An upper limit on nickel overabundance in the supercritical accretion disk wind of SS 433 from X-ray spectroscopy

TL;DR

This study uses X-ray spectroscopy to set an upper limit on nickel overabundance in SS 433's accretion disk wind, finding it to be less than 1.5 times the jet's nickel excess, refining our understanding of element distribution.

Contribution

First direct upper limit on nickel overabundance in SS 433's disk wind using X-ray spectral analysis, constraining models of element enrichment in the system.

Findings

Nickel overabundance in the wind is less than 1.5 times that in the jets.

Upper limits on Ni I Kα line flux were established at 0.9 and 2.5 x 10^-5 ph s^-1 cm^-2.

Measured Fe I Kα line flux is approximately 10^-4 ph s^-1 cm^-2.

Abstract

We take advantage of a long (with a total exposure time of 120 ks) X-ray observation of the unique Galactic microquasar SS 433, carried out with the XMM-Newton space observatory, to search for a fluorescent line of neutral (or weakly ionized) nickel at the energy 7.5 keV. We consider two models of the formation of fluorescent lines in the spectrum of SS 433: 1) due to reflection of hard X-ray radiation from a putative central source on the optically thick walls of the accretion disk "funnel"; and 2) due to scattering of the radiation coming from the hottest parts of the jets in the optically thin wind of the system. It is shown, that for these cases, the photon flux of Ni I K$_{\alpha}$ fluorescent line is expected to be 0.45 of the flux of Fe I K$_{\alpha}$ fluorescent line at 6.4 keV, for the relative nickel overabundance $Z_{Ni}/Z = 10$, as observed in the jets of SS 433. For the continuum model without the absorption edge of neutral iron, we set a 90 per cent upper limit on the flux of the narrow Ni I K$_{\alpha}$ line at the level of $0.9 \times 10^{-5}$ ph s$^{-1}$ cm$^{-2}$. For the continuum model with the absorption edge, the corresponding upper limit is $2.5 \times 10^{-5}$ ph s$^{-1}$ cm$^{-2}$. At the same time, for the Fe I K$_{\alpha}$ line, we measure the flux of $9.9_{8.4}^{11.2} \times 10^{-5}$ ph s$^{-1}$ cm$^{-2}$. Taken at the face value, the results imply that the relative overabundance of nickel in the wind of the accretion disc should be at least 1.5 times less than the corresponding excess of nickel observed in the jets of SS 433.