We present an investigation of the ultraviolet and X-ray spectra of the Seyfert 1.5 galaxy Markarian 817 .
The ultraviolet analysis includes two recent observations taken with the Cosmic Origins Spectrograph in August and December 2009 , as well as archival spectra from the International Ultraviolet Explorer and the Hubble Space Telescope .
Twelve Ly \alpha absorption features are detected in the 1997 GHRS and 2009 COS spectra – of these , four are associated with high-velocity clouds in the interstellar medium , four are at low-significance , and the remaining four are intrinsic features , which vary between the GHRS and COS observations .
The strongest intrinsic absorber in the 1997 spectrum has a systemic velocity of \sim - 4250 km s ^ { -1 } .
The corresponding feature in the COS data is five times weaker than the GHRS absorber .
The three additional weak ( equivalent width from 13–54 mÅ ) intrinsic Ly \alpha absorbers are at systemic velocities of -4100 km s ^ { -1 } , -3550 km s ^ { -1 } , and -2600 km s ^ { -1 } .
However , intrinsic absorption troughs from highly ionized C iv and N v , are not detected in the COS observations .
No ionized absorption signatures are detected in the \sim 14 ks XMM-Newton EPIC spectra .
The factor of five change in the intrinsic Ly \alpha absorber is most likely due to bulk motions in the absorber , since there is no drastic change in the UV luminosity of the source from the GHRS to the COS observations .
In a study of variability of Mrk 817 , we find that the X-ray luminosity varies by a factor of \sim 40 over 20 years , while the UV continuum/emission lines vary by at most a factor of \sim 2.3 over 30 years .
The variability of the X-ray luminosity is strongly correlated with the X-ray power-law index , but no correlation is found with the simultaneous optical/UV photometry .