We present the first non-LTE model atmospheres for neutron stars (NS). We study their structure and NLTE effects on the emergent thermal radiation of old isolated NS.

The degree of polarization of thermal radiation from a neutron star depends on photon energy, surface temperature and magnetic field, and it oscillates with the star rotation period. Observations of this polarization provide a new tool for investigating properties of these objects.

Using the Hubble Space Telescope we have successfully detected the ”dark” side of the optical companion to the eclipsing millisecond pulsar J2051–0827. These data were obtained over a number of system orbits and clearly show that there is variability in the optical flux, particularly at orbital phases after maximum light. Comparison with phases before maximum light suggests this variation is due to an additional component. The data are modelled by a gravitationally-distorted low-mass secondary star which is irradiated by the impinging pulsar wind. The quality of the model fits are however limited by the variability.

It is accepted that the formation of a millisecond binary pulsar (MBP) with a low–mass companion may be explained as the end–point of close binary evolution in which an old pulsar is spun–up by accretion from the red giant (Bhattacharaya & van den Heuvel 1991). In this paper we shall discuss the cooling properties of the helium white dwarfs (WD) in short orbital period MBP systems PSR J0437–4715, PSR J0751+1807 and PSR J1012+5307, without referring to the rotational history of neutron stars (NS).

Below we discuss observational data for several system for which the results of our calculations (Sarna, Antipova, & Muslimov 1998; Sarna, Ergma, & Antipova 1999) may be applied, by taking into account the orbital parameters of the system, the pulsar spin-down time, and the WD cooling time-scale.

We report on Keck and HST observations of the binary millisecond pulsar PSR B1855+09. We detect its white-dwarf companion and measure mF555W = 25.90 ± 0.12 and mF814W = 24.19 ± 0.11 (Vega system). From the reddening-corrected color we infer a temperature Teff = 4800 ± 800 K. The companion mass is known accurately from measurements of the Shapiro delay of the pulsar signal, . Given a cooling model, one can use the measured temperature to determine the cooling age. The main uncertainty in the cooling models for such low-mass white dwarfs is the amount of residual nuclear burning, which depends on the thickness of the hydrogen layer surrounding the helium core. For PSR B1855+09, such models lead to a cooling age of ∼10Gyr, which is twice the spin-down age of the pulsar. It may be that the pulsar does not brake (n=3.0) like a dipole rotating in vacuo. For other pulsar companions, however, ages well over lOGyr are inferred, indicating that the problem may lie with the cooling models. There is no age discrepancy for models in which the white dwarfs are formed with thinner hydrogen layers (< 3 × 10−4M⊙). See van Kerkwijk et al. ApJ (submitted) for more details.

Low-mass white dwarfs with helium cores (He-WDs) are known to result from mass loss and/or exchange events in binary systems where the donor is a low mass star evolving along the red giant branch (RGB). Therefore, He-WDs are common components in binary systems with either two white dwarfs or with a white dwarf and a millisecond pulsar (MSP). If the cooling behaviour of He-WDs is known from theoretical studies (see Driebe et al. 1998, and references therein) the ages of MSP systems can be calculated independently of the pulsar properties provided the He-WD mass is known from spectroscopy.

It is accepted that formation of a binary millisecond (or recycled) pulsar with a low–mass companion may be explained as the end–point of close binary evolution in which an old pulsar is spun–up by accretion from the secondary (Alpar et al., 1982). After detachment from the Roche lobe, the pulsar spin period starts to change due to magneto–dipole radiation and the white dwarf begins to cool down. In this paper we shall discuss the cooling history of helium core low–mass white dwarfs in the short orbital period millisecond binary pulsars PSR J0751+1807 and PSR J1012+5307 (Ergma, Sarna, & Antipova 1999).

The discovery is reported of the first accretion-powered millisecond pulsar, SAX J 1808.4–3658. This 2.5 millisecond pulsar has a magnetic field strength of 1–10108 Gauss and has all the characteristics of the long-predicted millisecond radio pulsar progenitor, a neutron star in an X-ray binary system where the process of recycling is taking place at this time.

Using the maximum likelihood method we show that the observed distribution of maximum frequencies of kilohertz quasi periodic oscillations (QPOs) in LMXBs agrees with the frequencies of orbital motion in the marginally stable orbit around compact objects with masses chosen randomly from some interval, which we find to be (1.55 M⊙, 2.40 M⊙) at the 99% confidence level. As this remarkable agreement with the expected mass range of neutron stars in LMXBs demonstrates, the assumption that the maximum-frequency QPOs occur in the marginally stable orbit is consistent with the current data set.

Neutron stars that have experienced episodes of mass transfer exhibit lower magnetic dipole moments μ than other neutron stars, e.g. binary radio pulsars with high-mass (μ ∼ 1028G cm3) and low-mass (μ ∼ 1027G cm3) companions, isolated millisecond pulsars (μ ∼ 1026G cm3), and accreting objects in low-mass X-ray binaries (μ < 1027G cm3). Indeed, the μ-versus-orbital-period and μ-versus-age relations for low-mass and high-mass binaries imply that μ decreases monotonically with accreted mass Ma (Taam & van den Heuvel 1986; van den Heuvel & Bitzaraki 1995). Simple Ohmic field decay cannot be responsible given the existence of cold and therefore old white-dwarf companions (Kulkarni 1986). Alternative mechanisms include accretion-induced heating of the crust (Konar & Bhattacharya 1997) and fluxoid-vortex interactions in the stellar interior (Srinivasan et al. 1990).

It is shown that X-ray binaries can be accelerated by their own radiation. It is possible if the magnetic field of a neutron star in a binary differs from the dipolar field. Asymmetric X-ray emission generated due to accretion of matter onto a neutron star surface creates an accelerating force. Its magnitude can be comparable or even larger than gravitational attraction of the binary to the Galaxy.

Starting from the nuclear interactions which are used to calculate the properties of atomic nuclei, we derive a new equation of state (EOS) for neutron star matter. With this realistic EOS, we calculate the structure of non-rotating neutron stars.

Strange stars could represent a new class of stellar compact objects, where a new phase of dense matter (deconfined strange quark matter) comes into play. Here, we discuss some possible observational evidence for their existence.

We study effects of the strange quark mass, ms, and of the QCD interactions, calculated to lowest order in αc, on the rapid rotation of strange stars (SS). The influence of rotation on global parameters of SS is greater than in the case of the neutron stars (NS). We show that independently of ms and αc the ratio of the rotational kinetic energy to the absolute value of the gravitational potential energy T/W for a rotating SS is significantly higher than for an ordinary NS. This might indicate that rapidly rotating SS could be important sources of gravitational waves.

We find constraints on minimum and maximum mass of ordinary neutron stars imposed by their early evolution (protoneutron star stage). We calculate models of protoneutron stars using a realistic standard equation of state of hot, dense matter valid for both supranuclear and subnuclear densities. Results for different values of the nuclear incompressibility are presented.

It is believed that pulsars are neutron stars or strange stars with crusts. However we suggest here that pulsars may be bare strange stars (i.e., strange stars without crust). Due to rapid rotation and strong emission, young strange stars produced in supernova explosions should be bare when they act as radio pulsars. Because of strong magnetic field, two polar-crusts would shield the polar caps of an accreting strange star. Such a suggestion can be checked by further observations.

I summarize recent observational and theoretical advances in the understanding of the Soft Gamma Repeaters and the Anomalous X-ray Pulsars. Several direct physical arguments point to very strong magnetic fields (B > 10 BQED = 4.4 × 1014 G) in SGR outbursts. The connection between these two classes of neutron stars is examined. Their persistent X-ray emission and spindown behavior are interpreted in the magnetar model, where a decaying magnetic field dominates all other sources of energy for radiative and particle emission. The response of a magnetic field to the violent motions in a supernova core is also examined, with a focus on mechanisms that may impart unusually large kicks.

The interpretation of Soft–Gamma–Repeaters (SGRs) and Anomalous X–Ray Pulsars (AXPs) as Magnetars (Thompson & Duncan 1996) raises again the issue of the generation of the ultra–strong magnetic fields (MFs) in neutron stars (NSs) and the related question of where these fields are anchored: in the core, penetrating the whole star, or confined to the crust. Recently, Heyl & Kulkarni (1998) considered the magneto–thermal evolution of magnetars with a core field. Since the assumption of a crustal field is at least not in disagreement with the observations of isolated pulsars (Urpin & Konenkov 1997) and of NSs in binary systems (Urpin, Geppert & Konenkov 1998, Urpin, Konenkov & Geppert 1998), here we would like to address the question whether the observations of SGRs and AXPs can be interpreted as magnetars having a crustal MF. Given the strength of the MF in magnetars we take into account, in an approximate manner, the strongly non–linear Hall effect on its decay. We intend to provide a contribution to an unified picture of NS MF evolution based on the crustal field hypothesis.

The soft gamma repeater SGR 1900+14 was observed in Pushchino observatory since 1988 December using BSA radio telescope operating at 111 MHz. We have detected the pulsed radio emission (Shitov 1999) with the same 5.16 s period that was reported earlier for this object (Hurley et al. 1998). The timing analysis has shown that this new radio pulsar PSR J1907+0919 associated with SGR 1900+14 has a superstrong magnetic field, which is 8.1 · 1014G, thereby confirming that it is a ”magnetar” (Duncan & Thompson 1992; Kouveliotou et al. 1999). The dispersion measure of PSR J1907+0919 is 281.4(9)pc · cm−3 which gives an estimate of the pulsar’s distance as about 5.8 kpc.

It is argued that bumps in the timing histories Ω(t) of the anomalous X-ray pulsars (AXPs) IE 1048.1-5937 and IE 2259+586 are the signature of a magnetar undergoing radiative precession, wherein the hydromagnetic deformation of the neutron star couples to an oscillating component of the vacuum-dipole radiation torque to produce an anharmonic wobble with period τpr ∼ 10 yr. An analysis of Euler’s equations of motion for a biaxial magnet reproduces the amplitude and recurrence time of the bumps for IE 1048.1-5937 and IE 2259+586, predicts Ω(t) for the next 20 years for both objects, and predicts a testable statistical relation between dΩ/dt and τpr for the AXP population overall. Radiative precession of soft gamma-ray repeaters is also discussed, together with implications for the internal (e.g. viscosity) and magnetospheric (e.g. e+e− pair currents) properties of magnetars.