# X-ray dips and a complex UV/X-ray cross-correlation function in the black hole candidate MAXI J1820+070

## TL;DR

This study analyzes X-ray and UV data from the black hole candidate MAXI J1820+070, revealing complex timing and spectral features including dips caused by ionized absorption and intricate cross-correlation patterns suggesting multiple UV emission sources.

## Contribution

It provides the first detailed spectral and timing analysis of simultaneous X-ray and UV data during the outburst rise, highlighting the complex interplay of emission mechanisms.

## Key findings

- X-ray spectrum fits a hard thermal Comptonization model
- Dipping intervals caused by ionized partial covering absorber
- Complex UV/X-ray cross-correlation with a positive lag peak and a pre-cognition dip

## Abstract

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.