XMM observations of three middle-aged pulsars

X-ray observations of middle-aged pulsars allow one to study nonthermal radiation from pulsar magnetospheres and thermal radiation from neutron star (NS) surfaces. In particular, from the analysis of thermal radiation one can infer the surface temperatures and radii of NSs, which is important for investigating evolution of these objects and constraining the equation of state of the superdense matter in the NS interiors. Here we present results of XMM observations of three middle-aged pulsars, J0538+2817, B0656+14 and J0633+1746 (Geminga), and briefly discuss mechanisms of their X-ray emission.Comment: 6 pages, 8 figures; to be published in Memorie della Societa' Astronomica Italiana, the Proceedings of the EPIC Consortium (held on Oct 14-16, 2003 in Palermo

Polarization of Thermal X-rays from Isolated Neutron Stars

Since the opacity of a magnetized plasma depends on polarization of radiation, the radiation emergent from atmospheres of neutron stars with strong magnetic fields is expected to be strongly polarized. The degree of linear polarization, typically ~10-30%, depends on photon energy, effective temperature and magnetic field. The spectrum of polarization is more sensitive to the magnetic field than the spectrum of intensity. Both the degree of polarization and the position angle vary with the neutron star rotation period so that the shape of polarization pulse profiles depends on the orientation of the rotational and magnetic axes. Moreover, as the polarization is substantially modified by the general relativistic effects, observations of polarization of X-ray radiation from isolated neutron stars provide a new method for evaluating the mass-to-radius ratio of these objects, which is particularly important for elucidating the properties of the superdense matter in the neutron star interiors.Comment: 7 figures, to be published in Ap

Mass-to-Radius Ratio for the Millisecond Pulsar J0437-4715

Properties of X-ray radiation emitted from the polar caps of a radio pulsar depend not only on the cap temperature, size, and position, but also on the surface chemical composition, magnetic field, and neutron star's mass and radius. Fitting the spectra and the light curves with neutron star atmosphere models enables one to infer these parameters. As an example, we present here results obtained from the analysis of the pulsed X-ray radiation of a nearby millisecond pulsar J0437-4715. In particular, we show that stringent constraints on the mass-to-radius ratio can be obtained if orientations of the magnetic and rotation axes are known, e.g., from the radio polarization data.Comment: 2 figures, aasms4.sty; accepted for publication in ApJLetter

Quantum nature of cyclotron harmonics in thermal spectra of neutron stars

Some isolated neutron stars show harmonically spaced absorption features in their thermal soft X-ray spectra. The interpretation of the features as a cyclotron line and its harmonics has been suggested, but the usual explanation of the harmonics as caused by relativistic effects fails because the relativistic corrections are extremely small in this case. We suggest that the features correspond to the peaks in the energy dependence of the free-free opacity in a quantizing magnetic field, known as quantum oscillations. The peaks arise when the transitions to new Landau levels become allowed with increasing the photon energy; they are strongly enhanced by the square-root singularities in the phase-space density of quantum states in the case when the free (non-quantized) motion is effectively one-dimensional. To explore observable properties of these quantum oscillations, we calculate models of hydrogen neutron star atmospheres with B \sim 10^{10} - 10^{11} G (i.e., electron cyclotron energy E_{c,e} = 0.1 - 1 keV) and T_{eff} = 1 - 3 MK. Such conditions are thought to be typical for the so-called central compact objects in supernova remnants, such as 1E 1207.4-5209 in PKS 1209-51/52. We show that observable features at the electron cyclotron harmonics form at moderately large values of the quantization parameter, b_{eff} = E_{c,e}/kT_{eff} = 0.5 - 20. The equivalent widths of the features can reach 100 - 200 eV; they grow with increasing b_{eff} and are lower for higher harmonics.Comment: 6 pages; shortened, references updated; published in Ap

X-ray Pulsations from the Central Source in Puppis A

There are several supernova remnants which contain unresolved X-ray sources close to their centers, presumably radio-quiet neutron stars. To prove that these objects are indeed neutron stars, to understand the origin of their X-ray radiation, and to explain why they are radio-quiet, one should know their periods and period derivatives. We searched for pulsations of the X-ray flux from the radio-quiet neutron star candidate RX J0822-4300 near the center of the Puppis A supernova remnant observed with the ROSAT PSPC and HRI. A standard timing analysis of the separate PSPC and HRI data sets does not allow one to detect the periodicity unequivocally. However, a thorough analysis of the two observations separated by 4.56 yr enabled us to find a statistically significant period $P\simeq 75.3$ ms and its derivative $\dot{P}\simeq 1.49\times 10^{-13}$ s s$^{-1}$. The corresponding characteristic parameters of the neutron star, age $\tau=P/(2\dot{P})=8.0$ kyr, magnetic field $B=3.4\times 10^{12}$ G, and rotational energy loss $\dot{E}=1.4\times 10^{37}$ erg s$^{-1}$, are typical for young radio pulsars. Since the X-ray radiation has a thermal-like spectrum, its pulsations may be due to a nonuniform temperature distribution over the neutron star surface caused by anisotropy of the heat conduction in the strongly magnetized crust.Comment: 9 pages, 2 postscript figures, to appear in ApJ Letters; an acknowledgment is adde

Thermal Radiation from Neutron Stars: Chandra Results

The outstanding capabilities of the Chandra X-ray observatory have greatly increased our potential to observe and analyze thermal radiation from the surfaces of neutron stars (NSs). Such observations allow one to measure the surface temperatures and confront them with the predictions of the NS cooling models. Detection of gravitationally redshifted spectral lines can yield the NS mass-to-radius ratio. In rare cases when the distance is known, one can measure the NS radius, which is particularly important to constrain the equation of state of the superdense matter in the NS interiors. Finally, one can infer the chemical composition of the NS surface layers, which provides information about formation of NSs and their interaction with the environment. We overview the recent Chandra results on the thermal radiation from various types of NSs -- active pulsars, young radio-quiet neutron stars in supernova remnants, old radio-silent ``dim'' neutron stars -- and discuss their implications.Comment: URL changed for Figures 1, 12 and 18: ftp://ftp.xray.mpe.mpg.de/people/zavli

Phase and Power Control in the RF Magnetron Power Stations of Superconducting Accelerators

Phase and power control methods that satisfy the requirements of superconducting accelerators to magnetron RF sources were considered by a simplified kinetic model of a magnetron driven by a resonant injected signal. The model predicting and explaining stable, low noise operation of the tube below the threshold of self-excitation (the Hatrree voltage in free run mode) at a highest efficiency, a wide range of power control and a wide-band phase control was well verified in experiments demonstrating capabilities of the magnetron transmitters for powering of state of the art superconducting accelerators. Descriptions of the kinetic model, the experimental verification and a conceptual scheme of the highly-efficient magnetron RF transmitter for the accelerators are presented and discussed.Comment: 10 pages, 15 figure