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Discovery of a new Galactic symbiotic X-ray binary directly associated with intense mass loss episodes of a heavily obscured Mira variable

Presentation #406.05 in the session “Stellar/Compact II (Oral)”.

Published onApr 01, 2022
Discovery of a new Galactic symbiotic X-ray binary directly associated with intense mass loss episodes of a heavily obscured Mira variable

The symbiotic X-ray binaries (SyXRBs) constitute a rare class of X-ray sources consisting of a compact object (neutron star [NS] or black hole) accreting from the wind of a highly evolved companion. Hosting NSs with some of the strongest known magnetic fields as well as representing a common pathway in the evolution of massive binaries towards gravitational wave sources, the demographics of SyXRBs offer unique insights into binary stellar evolution. It has been long suggested that SyXRBs become conspicuous in the X-ray sky only for a brief period in the binary lifetime when the donor reaches the very tip of the giant branch and undergoes intense mass loss. While the duration and characteristics of this stage are fundamental to understanding the active lifetimes of SyXRBs, conclusive evidence for X-ray enhancement coincident with donor activity remains lacking, particularly due to the heavily reddened spectrum of the donor stars that preclude detailed variability studies in the optical bands. In this talk, I will present SRGA J181414.6-225604, a Galactic X-ray transient discovered by the ART-XC instrument on board the SRG mission. With multi-epoch follow-up from Swift XRT, NICER and NuSTAR, the X-ray transient was found to exhibit a ~ 200 day long flare characterized by a highly absorbed X-ray spectrum consistent with an optically thick Comptonized plasma. Spatially and temporally coincident with the X-ray transient, we identified a heavily obscured variable infrared counterpart from the Palomar Gattini-IR near-infrared time domain survey. Combining infrared spectroscopy and time domain data from the last two decades, we classify the counterpart to be a very late-type (>M7), long-period (~1500 days) and luminous (MK ~ -10) pulsating Mira variable. The near-infrared data reveal a dramatic fading of the donor ~ 500 days prior to the X-ray flare, which we demonstrate to arise from an intense mass loss and dust obscuration episode. We thus propose that SRGA J181414.6-225604 represents the first observed case of a symbiotic X-ray binary directly triggered by an intense mass loss episode of an evolved donor star. I will end with an outlook highlighting how correlated X-ray activity and dust formation may be ubiquitous in the SyXRBs, and the potential for all-sky near-infrared and hard X-ray time domain surveys to complete our census of the Galactic SyXRB population.


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