X-ray brightening of 2SXPS J173508.4-292958, a candidate symbiotic star

ATel #13648; C. O. Heinke (U. Alberta), L. Rivera Sandoval, T. J. Maccarone (Texas Tech), J. Kennea (Penn State), A. Bahramian (Curtin U.), J. Strader (Michigan State U), & G. R. Sivakoff (U. Alberta), for the Swift Bulge Survey team
on 18 Apr 2020; 19:05 UT
Credential Certification: Craig Heinke (cheinke@virginia.edu)

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We have identified new very faint X-ray variability of the known X-ray source 2SXPS J173508.4-292958 using Swift XRT observations of the Swift Galactic Bulge Survey (Shaw et al. 2020, MNRAS, 492, 4344; ATels #10265, #10305, #10355, #10419, #10422, #10428, #12751, #12843). In a 120-second XRT exposure on 2020 April 16, we measured 8 counts (0.067 ct/s) from the position RA=17:35:08.7, Dec= -29:30:07, between 0.9 and 4.5 keV. Assuming the spectral fit by Chandra below gives an unabsorbed 0.3-10 keV flux of 4e-12 ergs/cm^2/s, or Lx=1.1e33 (D/1.5 kpc)^2 erg/s. This source was only marginally detected (10 counts) in our stacked Swift Galactic Bulge Survey observations from 2017 (Bahramian et al. in prep), with a countrate of 0.007 ct/s. The second Swift-XRT Point Source catalog records it as 2SXPS J173508.4-292958, at a countrate of 0.019 ct/s, with a first detection on 2008 February 16 (Evans et al. 2020, ApJS, 247, 54). The Chandra Galactic Bulge Survey (2 ks observations; Jonker et al. 2011, ApJS, 194, 18) also detected CXOGBS J173508.2-292957 (or CX8), at an unabsorbed flux of ~1.7e-12 ergs/cm^2/s (0.3 - 10 keV, assuming NH=2e21, and using a Gamma=2.4+0.3/-0.6 power-law fit to the spectra), from observations on 2009 February 1.

We obtained ~1000 second follow-up Swift observations on 2020 April 17 and April 18, where we detected the source at countrates of (3.9+/-0.7)*10^-2 cts/s and (2.2+/-0.6)*10^-2 cts/s (respectively), translating (for the same assumptions) to 0.3-10 keV unabsorbed fluxes of 2.3e-12 and 1.3e-12 erg/cm^2/s , or Lx=6e32 (D/1.5 kpc)^2 and 3e32 (D/1.5 kpc)^2 erg/s. While the uncertainty of the flux in the April 16 observation is large, this suggests a flux decay by a factor of 2 each day, for two days. The enhanced position (using the University of Leicester tool at https://www.swift.ac.uk) is:
RA: 17:35:08.42, Dec: -29:29:59.1
with an error radius of 3.2" (at 90% confidence).

The 2MASS star 17350831-2929580 with J=8.5, K=7.4, and Tycho magnitude V=11.6, is within this error circle, at 1.8" from the refined Swift position and 0.5" from the Chandra position. This star has a Gaia DR2 distance (Bailer-Jones et al. 2018, AJ, 156, 58) of 1.5+/-0.1 kpc. Using Swift UVOT, we measure U=14.83+-0.02 during the current outburst. Archival Swift UVOT images taken on 2019 May 9 measure UVM2=18.63 +-0.3, and four archival observations taken between 2017 April 13 and 2017 May 18 find UVW1=15.86+-0.08, with no signs of variability among them. ASAS-SN photometry (https://asas-sn.osu.edu) shows a periodic sinusoidal variation in the g' filter of ~0.1 mags every 32.3 days since Jan. 2019, along with a shallow but significant secular brightening of ~0.1 mag over the last ~2 years. The periodicity is not obvious in earlier ASAS-SN V and g' data.

The energies of the detected photons, and lack of a nearby bright star, indicate that this X-ray source is not likely to be a nearby, normal flaring star. The likely association with the bright giant star suggests it may be a symbiotic star, of which several have been detected in the Swift Bulge Survey (Shaw et al. 2020).

We have requested additional Swift observations, and encourage observations at other wavelengths. We thank the Swift team for their rapid support of these observations.