Short-lived episodes of emission line splitting in the candidate black hole X-ray binary MAXI 1820+070

ATel #11899; U. Munari and L. Zampieri (INAF Padova), P. Ochner (Univ. Padova), and F. Manzini (ANS Collaboration)
on 31 Jul 2018; 14:29 UT
Credential Certification: U. Munari (ulisse.munari@oapd.inaf.it)

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We are regularly collecting spectra and BVRI photometry of MAXI 1820+070 with the Asiago 1.22m, 1.82m and Schmidt 67/92 telescopes, and ANS Collaboration telescope ID 2300. These observations are performed in coordination with optical timing observations carried out by the AQUEYE+IQUEYE Collaboration (ATel #11723, #11824). MAXI 1820+070 is a bright and uncatalogued X-ray transient source first detected on 2018 March 11 by Kawamuro et al. (ATel #11399) and then identified with the optical transient ASASSN-18ey (ATel #11400, #11403, #11404, #11406). The source was soon proposed to be a candidate black hole X-ray binary by Baglio et al. (ATel #11418). A massive multi-wavelength observational effort is currently underway worldwide as indicated by the large number of ATel issued on this source so far. The source recently underwent a hard to soft accretion state transition (ATel #11820, #11823, #11827, #11831, #11833, #11855, #11887).

Here we describe the detection of changes in recent optical spectra of MAXI 1820+070 taken after the reported hard to soft accretion state transition. The observations have been obtained with the Asiago 1.22m telescope, at 2.31 Ang/pix dispersion over the 3300-8000 Ang range. The most recent observing dates have been centered on UT July 19.872, 23.883, 25.915, 27.878, 29.863 and 30.886. On all these dates the spectra are similar, with a strong blue continuum and superimposed broad emission lines (with an average FWHM of 1100 km/s) of - primarily - the Balmer series of hydrogen, HeII 4686 and 5412 Ang, various HeI, and the 4640 complex by NIII. On July 19 and 23 spectra, the Balmer emission lines appear single peaked and broadly Gaussian in shape. On July 25 the Balmer lines begin to show a hint of a double peaked profile, that matures into two components separated by about 850 km/s on July 27, and retrace back to just a double peaked profile by July 29, and finally to a single component by July 30. HeI and HeII lines show equivalent transitions between single/double profiles, apparently not in phase with those displayed by Balmer lines but with a similar velocity separation. We plan to continue the high cadence spectroscopic monitoring of MAXI 1820+070 and associated photometric and optical timing observations.