Optical spectroscopy and photometry of MAXI J1820+070 (ASASSN-18ey) during the large multi-wavelength re-brightening of March 2019

ATel #12608; U. Munari and L. Zampieri (INAF Padova), P. Ochner (Univ. Padova), F. Manzini (ANS Collaboration)
on 24 Mar 2019; 10:52 UT
Credential Certification: U. Munari (ulisse.munari@oapd.inaf.it)

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After a several-month-long outburst occurring last year, in October 2018 the transient black hole candidate X-ray binary MAXI J1820+070 (ASASSN-18ey) entered an almost quiescent phase characterized by low levels of the X-ray and optical emission (ATel #12157, #12534), that appeared to signal the end of the outburst. The faintest level recorded in our photometric monitoring was reached on March 3, 2019 at B=18.64 and V=18.00 (averaged over intra-night variability). Recently, between March 9 and 11, 2019, almost one year after its discovery (ATel #11399), the source started a significant re-brightening initially detected in the optical band (ATel #12567, #12596) and then also in the X-ray and radio bands (ATel #12573, #12577). The optical brightness of the source is now surpassing that of the first re-brightening observed in Sep-Oct 2018 (ATel #12128), occurring after the X-ray transition from the soft to hard state (ATel #12057): our photometry for March 24, 2019 gives in fact B=14.23, V=13.91, and R=13.60 (averaged over intra-night variability).

We are monitoring the evolution of this second powerful multi-wavelength re-brightening of MAXI J1820+070 with the Asiago 1.22m, 1.82m and Schmidt 67/92 telescopes, and ANS Collaboration telescope ID 2300. This monitoring is performed in coordination with the AQUEYE+IQUEYE Collaboration (ATel #11723, #11824, #11899, #11936, #12157). A low resolution optical spectrum of VES 263 was recorded on UT March 23.08, 2019 with the Asiago 1.22m telescope (3300-8000 Ang, 2.31 Ang/pix). On the blue continuum several emission lines stand out, in particular broad hydrogen Balmer, HeI and HeII 4686. A higher resolution spectrum was obtained on UT March 24.10, 2019 again with the Asiago 1.22m telescope (5950-7170 Ang, 0.60 Ang/pix). On this spectrum the Halpha emission has an equivalent width of -9.7 Ang, a FWZI span of 2500 km/s, a FWHM of 1100 km/s, and a distinctive triangular shape with a longer blue wing and a much sharper drop toward the red. The heliocentric radial velocity of the Halpha photo-center is measured at -58 km/s.