Bibcode
Cornelisse, R.; in't Zand, J. J. M.; Verbunt, F.; Kuulkers, E.; Heise, J.; den Hartog, P. R.; Cocchi, M.; Natalucci, L.; Bazzano, A.; Ubertini, P.
Bibliographical reference
Astronomy and Astrophysics, v.405, p.1033-1042 (2003)
Advertised on:
7
2003
Journal
Citations
132
Refereed citations
112
Description
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 (LX <~
2*E37 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 Lpers=2*E37 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.