Rapid X-ray variability in the I ZW 1 class object IRAS 13224-3809.
Abstract
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 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 10^44^ erg x s^-1^ X- ray luminosity in Seyfert galaxies. It is also the first reported X-ray variability in I Zw 1 class objects. The δt = 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. Only the Bremsstrahlung model gives an acceptable fit, but it must be rejected on the basis of the variability argument. Emission from a standard accretion disk model fits the data. Again, from the observed rapid variability we are forced to rule out this model. 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/Hβ ratio in active galactic nuclei.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- November 1993
- Bibcode:
- 1993A&A...279...53B
- Keywords:
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- 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;
- Astronomy