  Thermonuclear X-ray bursts from accreting neutron stars power brief but
strong irradiation of their surroundings, providing a unique way to study
accretion physics. We analyze MAXI/GSC and Swift/XRT spectra of a day-long
flash observed from IGR J17062-6143 in 2015. It is a rare case of recurring
bursts at a low accretion luminosity of 0.15% Eddington. Spectra from MAXI,
Chandra, and NuSTAR observations taken between the 2015 burst and the previous
one in 2012 are used to determine the accretion column. We find it to be
consistent with the burst ignition column of 5x10^10 g cm^-2, which indicates
that it is likely powered by burning in a deep helium layer. The burst flux is
observed for over a day, and decays as a straight power law: F~t^-1.15. The
burst and 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 burst peak, the Comptonized component disappears, when the burst may
dissipate the Comptonizing gas, and it returns in the burst tail. The
reflection signal suggests that the inner disk is truncated at ~100
gravitational radii before the burst, but may move closer to the star during
the burst. At the end of the burst, the flux drops below the burst cooling
trend for 2 days, before returning to the pre-burst level.
