A superburst from 4U 1254-690

We report the detection with the BeppoSAX Wide Field Cameras of a superburst from 4U 1254-690. The superburst is preceded by a normal type-I X-ray burst, has a decay time that is the longest of all eight superbursts detected so far and a peak luminosity that is the lowest. Like for the other seven superbursts, the origin is a well-known type-I X-ray burster with a persistent luminosity level close to one tenth of the Eddington limit. Based on WFC data of all persistently bright X-ray bursters, the average rate of superbursts is 0.51+/-0.25 per year per persistently bright X-ray burster. Some systems may have higher superburst rates. For all superbursters, we present evidence for a pure helium layer which is burnt in an unstable as well as a stable manner.Comment: Accepted by A&A Letter

Puzzling thermonuclear burst behaviour from the transient low-mass X-ray binary IGR J17473-2721

We investigate the thermonuclear bursting behaviour of IGR J17473-2721, an X-ray transient that in 2008 underwent a six month long outburst, starting (unusually) with an X-ray burst. We detected a total of 57 thermonuclear bursts throughout the outburst with AGILE, Swift, RXTE, and INTEGRAL. The wide range of inferred accretion rates (between <1% and about 20% of the Eddington accretion rate m-dot_Edd) spanned during the outburst allows us to study changes in the nuclear burning processes and to identify up to seven different phases. The burst rate increased gradually with the accretion rate until it dropped (at a persistent flux corresponding to about 15% of m-dot_Edd) a few days before the outburst peak, after which bursts were not detected for a month. As the persistent emission subsequently decreased, the bursting activity resumed with a much lower rate than during the outburst rise. This hysteresis may arise from the thermal effect of the accretion on the surface nuclear burning processes, and the timescale is roughly consistent with that expected for the neutron star crust thermal response. On the other hand, an undetected superburst, occurring within a data gap near the outburst peak, could have produced a similar quenching of burst activity.Comment: 18 pages, 12 figures, 1 table, accepted for publication in MNRA

Photospheric radius expansion X-ray bursts as standard candles

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)x10^{38} erg/s, 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.Comment: Accepted for publication in A&A, 20 pages, 7 figure

Binary systems and their nuclear explosions

Peer ReviewedPreprin

Habitat type as strongest predictor of the body size distribution of Chydorus sphaericus (O. F. Muller) in small water bodies

Chydorus sphaericus, a cladoceran characterized by a wide range of distribution, is often numerous in various water habitats. Its body size is a consequence of environmental characteristics and physiology. The aim of the study focused on the distribution of body size and densities of C. sphaericus in relation to environmental conditions represented by: (i) habitat types (elodeids, helophytes, and the open water); (ii) specific pond types (forest and field); (iii) pond size (surface area); (iv) the presence/lack of fish; and (v) physicochemical factors. Similar to large daphnids, in the case of the examined small water bodies, fish presence was responsible for a reduction of the body size of C. sphaericus in the zone of open water. More abundant crustacean communities and the presence of larger specimens were found among macrophytes, which indicated that aquatic vegetation offered optimal growth conditions as well as an effective refuge against fish predation. These facts reflect the necessity for maintaining a varied mosaic of habitats even in small water bodies such as the examined ponds. We found that not only the abundance of C. sphaericus but also its body size can be used as an bioindicator of environmental conditions as it preferred small and eutrophic ponds, particularly those with complex macrophyte cover (such as elodeids). Furthermore, the abundance pattern of zooplankton dominant species was affected by elodeids and fish presence as well as by the area of the ponds