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SOAR/Goodman spectroscopy of the possible optical counterpart to Swift J1658.2-4242

ATel #11342; A. Bahramian, C. Britt, J. Strader (MSU)
on 23 Feb 2018; 18:08 UT
Credential Certification: Arash Bahramian (bahramian@pa.msu.edu)

Subjects: Optical, X-ray, Binary, Black Hole

Referred to by ATel #: 11358

Swift J1658.2-4242 is a new transient discovered by Swift (GCN #22416 and #22417). NuSTAR observation of this indicates presence of a broad iron line and quasi-periodic oscillations (ATel #11321) and Radio observations by ATCA show a strong radio counterpart with a flat radio spectral slope (ATel #11322). These all suggest Swift J1658.2-4242 is likely to be a black hole X-ray binary.

Within the small (< 0.5") error circle of the ATCA position, there is a single optical source in archival imaging. The published mean magnitudes of this source are r=20.07 and i=18.65, from VPHAS+ DR2 (Drew et al 2014, MNRAS, 440, 2036). We observed this source on UT 2018 Feb 22.4 with the Goodman Spectrograph on the SOAR telescope.

We note that no new source beyond this archival source was present in acquisition images taken with a long-pass filter. This non-standard filter makes it challenging to determine whether the source has brightened compared to the archival data. Comparison with field stars suggests that any such brightening is < 1.5 mag, but a smaller change in its magnitude is not ruled out.

Our 1200-sec SOAR spectrum, covering the region of 4800 to 8800 A, is consistent with a normal mid to late K star, with an approximate Teff of 4000 to 4600 K. The most prominent absorption lines are those of the Ca triplet, and given a barycentric radial velocity of about -20 km/s. We see no evidence for H or He emission in any lines covered by our spectrum, nor evidence of a disk-like continuum. The reddening estimated from an SED fit to the spectrum is E(B-V)=1.4+/-0.1 mag.

We separately considered archival H and K photometry for this optical source as observed in the VVV survey DR2 (Minniti et al 2010, New Ast, 15, 433). Comparing these magnitudes to Padova solar metallicity isochrones for an age of 10 Gyr, the H-K vs. K color-magnitude diagram suggests E(B-V) = 1.3 for an assumed distance of 8 kpc and E(B-V) = 1.4 for a distance of 3 kpc (the minimum distance indicated by the radio/X-ray emission; ATel #11322). The resulting absolute magnitude of the optical source would imply classification as a red giant for any distance > 4 kpc; at 3 kpc the source could also be a subgiant.

Hence, if this source is the counterpart to the X-ray binary, it is likely to be an evolved star. However, the absence of accretion signatures in the spectrum, and the lack of evidence for a brighter optical counterpart, suggest that this optical source may well not be the counterpart to Swift J1658.2-4242. Deeper imaging could help determine whether there is another possible counterpart to the X-ray source.