Seyfert galaxies as X-ray sources

Using d a ta from th e L e ic e s te r U n iv e rs ity A r ie l V Sky Survey Instrum ent (SSl) th e work re p o rte d in t h i s th e s is e s ta b lis h e s th a t S e y fe r t g a la x ie s form a c la s s o f e x tr a g a la c t ic x -ray so u rc e . The p ro p e r tie s o f S e y fe r t g a la x ie s a re review ed and i t i s shown how s tan d a rd arguments le a d to a model of th e s tru c tu r e o f S e y fe r t galaxy n u c le i co n ta in in g th re e reg io n s c h a ra c te r is e d by th re e ty p es o f em ission: fo rb idden l i n e , p e rm itted l in e and * non-thermal* continuum. R adio, in f r a re d and o p t ic a l param eters b e liev ed to d eriv e from th e se th re e reg io n s a re then compared w ith x -ra y f lu x d e n s i t ie s from th e SSI in an a ttem p t to lo c a te th e source o f x -ra y em ission . I t i s found th a t x -ra y f lu x d e n s ity c o r re la te s only w ith param eters d e sc rib in g th e p e rm itted l in e and continuum re g io n s . S tandard argum ents p resen ted e a r l i e r show th a t th ese reg io n s a re very sm all («0.01 Ipc d iam e te r). O bservable x -ra y v a r i a b i l i t y i s th u s p o ss ib le and i s indeed seen . The v a r i a b i l i t y o f x -ra y em ission from x -ray a c t iv e g a la x ie s i s d esc rib ed u s in g , in th e main, SSI o b s e rv a tio n s . Some su g g estiv e s im i l a r i t i e s in t h e i r l i g h t curves Eire n o ted . An x -ra y lum inosity fu n c tio n f o r S e y fe r t g a la x ie s i s c o n s tru c ted by two methods whose r e s u l t s agree w e ll. Using t h i s lum inosity fu n c tio n th e S e y fe rt g a la x ie s a re found to c o n tr ib u te « 6% to th e d if fu s e x -ray background, w ithout ev o lu tio n . They can account f o r th e e n t i r e background i f on ly moderate ev o lu tio n i s allow ed. F in a l ly , re c e n t developm ents in th e f i e ld a re noted: th e d isco v ery o f more S e y fe r t g a la x ie s a s x -ray sou rces , v a r i a b i l i t y , and some f i r s t x ra y sp e c tra a re review ed. An ex tension to th e c la s s o f * x ra y a c t iv e g a la x ie s ' in th e form of high e x c ita t io n