Capturing a Pulsar Powering Up

X-ray transients, such as accreting neutron stars, can periodically undergo outbursts. The exact mechanism that causes the onset of outburst is unknown, but it is theorised that these kinds of outbursts are caused by a thermal-viscous instability in the accretion disk. Usually outbursts of accreting neutron stars are caught when the accretion disk has already undergone an instability, and the persistent flux has risen to a threshold detectable by all sky monitors on X-ray space observatories. In this presentation I will present the earliest known combined optical and X-ray monitoring observations of the lead-up and onset of outburst in an accreting neutron star system. We monitored the accretion-powered millisecond pulsar SAX J1808.4-3658 during the lead-up to the August 2019 outburst of the source using the Neil Gehrels Swift Observatory, the 2-m Faulkes Telescope South (at Siding Spring, Australia) and the Las Cumbres Observatory (LCO) network of 1-m robotic telescopes. We began our monitoring program 3 weeks before the outburst officially began, and observed a 12 day delay between the first optical rise and the first X-ray detection of the source. In this presentation I will discuss the implications of a 12 day delay between the optical and X-ray brightening, and relate this to the theory of outburst in soft X-ray transients such as SAX J1808.4-3658, including the disk instability model. Optical emission is thought to be produced by either the companion star or the outer disk, and X-ray emission is thought to be produced by the inner disk, where matter from the disk is transferred onto the neutron star. Theoretical calculations using the accretion rate and radius indicate that the viscous timescale of the disk in this system is approximately 12 days, coinciding with the optical to X-ray emission delay we observed.This work provides the earliest multiwavelength observations of an accreting neutron star coming into outburst that we are aware of, and is an important confirmation of the expected delay in optical to X-ray emission during the onset of outburst in soft X-ray transients. It also provides a new early indication of the onset of outburst in X-ray transients.