EDP Sciences
Free access
Volume 390, Number 1, July IV 2002
Page(s) 65 - 80
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20020689

A&A 390, 65-80 (2002)
DOI: 10.1051/0004-6361:20020689

A 10-day $\vec{ASCA}$ observation of the narrow-line Seyfert 1 galaxy IRAS 13224-3809

G. C. Dewangan1, Th. Boller2, K. P. Singh1 and K. M. Leighly3

1  Department of Astronomy & Astrophysics, Tata Institute of Fundamental Research, Mumbai 400 005, India
2  Max-Planck-Institute for Extraterrestrical Physics, Giessenbachstr., 85748 Garching, Germany

3  Department of Physics and Astronomy, The University of Oklahoma, 440 W. Brooks St., Norman, OK 73072, USA

(Received 12 Mars 2002 / Accepted 1 May 2002 )

We present an analysis of a 10-day continuous ASCA observation of the narrow-line Seyfert 1 galaxy IRAS 13224-3809. The total band ( $0.7{-}10~{\rm keV}$) light curve binned with 500 s reveals trough-to-peak variation by a factor $\ge$37. Rapid X-ray variability with a doubling timescale of 500 s has also been detected. The soft ( $0.7{-}1.3~{\rm keV}$) and hard ( $1.3{-}10~{\rm keV}$) X-ray band light curves binned to 5000 s reveal trough-to-peak variations by a factor $\ge$25 and ~20, respectively. The light curves in the soft and hard bands are strongly correlated without any significant delay. However, this correlation is not entirely due to changes in the power-law flux alone but also due to changes in the soft X-ray hump emission above the power law. The variability amplitude changes across the observation but is not correlated with the X-ray flux. The presence of a soft X-ray hump below ~ $2~{\rm keV}$, previously detected in ROSAT and ASCA data, is confirmed. Time resolved spectroscopy using daily sampling reveals changes in the power-law slope, with $\Gamma_{\rm X}$ in the range 1.74-2.47, however, day-to-day variations in $\Gamma_{\rm X}$ are not significant. The Soft hump emission is found to dominate the observed variability on a timescale of ~a week, but on shorter timescales (~20 000 s) the power-law component appears to dominate the observed variability. Flux resolved spectroscopy reveals that at high flux levels the power law becomes steeper and the soft hump more pronounced. This result is further confirmed using an earlier ASCA observation in 1994. The steepening of the photon-index with the fluxes in the soft and hard bands can be understood in the framework of disk/corona models in which accretion disk is heated by viscous dissipation as well as by reprocessing of hard X-rays following an X-ray flare resulting from coronal dissipation through magnetic reconnection events. Time dependent accretion disk-corona models are required to understand the observed correlation between the soft hump emission and the power-law flux.

Key words: galaxies: active -- galaxies: individual: IRAS 13224-3809 -- galaxies: nuclei -- galaxies: Seyfert -- X-rays: galaxies

Offprint request: G. C. Dewangan, gulab@tifr.res.in

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