2020ApJ...903...37L

The neutron star low-mass X-ray binary SAX J1712.6-3739 is known for its long and hard thermonuclear X-ray bursts from previous observations. Its thermonuclear bursts are so distinct as they can last for tens of minutes, as seen with Swift/BAT (E > 15 keV). To explore the origin of these extreme bursts and the nature of SAX J1712.6-3739, we analyzed the observations of all four bursts that were captured by Swift/BAT and derived the peak flux and the fluence of these bursts from joint studies with Swift/XRT and Swift/BAT. The derived bolometric peak fluxes observed by Swift set the distance of SAX J1712.6-3739 to be 4.6-5.6 kpc, while the derived absolute magnitude and average accretion rate agree with its ultracompact nature. Our measurements of the effective duration of these bursts conclude that the 2010 burst corresponds to a normal X-ray burst, the 2011 burst is consistent with an intermediate-duration burst, while the 2014 and the 2018 bursts are more energetic than common intermediate-duration bursts but less energetic than those known superbursts. We estimated that the average mass accretion rate of SAX J1712.6-3739 was about only 0.4%-0.7% {dot}M_Edd. Current theory predicts no carbon production in the bursters under such low accretion rate. If true, the 2014 and 2018 bursts are then deep helium bursts instead of carbon bursts. The thermonuclear bursts of SAX J1712.6-3739 have shown a very wide range of durations. The ignition model predicts that the diverse burst durations are induced by variable accretion rates, but current results provide only weak support to this inference.