Bursts of X-rays

An alternative track which has been pursued recently is time-resolved X-ray diffraction. An important goal is to improve the time resolution such that direct observation of the dynamics of the chemical bond is possible, corresponding to the observation of the time-dependent distribution of atomic positions. Pulsed X-rays are, e.g., obtained from synchrotron radiation or plasma sources. The temporal duration of these pulses is currently in the range of 100 ps-100 fs [4], Very recently, the first free-electron laser has produced short 100-10 fs coherent and highly intense bursts of X-rays [4—6]. 

Figure 18-4 Visible (blue) emission from peeling a roll of Scotch tape s in 1.3 pbar of air. Visible light is accompanied by bursts of X-rays. [Courtesy C Camara and S. Putterman, University of Cah lbmia. Los Angeles.]...

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

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.

A large array of X-ray detectors (total surface 60 cm2 at least), is needed with high segmentation and prompt enough to reject events from the earlier electrons contained in the burst (see Fig. 5). At present we are investigating various options to solve this problem. 

The origin of X-rays, 7-ray bursts and radiowaves that pervade the Galaxy is as mysterious as the cosmic-ray sources. Evidence is mounting that in most cases these phenomena are linked to violent explosions, thought to include neutron-star quakes, quark nuggets, supernovae, matter-antimatter annihilation, clumping of quasars and colliding stars. Of all space radiations radio phenomena have been studied in most detail. 

X-ray and optical follow-up studies reveal origin of y-ray bursts in powerful extragalactic sources ( hypernovae ) at cosmological distances. 

Nanosecond time-resolved crystallography of MbCO has been discussed in Section 3.7.2.3 of Chapter 3.46 After firing a 10-ns burst of laser light to break the CO-Fe bond, these researchers produced a diffraction image of the crystal through application of a 150-ps X-ray pulse. They are able to show release of the CO molecule, displacement of the Fe ion toward the proximal histidine, and recombination of the dissociated CO by about 100 ps. Essentially their results compare well with other spectroscopic studies of HbCO, MbCO and their models. 

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

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... 

Changes in mitochondrial stmcture are very relevant during X-ray induced apoptosis. A few hours after irradiation, a hyperpolarisation of A /m is noticed. This likely represents the attempt to restore the depleted ATP levels, stimulating the oxidative burst of surviving mitochondria. If this secondary oxidative stress overcomes the threshold given by mitochondrial thiols, mitochondrial cardiolipin is oxidized and mitochondrial inner membrane allows the leakage of A /m with the consequent initiation of the execution phase. 

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