Astronomy and Astrophysics – Astronomy
Scientific paper
Nov 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993a%26a...279...53b&link_type=abstract
Astronomy and Astrophysics (ISSN 0004-6361), vol. 279, no. 1, p. 53-60
Astronomy and Astrophysics
Astronomy
62
Active Galactic Nuclei, Emission Spectra, Line Spectra, Seyfert Galaxies, Variability, Visible Spectrum, X Ray Astronomy, X Ray Spectra, X Rays, H Beta Line, Infrared Astronomy Satellite, Iron, Rosat Mission, Spaceborne Astronomy
Scientific paper
X-ray variability in the 0.1 - 2.4 keV ROSAT energy band with a doubling timescale of 800 s and a factor of 4 within a few hours has been detected in the IRAS active galactic nucleus (AGN) 13224-3809. The optical spectrum indicates that IRAS 13224-3809 is a narrow-line Seyfert 1 galaxy with strong permitted Fe II emission, a member of the unusual I Zw 1 class objects. IRAS 13224-3809 appears to be one of the most rapidly variable AGN known so far. This is the first time that variability on a timescale smaller than 1000 s is reported at such high L(0.1 - 2.4 keV) = 3 x 1044 ergs/s X-ray luminosity in Seyfert galaxies. It is also the first reported X-ray variability in I Zw 1 class objects. The Delta t = 800 s variation indicates that the X-rays come from a compact region of about 17 light minutes in size. Whereas the X-ray emission varies both in the soft S(0.1 - 0.4 kev) and in the hard H(0.4 - 2.4 keV) band, the hardness ratio (H-S)/(H+S) remains constant implying that no significant spectral changes occur during the X-ray flux variation. The observed steep X-ray spectrum was compared to powerlaw, blackbody and Bremsstrahlung models. A scenario in which a hard X-ray source irradiates the accretion disk which reemits at soft X-ray energies can explain both, the steep X-ray spectrum and the variability. We speculate that rapid X-ray variability could be a common feature in I Zw 1 type objects and that it may be related to the unusual optical spectra of these sources. This may help to solve the open problem of the Fe II/hydrogen beta ratio in active galactic nuclei.
Boller Th.
Caulet Adeline
Dennefeld Michel
Fink Henner
Molendi Silvano
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