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SAX J1748.9-2021 is a transiently accreting X-ray millisecond pulsar. It isalso known as an X-ray burster source discovered by Beppo-SAX. We analysed thepersistent emission and type-I X-ray burst properties during its 2015 outburst.The source varied from hard to soft state within half day. We modeled thebroad-band spectra of the persistent emission in the 1 - 250 keV energy bandfor both spectral states using the quasi-simultaneous INTEGRAL and Swift data.The broad-band spectra are well fitted by an absorbed thermal Componizationmodel, compps, in a slab geometry. The best-fits for the two states indicatesignificantly different plasma temperature of 18 and 5 keV and the Thomsonoptical depth of 3 and 4, respectively. In total, 56 type-I X-ray bursts wereobserved during the 2015 outburst, of which 26 detected by INTEGRAL in the hardstate, 25 by XMM-Newton in the soft state, and 5 by Swift in both states. Asthe object transited from the hard to the soft state, the recurrence time forX-ray bursts decreased from $\approx 2$ to $\approx1$ hr. The relation betweenthe recurrence time, $ \Delta t_{\rm rec}$, and the local mass accretion rateper unit area onto the compact object, $\dot m$, is fitted by a power-lawmodel, and yielded as best fit at $\Delta t_{\rm rec} \sim \langle \dot{m}\rangle^{-1.02\pm0.03}$ using all X-ray bursts. In both cases, the observedrecurrence times are consistent with the mixed hydrogen/helium bursts. We alsodiscuss the effects of type-I X-ray bursts prior to the hard to softtransition.

Mixed H/He bursts in SAX J1748.9-2021 during the spectral change of its 2015 outburst