The Physical Conditions of the X‐Ray Emission Line Regions in the Circinus Galaxy

Abstract

We present a detailed X-ray spectral analysis of the Circinus galaxy using archival data obtained with the ASCA satellite. The spectrum shows numerous emission lines in the soft X-ray band from highly ionized ions, as well as Compton reÑection and Ñuorescent lines from neutral or near-neutral matter. We analyze the spectrum in the context of a self-consistent recombination cascade model and Ðnd that a nearly Ñat di †erential emission measure (DEM) distribution in ionization parameter Ðts the data. For a Ðxed solid angle distribution of matter surrounding a point source, this corresponds to a run of electron density of the form n(r) D r~3@2, which is suggestive of Bondi accretion onto a central compact mass. Using this density proÐle and comparing the resulting emission spectra with the ASCA data, the size of the X-ray emission line region is estimated to be kpc. We also show that the derived density as a [1 function of radius is compatible with the X-ray recombination line emission region being the conÐning medium of the coronal line regions.

AChandraObservation of the Circinus Galaxy

“…However, the temperature of the hotter MEKAL component is very high keV, which is the hottest (kT h \ 80 temperature at which the model is tabulated) and unconstrained, making it impossible to distinguish the M2]M2 from the M2]P model. The temperature of the cooler MEKAL (kT D 0.6 keV) agrees well with the collisional ionization equilibrium model of Sako et al (2000). The addition of a second MEKAL component to either the M1]P or M2]P model does not signiÐcantly (at Z70% conÐdence) improve the Ðt.…”

“…A hard (! D 0) power-law continuum and strong Fe Ka emission were found in earlier ASCA (Matt et al 1996 ;Sako et al 2000) and BeppoSAX (Guainazzi et al 1999) observations, as well as in the Chandra HETGS observation (Sambruna et al 2001b). These properties are expected if we see the nuclear spectrum Compton reÑected from the far side of a putative dense torus and do not directly view the nucleus itself.…”

“…Thus, we conclude that there is evidence in the ACIS data for soft X-ray emission in excess of the absorbed power law (cf. Matt et al 1996 ;Sako et al 2000). The energy and equivalent width of the Fe Ka emission line are 6.38 and D3 keV, respectively.…”

“…Superposed on this spectrum are a number of emission lines from "" neutral ÏÏ and ionized states of Ne, Mg, Si, S, and Fe Kb (Matt et al 1996 ;Guainazzi et al 1999). The energies and equivalent widths of the soft (0.5È3 keV) X-ray lines indicate that they cannot be produced in the material responsible for the Compton reÑection but must instead originate from highly ionized gas, possibly photoionized by the Seyfert nucleus (Sako et al 2000). A recent Chandra High Energy Transmission Grating Spectrometer (HETGS) observation supports a two-component, centrally illuminated model (Sambruna et al 2001b).…”

Flux and spectral variations in the Circinus Galaxy

“…The line spectrum above ∼2 keV clearly originates from low ionized matter (Matt et al 1996;Netzer et al 1998;Sako et al 2000). It was shown that it is fully compatible with reflection from the inner surface of a mildly ionized torus, the same matter likely responsible for the absorption of the nuclear radiation: this interpretation leads to an estimate of the inner radius of the torus of ∼0.2 pc (Bianchi et al 2001).…”