Pulsar Topics:

• General Relativity
  
  Binary pulsars give us the only strong-field tests of gravitational theories. Russel Hulse and Joe Taylor earned the Nobel Prize in Physics in 1993 for their discovery of the first binary pulsar, PSR B1913+16. Precise measurements of the orbital motion of this object with the Arecibo radio telescope confirmed the existence of gravitational waves (GWs), a fundamental prediction of general relativity (GR) and many other theories of gravity.
  
  
  
  
  
  A new binary system consisting of two pulsars, PSR J0737-3039 (A and B, see artistic depiction above) has recently provided a total of five tests of general relativity, including the most precise ever made in the strong-field regime (Kramer ey al. 2006). The precision of some of these tests keeps increasing fast with time. This will become a real test of our understanding of the fundamental nature of space and time.
  
  
• Gravitational waves
  
  Fast-spinning pulsars could provide the terrestrial GW detectors like LIGO and VIRGO with a detectable source, particularly if the very fast rotators lose energy via the GW-induced Rossby instability.
  
  
  
  
  
  Precise timing of an array of bright millisecond pulsars can be used to detect low-frequency GWs (picture above). The basic idea is that the distances from distant pulsars to the solar system are affected by the passage of a gravitational wave through the solar system. These changing distances can in principle be measured using pulsar timing.
  
  
• Equation of state of super-dense matter
  
  
  
  
  Given the large mass (~ 1.4 solar masses) and small radius (10-16 km) of neutron stars, matter at their centers is one or two orders of magnitude denser than atomic nuclei. The composition and behavior of matter in these conditions is not known. Measurements of the physical characteristics of neutron stars (mass, radius, spin frequency, moment of inertia) can constrain the equation of state of super-dense matter.
  
  
• Pulsar Surveys
  
  We are carrying out several surveys of the neutron star population of the Milky Way. Most of these are blind surveys, like the ALFA pulsar survey and (soon) the Effelsberg all-sky survey.
  
  Others are directed surveys, like the Fermi unidentified source survey.
  This has just found one new millisecond pulsar! (see German press release here)
  
  

Research topics on cosmic magnetism:

• Magnetic fields in the Milky Way
  
  The overall structure of the magnetic field of the Milky Way is much more difficult to determine than in nearby galaxies, because of the position of the solar system in the disk of the Milky Way. Faraday rotation measurements from pulsars with known distances and from extragalactic sources shining through the Milky Way allow to investigate its magnetic field in three dimensions. The overall field structure follows the optical spiral arms, like in other galaxies. Magnetic field studies of the Milky Way reveal much more spatial details than possible in any nearby galaxy. One large-scale field reversal in the disk inside the solar radius has been found, which is so far unique in respect to what is known about nearby galaxy fields. The regular and turbulent magnetic field components in the disk and the halo are of the same order of a few micro-Gauss. The magnetic field is significantly higher in the Galactic Centre region. The disk field is symmetric with respect to the Galactic plane, while the toroidal halo field is antisymmetric (i.e. with different directions above and below the plane).
  
  
• Magnetic fields in nearby galaxies
  
  The magnetic field traced by radio polarization observations forms nice spiral patterns in almost every galaxy, even in flocculent and irregular types which lack any spiral optical structure. Spiral fields are also observed in the central regions of galaxies and in circum-nuclear rings of gas. In galaxies with massive spiral arms, the magnetic field lines run mostly parallel to the optical arms, but are concentrated at the inner edge of the spiral arms or between the spiral arms. In several galaxies, the field forms independent ''magnetic arms'' between the arms, in some they are crossing the spiral arms.
  
  
  
  Polarized emission emerges from ordered fields, but polarization "vectors'' are ambiguous by multiples of 180 degrees. Only measurements of Faraday rotation from multi-wavelength radio polarization observations allow to determine the direction of the regular field component along the line of sight. Large-scale patterns of Faraday rotation observed in a few spiral galaxies reveal regular fields with a large-scale constant direction, as predicted by dynamo models. The field structures of several other galaxies can be described by a superposition of two or more dynamo modes. However, in many galaxies observed no clear patterns of the regular fields were found. Either the field structure is not resolved of present-day telescopes, or most of the spiral fields seen in polarization are anisotropic (with frequent small-scale reversals) due to shearing or compressing gas flows, or many galaxies are still too young to have developed fully regular fields.
  
  
• Radio halos
  
  Galaxies seen in edge-on view possess radio halos with similar exponential scale heights of 1.8 +/- 0.3 kpc. The magnetic field orientations are mainly parallel to the disk near the plane, while vertical components are visible at above and below the plane and also at large distances from the center, forming an X-shaped structure. These observations support the idea of a ''galactic wind'' which is driven by star formation in the disk and transports gas, magnetic fields and cosmic-ray particles into the halo. The outflow speed of the cosmic-ray bulk velocity can be measured from radio observations.
  
  
• Evolution of magnetic fields in galaxies
  
  The origin of the first magnetic fields in the early Universe is still a mystey. A large-scale primordial field is hard to maintain in a young galaxy because the galaxy rotates differentially, so that field lines get strongly wound up during galaxy evolution, while observations show significant pitch angles. "Seed'' fields could also originate from the time of cosmological structure formation, e.g. by the Weibel instability in shocks, or could have been injected by the first stars or jets generated by the first black holes, followed by a mechanism to amplify and organize the magnetic field. The most promising mechanism to sustain magnetic fields in the interstellar medium of galaxies is the dynamo. In young galaxies without ordered rotation a small-scale dynamo amplified the seed fields within about 100 million years. The mean-field dynamo in galaxy disks generated large-scale fields within a few billion years. This scenario will be tested with the SKA and its precursor telescopes.
  
  
• Simulation of the polarized sky seen with the SKA
  
  
  
  All-sky maps in total intensity at low and high freqencies and polarized emission at 1.4~GHz (DRAO 26-m and Villa Elisa 30-m telescopes) and 22.8~GHz (WMAP), in combination with extragalactic RMs in the Galactic plane from the CGPS and high latitudes from a new Effelsberg L-band RM survey were used to model the large-scale Galactic field, as part of the SKADS science simulations. Based on these simulated low resolution sky maps, arcsec simulations of the diffuse Galactic emission and the RM distribution for extragalactic sources were made towards various Galactic directions. These simulations allow an estimate of the Galactic foreground influence on sensitive SKA observations of extragalactic objects. They will also be useful for the foreground subtraction of future low frequency experiments to detect signals from the Epoch of Reionization. As another part of the SKADS science simulations, dynamo theory was used to derive the timescales of amplification and ordering of magnetic fields in galaxies. Based on models describing the formation and evolution of dwarf and disk galaxies, an evolutionary model of turbulent and regular magnetic fields was developed, with several predictions that can be tested observationally.