The Feasibility of Using Fe <scp>XXIII</scp> Metastable Transitions as a Density Diagnostic for Low-mass X-Ray Binary Disk Winds

Abstract Low-mass X-ray binaries (LMXBs) occasionally show signs of outflowing material from the accretion disk. Studying these outflows can inform the understanding of the geometry of the systems, as well as the dynamics and energetics of accretion. One key variable for determining the location of these disk winds is the density of the outflowing material. In this paper we explore a density diagnostic based upon the absorption of ionizing photons by density-sensitive metastable states of Fe XXIII . This can yield a blueshifted complex of absorption features in the region of 6.61–6.64 keV. We use the photoionization code pion to test how varying the ionizing spectrum affects the detectability and interpretation of these features. We base these ionizing spectral energy distributions on GX 13+1 to represent a bright thermally dominated spectrum, 4U 1735−44 to represent a harder, fainter LMXB spectrum, and MAXI J1820+070 to represent a black hole (BH) LMXB spectrum completely dominated by Comptonized emission. For each of these, we find that the regime where Fe XXIII can be used as a density diagnostic is with an ionization parameter <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mi>ξ</mml:mi> <mml:mo>/</mml:mo> <mml:mi mathvariant="normal">erg</mml:mi> <mml:mspace width="0.25em"/> <mml:mi mathvariant="normal">cm</mml:mi> <mml:mspace width="0.25em"/> <mml:msup> <mml:mrow> <mml:mi mathvariant="normal">s</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> <mml:mo stretchy="false">)</mml:mo> <mml:mo>∼</mml:mo> <mml:mn>2</mml:mn> <mml:mspace width="0.1em"/> <mml:mtext>–</mml:mtext> <mml:mspace width="0.1em"/> <mml:mn>3</mml:mn> </mml:math> and an outflow density <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:msub> <mml:mi>n</mml:mi> <mml:mi mathvariant="normal">H</mml:mi> </mml:msub> <mml:mo>/</mml:mo> <mml:msup> <mml:mi>cm</mml:mi> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>3</mml:mn> </mml:mrow> </mml:msup> <mml:mo stretchy="false">)</mml:mo> <mml:mo>≳</mml:mo> <mml:mn>14</mml:mn> </mml:math> . The typical range of ionization parameters for LMXBs indicates that this technique is more feasibly achieved with BH LMXBs than their neutron star counterparts.