INTEGRAL detects a rebrightening of the accreting neutron star XTE J1739-285

During observations of the Galactic Center region, INTEGRAL has detected renewed activity of the neutron star X-ray binary XTE J1739-285, which was suggested to have the fastest spin rate, based on burst oscillations (Kaaret et al., 2007, ApJL 657, 2, L97-L100). The most recent outburst from this system was reported in September 2019 (ATel #13138 ). The source was observed by INTEGRAL from February 8 at 9:23 to Feb 9 at 00:07 (UT), and from February 12 at 14:42 to Feb 13 at 5:25 (UT). Between these two observations the source flux as seen by IBIS/ISGRI in the 28-40 keV energy band increased from 15+/-2 mCrab to 20+/-2 mCrab, which suggests we are observing the onset of a new outburst of this system. Fitting the combined source spectrum in the 20-100 keV energy with a power law model, we derive a photon index = 4.2 (-1.2/+1.6) and a 20-100 keV flux =2.0 ± 0.4 1e-10 erg/s/cm2 (net exposure of 54 ks). The source was detected at the edge of the JEM-X field of view of two science windows (3 ks total) during these observations, from which we derive a preliminary 3-sigma upper limit of about 10 mCrab in the 3-20 keV energy range. No Type-I X-ray bursts have been detected by JEM-X. Additional observations of the Galactic Center region by INTEGRAL will be performed in the coming days. We encourage follow up of this source at all wavelengths.