SOAR/Goodman optical spectroscopy of MAXI J1820+070

ATel #11424; A. Bahramian, J. Strader, K. Dage (MSU)
on 15 Mar 2018; 18:03 UT
Credential Certification: Arash Bahramian (bahramian@pa.msu.edu)

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On 2018 Mar 6, ASAS-SN discovered an optical transient toward the inner galaxy. Subsequently MAXI J1820+070 was discovered as a new X-ray transient (ATel #11399), and D. Denisenko suggested the two transients were associated (ATel #11400). Subsequent X-ray and optical observations strengthened this association, and led to the conclusion that this source is likely a new black hole X-ray binary (ATels #11406, #11418, 11420, 11421 , 11423).

We obtained optical spectroscopy of this source on UT 2018 March 15.4 with the Goodman Spectrograph on the SOAR telescope. We obtained both a low resolution spectrum (wavelength range 3000-7000 A, resolution 5.6 A) and a moderate resolution spectrum (5500-6750 A, resolution 1.7 A). The low resolution spectrum shows strong, broad H-alpha emission and weaker emission from He I 5875 and the Bowen blend. In the medium resolution spectrum, the H-alpha has a FWHM of about 1150 km/s. It is asymmetric but not clearly double-peaked, possibly due to the modest S/N of our spectrum. This FWHM is relatively low for a black hole X-ray transient, suggesting either that the system has a long period associated with an evolved donor or that it is relatively face-on. There are no obvious absorption features in the spectrum, indicating that continuum is dominated by the accretion disk.

The optical position of ASASSN-18ey/MAXI J1820+070 is consistent with a star in archival data, and this star has a Gaia DR1 J2000 position of 18:20:21.9393, +07:11:07.190, with a nominal astrometric uncertainty of a few milliarcseconds per coordinate.