The energetic thermonuclear bursts in SAX J1712.6-3739

Abstract: The neutron star low-mass X-ray binary SAX J1712.6-3739 is known for its very long and hard thermonuclear bursts in past X-ray observations. Its thermonuclear bursts are so energetic that they can even last for tens of minutes in the hard X-ray observations of Swift/BAT (E > 15 keV). For exploring the natures of SAX J1712.6-3739 and its thermonuclear bursts, we analyzed its four bursts which were all captured by Swift/BAT. The peak flux and fluence for these bursts are derived by applying the archive observation data of Swift/BAT and Swift/XRT. Based on the peak fluxes observed by Swift, the distance of SAX J1712.6-3739 is estimated as 4.6 kpc, which is much closer than the estimate by the BeppoSAX observations. The corresponding absolute magnitude and average accretion rate agree its ultra-compact X-ray binary nature. By the measurements of effective duration, we determined 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 superbursts. Since the average mass accretion rate of SAX J1712.6-3739 was only 0.6% Eddington accretion rate, and current theory predict no carbon production in the bursters under very low accretion rate. Therefore, the 2014 and the 2018 bursts are very likely to be deep helium bursts. In the Swift observations, SAX J1712.6-3739 has shown at least three different levels of duration in thermonuclear bursts. They were possibly induced by variable accretion rates, because their ignition column depths roughly follow the prediction of ignition models for pure helium bursts at corresponding accretion rates.