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Motivated by the recent observation by NICER of a type I X-ray burst from SAXJ1808.4-3658 with a distinct "pause" feature during its rise (Bult et al.2019), we show that bursts which ignite in a helium layer underneath ahydrogen-rich shell naturally give rise to such pauses, as long as enoughenergy is produced to eject the outer layers of the envelope by super-Eddingtonwinds. The length of the pause is determined by the extent of the convectiongenerated after ignition, while the rate of change of luminosity following thepause is set by the hydrogen gradient left behind by convection. Using the MESAstellar evolution code, we simulate the accumulation, nuclear burning andconvective mixing prior to and throughout the ignition of the burst, followedby the hydrodynamic wind. We show that the results are sensitive to thetreatment of convection adopted within the code. In particular, the efficiencyof mixing at the H/He interface plays a key role in determining the shape ofthe lightcurve. The data from SAX J1808.4-3658 favors strong mixing scenarios.Multidimensional simulations will be needed to properly model the interactionbetween convection and nuclear burning during these bursts, which will thenenable a new way to use X-ray burst lightcurves to study neutron star surfaces.

The imprint of convection on Type I X-ray bursts: Pauses in photospheric radius expansion lightcurves