Taking a break: Paused accretion in the symbiotic binary RT Cru

¹ CONICET-Instituto de Astronomía y Física del Espacio Ciudad Universitaria – Pabellon 2 Intendente Güiraldes 2160, C1428EGA Ciudad Autónoma de Buenos Aires, Argentina
e-mail: gjmluna@iafe.uba.ar
² Universidad de Buenos Aires, Facultad de Ciencias Exactas y Naturales, Buenos Aires, Argentina
³ Universidad Nacional de Hurlingham, Av. Gdor. Vergara 2222, Villa Tesei, Buenos Aires, Argentina
⁴ CRESST and X-ray Astrophysics Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁵ Department of Physics, University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA
⁶ Columbia Astrophysics Lab. 550 W120th St., 1027 Pupin Hall, MC 5247 Columbia University, New York, NC 10027, USA
⁷ Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, Surrey RH5 6NT, UK
⁸ Department of Physics & Astronomy, Stony Brook University, Stony Brook, NY 11794-3800, USA
⁹ Gemini Observatory/NSF’s NOIRLab, Casilla 603, La Serena, Chile
¹⁰ Institute of Astronomy and National Astronomical Observatory, Bulgarian Academy of Sciences, 72 Tsarigradsko Chaussee Blvd., 1784 Sofia, Bulgaria
¹¹ Departamento de Física, Universidade Federal de Sergipe, Av. Marechal Rondon, s/n, 49100-000 São Cristóvão, SE, Brazil
¹² Observatório Nacional, Rua Gal. José Cristino 77, 20921-400 Rio de Janeiro, RJ, Brazil
¹³ Instituto de Ciencias Astronómicas, de la Tierra y del Espacio (ICATE-CONICET), Av. España Sur 1512, J5402DSP San Juan, Argentina
¹⁴ Instituto de Investigación Multidisciplinar en Ciencia y Tecnología, Universidad de La Serena, Av. R. Bitrán 1305, La Serena, Chile
¹⁵ Departamento de Astronomía, Universidad de La Serena, Av. J. Cisternas 1200, La Serena, Chile
¹⁶ Universidad Nacional de Córdoba, Observatorio Astronómico de Córdoba, Laprida 854, 5000 Córdoba, Argentina
¹⁷ Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Godoy Cruz 2290, Ciudad Autónoma de Buenos Aires, Argentina

Received: 13 September 2022
Accepted: 23 November 2022

Abstract

Symbiotic binaries sometimes hide their symbiotic nature for significant periods of time. There is mounting observational evidence that, in symbiotics that are powered solely by the accretion of the red giant’s wind material onto a white dwarf, without any quasi-steady shell burning on the surface of the white dwarf, the characteristic emission lines in the optical spectrum can vanish, leaving the semblance of an isolated red giant spectrum. Here we present compelling evidence that this disappearance of optical emission lines from the spectrum of RT Cru in 2019 was due to a decrease in the accretion rate, which we derived by modeling the X-ray spectrum. This drop in accretion rate leads to a lower flux of ionizing photons and thus to faint or absent photoionization emission lines in the optical spectrum. We observed the white dwarf symbiotic RT Cru with XMM-Newton and Swift in X-rays and UV and collected ground-based optical spectra and photometry obtained over the last 33 yr. This long-term coverage shows that, during most of the year 2019, the accretion rate onto the white dwarf was so low, Ṁ = (3.2 ± 0.06) × 10⁻¹¹ M_⊙ yr⁻¹ (d/2.52 kpc)², that the historically detected hard X-ray emission almost vanished, the UV flux faded by roughly 5 mag, the U, B, and V flickering amplitude decreased, and the Balmer lines virtually disappeared from 2019 January through March. Long-lasting low-accretion episodes such as the one reported here may hamper the chances of RT Cru experiencing a nova-type outburst despite the high mass of the accreting white dwarf.