Long Term Wind-Driven X-Ray Spectral Variability of NGC 1365 with Swift

Abstract

We present long-term (months-years) X-ray spectral variability of the Seyfert 1.8 galaxy NGC 1365 as observed by Swift, which provides well sampled observations over a much longer timescale (6 years) and a much larger flux range than is afforded by other observatories. At very low luminosities the spectrum is very soft, becoming rapidly harder as the luminosity increases and then, above a particular luminosity, softening again. At a given flux level, the scatter in hardness ratio is not very large, meaning that the spectral shape is largely determined by the luminosity. The spectra were therefore summed in luminosity bins and fitted with a variety of models. The best fitting model consists of two power laws, one unabsorbed and another, more luminous, which is absorbed. In this model, we find a range of intrinsic 0.5-10.0 keV luminosities of approximately 1.1-3.5 ergs/s, and a very large range of absorbing columns, of approximately 10^22 - 10^24 cm^-2. Interestingly, we find that the absorbing column decreases with increasing luminosity, but that this result is not due to changes in ionisation. We suggest that these observations might be interpreted in terms of a wind model in which the launch radius varies as a function of ionising flux and disc temperature and therefore moves out with increasing accretion rate, i.e. increasing X-ray luminosity. Thus, depending on the inclination angle of the disc relative to the observer, the absorbing column may decrease as the accretion rate goes up. The weaker, unabsorbed, component may be a scattered component from the wind.

9 pages, 7 figures, accepted for publication in the Monthly Notices of the Royal Astronomical Society

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• Occultation measurement of the size of the X-ray emitting region in the Active Galactic Nucleus of NGC 1365
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• WISE view of changing-look AGNs: evidence for a transitional stage of AGNs
• Detection of the high energy cut-off from the Seyfert 1.5 galaxy NGC 5273
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• Transient obscuration event captured in NGC 3227 IV. Origin of the obscuring cloud variability
• X-ray narrow emission lines from the nuclear region of NGC 1365
• The largest Swift AGN monitoring campaign: UV/optical variability in NGC 5548
• Long-Term X-ray Spectral Variability of Seyfert Galaxies with Swift