NICER observations the new X-ray transient SWIFT J174038.1-273712

Referred to by ATel #: 14550, 14552Following the discovery of the new X-ray transient SWIFT J174038.1-273712 by the Neil Gehrels Swift Observatory (ATels #14536), we triggered observations with the Neutron Star Interior Composition Explorer (NICER) that started on 2021 April 9 at 14:52 UT (total exposure of ~27 ksec).NICER's average source count rate was 4.2 cts/sec (0.5-10 keV). The 0.5-10 keV power spectrum does not present significant periodic or quasi-periodic signals in the frequency range 0.005Hz - 2.500 Hz. The power spectrum shows low-frequency variability (red noise) with a fractional rms amplitude of 28+/-1% (0.5-10 keV as measured in the 0.007-1.7 Hz range).The continuum X-ray spectrum extracted from the NICER data in the 0.5-10 keV energy range is well described (red. chi^2 1.24 for 639 d.o.f.) by an absorbed blackbody and a thermally comptonized continuum ((Tbabs(bbody+nthcomp) in XSPEC). The absorption column density derived from the spectral fit is N_H = 0.93(2)E22 cm^-2. The blackbody component is described by a temperature kT=0.39(2) keV while the thermally comptonized component is characterized by a photon index Gamma=2.3(3) produced by an electron population with temperature kTe<=4 keV and seed photon temperature linked to the blackbody component. There is marginal evidence of a broad (sigma 0.54(13) keV) Fe line feature at 6.64(14) keV. The absorbed 0.5-10 keV source flux is 1.5(2)E-11 erg cm^-2 s^-1, that implies a relatively low-luminosity (L~2e35 erg/s) system, even assuming a source distance of 10 kpc.Currently, the temporal and spectral properties of SWIFT J174038.1-273712 do not allow us to unambiguously identify the compact object's nature in the binary X-ray system. The observed source flux is either compatible with the beginning/ending of a standard outburst from a Low-mass X-ray binary or the beginning of the outburst of a so-called very faint X-ray transient (Wijnands et al. 2006, A&A, 449, 1117). Therefore, further X-ray as well as multi-wavelength observations are strongly 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.