NICER observes increasing luminosity from XTE J1739-285

On 2019 September 27, routine monitoring of the Galactic Center with INTEGRAL revealed renewed activity from the neutron star X-ray transient XTE J1739-285 (ATel #13138). Follow-up observations with NICER commenced on September 27, collecting 19 ks of exposure between Sep 27 22:41 UTC and Sep 30 12:54 UTC. During this timespan we observed the source count-rate increase steadily from 200 c/s to 540 c/s. The 0.5-10 keV continuum emission could be reasonably well described using an absorbed blackbody plus power-law, for absorption column density of N_H = (2.5 +/ 0.1)e22 cm^-2. Over the course of our observations we see the 1-10 keV source flux increase from 1.9e-9 erg/cm^2/s to 5.0e-9 erg/cm^2/s. In parallel, we observe a modest but significant shift in spectral shape, with the blackbody temperature evolving from 0.960 +/- 0.013 keV to 1.113 +/- 0.009 keV; and the power-law photon index holding at Gamma = 1.84 +/- 0.02. Additionally, we observe complex residuals around 0.8 and 6.7 keV; however, more detailed spectral analysis is required to interpret these features. The 0.5-10 keV power spectrum of the source shows very low frequency noise (<0.5 Hz), with an integrated 0.01-100 Hz fractional rms of about 2%. No high frequency (>10 Hz) power was observed. Additionally, an acceleration search did not reveal any significant coherent periodicities above 50 Hz. No X-ray bursts have been observed thus far, but further NICER monitoring of this source is underway. 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.