期刊
ASTRONOMY & ASTROPHYSICS
卷 388, 期 3, 页码 787-792出版社
EDP SCIENCES S A
DOI: 10.1051/0004-6361:20020471
关键词
X-rays : galaxies; galaxies : active; galaxies : Seyfert; galaxies : individual : UGC 4203; galaxies : nuclei
We report on a dramatic transition between a Compton-thick, reflection-dominated state and a Compton-thin state in the Seyfert 2 galaxy UGC 4203, discovered by comparing a recent (May 2001) XMM-Newton observation with ASCA observations performed about six years earlier. This transition can be explained either as a change in the column density of the absorber, maybe due to moving clouds in a clumpy torus, or as the revival of a transient active nucleus, which was in a phase of very low activity when observed by ASCA. If the latter explanation is correct, spectral transitions of this kind provide observational support to the idea that Compton-thick and Compton-thin regions coexist in the same source, the former likely to be identified with the torus, the latter with dust lanes on much larger scales.
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