Spectroscopy and photometry of the optical counterpart to 4U 1730-22 with SOAR

ATel #14694; J Strader, K. Sokolovsky, E. Aydi (MIchigan St), T Maccarone (Texas Tech), C Heinke, G Sivakoff (Alberta), A Bahramian (Curtin)
on 9 Jun 2021; 21:04 UT
Credential Certification: Jay Strader (strader@pa.msu.edu)

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MAXI recently observed an X-ray outburst (ATel #14683) from a source subsequently identified with Swift/XRT (ATel #14686, ATel #14688) as likely to be 4U 1730-22, which has not been observed in outburst since 1972.

We obtained i-band imaging of this region with the Goodman Spectrograph on the SOAR Telescope on UT 2021-06-09.12. We identified a new optical source with i = 19.0 +/- 0.1 mag (calibrated to Pan-STARRS, Chambers et al. 2016, arXiv:1612.05560) at an ICRS position of (R.A., Dec.) = (17:33:57.551, -22:01:56.79). Calibrated to Gaia EDR3, the per-coordinate uncertainty in this position is about 0.07".

This new source is at nearly the exact location (within ~ 0.1") of the candidate Chandra quiescent X-ray counterpart to 4U 1730-22 (Tomsick et al 2007, ApJ, 663, 461), and also consistent with the revised Swift/XRT error circle for the new X-ray outburst (ATel #14688). This source was neither present in Pan-STARRS (down to an approximate limit of i<21) nor in the deep R-band imaging presented by Tomsick et al. This source is the same optical counterpart described by Russell et al (ATel #14693) and the position and photometry reported here are generally of higher precision, but entirely consistent with, the values reported in that ATel.

We obtained an 1800 sec spectrum of this newly identified optical source with SOAR/Goodman, covering the wavelength range ~ 4800 to 8800 Ang at a FWHM resolution of ~ 5.4 Ang. This spectrum shows a flat (in F_lambda) to mildly blue continuum without a correction for foreground reddening. The most prominent emission lines are from hydrogen (Balmer and Paschen), along with some weaker He I lines. There are no clear stellar absorption lines, though there is a possible DIB around 6280 Ang.

These spectral properties are consistent with a low-mass X-ray binary in outburst, confirming this source as the counterpart to 4U 1730-22, and confirm the Tomsick et al. identification of CXOU J173357.5-220156 as the quiescent neutron star counterpart. We note the FWHM of the H-alpha emission is about 860 km/s. This is relatively low for a neutron star low-mass X-ray binary. For an assumed distance of 10 (8) kpc, the approximate reddening-corrected M_i ~ 3.5 (4.0). Given an X-ray luminosity of around 1e36 erg/s at this distance (ATel #14686), the van Paradijs & McClintock (1994, A&A, 290, 133) relation between orbital period and X-ray and optical luminosity suggests a relatively short orbital period of <~ 2-3 hr. Hence the low FWHM of H-alpha is unlikely due to a long period, but instead to a face-on orientation for the binary.