Delving into the depths of NGC 3783 with XRISM III. Birth of an ultrafast outflow during a soft flare

TL;DR

This study reports the detection of an ultrafast outflow in NGC 3783 during a soft flare, suggesting magnetic driving mechanisms similar to solar coronal mass ejections, based on XRISM observations.

Contribution

It presents the first observation of an ultrafast outflow launched during a soft flare in NGC 3783, highlighting magnetic driving as a key mechanism.

Findings

Detection of a 0.19c ultrafast outflow synchronized with a soft flare.

Hints of evolving outflow velocities from 0.05c to 0.3c over three days.

Outflow acceleration likely driven by magnetic reconnection, not radiation pressure.

Abstract

The 2024 X-ray/UV observation campaign of NGC 3783, led by XRISM, revealed the launch of an ultrafast outflow (UFO) with a radial velocity of 0.19c (57000 km/s). This event is synchronized with the sharp decay, within less than half a day, of a prominent soft X-ray/UV flare. Accounting for the look-elsewhere effect, the XRISM Resolve data alone indicate a low probability of 2e-5 that this UFO detection is due to random chance. The UFO features narrow H-like and He-like Fe lines with a velocity dispersion of 1000 km/s, suggesting that it originates from a dense clump. Beyond this primary detection, there are hints of weaker outflow signatures throughout the rise and fall phases of the soft flare. Their velocities increase from 0.05c to 0.3c over approximately three days, and they may be associated with a larger stream in which the clump is embedded. The radiation pressure is insuffcient to drive the acceleration of this rapidly evolving outflow. The observed evolution of the outflow kinematics instead closely resembles that of solar coronal mass ejections, implying magnetic driving and, conceivably, reconnection near the accretion disk as the likely mechanisms behind both the UFO launch and the associated soft flare.