INTEGRAL & XMM-Newton observations of GS 1826-238: the clock wagging its tail
Combining the spectral capabilities of ESA's missions INTEGRAL
(IBIS/ISGRI and JEM-X) and XMM-Newton (RGS) one is able to disentangle
the spectra of the burst and persistent emission of the popular X-ray
'clocked' burster GS 1826-238.
Type-I X-ray bursts are thermonuclear explosions in the surface layers
of weakly magnetized accreting neutron stars (NS) in low mass X-ray
binary systems. As the accreted material accumulates on the surface of
the NS, the density and temperature increase until eventually ignition
temperatures are reached and a thermonuclear runaway is triggered. The
release of nuclear energy heats up the NS ocean and atmosphere in a few
seconds and increases the system luminosity by various orders of magnitude.
In 15 years of INTEGRAL observations of GS 1826-238, bursts were
identified which occurred when the source was in the hard state. The
stable spectral and bursting properties of the system in the hard state
allowed to stack spectra during these bursts and perform a time-resolved
burst-spectral analysis. These data were complemented with the available
XMM-Newton observations of the source.
The evolution of the average burst light curve is shown in Figure 1.
Simultaneously at the time of the peak of the burst detected in soft
X-rays, a drop in the source's hard X-ray emission (i.e., above 35 keV)
is observed. This drop is also seen in the source's X-ray spectrum,
shown in Figure 2. The emission can be modelled in terms of a cooling of
the coronal electrons in response to the intense burst photon shower. As
the burst evolves, the hard X-ray emission and coronal electron
temperatures slowly return to pre-burst values.
At the same time a soft X-ray excess with respect to the burst
black-body emission can be seen. This excess can be modelled by fitting
the burst emission with, a, more realistic, atmosphere model, instead of
a simple black-body model.