English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Tools annual

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Tools monthly

Global: Zendy Plus monthly

Presents the access of premium content as premium feature

Premium Content

Global: Zendy Plus annual

Global: Zendy Free Plan

We report on high-resolution X-ray spectroscopy of the ultracompact X-raybinary pulsar 4U 1626-67 with Chandra/HETGS acquired in 2010, two years afterthe pulsar experienced a torque reversal. The well-known strong Ne and Oemission lines with Keplerian profiles are shown to arise at the inner edge ofthe magnetically-channeled accretion disk. We exclude a photoionization modelfor these lines based on the absence of sharp radiative recombination continua.Instead, we show that the lines arise from a collisional plasma in theinner-disk atmosphere, with $T\simeq 10^7$ K and $n_e \sim 10^{17}$ cm^(-3). Wesuggest that the lines are powered by X-ray heating of the optically-thick diskinner edge at normal incidence. Comparison of the line profiles in HETGSobservations from 2000, 2003, and 2010 show that the inner disk radiusdecreased by a factor of two after the pulsar went from spin-down to spin-up,as predicted by magnetic accretion torque theory. The inner disk is well insidethe corotation radius during spin-up, and slightly beyond the corotation radiusduring spin-down. Based on the disk radius and accretion torque measured duringsteady spin-up, the pulsar's X-ray luminosity is $2\times 10^{36}$ erg/s,yielding a source distance of 3.5(+0.2-0.3) kpc. The mass accretion rate is anorder of magnitude larger than expected from gravitational radiation reaction,possibly due to X-ray heating of the donor. The line profiles also indicate abinary inclination of 39(+20-10) degrees, consistent with a 0.02 Msun donorstar. Our emission measure analysis favors a He white dwarf or a highly-evolvedH-poor main sequence remnant for the donor star, rather than a C-O or O-Newhite dwarf. The measured Ne/O ratio is 0.46+-0.14 by number. In an appendix,we show how to express the emission measure of a H-depleted collisional plasmawithout reference to a H number density.

Collisional Ionization in the X-ray Spectrum of the Ultracompact Binary 4U 1626-67