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tokens: keV counts cm −2 s −1 , where the effective area of the XRT and NICER were taken as 110 and 1900 cm 2 , respectively. The optical and UV magnitudes have been dereddened. The first clear detection of SAX J1808.4−3658 with NICER during the rise was made on August 6, 22:00 utc , when the net (background-subtracted) count rate was 9.16 ± 0.16 counts s −1 . We fit the spectrum of that GTI with an absorbed power law ( tbnew × pegpwrlw in xspec ), with the photoionization cross-section set to vern and the abundances set to wilm . Our best fit (χ 2 = 88 for 108 degrees of freedom) yields an N H of 0.33 ± 0.03 atoms cm −2 , a power-law photon index of 2.43 ± 0.07, and an unabsorbed 0.5–10 keV flux of (2.91 ± 0.11) × 10 −11 erg cm −2 s −1 . A potential detection was made one GTI earlier, on August 5 22:46 utc , when the net count rate (1.44 ± 0.06 counts s −1 ) was higher than during all observations preceding it (maximum of 0.85 counts s −1 ). However, comparison to contemporaneous background measurements indicated that the flux was still background dominated, and there was no statistically significant detection. Between July 30 and August 5, the net count rates varied between −0.22 and 1.44 counts s −1 , with an average of 0.402 ± 0.003 counts s −1 . This rate is significantly above zero, possibly due to contaminating sources in NICER’s ∼30 arcmin 2 field of view. The variations in the pre-outburst net count rate are likely due to limitations of the background model (which is still being refined). The 1σ standard deviation in the net count rate before the first detection is 0.32 counts s −1 . We consider the average pre-outburst net count rate plus three times the 1σ standard deviation, 1.37 counts s −1 , to be a good general estimate of the NICER detection limit of SAX J1808.4−3658. Using the best-fitting model to the spectrum of the first detection, this count rate corresponds to an unabsorbed 0.5–10 keV flux of ∼4 × 10 −12 erg cm −2 s −1 . 3 RESULTS The multiwavelength light curve of the 2019 rise to outburst of SAX J1808.4−3658 is shown in Fig. 1 . Activity of the source was first noticed in the optical i ′ -band observations, beginning July 25 (MJD 58689), as the i ′ -band magnitude fluctuated between 0.7 and 1.0 mag brighter than the quiescent level of approximately 19.8 mag. Then, between August 3.85 and 5.98 (MJD 58698.85 and 58700.98), the i ′ -band intensity increased to approximately 2 mag brighter than the quiescent level, and continued to increase over the coming days as the outburst progressed. The peak of the 2019 outburst was reached at around August 10 (MJD 58705) in optical, and August 14 (MJD 58709) in X-ray (Fig. 1 ). The first Swift /UVOT detection occurred on August 4 (MJD 58699.71), 10 d after the first sign of increased optical activity in the source, and 2 d before the first X-ray detection. The UV magnitude in all bands rose sharply by approximately 3 mag over the next 4 d until the outburst peak was reached. We note that