Projects

SFB / TR 7 "Gravitational Wave Astronomy"

Gravitational Waves from Magnetar Seismology

We participate in project B9 "Gravitational Waves from Magnetar Seismology".

Magnetars are neutron stars with immense magnetic fields: B > 1014 Gauss. To date, three giant flares from magnetars have been observed, each yielding peak luminosities of an astonishing few times 1047 erg/s. Timing analysis of the tails of these flares reveal quasiperiodic oscillations (QPOs), whose frequencies are consistent with Alfvén waves associated with the extremely strong magnetic fields in both the interior and crust of the star. It is an open question whether the GW output from such events provides viable sources for current and/ or future generation GW detectors.

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       QPO

Interpretation of Gravitational Wave Signals                                                                  

We also participate in project C2 "Interpretation of Gravitational Wave Signals".

In the observational part of this project we investigate very young, oscillating neutron stars, precessing neutron stars and old binary neutron stars that are the most prominent sources of gravitational waves. We also investigate the birth rate of neutron stars that is enhanced in the so-called Gould Belt, a torus-like structure around the Sun younger than 50 million years with several thousand stars including both young neutron stars and supernova progenitors. Here, the goal is a clear prediction of the neutron stars birth rate and the fraction of neutron star-neutron star binaries in the Gould Belt.

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Populations of Astrophysical Sources

Here, we participate in project C7 "Populations of Astrophysical Sources".

Isolated neutron stars are potential sources for constraining the equation-of-state of matter at ultra-high densities. The determination of the mass can be achieved by measuring the surface gravity or gravitational redshift from absorption lines. Here, strong magnetic fields and polarization effects become important, so that we will investigate radiation transfer through the geometrically thin neutron star atmosphere. The goal is to construct a new generation of non-LTE model atmospheres for neutron stars.

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Constraining the Equation of State of neutron stars from X-ray observation

   

Matter at high pressure and densities (1015 g/cm3), large magnetic fields (1013 G) and high temperatures (106 K) cannot be generated in laboratories. To study the properties of matter under such extreme conditions, in particular the equation of state (EoS) at super nuclear densities, one needs suitable objects, hence neutron stars (NSs). The EoS can be constrained by the mass-radius relation (the compactness), period and long term evolution of the period (glitch, free precession) and cooling behaviour. Current NS models deal with various families of EoS, i.e. with strange stars (quark stars or stars with pion and/or kaon condensates), NSs with ordinary neutrons mixed with protons and electrons (stiff EoSs yielding larger radii and soft EoSs yielding smaller radii; both in conjunction with or without superfluid matter).

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     M R 2 FSU SLy BPS 3

Kinematics of young neutron stars

   

There are many young, nearby neutron stars (NSs) with distance and proper motion measurements known, most of them radio pulsars, plus a few radio-quiet thermal X-ray emitters. By tracing back the 3D motion of all those NSs, nearby young stellar associations and clusters (later also bubbles), and also all known young runaway stars, we can find close encounters in space and time, i.e. events, where NSs intersected with an association and/or a runaway star. Then, the NS may have been born in that association at that time in a supernova. An additional encounter with a runaway star would provide additional evidence.

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Search for new neutron stars

   

Finding new neutron stars is a crucial topic as the expected total number of neutron stars in our Galaxy was predicted to be ~ 1 billion  of which isolated NS should form the majority. Until today there are ~ 2000 known radio pulsars and only seven known isolated thermally emitting NS (called the magnificent seven). Since the discovery of the first INS (RX J1856.5-3754) in 1996 the search for more thermal NS is an ongoing process. Those seven objects have been recognized by their high X-ray to optical flux ratio and their rather soft X-ray emission represented by low X-ray hardness ratios.

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   HR1_HR2_bb_different_types

 

Optical Investigations

Since the first optical detection of the brightest M7, RX J1856.5-3754 (RXJ1856 for short), using the Hubble Space Telescope (HST) by Walter, Wolk & Neuhäuser (1996), for six of the M7 faint optical counterparts could be observed with HST or large aperture (3.6m to 8m and more telescope diameter) ground based facilities, such as the Very Large Telescope (VLT) on Cerro Paranal in Chile. The investigation of the optical emission, coming from those objects offers a variety of new scientific possibilities.

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