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Abstract:

The use of mathematical and computational models to explore, analyze and predict the behavior of biological systems is rarely successful and widely mistrusted. This is in part due to the naive assumption that methods which are conspicuously successful in the physical sciences, will work equally well in biology. It appears, instead, that useful biological models require relatively little applied mathematics, but very much more investment in software engineering. Some of the technologies, such as threading, object reflection, and graphical interfaces are well established. Others, such as markup languages for describing models in a standardised, machine-readable way, are just emerging. Once harnessed in fields where their use is almost essential, these technologies may also find uses in fields where traditional methods have been successful. Eggleton's stellar evolution code is taken as an example to examine what, if anything, biological methods can bring to computational physics.

Abstract:

Globular clusters are overabundant per unit mass in neutron-star binaries, including low-mass X-ray binaries and radio pulsars. We report here on two complementary population synthesis studies which relate directly to the formation and evolution of neutron star binaries in globular clusters. In the first we consider an early population of massive stars, including those in binary systems, which produce the original population of neutron stars. We compute the probability of retention of these neutron stars, and quantitatively confirm the idea that the retention fraction for neutron stars born in binary systems is greatly enhanced over those born in isolated stars. However, with the currently fashionable natal kick velocities given to neutron stars, the retention fraction may well be insufficient to explain the current population of neutron star binaries. In the second study, we follow a large population of primordial binaries and neutron stars throughout the lifetime of a globular cluster whose properties may be similar to 47 Tuc. We directly compute all 3-body interactions among binary systems, neutron stars, and isolated field stars throughout the history of the cluster. The evolution of certain types of neutron star binaries is followed up to the current epoch. The numbers of close, recycled, binary radio pulsars are evaluated and compared with the results of radio observations. We find that for an initial population of some 10(6) primordial binaries and similar to 10(4) neutron stars, we can plausibly account for the large numbers of binary radio pulsars that are inferred to exist in clusters such as 47 Tuc.

Abstract:

The relativistic precession model for quasi-periodic oscillations (QPOs) in low-mass X-ray binaries is reviewed. The behaviour of three simultaneous types of QPOs is well matched in terms of the fundamental frequencies for geodesic motion in the strong field of the accreting compact object for reasonable star masses and spin frequencies. The model works for neutron star as well as black hole candidate systems, as it ascribes the higher frequency kHz QPOs, the lower frequency kHz QPOs and the horizontal branch oscillations to the Keplerian, periastron precession and nodal precession frequencies of matter orbiting close to the inner edge of the accretion disk. The remarkable correlation between the centroid frequency of QPOs in both neutron star and black hole candidate low-mass X-ray binaries is very well fit by the model. Some testable predictions of the model are described. QPOs from low-mass X-ray binaries might provide an unprecedented laboratory to test general relativity in the strong field regime.

Abstract:

We have detected a broadening in the wings of the first harmonic peak in the power spectrum of the X-ray millisecond binary pulsar SAX J1808.4-3658. This feature results from the convolution of the red noise present in the power spectrum with the harmonic line. This demonstrates that the aperiodic variability (that causes the red noise) is modulated at the spin period, therefore suggesting that it originates at the magnetic caps close to the neutron star surface. This is consistent with what has been found in some high mass X-ray pulsators.

Abstract:

The dozen known black-hole X-ray binaries are our sole source of information on the stellar-mass black-hole population in our Galaxy. It is shown, from a combination of evolutionary considerations and the observed galactic incidence of low-mass X-ray binaries that contain neutron stars and black holes, that the total number of stellar-mass black holes in the Galaxy must be of order 10(8). The very peculiar supernova SN 1998 bw that coincided with Gamma Ray burst GRB980425 was the first birth event of a stellar black hole ever observed. This supernova together with the large overabundances of alpha-elements observed in the companion stars of the black-holes in the X-ray binaries GRO J1655-40 and V4641 Sgr and the observed runaway velocities of the black-hole Xray binaries show that stellar-mass black holes are formed in supernova events in which a considerable amount of mass (at least several Mcircle dot) is explosively ejected, most probably in a symmetric way.