by the burst [@He2016; @Fragile2018].

Spectral evolution
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The X-ray burst light curve shows a double-peaked structure. Given that the second peak in count-rate occurs after the PRE phase, and that this peak is reproduced (albeit less prominently) in the bolometric flux, we conclude that this feature is astrophysical in origin. A very similar double-peaked structure in an X-ray burst from 4U 1608–52 was recently observed with [*NICER*]{}[@Jaisawal2019]. Although that burst showed a hotter photosphere and lacked the strong soft excess that we detect in [SAX J1808]{}, the rebrightening phase is nearly identical in both bursts. In each case, the end of the PRE phase coincides with a pronounced dip in count-rate, and is followed by a secondary peak.

@Jaisawal2019 considered a number of plausible origins for the rebrightening of the burst flux, including the ignition of fresh material [@Keek2017b], stalled thermonuclear flame spreading [@Bhattacharyya2006], and waiting points in the rp-process [@Fisker2004]. Our observation of rebrightening in the X-ray burst from [SAX J1808]{}does not rule out any of these proposed explanations. It does, however, add two new perspectives: first, in [SAX J1808]{}the rebrightening coincides with the onset of burst oscillations, which may be difficult to reconcile a flame spreading model. 