Publication Date

April 2018

Advisor(s)

Edward Moran

Major

Astronomy

Language

English (United States)

Abstract

Recently, archival data have revealed dramatic X-ray and optical variability in a handful of radio-quiet quasars over a span of 10 years. In some cases, this variability has been accompanied by changes in spectral characteristics in the optical band, which may provide clues regarding how massive black holes are fueled. To explore this issue further, we have used X-ray data obtained with the Einstein and ROSAT observatories to identify objects that dimmed or brightened tremendously over the 10-year span between these missions. Some of the objects we have found were observed multiple times with ROSAT; thus, we have measured long-term X-ray light curves for these sources to confirm their extreme variability. From a sample of 40 such objects, we have focused on 10 AGNs, which dimmed by factors of 8 to 76 between 1980 and 1990. The optical data from the Palomar 5m telescope confirms seven quasars, one Narrow-line Seyfert 1s, a Seyfert 1 galaxy and a Seyfert 2 galaxy. The X-ray soft (0.5-2.0 keV) luminosity remains consistent for these optical classifications. However, the Seyfert 2 galaxy had an X-ray luminosity greater than 1E43 ergs/s limit for Seyfert 1 galaxies. Thus, this source is an example of a "Changing-Look AGN." CLAGNs are objects that have transition from one spectral type (say 1) to another (2). These sources have not totally "turned off" despite the dramatic decrease in their accretion luminosity. From other permutations of X-ray observatories to compare, we may find objects in our expanded sample that could potentially reveal a population whose mass-accretion rates have declined, providing insight into the origin and duration of black-hole fueling mechanisms.

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