K2 & TESS observations of symbiotic X-ray binaries: GX 1+4 and IGR J16194-2810

TL;DR

This study analyzes K2 and TESS data of symbiotic X-ray binaries GX 1+4 and IGR J16194-2810, revealing their spin periods, variability, and potential orbital characteristics, advancing understanding of their accretion processes and system dynamics.

Contribution

It provides the first optical detection of the neutron star spin period in IGR J16194-2810 and confirms the ongoing spin-down of GX 1+4 using space-based photometry.

Findings

GX 1+4's spin period is 180.426 seconds, continuing to spin down.

IGR J16194-2810's neutron star has a spin period of 242.837 minutes.

Optical and X-ray variability are correlated in GX 1+4.

Abstract

I analyze the K2 and TESS data taken in 2016, 2019 and 2021 of the symbiotic X-ray binaries GX 1+4 and IGR J16194-2810. GX 1+4 consists of a pulsar accreting from a red giant companion in a 1160 days orbit. Since 1984, the pulsar has shown a continuous spin-down rate of $\dot{P}$=-0.1177(3) mHZ/yr. I report the detection of the spin period at an average value of 180.426(1) seconds as observed with the K2 mission and confirm that the spin period continues to increase at a rate of $\sim$1.61$\times$10$^{-7}$ s/s. The K2 and hard X-rays, as observed with Swift/BAT, varied in tandem, in agreement with other authors who proposed that the optical light arise from reprocessed X-ray emission. In the case of IGR J16194-2810, the X-ray and optical spectroscopy have been interpreted as arising from a neutron star accreting from a M2 III red giant companion. Its orbital period is unknown, while I report here the detection of a modulation with a period of 242.837 min, interpreted as the neutron star spin period. IGR J16194-2810 is thus the second symbiotic X-ray binary where the spin period is detected in optical wavelengths. This period, however, was only detected during the TESS observations of Sector 12 in 2019. The non-detection of this modulation during the observations of Sector 39 in 2021 is perhaps related with the orbital modulation, i.e. a low inclination of the orbit.