Astronomy and Astrophysics – Astrophysics
Scientific paper
1999-11-19
Mon.Not.Roy.Astron.Soc. 315 (2000) 77
Astronomy and Astrophysics
Astrophysics
5 pages, submitted to the MNRAS
Scientific paper
10.1046/j.1365-8711.2000.03379.x
I present ASCA observations of IRAS00317-2142, the most luminous (Lx~10^43 erg s-1 0.1-2 keV) of the 'composite' class of galaxies. This enigmatic class of objects presents narrow-emission line optical spectra classifying these galaxies as star-forming on the basis of the diagnostic emission line ratios; yet, the presence of weak Halpha broad wings also suggests the presence of a weak or obscured AGN. The ASCA spectrum can be represented with a power-law with a photon index of 1.76+-0.08. Strong variability is detected (by about a factor of three) between the ROSAT and ASCA 1-2 keV flux. These characteristics clearly suggest an AGN origin for the X-ray emission. However, the precise nature of this AGN remains still uncertain. There is no evidence for a high absorbing column density above the Galactic. Moreover, there is no strong evidence for an Fe line at 6.4 keV, with the 90 per cent upper limit on the equivalent width being 0.9 keV. Thus the X-ray spectrum is consistent with an unobscured Seyfert-1 interpretation. This discrepancy with the optical spectrum, may be explained by either a strong star-forming component or a 'dusty' ionised absorber. Finally, the possibility that IRAS00317-2142 may harbour a heavily obscured AGN where the X-ray emission is mainly due to scattered light, appears less plausible due to the high value of the fx/f[OIII] ratio which is more indicative of unobscured Seyfert-1 type AGN.
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