241 research outputs found
A 10-day ASCA Observation of the Narrow-line Seyfert~1 galaxy IRAS 13224-3809
(Abridged) We present an analysis of a 10-day continuous ASCA observation of
the narrow-line Seyfert 1 galaxy IRAS 13224-3809. The soft (0.7-1.3 keV) and
hard (1.3-10 keV) X-ray band light curves binned to 5000s reveal trough-to-peak
variations by a factor >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
presence of a soft X-ray hump below 2 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 in the range 1.74-2.47,
however, day-to-day variations in Gamma are not significant. The Soft hump
emission is found to dominate the observed variability on a timescale of a
week, but on shorter timescales (20000s) 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. 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.Comment: 29 pages, 16 figures, To apear in A&
The X-Ray Spectral Variability of Mrk 766
Analysis results from ASCA and ROSAT observations of the narrow-line Seyfert
1 galaxy Mrk 766 are reported. In the ASCA observation we observed rapid
variability with a doubling time scale of 1000 seconds. A spectral variability
event was observed in which the spectrum softened and hardened above and below
~1 keV, respectively, as the flux increased. The spectra could be modeled with
5 components: a power law, warm absorber, iron K(alpha) line and soft excess
component flux. The spectral variability resulted from a highly significant
change in the intrinsic photon law index from Gamma ~1.6 to ~2.0, an increase
in the warm absorber ionization, and a marginally significant decrease in the
soft component normalization. A ~100 eV equivalent width narrow iron K(alpha)
line was detected in the high state spectrum. Spectral hardening during flux
increases was observed in three ROSAT observations.
The change in intrinsic photon index and disappearance of the soft excess
component in the ASCA spectra can be explained as a transition from a first
order pair reprocessed spectrum to a pair cascade brought about by a sudden
increase in the injected electron Lorentz factor. The change in the ionization
of the warm absorber, though model dependent, could correspond to the increase
in flux at the oxygen edges resulting from the spectral index change. The ROSAT
spectral variability can be interpreted by variable intensity hard power law
and a relatively nonvarying soft component, possibly primary disk emission.
These results are compared with those reported from other narrow-line Seyfert 1
galaxies.Comment: 29 pages using (AASTeX) aaspp4.sty and 18 Postscript figures. To
appear in the September 1, 1996, issue of The Astrophysical Journa
- …