X-ray astrophysics of compact objects

Anna Marishka Krickovich Leroux

Seyfert galaxies have long been known to exhibit rapid X- ray variability. As yet, no definitive mechanism for this variability has been identified, but shocks and flare- like events have been suggested as viable possibilities. An ASCA observation of the galaxy NGC 3227 had flares which followed a linear increase and exponential decrease, which is the same pattern followed by solar flares. Using the same techniques previously used to derive scaling laws for solar flares, a scaling law for flares on the accretion disks of AGN is derived. The law relates the loop length to the loop apex temperature, rise time, and decay time of the flare in the light curve, and from the data obtained from the observation of NGC 3227, the flarelike events are consistent with a physically viable region in the parameter space of loop length and apex temperature. Thus, at least by this measure, flares caused by magnetic loops remain a physically viable mechanism for AGN variability. Observations of Seyfert Galaxies, whether by ASCA or by other satellites, have previously divided the data of long observations by observation time or by luminosity to create spectra which average over one of these. Here an alternative method for classifying bins in the light curve according to hardness is developed, and correlated to the phenomenology of the light curve. This method is applied to the 1994 150 ks exposure ASCA observation of MCG-6-30-15. The results are that, when coupled with very conservative spectral fitting techniques, the new extraction technique suggests that local minima and maxima are spectrally different from other regions of the light curve with the same luminosity. The Seyfert galaxy Arkarian 120 is also analyzed, but has too short an exposure to extract statistically viable spectra. The spectral fitting of this source shows no evidence for a warm absorber, but suggests a soft energy excess due to blackbody radiation.

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