tokens: of the burst spectrum (see Lewin et al. 1993 ; Strohmayer et al. 2006 , for a review). We scrutinised the NICER, XMM-Newton EPIC-MOS/PN, Swift -XRT, and NuSTAR data for the presence of thermonuclear bursts at different time scales (1, 10, and 100 s), but we found none. Next we focused on INTEGRAL JEM-X (3–35 keV) data and identified one thermonuclear burst candidate. We describe the analysis of the JEM-X data covering the event below. 6.1. Properties of the type-I X-ray burst We searched INTEGRAL/JEM-X data for bursts within revolutions 1986–2009, following the same approach as Mereminskiy et al. (2017) . We focussed on one of the two JEM-X cameras, JEM-X1; JEM-X2 was operating simultaneously, but this instrument seems to be experiencing reduced sensitivity in recent years. We identified one candidate thermonuclear (type-I) burst from IGR J17591−2342, in Scw-500010 of INTEGRAL revolution 2001 on Sept. 19, 2018; 23:07:33 (MJD 58380.96358 TT). The key properties of the burst are listed in Table 3 . Based on its intensity, this burst should have also been detectable by ISGRI; however, due to the loss of sensitivity over time in the lower ISGRI energy band, 18–27 keV, we did not detect this event in those data. Table 3. Parameters of the type-I X-ray burst observed by INTEGRAL JEM-X during the outburst of IGR J17591−2342 in 2018. Notes. (a) 58380.96358 MJD. (b) The τ fit has been measured after a plateau of ∼10 s. (c) Burst peak flux in 0.1–40 keV energy band. (d) Burst peak temperature. (e) Pre-burst unabsorbed flux in 0.1–250 keV energy range. (f) Burst fluence in 0.1–40 keV energy band. The type-I X-ray burst occurred ∼60 d after the outburst commenced, just after the source was at the highest X-ray luminosity (see Fig. 2 ). We defined the burst start time as the time at which the X-ray intensity of the source first exceeded 10% of the burst peak flux (above the persistent intensity level). The event showed a fast (∼1–2 s) rise time, followed by a ∼10 s long plateau at its peak (Fig. 8 ). Therefore, we conclude that most likely a photospheric radius expansion took place (see e.g. Falanga et al. 2007 ; Galloway et al. 2008 ; Bozzo et al. 2010 ). After the plateau, the burst decay profile could be well fitted with an exponential function and the correspondingly derived e-folding time is τ fit ∼ 9 s. The total duration of the burst was ∼40 s. Fig. 8. Light curve of the type-I X-ray burst detected by JEM-X and reported for the first time in this paper. The burst start time was 58380.96358 MJD. The JEM-X light curve was extracted in the 5–25 keV energy range with a time bin of 2 s. For the burst spectral analysis, we first determined the persistent flux before the burst onset by converting the ISGRI counts from Rev-2001 to the total broadband spectrum obtained from Revs. 1994–1995 (see Sect. 5 ). We note that during the outburst, the ISGRI data in the 20–150 keV band alone were fitted by a power-law model, and the spectral parameters were
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