Abstract
We present and discuss the results of our photometric and spectroscopic monitoring of 2SXPS J173508.4-292958 carried out from April to August 2020. This X-ray source, in the foreground with respect to the Galactic center, brightened in X-rays during 2020, prompting our follow-up optical observations. We found the star to contain a K4III giant with a modest but highly variable Halpha emission, composed by a ~470 km/s wide component with superimposed a narrow absorption, offset by a positive velocity with respect to the giant. No orbital motion is detected for the K4III, showing an heliocentric radial velocity stable at -12(+/-1) km/s. No flickering in excess of 0.005 mag in B band was observed at three separate visits of 2SXPS J173508.4-292958. While photometrically stable in 2016 through 2018, in 2019 the star developed a limited photometric variability, that in 2020 took the form of a sinusoidal modulation with a period of 38 days and an amplitude of 0.12 mag in V band. We argue this variability cannot be ascribed to Roche-lobe filling by the K4III star. No correlation is observed between the photometric variability and the amount of emission in Halpha, the latter probably originating directly from the accretion disk around the accreting companion. While no emission from dust is detected at mid-IR wavelengths, an excess in U-band is probably present and caused by direct emission from the accretion disk. We conclude that 2SXPS J173508.4-292958 is a new symbiotic star of the accreting-only variety (AO-SySt).
Highlights
Attention to the X–ray source 2SXPS J173508.4-292958 has been recently driven by a report from Heinke et al (2020) about a brightening of the object they observed in X–rays with the Swift satellite in April 2020
We have dereddened our BVRI photometry combined with 2MASS JHKs data and found an excellent match with the spectral energy distribution of a K4III giant (Fig. 1)
Supporting the association of the K4III giant with the X–ray source 2SXPS is the presence on optical spectra of emission in Hα, not expected in normal and single K4III stars belonging to the field
Summary
Attention to the X–ray source 2SXPS J173508.4-292958 has been recently driven by a report from Heinke et al (2020) about a brightening of the object they observed in X–rays with the Swift satellite in April 2020. Noting the positional coincidence with the cool 2MASS star 17350831-2929580 of Ks=7.4 mag, Heinke et al suggested that 2SXPS J173508.4-292958 could be a previously unknown symbiotic star. Frigo near-UV excess and Hα in emission, the latter showing a broad profile with a central absorption. Such properties confirm a probable symbiotic nature for 2SXPS J173508.4-292958 (for a recent global review of symbiotic stars see Munari 2019). In this paper we report about the results of our follow-up photometric and spectroscopic campaign to monitor 2SXPS J173508.4-292958 (2SXPS for short in the rest of this paper) during the period of seasonal visibility in 2020
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