RT Cru: a look into the X-ray emission of a peculiar symbiotic star

TL;DR

This study analyzes extensive X-ray data of RT Cru, a peculiar symbiotic star, to understand its emission mechanisms and the nature of its white dwarf, suggesting it may be a magnetic white dwarf with a high accretion rate.

Contribution

The paper provides the first detailed spectral analysis of RT Cru using multiple X-ray observatories, proposing it as a magnetic white dwarf with specific mass estimates and accretion characteristics.

Findings

X-ray spectrum fits a cooling flow model with absorbers

White dwarf mass estimated at ~1.2 M_Sun or alternatively 0.9-1.1 M_Sun if magnetic

Possible magnetic white dwarf nature with high accretion rate

Abstract

Symbiotic stars are a heterogeneous class of interacting binaries. Among them, RT Cru has been classified as prototype of a subclass that is characterised by hard X-ray spectra extending past ~20 keV. We analyse ~8.6 Ms of archival INTEGRAL data collected in the period 2003-2014, ~140 ks of Swift/XRT data, and a Suzaku observation of 39 ks, to study the spectral X-ray emission and investigate the nature of the compact object. Based on the 2MASS photometry, we estimate the distance to the source of 1.2-2.4 kpc. The X-ray spectrum obtained with Swift/XRT, JEM-X, IBIS/ISGRI, and Suzaku data is well fitted by a cooling flow model modified by an absorber that fully covers the source and two partial covering absorbers. Assuming that the hard X-ray emission of RT Cru originates from an optically thin boundary layer around a non-magnetic white dwarf, we estimated a mass of the WD of about 1.2 M_Sun. The mass accretion rate obtained for this source might be too high for the optically thin boundary layer scenario. Therefore we investigate other plausible scenarios to model its hard X-ray emission. We show that, alternatively, the observed X-ray spectrum can be explained with the X-ray emission from the post-shock region above the polar caps of a magnetised white dwarf with mass ~0.9-1.1 M_Sun.