19 research outputs found
Large frequency drifts during Type I X-ray bursts
We study the spin-down of a neutron star atmosphere during the Type I X-ray
burst in low mass X-ray binaries. Using polar cap acceleration models, we show
that the resulting stellar ``wind'' torque on the burning shell due to the
flowing charged particles (electrons, protons and ions) from the star's polar
caps may change the shell's angular momentum during the burst. We conclude that
the net change in the angular momentum of the star's atmosphere can account for
rather large frequency drifts observed during Type I X-ray burst.Comment: 8 pages, more discussion adde
Discovery of type-I X-ray bursts from the low-mass X-ray binary 4U 1708-40
We report the discovery of type-I X-ray bursts from the low-mass X-ray binary
4U 1708-40 during the 100 ks observation performed by BeppoSAX on 1999 August
15-16. Six X-ray bursts have been observed. The unabsorbed 2-10 keV fluxes of
the bursts range from ~ (3-9)x10^(-10) erg cm^(-2)s^(-1). A correlation between
peak flux and fluence of the bursts is found, in agreement with the behaviour
observed in other similar sources. There is a trend of the burst flux to
increase with the time interval from the previous burst. From the value of the
persistent flux we infer a mass accretion rate Mdot~7x10^(-11) Msun/yr, that
may correspond to the mixed hydrogen/helium burning regime triggered by
thermally unstable hydrogen. We have also analysed a BeppoSAX observation
performed on 2001 August 22 and previous RXTE observations of 4U 1708-40, where
no bursts have been observed; we found persistent fluxes of more than a factor
of 7 higher than the persistent flux observed during the BeppoSAX observation
showing X-ray bursts.Comment: accepted for publication in MNRA