X-ray Reflection and An Exceptionally Long Thermonuclear Helium Burst from IGR J17062-6143

Thermonuclear X-ray burstsfrom accreting neutron starspower brief but strong irradiation of their surroundings, providing a unique way to study accretion physics. We analyze MAXI/GSC and Swift/XRT spectra of an exceptionally long flash observed from IGR J17062-6143 in 2015. It is a rare case of recurring burstsat the low accretion luminosity of 0.15% Eddington. Spectra from MAXI, Chandra, and NuSTAR observations taken between the 2015 burstand the previous one in 2012 are used to determine the accretion column. We find it to be consistent with the burstignition column of 5x10^10 g cm^-2, which indicates that it is likely powered by burning in a deep helium layer. The burstflux is observed for hours, and decays as a straight power law: F~t^-1.15. The burstand persistent spectra are well described by thermal emission from the neutron star, Comptonization of this emission in a hot optically thin medium surrounding the star, and reflection off the photoionized accretion disk. At the burstpeak, the Comptonized component disappears, when the burstmay dissipate the Comptonizing gas, and it returns in the bursttail. The reflection signal indicates that the inner disk is truncated at ~100 R_g before the burst, but moves closer to the star during the burst.