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X-ray transients, such as accreting neutron stars, periodically undergooutbursts, thought to be caused by a thermal-viscous instability in theaccretion disk. Usually outbursts of accreting neutron stars are identifiedwhen the accretion disk has undergone an instability, and the persistent X-rayflux has risen to a threshold detectable by all sky monitors on X-ray spaceobservatories. Here we present the earliest known combined optical, UV, andX-ray monitoring observations of the outburst onset of an accreting neutronstar low mass X-ray binary system. We observed a significant, continuingincrease in the optical i'-band magnitude starting on July 25, 12 days beforethe first X-ray detection with Swift/XRT and NICER (August 6), during the onsetof the 2019 outburst of SAX J1808.4-3658. We also observed a 4 day optical toX-ray rise delay, and a 2 day UV to X-ray delay, at the onset of the outburst.We present the multiwavelength observations that were obtained, discussing thetheory of outbursts in X-ray transients, including the disk instability model,and the implications of the delay. This work is an important confirmation ofthe delay in optical to X-ray emission during the onset of outbursts in lowmass X-ray binaries, which has only previously been measured with lesssensitive all sky monitors. We find observational evidence that the outburst istriggered by ionisation of hydrogen in the disk.

Enhanced optical activity 12 days before X-ray activity, and a 4 day X-ray delay during outburst rise, in a low-mass X-ray binary