Symbiotic stars in X-rays

IV. XMM-Newton, Swift, and TESS observations

¹ CONICET-Universidad de Buenos Aires, Instituto de Astronomía y Física del Espacio (IAFE), Av. Inte. Güiraldes 2620, C1428ZAA Buenos Aires, Argentina
² Universidad Nacional de San Juan, Facultad de Ciencias Exactas, Físicas y Naturales, Av. Ignacio de la Roza 590 (O), Complejo Universitario “Islas Malvinas”, Rivadavia, J5402DCS San Juan, Argentina
³ Universidade Estadual Paulista “Júlio de Mesquita Filho”, UNESP, Campus of Guaratinguetá, Av. Dr. Ariberto Pereira da Cunha, 333 – Pedregulho, Guaratinguetá, SP 12516-410, Brazil
⁴ CONICET-Universidad Nacional de Hurlingham, Av. Gdor. Vergara 2222, Villa Tesei, Buenos Aires, Argentina
⁵ CRESST and X-ray Astrophysics Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁶ Department of Physics, University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA
⁷ IP&D, Universidade do Vale do Paraíba, 12244-000 São José dos Campos, SP, Brazil
⁸ Columbia Astrophysics Lab., 550 W120th St., 1027 Pupin Hall, MC 5247 Columbia University, New York, NC 10027, USA
⁹ Department of Physics & Astronomy, Stony Brook University, Stony Brook, NY 11794-3800, USA

Received: 8 March 2024
Accepted: 29 April 2024

Abstract

White dwarf symbiotic binaries are detected in X-rays with luminosities in the range of 10³⁰–10³⁴ ergs s⁻¹. Their X-ray emission arises either from the accretion disk boundary layer, from a region where the winds from both components collide, or from nuclear burning on the surface of the white dwarf (WD). In our continuous effort to identify X-ray-emitting symbiotic stars, we studied four systems using observations from the Neil Gehrels Swift Observatory and XMM-Newton satellites in X-rays and from Transiting Exoplanet Survey Satellite (TESS) in the optical. The X-ray spectra were fit with absorbed optically thin thermal plasma models that are either single- or multitemperature with kT < 8 keV for all targets. Based on the characteristics of their X-ray spectra, we classified BD Cam as possible β-type, V1261 Ori and CD −27 8661 as δ-type, and confirmed NQ Gem as β/δ-type. The δ-type X-ray emission most likely arises from the boundary layer of the accretion disk, while in the case of BD Cam, its mostly soft emission originates from shocks, possibly between the red giant and WD and disk winds. In general, we find that the observed X-ray emission is powered by accretion at a low accretion rate of about 10⁻¹¹ M_⊙ yr⁻¹. The low ratio of X-ray to optical luminosities, however indicates that the accretion-disk boundary layer is mostly optically thick and tends to emit in the far or extreme UV. The detection of flickering in optical data provides evidence of the existence of an accretion disk.