Watching gravitational waves
نویسنده
چکیده
In this thesis the interaction of gravitational waves (gws) with electromagnetic waves (emws) in a static magnetic background field is considered. This interaction is a consequence of the general relativistic Einstein field equations. These equations dictate the excitation of electromagnetic waves when a gravitational wave interferes with a static electromagnetic background field and a similar reverse process. Such interactions can become effective, close to very energetic sources such as colliding neutron star binaries (i.e. gamma ray bursts), and quacking supernova remnants (magnetars), or in the large scale magnetic fields of the early universe. The former two sources have strong, localized and the latter weak, but extended magnetic fields. This is important because the energy transfer efficiency of gw ⇔ emw conversions appears to be quadratically proportional to the background field strength and the extension of the interaction region. All calculations in this thesis are done in a general relativistic, space+time, noncoordinate formalism. The reason for this is that in such tedrad systems, equations remain transparent and facilitate easy physical interpretation and connection to measurements. In this framework, the conversion efficiency of gws to light and vice versa in a vacuum is considered, first in an estimate and then in a more elaborate and general, exact calculation. The gw⇒emw conversion is proposed as a possible indirect detection device for gamma ray bursts and magnetars. Also, the possibility is considered of explaining the small fluctuations in the cosmic background radiation by the conversion of gws in the early universe to emws superposed on the homogeneous background radiation. Next, to obtain more realistic results, the same process is examined in a thin plasma which leads to the same emws as generated in a vacuum. The importance of the presence of this plasma, though, is that it might damp the generated radio waves before they can travel over astromical distances, unless non-linear effects lead to higher frequencies of the emws. If radio waves with large enough frequencies are generated, gamma ray bursts and supernovæ might be detectable with (space based) radio detectors such as the proposed Astronomical Low Frequency Array (alfa) as well as with gw detectors such as Laser Interferometer Gravitational wave Observatory (ligo) with a event rate of as many as a few per year in our local galaxy group and the Virgo cluster. In the last chapter, an entirely different interaction is proposed, in which the gravitational wave interacts with the plasma and generates fast magneto-acoustic plasma waves, thus dissipating its energy into the plasma. The plasma can then emmit the energy as electromagnetic radiation. Theoretical models for gamma ray burst could be improved a lot if even a small fraction of the gw energy could be converted into emws in this fashion. The reason for this is that the energy released in a neutron star binary merger is expected to be released mainly in gws, whereas the obeserved energy flux is mostly electromagnetic. The dispersion relation derived for this interaction is the most interesting, new, result of this thesis work.
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