Mathematics – Logic
Scientific paper
Oct 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000phdt.........1w&link_type=abstract
Thesis (PhD). UNIVERSITY OF ALABAMA IN HUNTSVILLE, Source DAI-B 61/04, p. 1994, Oct 2000, 162 pages.
Mathematics
Logic
1
Scientific paper
Following the discovery that some (and possibly all) gamma-ray bursts (GRBs) come from cosmological distances, it is now apparent that active Galactic sources of high- energy (>25 keV) bursts are rare. The rarity of these sources warrants a detailed study of their observed properties in order to better understand their nature. Observations of two different types of magnetized neutron stars which produce super-Eddington bursts are presented in this dissertation. The Bursting Pulsar (GRO J1744-28) is a transient low- mass X-ray binary (LMXB) near the Galactic center that emits both Type II bursts and periodic pulsations. It is unique in that it is the only known binary to exhibit this behavior. GRO J1744-28 has gone into outburst twice since its discovery and we show the second outburst is nearly identical to the first with the exception of a decrease in intensity by a factor ~1.7 of both the burst and persistent emission. During the bursts of the first outburst, the pulsations are observed to lag behind their expected arrival times based upon the persistent emission pulsar ephemeris. The magnitude of the pulse time lags during and after bursts are quantified with greater precision than before for a large subset of events from the first and second outbursts. The pulse time delay during bursts of the first outburst is found to be correlated with the peak intensity of the bursts. Soft gamma repeaters (SGIRs) are transient sources of brief, intense hard X-rays and low-energy γ-rays, very likely neutron stars with superstrong magnetic fields of order 1014-1015 G, i.e., magnetars. This dissertation presents the discovery of one of the four known SGRs, 1627-41. We have discovered the probable persistent X-ray counterpart to this SGR whose characteristics are consistent with the other SGR counterparts. Observations of the persistent X-ray emission from SGR 1900+14 show that the spectrum is best fit with a two-component model (blackbody + power law) during quiescence. This two-component model is a common trait of anomalous X-ray pulsars, a group of isolated X- ray sources (i.e., no companion) that also show strong evidence for a magnetar origin. The persistent flux is found to increase during a burst active interval for this source, and we find this increase lies exclusively in the non-thermal component of the spectrum. Next, we study the spin history of SGR 1900+14 over 2.5 yrs and find significant deviations from a constant spin down both during quiescence and a burst active interval. We find circumstantial evidence for a discrete spin-down event, i.e., a `braking' glitch, possibly associated with a giant flare recorded on 27 August 1998. Finally, we report the detection of two unusually hard bursts from SGR 1900+14. These events are distinctly different from typical bursts both temporally and spectrally. In fact, aside from their spectral evolution, they more closely resemble GRBs.
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