A wide field view of the population of X-ray bursters in the Galaxy

Abstract

In this thesis several discoveries done with the Wide Filed Cameras (WFC) onboard the BeppoSAX satellite are presented. The main theme is about the low mass X-ray binaries and in particular the type I X-ray bursters. The population of low mass X-ray binaries in our Galaxy are concentrated towards the galactic center. Therefore, due to their large field of view (40o x 40o), the WFC are excellent instruments to observe a large fraction ( ?50%) of the population, simultaneously. In total, the WFC have observed the Galactic center region for 2 months over a period of six years. During the period over 1800 type I X-ray bursts have been observed from 37 sources. Also, over a dozen new transients have been detected, most of them also showing X-ray bursts. In this thesis four different subjects are addressed in the area of type I X-ray bursters. The first subject describes the discovery of SAX J1750.8-2900, a new member of the class of faint transients. The outburst lasted for -2 weeks before the source disappeared below the detection limit of the WFC. Seven type I X-ray bursts were observed during the outburst and two after the end of the outburst, proving that the compact object is a neutron star. The discovery of the first so-called superburst is the second subject. An hours long flare-like event was detected from the known X-ray burster 4U 1735-44. This event showed all the characteristics of thermonuclear X-ray bursts. However, no explanation could be given for the long duration and the large amount of energy released: both are an order of magnitude larger than typical type I X-ray bursts. The discovery of this first superburst triggered an archival search for other such events in the WFC database. Two such superbursts were found, and the one in Serpens X-1 which is described in this thesis. In the meantime two theoretical models were developed to explain these superbursts: unstable carbon burning in the deeper layers of a neutron star and electron capture on heavy elements in the deeper layers of the neutron star. Interestingly, according to the newest ideas most of the energy of a superburst is released due to photo-dissociation of heavy elements, making superbursts the only known process in astronomy where nuclear fission is the dominant energy source! Five new discoveries of burst sources are reported in the third part of this thesis. Four of these sources are burst-only sources, strengthening the proof of existence of this class. Follow-up observations with the Chandra X-ray satellite were performed for five of these burst-only sources. No X-ray counterparts could unambiguously be identified, and the inferred persistent luminosity of ~ 10 32 erg s-1 shows that these sources are probably connected with the soft X-ray transients in quiescence. In the final part of this thesis general properties of the population of X-ray bursters are derived. The nine of the most frequent bursters, as observed with the WFC are discussed. They appear to show the same burst behaviour when they are at the same luminosity. At the lowest luminosities there is a linear increase in burst rate with increasing luminosity. At 3 x 10 37 erg s-1 there is a drop by a factor of five in burst rate observed, connected to the transition of burst in a hydrogen-rich environment to pure hellium bursts. At higher luminosities the burst rate becomes irregular and no clear trend is observed with increasing luminosities