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

Abstract

We report on high-resolution X-ray spectroscopy of the ultracompact X-ray binary pulsar 4U 1626−67 with Chandra/HETGS acquired in 2010, two years after the pulsar experienced a torque reversal. The well-known strong Ne and O emission lines with Keplerian profiles are shown to arise at the inner edge of the magnetically-truncated accretion disk. We exclude a photoionization model for these lines based on the absence of sharp radiative recombination continua. Instead, we show that the lines arise from a collisional plasma in the inner-disk atmosphere, with T ≃ 10 7 K and n e ∼ 10 17 cm −3 . We suggest that the lines are powered by X-ray heating of the optically-thick disk inner edge at normal incidence. Comparison of the line profiles in HETGS observations from 2000, 2003, and 2010 show that the inner disk radius decreased 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 inside the corotaton radius during spin-up, and slightly beyond the corotation radius during spin-down. Based on the disk radius and accretion torque measured during steady spin-up, the pulsar's X-ray luminosity is (2.0 +0.2 −0.4 ) × 10 36 erg s −1 , yielding a source distance of 3.5 +0.2 −0.3 kpc. The mass accretion rate is an order of magnitude larger than expected from gravitational radiation reaction, possibly due to X-ray heating of the donor. The line profiles also indicate a binary inclination of 39 +20 −10 degrees, consistent with a ≃0.02 M ⊙ donor star. Our emission measure analysis favors a He white dwarf or a highly-evolved H-poor main sequence remnant for the donor star, rather than a C-O or O-Ne white 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 hydrogen-depleted collisional plasma without reference to a hydrogen number density.

The Ups & Downs of Accreting X-Ray Pulsars: Decade-long observations with the Fermi Gamma-Ray Burst Monitor

“…The optical companion is a very low-mass star (<0.1 M ; McClintock et al 1977aMcClintock et al , 1980. The Gaia measured distance to the star is 3.5 +2.3 −1.3 kpc, consistent with more recent measurements of 3.5 +0.2 −0.3 kpc by Schulz et al (2019). Since its discovery, the source has shown two major torque reversal episodes.…”

Chandra/LETGS Studies of the Collisional Plasma in 4U 1626–67

“…The only other detected emission feature in 4U 1626-67 is a weak iron line complex around 6.4 keV (Camero-Arranz et al 2012). The 2006 Suzaku and 2010 Chandra/HETGS observations report a near-neutral iron fluorescence line at ∼6.4 keV, while the 2010 Suzaku observation and 2015 NuSTAR observations see a 6.7 keV feature but no evidence for the neutral line (Camero-Arranz et al 2012;Koliopanos & Gilfanov 2016;D'Aì et al 2017;Schulz et al 2019). It remains unclear why the iron complex appears as different as it does between these studiesthere were only ∼9 months between the 2010 Chandra and Suzaku observations.…”

Pulse Frequency Fluctuations of Persistent Accretion Powered Pulsars

“…This source is a well-known ultra-compact system, containing a very low mass companion (Giacconi et al 1971;McClintock et al 1977). Moreover, it is the only ultracompact system that hosts a strongly magnetized pulsar (Schulz et al 2019). The unique case of 4U 1626-67 suggests that there should be different factors to consider for the torque noise processes in accreting systems besides the presence of wind/disk accretion and their magnetic field strengths, such as variations of the inner disk radius (Gençali et al 2022).…”

“…4U 1626-67 is an ultra-compact LMXB system composed of a pulsar and a very-low mass companion star (< 0.1𝑀 ), called Kz TrA (Giacconi et al 1971;McClintock et al 1977). The accreting pulsar has a spin-period of ∼7.68 s, a rapid orbital period of only 42 min and an estimated distance of 3.5 +2.3 −1.3 kpc (Middleditch et al 1981;Chakrabarty 1998;Schulz et al 2019;Malacaria et al 2020). The detailed spectral studies of 4U 1626-67 with BeppoSAX reveals the existence of cyclotron resonant scattering feature (CRSF) around ∼37 keV which leads to an inferred dipole magnetic field strength of 3.2 × (1 + 𝑧) 10 12 G (Orlandini et al 1998).…”