X-ray transients, such as accreting neutron stars, periodically undergo outbursts, thought to be caused by a thermal-viscous instability in the accretion disk. Usually outbursts of accreting neutron stars are identified when the accretion disk has un
dergone an instability, and the persistent X-ray flux has risen to a threshold detectable by all sky monitors on X-ray space observatories. Here we present the earliest known combined optical, UV, and X-ray monitoring observations of the outburst onset of an accreting neutron star low mass X-ray binary system. We observed a significant, continuing increase in the optical i-band magnitude starting on July 25, 12 days before the first X-ray detection with Swift/XRT and NICER (August 6), during the onset of the 2019 outburst of SAX J1808.4-3658. We also observed a 4 day optical to X-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 the theory of outbursts in X-ray transients, including the disk instability model, and the implications of the delay. This work is an important confirmation of the delay in optical to X-ray emission during the onset of outbursts in low mass X-ray binaries, which has only previously been measured with less sensitive all sky monitors. We find observational evidence that the outburst is triggered by ionisation of hydrogen in the disk.
Observations of the accretion powered millisecond pulsar SAX J1808.4-3658 have revealed an interesting binary evolution, with the orbit of the system expanding at an accelerated rate. We use the recent finding that the accreted fuel in SAX J1808.4-36
58 is hydrogen depleted to greatly refine models of the progenitor and prior evolution of the binary system. We constrain the initial mass of the companion star to 1.0-1.2 M$_{mathrm{odot}}$, more massive than previous evolutionary studies of this system have assumed. We also infer the system must have undergone strongly non-conservative mass transfer in order to explain the observed orbital period changes. Following Jia & Li (2015), we include mass loss due to the pulsar radiation pressure on the donor star, inducing an evaporative wind which is ejected at the inner Lagrangian point of the binary system. The resulting additional loss of angular momentum resolves the discrepancy between conservative mass transfer models and the observed orbital period derivative of this system. We also include a treatment of donor irradiation due to the accretion luminosity, and find this has a non-negligible effect on the evolution of the system.
We report the discovery of Type I (thermonuclear) X-ray bursts from the transient source XMMU J181227.8-181234 = XTE J1812-182. We found 7 X-ray bursts in Rossi X-ray Timing Explorer observations during the 2008 outburst, confirming the source as a n
eutron star low mass X-ray binary. Based on the measured burst fluence and the average recurrence time of 1.4$^{+0.9}_{-0.5}$ hr, we deduce that the source is accreting almost pure helium ($X leq 0.1$) fuel. Two bursts occurred just 18 minutes apart; the first short waiting time bursts observed in a source accreting hydrogen-poor fuel. Taking into consideration the effects on the burst and persistent flux due to the inferred system inclination of $30pm{10}$ degrees, we estimate the distance to be $14pm{2}$ kpc, where we report the statistical uncertainty but note that there could be up to $20%$ variation in the distance due to systematic effects discussed in the paper. The corresponding maximum accretion rate is $0.30pm0.05$ times the Eddington limit. Based on the low hydrogen content of the accreted fuel and the short average recurrence time, we classify the source as a transient ultracompact low-mass X-ray binary.
