Detection of a 20 minute time lag observed from Sgr A* between 8 and 10 GHz with the VLA

Abstract

We report the detection and analysis of a radio flare observed on 17 April 2014 from Sgr A* at $9$ GHz using the VLA in its A-array configuration. This is the first reported simultaneous radio observation of Sgr A* across $16$ frequency windows between $8$ and $10$ GHz. We cross correlate the lowest and highest spectral windows centered at $8.0$ and $9.9$ GHz, respectively, and find the $8.0$ GHz light curve lagging $18.37_{-2.18}^{+2.17}$ minutes behind the $9.9$ GHz light curve. This is the first time lag found in Sgr A*'s light curve across a narrow radio frequency bandwidth. We separate the quiescent and flaring components of Sgr A* via flux offsets at each spectral window. The emission is consistent with an adiabatically-expanding synchrotron plasma, which we fit to the light curves to characterize the two components. The flaring emission has an equipartition magnetic field strength of $2.2$ Gauss, size of $14$ Schwarzschild radii, average speed of $12000$ km s${}^{-1}$, and electron energy spectrum index ($N(E)\propto E^{-p}$), $p=0.18$. The peak flare flux at $10$ GHz is approximately $25$% of the quiescent emission. This flare is abnormal as the inferred magnetic field strength and size are typically about $10$ Gauss and few Schwarzschild radii. The properties of this flare are consistent with a transient warm spot in the accretion flow at a distance of $10$-$100$ Schwarzschild radii from Sgr A*. Our analysis allows for independent characterization of the variable and quiescent components, which is significant for studying temporal variations in these components.

9 pages, 6 figures, accepted to the Monthly Notices of the Royal Astronomical Society

References in corpus (6)

• Flaring Activity of Sgr A* at 43 and 22 GHz: Evidence for Expanding Hot Plasma
• Simultaneous Chandra, CSO and VLA Observations of Sgr A*: The Nature of Flaring Activity
• The Surprisingly Small Impact of Magnetic Fields On The Inner Accretion Flow of Sagittarius A* Fueled By Stellar Winds
• Simultaneous NIR/sub-mm observation of flare emission from SgrA*
• Improved AGN light curve analysis with the z-transformed discrete correlation function
• Coordinated NIR/mm observations of flare emission from Sagittarius A*

Cited by in corpus (5)

• Magnetic reconnection plasmoid model for Sagittarius A* flares
• Two-Temperature GRMHD Simulations of Black Hole Accretion Flows with Multiple Magnetic Loops
• Polarized signatures of adiabatically expanding hotspots in Sgr A*'s accretion flow
• Time delays between radio and X-ray and between narrow radio bands of Sgr A* flares in the shock oscillation model
• Variability of Sagittarius A* at 3 GHz on minute-scale with MeerKAT