Abstract. We examined the maximum bolometric peak luminosities during type I X-ray bursts from the persistent or transient luminous X-ray sources in globular clusters. We show that for about two thirds of the sources the maximum peak luminosities during photospheric radius expansion X-ray bursts extend to a critical value of 3.79±0.15×10 38 erg s −1 , assuming the total X-ray burst emission is entirely due to black-body radiation and the recorded maximum luminosity is the actual peak luminosity. This empirical critical luminosity is consistent with the Eddington luminosity limit for hydrogen poor material. Since the critical luminosity is more or less always reached during photospheric radius expansion X-ray bursts (except for one source), such bursts may be regarded as empirical standard candles. However, because significant deviations do occur, our standard candle is only accurate to within 15%. We re-evaluated the distances to the twelve globular clusters in which the X-ray bursters reside.

We report the first high-energy survey catalog obtained with the IBIS gamma-ray imager on board INTEGRAL. The analysis has been performed on the first-year Core Program ISGRI data comprising both Galactic Plane Scan and Galactic Centre Deep Exposure pointings for a total exposure time exceeding 5 Ms. This initial survey has revealed the presence of ∼120 sources detected with the unprecedented sensitivity of ∼1 mcrab in the energy range 20-100 keV. Each source is located to an accuracy between 1Ј and 3Ј, depending on its brightness. The outstanding IBIS capability to locate soft g-ray emitters has allowed us to identify most of the detected sources with already known Galactic X-ray binary systems, while 28 of the objects are of unknown nature.

Abstract. We present an overview of BeppoSAX Wide Field Cameras observations of the nine most frequent type I X-ray bursters in the Galactic center region. Six years of observations (from 1996 to 2002) have amounted to 7 Ms of Galactic center observations and the detection of 1823 bursts. The 3 most frequent bursters are GX 354-0 (423 bursts), KS 1731-260 (339) and GS 1826-24 (260). These numbers reflect an unique dataset. We show that all sources have the same global burst behavior as a function of luminosity. At the lowest luminosities (L X < ∼ 2 × 10 37 erg s −1 ) bursts occur quasi-periodically and the burst rate increases linearly with accretion rate (clear in e.g. GS 1826-24 and KS 1731-260). At L pers = 2 × 10 37 erg s −1 the burst rate drops by a factor of five. This corresponds to the transition from, on average, a hydrogen-rich to a pure helium environment in which the flashes originate that are responsible for the bursts. At higher luminosities the bursts recur irregularly; no bursts are observed at the highest luminosities. Our central finding is that most of the trends in bursting behavior are driven by the onset of stable hydrogen burning in the neutron star atmosphere. Furthermore, we notice three new observational fact which are difficult to explain with current burst theory: the presence of short pure-helium bursts at the lowest accretion regimes, the bimodal distribution of peak burst rates, and an accretion rate that is ten times higher than predicted at which the onset of stable hydrogen burning occurs. Finally, we note that our investigation is the first to signal quasi-periodic burst recurrence in KS 1731-260, and a clear proportionality between the frequency of the quasi-periodicity and the persistent flux in GS 1826-24 and KS 1731-260.

Abstract.We have discovered three certain (SAX J1324.5−6313, 2S 1711−339 and SAX J1828.5−1037) and two likely (SAX J1818.7+1424 and SAX J2224.9+5421) new thermonuclear X-ray burst sources with the BeppoSAX Wide Field Cameras, and observed a second burst ever from a sixth one (2S 0918−549). Four of them (excluding 2S 1711−339 and 2S 0918−549) are newly detected X-ray sources from which we observed single bursts, but no persistent emission. We observe the first 11 bursts ever from 2S 1711−339; persistent flux was detected during the first ten bursts, but not around the last burst. A single burst was recently detected from 2S 0918−549 by Jonker et al. (2001); we observe a second burst showing radius expansion, from which a distance of 4.2 kpc is derived. According to theory, bursts from very low flux levels should last ∼ > 100 s. Such is indeed the case for the last burst from 2S 1711−339, the single burst from SAX J1828.5−1037 and the two bursts from 2S 0918−549, but not for the bursts from SAX J1324.5−6313, SAX J1818.7+1424 and SAX J2224.9+5421. The bursts from the latter sources all last ∼20 s. We suggest that SAX J1324.5−6313, SAX J1818.7+1424, SAX J1828.5−1037 and SAX J2224.9+5421 are members of the recently proposed class of bursters with distinctively low persistent flux levels, and show that the galactic distribution of this class is compatible with that of the standard low-mass X-ray binaries.

Abstract.We report on an approximately twelve hour long X-ray flare from the low-mass X-ray binary KS 1731−260. The flare has a rise time of less than 13 min and declines exponentially with a decay time of 2.7 hours. The flare emission is well described by black-body radiation with peak temperature of 2.4 keV. The total energy release from the event is 10 42 erg (for an assumed distance of 7 kpc). The flare has all the characteristics of thermo-nuclear X-ray bursts (so-called type I X-ray bursts), except for its very long duration and therefore large energy release (factor of 1500-4000 longer and 250-425 more energy than normal type I X-ray bursts from this source). The flare is preceded by a short and weak X-ray burst, possibly of type I. Days to weeks before the flare, type I X-ray bursts were seen at a rate of ∼3 per day. However, after the flare type I X-ray bursting ceased for at least a month, suggesting that the X-ray flare affected the type I bursting behaviour. The persistent emission is not significantly different during the non-bursting period. We compare the characteristics of this event with similar long X-ray flares, so-called superbursts, seen in other sources (4U 1735−44, 4U 1820−30, 4U 1636−53, Ser X-1, GX 3+1). The event seen from KS 1731−260 is the longest reported so far. We discuss two possible mechanisms that might cause these superbursts, unstable carbon burning (as proposed recently) and electron capture by protons with subsequent capture of the resulting neutrons by heavy nuclei.