X-Ray Observation of the Roche-lobe-filling White Dwarf plus Hot Subdwarf System ZTF J213056.71+442046.5

• Published in: The Astrophysical journal 2022-05, Vol.931 (1), p.13
• Main Authors: Mereghetti, S., La Palombara, N., Kupfer, T., Marsh, T. R., Copperwheat, C. M., Deshmukh, K., Esposito, P., Maccarone, T., Pintore, F., Rigoselli, M., Rivera Sandoval, L., Tiengo, A.
• Format: Article
• Language: English
• Subjects: Accretion disks; Astrophysics; B subdwarf stars; Compact binary stars; Light curve; Luminosity; Orbits; Stellar accretion; Subdwarf stars; Wavelengths; White dwarf stars; X-ray astronomy; X-ray emissions; X-rays; XMM (spacecraft)
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/ac686e

ZTF J213056.71+442046.5 is the prototype of a small class of recently discovered compact binaries composed of a white dwarf and a hot subdwarf that fills its Roche lobe. Its orbital period of only 39 minutes is the shortest known for the objects in this class. Evidence for a high orbital inclination ( i = 86°) and for the presence of an accretion disk has been inferred from a detailed modeling of its optical photometric and spectroscopic data. We report the results of an XMM-Newton observation carried out on 2021 January 7. ZTF J213056.71+442046.5 was clearly detected by the Optical Monitor, which showed a periodic variability in the UV band (200–400 nm), with a light curve similar to that seen at longer wavelengths. Despite accretion on the white dwarf at an estimated rate of the order of 10 −9 M ⊙ yr −1 , no X-rays were detected with the EPIC instrument, with a limit of ∼10 30 erg s −1 on the 0.2–12 keV luminosity. We discuss possible explanations for the lack of a strong X-ray emission from this system.