AIU Neutron Star Group

 

Introduction

We mainly focus our investigations on a special kind of radio-quiet X-ray pulsars – the so called “Magnificent Seven” (M7 for short). They are highly magnetised (B~1012 – 1013 Gauss) and exhibit long pulse periods (3 - 12 s). Their soft X-ray spectra are best modelled with black body radiation (Teff=40-120 eV). They are nearby and young.

One of the main goals of neutron star science is constraining the Equation of State (EoS) of matter under extreme conditions (densities around nuclear density or even beyond and large magnetic fields). One observable of the EoS is the compactness, i.e. the ratio of mass and radius of the neutron star. Observed neutron star masses in binaries range from 1.1 MSun - 2.0 MSun and are accurately measured from binary pulsars or derived directly from the Shapiro delay. However, the radii are rather unknown.

In principle the radius can be determined from the flux and the luminosity (if the distance is known); for determining the luminosity from the flux, one needs a good fitting model, for example - but not neccessarily - a blackbody model, which would give the temperature and, hence, luminosity. Atomic lines (if identified correctly)
in the X-ray spectra can yield the compactness from gravitational redshift. Age and temperature enable us to draw conclusions on the cooling mechanism, hence lead also to constrains on the EoS. Therefore the M7 are suitable targets for such investigations.

Among the M7, the neutron star RX J0720.4-3125 may show free precession that is only possible for certain parameters of the superfluid interior, i.e. again constrains possible EoS. Free precession requires a deformation (that influences the precession period) of the neutron star, so that we can measure the value of deformation (ellipticity). Knowledge of the ellipticity and the EoS are required for the population synthesis of detectable gravitational waves, thus we are involved in the SFB for “Gravitational Wave Astronomy”.

Fig_1

Phase residuals of RX J0720.4-31.25 derived with the updated timing solution in Hohle et al. 2010, A&A 521, 11. The phase residuals seem to exhibit a periodic pattern.

Fig_2

Variation of the black body temperature of RX J0720.4-31.25, see also Hohle et. al 2009, A&A 498, 811.

 

 

Our group performs X-ray observations with the X-ray satellites XMM-Newton and Chandra for X-ray spectroscopy, as well as deep optical (UBV Halpha with the VLT and/or Hubble) observations (to measure magnitudes, distances and proper motions) and observations in the near infrared (Spitzer). Tracing back neutron stars to their origin by using comprehensive Monte-Carlo simulations delivers their true (kinematic) age. Furthermore, we also investigate the population of massive stars (supernova progenitors) in the solar vicinity.

 

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