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The spectral energy distribution of PKS 2004-447: a compact steep-spectrum source and possible radio-loud narrow-line Seyfert 1 galaxy

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The spectral energy distribution of PKS 2004-447: a compact steep-spectrum source and possible radio-loud narrow-line Seyfert 1 galaxy

L. C. Gallo, P. G. Edwards, E. Ferrero, J. Kataoka, D. R. Lewis, S. P. Ellingsen, Z. Misanovic, W. F. Welsh, M. Whiting, Th. Boller, W. Brinkmann, J. Greenhill and A. Oshlack, 2006; MNRAS, in press

An investigation of the spectral energy distribution (SED) of the compact steep-spectrum (CSS) source and possible radio-loud narrow-line Seyfert 1 galaxy (NLS1), PKS 2004-447, is presented. Five out of six well-studied radio-loud NLS1 share this dual classification (optically defined as a NLS1 with radio definition of a CSS or giga-hertz peaked spectrum (GPS) source), suggesting that the connection could have a physical origin. The SED is created from simultaneous observations (within 24 hours) at radio (from ATCA), optical/NIR (from Siding Spring) and UV/X-ray (from XMM-Newton) wavelengths. The X-ray data show evidence of short-term variability (primarily a 30 per cent increase in the final 4 ks of the observation), a possible soft excess, and negligible absorption. Together with the rest of the SED, the Xray emission is excessive in comparison to synchrotron plus synchrotron self-Compton (SSC) models. The SED can be described with a two component model consisting of extended synchrotron/ SSC emission with Comptonisation in the X-rays, though SSC models with a very high electron-to-magnetic energy density ratio cannot be excluded either. The peak emission in the SED appears to be in the near infrared, which can be attributed to thermal emission (T ∼ 1000K) from a dusty torus. Analysis of a non-contemporaneous, low-resolution optical spectrum suggests that the narrow-line region (NLR) is much more reddened than the X-ray emitting region suggesting that the gas-to-dust ratio in PKS 2004-447 may be very different than in our own Galaxy. This could be achieved if the radio jets in PKS 2004-447 deposits material from the nucleus into the NLR. Long-term radio monitoring of PKS 2004-447 shows a rather constant light curve over nearly a six month period with the exception of one outburst when the 6.65 GHz flux increased by 35 per cent over 19 days. It is not possible to differentiate between intrinsic or extrinsic (i.e., interstellar scintillation) origins for this outburst, but the detection of the rare event demonstrates the importance of intensive monitoring campaigns. In comparison to general samples of GPS sources, which appear to be X-ray weak, NLS1-CSS/GPS sources possess stronger X-ray emission relative to radio (comparable to normal radio-loud AGN). In addition, NLS1-CSS/GPS sources also exhibit lower intrinsic absorption than GPS sources of similar X-ray luminosity. This is consistent with the additional X-ray component required for PKS 2004-447, but larger samples of NLS1-CSS/GPS are needed before any conclusive remarks can be made.

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