MAXI J1820+070: VLT and GTC spectroscopic follow-up shows a significant spectral evolution from the early stages of the outburst

ATel #11481; T. Munoz-Darias, F. Jimenez-Ibarra, M. Armas Padilla, J. Casares, M. A. P. Torres (IAC-Tenerife)
on 27 Mar 2018; 18:05 UT
Credential Certification: Teo Muñoz-Darias (teo.munoz-darias@iac.es)

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The detection of the new black hole candidate MAXI J1807+132 has triggered a worldwide follow-up campaign from high energies to radio wavelengths (ATel #11400, #11403, #11418, #11421, #11423, #11424, #11425, #11426, #11427, #11432, #11437, #11439, #11440, #11445, #11451, #11458 and #11458). As part of this observational effort, we have performed spectroscopic observations under our Target of Opportunity programs at both the Gran Telescopio Canarias (GTC) and the Very Large Telescope (VLT). These observations are being performed with the OSIRIS (GTC) and X-Shooter (VLT) spectrographs since March 16, 2018.

As previously reported (ATels #11424 and #11425), the optical spectrum shows the Bowen blend, hydrogen (Balmer and Paschen series) He-I and He-II emission lines characteristic of low-mass X-ray binaries in outburst. Our preliminary analysis of the data taken over 12 different epochs (March 16 to March 26) reveals a clear evolution in the optical spectra. The March 16 observations show asymmetric Balmer and He-I emission profiles, which are clearly skewed towards the red (see also ATels #11424 and #11425), while more symmetric profiles are observed from March 17 onwards (see linked figure below). This, together with the detection of shallow blue-shifted He-I absorption components in our high signal-to-noise (March 16) X-Shooter spectra, suggest the presence of a wind outflow with a terminal velocity of ~1500 km/s during the early stages of the outburst. A similar phenomenology was observed in the 2015 outbursts of V404 Cyg (Muñoz-Darias et al. 2016, Nature, 534, 75; MNRAS 2017, 465 L124).

The brightness of the source, which has gradually increased from g=12.9 on March 16 to g=12.0 on March 26, allowed us to take spectra with time exposures as short as 120s, but only relatively minor variations from spectrum to spectrum are observed within every epoch. For instance, the H-alpha equivalent width is observed to vary within ~0.5 Ang over 40-min time scales, while there is an overall increase from ~4 Angs on March 17 to ~9 Angs on March 26.

Contemporaneous Swift/XRT X-ray observations show a significant softening over the above period of time with hardness [(2-10 keV)/ (0.5-2 keV)] evolving from 0.65 to 0.45. We note that the observations are heavily affected by pile-up. This was preliminary corrected by excluding the central region of the aperture from the analysis.

We are grateful to the GTC and VLT teams for their fast and efficient response to our ToO triggers.

Evolution of the H-alpha line profile (VLT+X-Shooter)