Simultaneous JWST, NuSTAR, and VLA Monitoring of Sgr A*: A Unified Picture of the Variable IR, X-ray and Radio Emission

TL;DR

The study investigates correlated variability of Sgr A* across IR, X-ray, and radio bands using a simultaneous JWST-NIR, NuSTAR, and VLA campaign, revealing a bright X-ray flare coincident with an IR flare and a delayed radio peak. By testing synchrotron and inverse Compton scenarios with disk and flare electron populations, the authors argue that the X-ray flare is best produced by inverse Compton scattering of IR photons by thermal electrons in the disk, requiring relativistic beaming with a bulk velocity around $v\approx0.7c$ of the IR-emitting plasma toward the disk. They develop a two-zone physical picture involving a reconnection-driven current sheet and a ejected flux rope, predicting IR beaming toward the disk, disk up-scattering, and a delayed, expanding radio/submm flare. The results constrain disk magnetic fields to $B_d\sim20$–$30$ G, derive radio flare parameters consistent with an expanding hotspot ($R_0\approx5.3\,r_g$, $B_0\approx23$ G, $v_{\rm exp}\approx0.018c$), and offer a unified framework for IR, X-ray, and radio variability in Sgr A*. This work has implications for understanding MAD accretion, reconnection physics near event horizons, and the coupling between disk and outflow in low-luminosity active nuclei.

Abstract

Flux variability is a fundamental channel of information from Sgr A* because of its direct probe of processes occurring within an accretion disk under strong gravity. We present simultaneous IR, X-ray and radio observations of Sgr A* on 2024 Apr 05 using JWST, NuSTAR, and VLA. We report the detection of a strong X-ray flare with a luminosity of $\sim5.2x10^{35}$ erg/s coincident with a bright near-IR flare, and a brightening in radio about an hour later. We investigate the candidate physical mechanisms for the X-ray flare emission and conclude that this can best be explained by inverse Compton scattering of near-IR flare radiation. We propose a dynamic scenario analogous to a coronal mass ejection in which a magnetic flux rope is ejected from Sgr A*'s inner accretion flow with a current sheet extending down from the rope to the bulk of the accretion flow. Reconnection within the sheet produces oppositely directed flows of accelerated particles moving upwards towards the rope and downwards towards the accretion flow. Infrared radiation from the approaching energetic electrons is enhanced by beaming and up-scattered by thermal electrons in the accretion flow to produce the strong X-ray flare. Meanwhile, the relativistic electrons moving in the opposite direction away from the disk experience weaker magnetic fields so radiate at longer wavelengths by feeding into the magnetic flux tube and adiabatically cooled during its subsequent expansion. This physical picture attempts to unify the origin of the variable emission from Sgr A* at IR, X-ray and radio/submm wavelengths.

Figures (14)

• Figure 1: (a) The normal stellar field at 2.1 $\mu$m of NIRCam of JWST surrounding Sgr A* (top left), whereas a version that has had a baseline image (when Sgr A* was not flaring) subtracted from it (top right). This allows Sgr A* to be seen without the overlapping nearby stars. Note that the cores of bright stars in the field, such as the IRS16 members and IRS29N, are saturated and hence appear black. (b) An X-ray image showing $34.7"$ extraction circles around Sgr A* (yellow), the nearby bright LMXB AX J1745.6-2901 (cyan), and the background regions used for Sgr A* (white, chosen to be equidistant from the LMXB as Sgr A*. The images show when Sgr A* is not flaring (middle left) and when flaring (middle right, shorter 1000 s exposure time). (c) A VLA image of the inner $\sim30\times40"$ of Sgr A* showing the mini-spiral ionized gas at 34 GHz (bottom), taken simultaneously on April 5, 2004. The bright compact source is Sgr A*. At this frequency the quiescent flux dominates over the variable emission.
• Figure 2: (a) Top Left The 2.1 $\mu$m (black) and 3-10 keV NuSTAR (green) light curves of Sgr A* show a strong X-ray flare coincident with a NIR flare. (b) Top Right A close-up view of the NIR light curve of the flare at 2.1 $\mu$m is shown against the X-ray light curve but normalized such that both light curves rise together. The X-ray flare is detected within the envelope of the NIR flare. (c) Bottom Similar to (b) except at 4.8$\mu$m.
• Figure 3: (a) Top Left: Correlation of X-ray flare emission detected by NuSTAR with NIR flare emission at 2.1 $\mu$m, illustrating their similar origin. (b) Top Right: Similar to (a), except at 4.8 $\mu$m. (c) Bottom: A plot of the spectral index of NIR flare emission against the X-ray flux density. A trend is noted in that the NIR spectral index becomes shallower as the X-ray flux increases. The colored line is a linear fit to the full data set, although the correlation seems to cease at $\gtrsim$ 9 and $\gtrsim$ 20 mJy in the IR fluxes.
• Figure 4: (a) Top Left: Light curve of Sgr A* at 2.1 $\mu$m taken on 2024, April 5. (b) Top Right: Same as (a), except the NIR spectral index of Sgr A* as a function of time. (c) Bottom: Same as (a), except that normalized values of the time derivative of the flux at 2.1 (blue) and 4.8$\mu$m (red) are displayed as a function of time.
• Figure 5: (a) Left: Superimposed on the NIR light curve at 2.1 $\mu$m are the light curves of Sgr A* at 1 cm, ranging from frequencies 29 to 37 GHz with colors from red to blue to violet, respectively. Note that the scale for the radio flux density does not start at zero (i.e. the contrast in the variation of the radio emission is much lower than in the NIR.) (b) Right: The cross correlation coefficient between NIR and radio as a function of lag time. There are two peaks shown with dotted lines at 1 and 3.5 h lag times.