  Type I X-ray bursts in GS 1826-24, and in several other systems, may induce
cooling of the hot inner accretion flow that surrounds the bursting neutron
star. Given that GS 1826-24 remained persistently in the hard state over the
period 2003-2008 and presented regular bursting properties, we stacked the
spectra of the X-ray bursts detected by INTEGRAL (JEM-X and ISGRI) and
XMM-Newton (RGS) during that period to study the effect of the burst photons on
the properties of the Comptonizing medium. The extended energy range provided
by these instruments allows the simultaneous observation of the burst and
persistent emission spectra. We detect an overall change in the shape of the
persistent emission spectrum in response to the burst photon shower. For the
first time, we observe simultaneously a drop in the hard X-ray emission,
together with a soft X-ray excess with respect to the burst blackbody emission.
The hard X-ray drop can be explained by burst-induced coronal cooling, while
the bulk of the soft X-ray excess can be described by fitting the burst
emission with an atmosphere model, instead of a simple blackbody model.
Traditionally, the persistent emission was assumed to be invariant during X-ray
bursts, and more recently to change only in normalization but not in spectral
shape; the observed change in the persistent emission level during X-ray bursts
may thus trigger the revision of existing neutron star mass-radius constraints,
as the derived values rely on the assumption that the persistent emission does
not change during X-ray bursts. The traditional burst fitting technique leads
to up to a 10% overestimation of the bolometric burst flux in GS 1826-24, which
significantly hampers the comparisons of the KEPLER and MESA model against this
'textbook burster'.
