 914 d.o.f.) by an absorbed thermally comptonized continuum (nthcomp in xspec). The Crab correction was applied to the data in order to deal with unmodelled instrument systematics (see, e.g., Ludlam et al. 2018, ApJL 858, L5). The absorption column density derived from the spectral fit is N_H = 7.4(1)E22 cm^-2, comparable to the integrated H column density obtained from the Chandra observation of the source in quiescence (Wijnands, Miller & Wang 2002, ApJ 579, 422). The thermally comptonized component is characterized by a photon index Gamma=2.4(2) produced by an electron population with temperature kTe=2.5(4) keV and a black-body seed photon temperature kTb=0.7(1) keV. The absorbed 0.6-10 keV source flux is 1.3(2)E-9 erg cm^-2 s^-1. There is no evidence of a spectral Fe line feature in the energy range 6.4-7 keV. Multiwavelength observations of this ongoing transient activity are encouraged. NICER is a 0.2-12 keV X-ray telescope operating on the International Space Station. The NICER mission and portions of the NICER science team activities are funded by NASA.