Chandra high-resolution X-ray spectroscopy of AM Herculis

Abstract

We present the results of high-resolution spectroscopy of the prototype Polar AM Herculis observed with Chandra High Energy Transmission Grating . The X-ray spectrum contains hydrogen-like and helium-like lines of Fe, S, Si, Mg, Ne, and O, with several Fe L-shell emission lines. The forbidden lines in the spectrum are generally weak, whereas the hydrogen-like lines are stronger suggesting that emission from a multitemperature, collisionally ionized plasma dominates. The helium-like line flux ratios yield a plasma temperature of 2 MK and a plasma density 1-9×10¹² cm⁻³, whereas the line flux ratio of Fe XXVI to Fe XXV gives an ionization temperature of 12.4^+1.1_−1.4keV. We present the differential emission measure distribution of AM Her, whose shape is consistent with the volume emission measure obtained by a multitemperature APEC model. The multitemperature plasma model fit to the average X-ray spectrum indicates the mass of the white dwarf to be ~1.15 M_⊙. From phase-resolved spectroscopy, we find the line centers of Mg XII, S XVI, resonance line of Fe XXV, and Fe XXVI emission modulated by a few hundred to 1000 km s⁻¹ from the theoretically expected values, indicating bulk motion of ionized matter in the accretion column of AM Her. The observed velocities of Fe XXVI ions are close to the expected shock velocity for a 0.6 M_⊙ white dwarf. The observed velocity modulation is consistent with that expected from a single pole accreting binary system.