An Upper Limit on Nickel Overabundance in the Supercritical Accretion Disk Wind of SS 433 from X-ray Spectroscopy

Abstract

We analyze a long (with a total exposure time of 120 ks) X-ray observation of the unique Galactic microquasar SS 433 carried out by the XMM-Newton space observatory with the goal of searching for the fluorescent line of neutral (or weakly ionized) nickel at energy 7.5 keV. We consider two models for the formation of fluorescent lines in the spectrum of SS 433: (1) through the reflection of radiation from a putative central X-ray source off the optically thick neutral gas of the supercritical disk “funnel” walls; and (2) due to the scattering of the radiation coming from the hottest parts of the jets in the optically thin wind of the system. We show that for these two cases the flux of the Ni I K_α fluorescent line is expected to be 0.45 of the flux of the Fe I K_α fluorescent line at 6.4 keV for the relative nickel overabundance Z_Ni/Z = 10 observed in the jets of SS 433. For the continuum model without the absorption edge of neutral iron, we have found an upper limit on the flux of the narrow Ni I K_α fluorescent line of 0.9 × 10⁻⁵ phot s⁻¹ cm⁻² (90% confidence level). In the continuum model with the absorption edge we have determined an upper limit on the flux of the Ni I K_α line at the level of 2.5×10⁻⁵ phot s⁻¹ cm⁻². At the same time, the flux of the fluorescent iron line has been measured to be 9.9_8.4^11.2 × 10⁻⁵ phot s⁻¹ cm⁻². This result implies that the nickel overabundance in the accretion disk wind should be at least a factor of 1.5 times smal than the corresponding nickel overabundance observed in the jets of SS 433.