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

TL;DR

The paper reports the birth of an ultrafast outflow in NGC 3783 during a soft X-ray/UV flare detected with XRISM Resolve, identifying a UFO at v approx 0.19c (57000 km/s) via an 8.4 keV absorption feature and corroborated by cross-instrument analysis. Time-resolved spectroscopy shows the outflow emerges during the flare decay, with phase-resolved signatures suggesting a clumpy dense component embedded in a broader outflow and hints of additional faster signatures in other flare phases. The inferred launching mechanism is magnetic, likely tied to reconnection near the accretion disk, as radiation pressure cannot account for the observed impulsive acceleration (max ~600 m s^-2) and CME-like evolution is invoked. The work provides stringent density, ionization, and location constraints (e.g., n and R bounds, launch around ~50 R_g) and emphasizes a magnetic driving scenario with implications for AGN feedback and disk-jet magnetohydrodynamics. The results demonstrate XRISM’s power in resolving fast, highly ionized winds and motivate broader phase-resolved UFO studies across AGN samples.

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.

Figures (12)

• Figure 1: XRISM Xtend light curves from the NGC 3783 campaign. Left: Soft- and hard-band light curves, shown in black and red, respectively. The light curve has been binned to multiples of the XRISM orbit (5747 s), and in this paper we count time since the start of the XRISM observation. Right: X-ray variability surrounding the main soft flare at $t \sim 2.8 \times 10^5$ s.
• Figure 2: Hardness ratio, defined by the count rates in the $3.0-6.0$ keV and $0.3-0.6$ keV bands, plotted against their combined count rate. Each data point represents a single XRISM orbit (5747 s). The data are color-coded by flare phase: pre-flare (black), rise (red), decay (blue), after-flare (magenta), and post-flare (orange). Gray points indicate observations outside the main soft flare.
• Figure 3: Time evolution of the NGC 3783 Resolve spectra. Top: Average spectrum over all flare phases. Middle: Spectrum during the decay phase only. Bottom: Average spectrum combining all periods outside the soft flare, shown for comparison. The dip at $\sim 8.4$ keV is unique to the decay phase. The light blue curve in the middle panel shows the best-fit model excluding both the UFO and the 3700 km s$^{-1}$ components.
• Figure 4: Detailed look at the dip at 8.4 keV and the LEE. Left: Resolve, Xtend, NuSTAR, and XMM-Newton pn spectra from the decay phase. The red line shows the best-fit model to the Resolve data and including the UFO component, while the light blue line represents the model without it. Right: $\Delta C$ distribution from $1.3 \times 10^{5}$ runs of the Monte Carlo simulation of the Resolve spectrum, and $1 \times 10^4$ times for each of other three instruments (black histograms) and the observed $\Delta C$ distribution for the Resolve data (red histograms). Vertical lines mark the observed $\Delta C$ values for the other three instruments.
• Figure 5: Phase-resolved Resolve spectra during the soft flare. The best-fit models are shown in red. The light blue curves show the model that excludes both the UFO and the 3700 km s$^{-1}$ component.