The optical counterpart of the X-ray source 2SXPS J173508.4-292958 is a new, yellow-type symbiotic star

ATel #13660; U. Munari (INAF Padova), P. Valisa, S. Dallaporta, A. Vagnozzi, V. Andreoli, F. Castellani (ANS Collaboration)
on 24 Apr 2020; 10:29 UT
Credential Certification: U. Munari (ulisse.munari@oapd.inaf.it)

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With an assortment of telescopes operated by ANS Collaboration, we have obtained optical spectroscopy (low and high resolution) and BVRI photometry of the optical counterpart of 2SXPS J173508.4-292958, the source recently reported by Heinke et al. (ATel #13648) to have brightened in the X-rays.

We have recorded BVRI photometry on Apr 23.067 and 24.074 UT. Given the presence of nearby field stars, data reduction in the Landolt system has been performed in PSF-fitting mode. The results are identical within 0.004 mag on both nights at V=11.849, R=10.909, I=9.917, while results for B band are 13.490 and 13.444 respectively. The 0.046 mag difference is vastly larger than errors, and points to real short-term variability at the shortest wavelengths.

Even if projected close to the Galactic Center, the object is in the foreground at 1.58 kpc according to Gaia parallax. From Green et al. (2019) 3D extinction maps, it should suffer from a modest E(B-V)=0.375. 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. Fitting instead with K3III or K5III distributions would require to change E(B-V) to 0.49 or 0.28, respectively. Combining distance, reddening and observed V=11.849, an absolute magnitude M(V)=-0.55 is derived, matching a K4III/II luminosity class according to Sowell et al. (2007, AJ 134, 1089). An excess flux of 0.20 and 0.29 mag at B and U bands is observed (U-band mag from UVOT data by Heinke et al.).

Low resolution and Echelle spectroscopy have been obtained on both Apr 23.10 and 24.10 UT with Stroncone 50cm and Varese 84cm telescopes. The K4III/II absorption spectrum shows an heliocentric radial velocity HRV=-9 +/-1 km/s. Halpha appears in weak emission (E.W.=-1.53 Ang), with a broad profile of FWHM=360 km/s (corrected for instrumental resolution) and HRV=+50 km/s, with superimposed a sharp and deep absorption component at HRV=+5 km/s (FWHM equal to instrumental resolution). No other emission lines are obvious in our spectra.

The presence of Halpha in emission proves the link with the X-ray source and leads to classify 2SXPS J173508.4-292958 as a new, yellow-type symbiotic star of the accreting-only variety. The short-term variability in B band and the weakness of emission lines suggest that the flux excess at bluest wavelengths comes directly from the accreting companion more than from any ionized fraction of the K giant stellar wind. In this respect, the difference in HRV between the K giant and the Halpha emission should relate to orbital motion within the system.