Winds from Neutron Stars and Strong Type I X--Ray Bursts

A model for stationary, radiatively driven winds from X--ray bursting neutron stars is presented. General relativistic hydrodynamical and radiative transfer equations are integrated from the neutron star surface outwards, taking into account for helium nuclear burning in the inner, dense, nearly hydrostatic shells. Radiative processes include both bremsstrahlung emission--absorption and Compton scattering; only the frequency--integrated transport is considered here. It is shown that each solution is characterized by just one parameter: the mass loss rate \Mdot, or, equivalently, the envelope mass \Menv. We found that, owing to the effects of Comptonization, steady, supersonic winds can exist only for \Mdot larger than a limiting value \Mdot_{min} \approx\Mdot_{E}. Several models, covering about two decades in mass loss rate, have been computed for given neutron star parameters. We discuss how the sequence of our solutions with decreasing \Menv can be used to follow the time evolution of a strong X--ray burst during the expansion/contraction phase near to the luminosity maximum. The comparison between our numerical results and the observational data of Haberl {\it et al.\/} (1987) for the bursts in 4U/MXB 1820-30 gives an estimate for both the spectral hardening factor and the accretion rate in this source.Comment: 32 pages (10 postsript figures available on request), PlainTe

Accretion Rates in X--Ray Bursting Sources

We present estimates for the accretion rates in 13 X--ray bursting sources which exhibit photospheric expansion, basing on theoretical models of stationary, radiatively driven winds from neutron stars. The relatively high values obtained, \Mdot_{acc}\magcir 10^{-9} \MS, are in accordance with theoretical limits for unstable helium burning, and, at the same time, almost never exceed the ``dynamical'' limit for stationary accretion, \sim 10 \Mdot_{Edd}. The only exceptions are 1820-30, already known to be a very peculiar object, and 1608-522; there are indications, however, that in both sources, accretion could be non--stationary.Comment: 21 pages, PlainTe

On the Nature of Photospheric Oscillations in Strong X--Ray Bursts

A possible sound origin for the photospheric oscillations in the X--ray bursting sources 1608-522 and 2127+119 is suggested. It is shown that standing sound waves in an expanding spherical envelope can have periods very close to the observed ones. The quite large ratio, $\sim$ 10, of the periods in the two sources is explained in terms of different wave regimes. The relevance of sound oscillations to the observed QPO in type II bursts of the Rapid Burster is also discussed.Comment: 14 pages, PlainTe

On the Amplitude of Burst Oscillations in 4U 1636-54: Evidence for Nuclear Powered Pulsars

We present a study of 581 Hz oscillations observed during a thermonuclear X-ray burst from the low mass X-ray binary (LMXB) 4U 1636-54 with the Rossi X-ray Timing Explorer (RXTE). We argue that the combination of large pulsed amplitudes near burst onset and the spectral evidence for localized emission during the rise strongly supports rotational modulation as the mechanism for the oscillations. We discuss how theoretical interpretation of spin modulation amplitudes, pulse profiles and pulse phase spectroscopy can provide constraints on the masses and radii of neutron stars. We also discuss the implication of these findings for the beat frequency models of kHz X-ray variability in LMXB.Comment: AASTEX Latex, 13 pages including 5 figures. Accepted for publication in the Astrophysical Journal Letter

The Long Term Stability of Oscillations During Thermonuclear X-ray Bursts: Constraining the Binary X-ray Mass Function

We report on the long term stability of the millisecond oscillations observed with the Rossi X-ray Timing Explorer (RXTE) during thermonuclear X-ray bursts from the low mass X-ray binaries (LMXB) 4U 1728-34 and 4U 1636-53. We show that bursts from 4U 1728-34 spanning more than 1.5 years have observed asymptotic oscillation periods which are within 0.2 microsec. of each other, well within the magnitude which could be produced by the orbital motion of the neutron star in a typical LMXB. This stability implies a timescale to change the oscillation period of > 23,000 years, suggesting a highly stable process such as stellar rotation as the oscillation mechanism. We show that period offsets in three distinct bursts from 4U 1636-53 can be plausibly interpreted as due to orbital motion of the neutron star in this 3.8 hour binary system. We discuss the constraints on the mass function which can in principle be derived using this technique.Comment: 11 pages, 4 figures. AASTeX, to be published in the Astrophysical Journal Letter

Mechanisms for High-frequency QPOs in Neutron Star and Black Hole Binaries

We explain the millisecond variability detected by Rossi X-ray Timing Explorer (RXTE) in the X-ray emission from a number of low mass X-ray binary systems (Sco X-1, 4U1728-34, 4U1608-522, 4U1636-536, 4U0614+091, 4U1735-44, 4U1820-30, GX5-1 and etc) in terms of dynamics of the centrifugal barrier, a hot boundary region surrounding a neutron star. We demonstrate that this region may experience the relaxation oscillations, and that the displacements of a gas element both in radial and vertical directions occur at the same main frequency, of order of the local Keplerian frequency. We show the importance of the effect of a splitting of the main frequency produced by the Coriolis force in a rotating disk for the interpretation of a spacing between the QPO peaks. We estimate a magnitude of the splitting effect and present a simple formula for the whole spectrum of the split frequencies. It is interesting that the first three lowest-order overtones fall in the range of 200-1200 Hz and match the kHz-QPO frequencies observed by RXTE. Similar phenomena should also occur in Black Hole (BH) systems, but, since the QPO frequency is inversely proportional to the mass of a compact object, the frequency of the centrifugal-barrier oscillations in the BH systems should be a factor of 5-10 lower than that for the NS systems. The X-ray spectrum formed in this region is a result of upscattering of a soft radiation (from a disk and a NS surface) off relatively hot electrons in the boundary layer. We also briefly discuss some alternative QPO models, including a possibility of acoustic oscillations in the boundary layer, the proper stellar rotation, and g-mode disk oscillations.Comment: The paper is coming out in the Astrophysical Journal in the 1st of May issue of 199