Limiting the accretion disk light in two mass transferring hot subdwarf binaries

TL;DR

This study investigates two hot subdwarf binaries with white dwarf companions, aiming to detect accretion disks through spectral and polarimetric analysis, and examines orbital decay via timing analysis, finding no direct evidence of accretion disks or decay yet.

Contribution

The paper provides the first detailed spectral and polarimetric constraints on accretion disks in these short-period binaries, and presents an extensive timing analysis for orbital decay.

Findings

No accretion disk signatures detected in spectra or polarization.

Upper limits placed on accretion disk flux and polarization variability.

No conclusive evidence of orbital decay observed.

Abstract

We report the results from follow-up observations of two Roche-lobe filling hot subdwarf binaries with white dwarf companions predicted to have accretion disks. ZTF J213056.71+442046.5 (ZTF J2130) with a 39-minute period and ZTF J205515.98+465106.5 (ZTF J2055) with a 56-minute period were both discovered as subdwarf binaries with light curves that could only be explained well by including an accretion disk in their models. We performed a detailed high-resolution spectral analysis using Keck/ESI to search for possible accretion features for both objects. We also employed polarimetric analysis using the Nordic Optical Telescope (NOT) for ZTF J2130. We did not find any signatures of an accretion disk in either object, and placed upper limits on the flux contribution and variation in degree of polarisation due to the disk. Owing to the short 39-minute period and availability of photometric data over six years for ZTF J2130, we conducted an extensive $O - C$ timing analysis in an attempt to look for orbital decay due to gravitational wave radiation. No such decay was detected conclusively, and a few more years of data paired with precise and consistent timing measurements were deemed necessary to constrain $\dot P$ observationally.