X-ray burst

Observations of X-ray bursts from AXPs which are very similar to the ones from SGRs. 

Cottam, J., Paerels, F., Mendez, M. (2002), Gravitationally redshifted absorption lines in the X-ray burst spectra of a neutron star , Nature 420, 51. 

Gavriil, F.P., Kaspi, V.M., Woods, P.M. (2002), Magnetar-like X-ray bursts from an anomalous X-ray pulsar , Nature 419, 142. 

Next we consider the compact star in the low mass X-ray binary 4U 1728-34. In a very recent paper Shaposhnikov et al. (2003) (hereafter STH) have analyzed a set of 26 Type-I X-ray bursts for this source. The data were collected by the Proportional Counter Array on board of the Rossi X-ray Timing Explorer (RXTE) satellite. For the interpretation of these observational data Shaposhnikov et al. 2003 used a model of the X-ray burst spectral formation developed by Titarchuk (1994) and Shaposhnikov Titarchuk (2002). Within this model, STH were able to extract very stringent constrain on the radius and the mass of the compact star in this bursting source. The radius and mass for 4U 1728-34, extracted by STH for different best-fits of the burst data, are depicted in Fig. 6 by the filled squares. Each of the four MR points is relative to a different value of the distance to the source (d = 4.0, 4.25, 4.50, 4.75 kpc, for the fit which produces the smallest values of the mass, up to the one which gives the largest mass). The error bars on each point represent the error contour for 90% confidence level. It has been pointed out (Bombaci 2003) that the semi-empirical MR relation for the compact star in 4U 1728-34 obtained by STH is not compatible with models pure hadronic stars, while it is consistent with strange stars or hybrid stars. 

Decisive informations on the mass-to-radius ratio can be provided by measuring the gravitational redshift of lines in the spectrum emitted from the compact star atmosphere. Very recently, redshifted spectral lines features have been reported for two different X-ray sources (Cottam et al. 2002 Sanwal et al. 2002). The first of these sources is the compact star in the low mass X-ray binary EXO 0748-676. Studying the spectra of 28 type-I X-ray bursts in... 

This equation has been widely applied to X-ray bursts and nova outbursts in which the acceleration occurs inside the photosphere (Kato 1983a, b, 1986). There is, however, a problem that the velocity could not be zero at the surface of the degenerate stars. Equation (2) gives the finite velocity at the surface of the degenerate stars, in spite of no matter actually flowing out from the interior of the degenerate stars. 

The inner boundary radius R is the radius of the neutron star for X-ray bursts and is zero in tfie normal stars. 

However, the situation is not so simple when the heat transport is coupled with stellar structure. Here, I would give another example. It is a problem of X-ray bursting neutron star. The X-ray burst proceeds in tens of second and it might be much shorter as compared with the time scale of heat transport in the envelope of the neutron star. During the burst the X-ray luminosity of the neutron star becomes very close to the Eddington luminosity and the outer layers of the envelope are pushed up by the radiation coming from the interior. Then the neutron star is puffed up and the time scale of heat transport becomes shorter and shorter. Finally, the envelope solution with steady mass flow in thermal equilibrium becomes a good approximation and such situation is also observation-ally confirmed. Before this has become understood, a specialist tried to calculate such expansion of the envelope all the way as an initial value problem by means of stellar evolution code, but it was found impracticable. 

Figure 4 Regions of the density vs. temperature plane in which the various hydrogen-burning processes are dominant [MAT84c]. The normal CNO cycle occurs in stars slightly larger than the sun. The hot (beta-limited) CNO cycle is particularly important in supermassive stars. The rp-process is important during the thermonuclear runaways on accreting neutron stars which may be the source of X-ray bursts.

Fig. 17. Gated viewing recordings of a small lead object using short-pulse X-ray bursts passing through water layers of different thicknesses. In the lower part of the figure time-integrated representations are shown clearly demonstrating the loss of contrast for thicker samples (From Ref. [70]).

Woosley, S. E. et al. Models for Type I X-Ray Bursts with Improved Nuclear Physics. Astrophys. J. Suppl. 2004,151, 75-102. 

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