New insights into the quasi-periodic X-ray burster GS 0836-429

GS 0836-429 is a neutron star X-ray transient that displays Type-I X-ray bursts. In 2003 and 2004 it experienced two outbursts in X-rays. We present here an analysis of the system bursting properties during these outbursts. We studied the evolution of the 2003-2004 outbursts in soft X-rays using RXTE (2.5-12 keV; ASM), and in hard X-rays with INTEGRAL (17-80 keV, IBIS/ISGRI). Using data from the JEM-X monitor onboard INTEGRAL we detected 61 Type-I X-ray bursts, and confirm that the source displayed a quasi-periodic burst recurrence time of about 2.3 hours. We improve the characterization of the fuel composition, as well as the description of the typical burst durations and fluences. We estimate the average value of $\alpha$ to be $49\pm\,3$. This value together with the observed burst profiles indicate a regime of a mixed He/H runaway triggered by unstable helium ignition. In addition, we report the detection of four series of double bursts, with burst recurrence times of $\leq\,20$ minutes. The measured recurrence time in double bursts is too short to allow the accretion of enough fresh material, necessary to trigger a Type-I X-ray burst. This suggests the presence of left-over, unburned material from the preceding burst which gets ignited in a time scale of minutes. The energies and time scales of the secondary bursts suggest a lower fraction of hydrogen compared to that estimated for the primary bursts. The persistent emission was roughly constant during the period when the Type I X-ray bursts were detected. We derive an average accretion rate during our observations of $\dot{m}\sim\,8\,\%\,\dot{m}_{Edd}$. The spectrum of the persistent emission can be fit with a non-thermal component, indicative for the source to be in a hard state when the INTEGRAL observations were performed.