Broadband spectral investigations of magnetar bursts

Official URL: http://risc01.sabanciuniv.edu/record=b1655307 (Table of Contents)

Magnetars are neutron stars whose variety of energetic emission mechanisms are thought to be governed by the decay of their extremely strong magnetic fields (B ⇠1014G) . Studies on radiative magnetar behaviour promise insight into emission mechanisms in highly magnetized regions as well as the formation, evolution and structure of neutron stars. In this thesis, we present our broadband (2-250 keV) spectral analysis of 42, 125 and 221 bursts from magnetar sources SGR J1550-5418, SGR 1900+14 and SGR 1806-20, respectively, detected with the Rossi X-ray Timing Explorer (RXTE) mission. We find that two blackbody functions (BB+BB), sum of two modified blackbody functions (LB+LB), sum of blackbody and powerlaw functions (BB+PO) and a power law with a high energy exponential cut-off (COMPT) all provide acceptable fits at similar levels. We report that when a c2 comparison test is employed, 258 out of 388 bursts examined provided better fit statistics (lower c2 within 0.05 significance) when fitted with the COMPT while 28 were better fitted with a sum of two blackbody functions. We performed numerical simulations to further constrain the best fitting model for each burst spectrum, and found that 69 out of 102 burst spectra with well-constrained parameters are significantly better described by the Comptonized model. We also found that 66 out of 102 these burst spectra are better described with LB+LB, which is employed in X-ray spectral modeling for the first time here, than BB+BB and BB+PO. We also show a significant correlation between burst emission area and blackbody temperatures when BB+BB fits are employed. We expand on the physical interpretation of these models and discuss our results in the framework of strongly magnetized neutron star case.