Comparison of high- and low-state X-ray spectra in the type 1.5 quasi-stellar object 2MASS 0918+2117

Abstract

When observed by XMM-Newton in 2003, the type 1.5 quasi-stellar object 2MASS 0918+2117 was found to be in a low state, with an X-ray flux approximately four to five times fainter than during an earlier Chandra observation. The 2-6 keV spectrum was unusually hard (photon index Γ ~ 1.25), with evidence for a reflection-dominated continuum, while a soft excess visible below ~1 keV prevented confirmation of the anticipated low energy absorber. In a second XMM-Newton observation in 2005, the X-ray flux is found to have recovered, with a 2-10 keV continuum spectrum now typical of a broad-line active galaxy (Γ ~ 2) and a deficit of flux below ~1 keV indicative of continuum absorption in a column N_H ~ 4 × 10²¹cm^-2. We find the preferred ionization state of the absorbing gas to be low, which then leaves a residual soft excess of similar spectral form and flux to that found in the 2003 XMM-Newton observation. Although observed at different epochs, we note that dust in the absorbing column could also explain the red nucleus and strong optical polarization of 2MASS 0918+2117.