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The neutron star low-mass X-ray binary SAX J1712.6-3739 is known for its verylong and hard thermonuclear bursts in past X-ray observations. Itsthermonuclear bursts are so energetic that they can even last for tens ofminutes in the hard X-ray observations of Swift/BAT (E > 15 keV). For exploringthe natures of SAX J1712.6-3739 and its thermonuclear bursts, we analyzed itsfour bursts which were all captured by Swift/BAT. The peak flux and fluence forthese bursts are derived by applying the archive observation data of Swift/BATand Swift/XRT. Based on the peak fluxes observed by Swift, the distance of SAXJ1712.6-3739 is estimated as 4.6 kpc, which is much closer than the estimate bythe BeppoSAX observations. The corresponding absolute magnitude and averageaccretion rate agree its ultra-compact X-ray binary nature. By the measurementsof effective duration, we determined that the 2010 burst corresponds to anormal X-ray burst, the 2011 burst is consistent with an intermediate-durationburst while the 2014 and the 2018 bursts are more energetic than commonintermediate-duration bursts but less energetic than superbursts. Since theaverage mass accretion rate of SAX J1712.6-3739 was only 0.6% Eddingtonaccretion rate, and current theory predict no carbon production in the burstersunder very low accretion rate. Therefore, the 2014 and the 2018 bursts are verylikely to be deep helium bursts. In the Swift observations, SAX J1712.6-3739has shown at least three different levels of duration in thermonuclear bursts.They were possibly induced by variable accretion rates, because their ignitioncolumn depths roughly follow the prediction of ignition models for pure heliumbursts at corresponding accretion rates.

The energetic thermonuclear bursts in SAX J1712.6-3739