MAXI J1820+070, a black hole candidate first detected in early March 2018, was observed by XMM-Newton during the outburst rise. In this letter we report on the spectral and timing analysis of the XMM-Newton X-ray and UV data, as well as contemporaneous X-ray data from the Swift satellite. The X-ray spectrum is well described by a hard thermal Comptonization continuum. The XMM-Newton X-ray light curve shows a pronounced dipping interval, and spectral analysis indicates that it is caused by a moderately ionized partial covering absorber. The XMM-Newton/OM U-filter data does not reveal any signs of the 17 hr orbital modulation that was seen later on during the outburst decay. The UV/X-ray cross correlation function shows a complex shape, with a peak at positive lags of about 4 seconds and a pre-cognition dip at negative lags, which is absent during the X-ray dipping episode. Such shape could arise if the UV emission comes partially from synchrotron self-Compton emission near the black hole, as well as from reprocessing of the X-rays in the colder accretion disc further out.
@article{arxiv.1906.06519,
title = {X-ray dips and a complex UV/X-ray cross-correlation function in the black hole candidate MAXI J1820+070},
author = {J. J. E. Kajava and S. E. Motta and A. Sanna and A. Veledina and M. Del Santo and A. Segreto},
journal= {arXiv preprint arXiv:1906.06519},
year = {2019}
}