Unprecedented variability of Sgr A* in NIR

Abstract

The electromagnetic counterpart to the Galactic center supermassive black hole, Sgr A*, has been observed in the near-infrared for over 20 years and is known to be highly variable. We report new Keck Telescope observations showing that Sgr A* reached much brighter flux levels in 2019 than ever measured at near-infrared wavelengths. In the K${}^{\prime }$ band, Sgr A* reached flux levels of $\sim 6$ mJy, twice the level of the previously observed peak flux from $>13,000$ measurements over 130 nights with the VLT and Keck Telescopes. We also observe a factor of 75 change in flux over a 2-hour time span with no obvious color changes between 1.6 $\mu$m and 2.1 $\mu$m. The distribution of flux variations observed this year is also significantly different than the historical distribution. Using the most comprehensive statistical model published, the probability of a single night exhibiting peak flux levels observed this year, given historical Keck observations, is less than $0.3\%$. The probability to observe the flux levels similar to all 4 nights of data in 2019 is less than $0.05\%$. This increase in brightness and variability may indicate a period of heightened activity from Sgr A* or a change in its accretion state. It may also indicate that the current model is not sufficient to model Sgr A* at high flux levels and should be updated. Potential physical origins of Sgr A*'s unprecedented brightness may be from changes in the accretion-flow as a result of the star S0-2's closest passage to the black hole in 2018 or from a delayed reaction to the approach of the dusty object G2 in 2014. Additional multi-wavelength observations will be necessary to both monitor Sgr A* for potential state changes and to constrain the physical processes responsible for its current variability.

9 pages, 4 figures, accepted to ApJ Letters

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Cited by in corpus (69)

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• Time Variations in the Flux Density of Sgr A* at 230 GHz Detected with ALMA
• Second Scale Submillimeter Variability of Sagittarius A* during flaring activity of 2019: On the Origin of Bright Near Infrared Flares
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• Accretion Flow Morphology in Numerical Simulations of Black Holes from the ngEHT Model Library: The Impact of Radiation Physics
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• S0-2 star, G1- and G2-objects and flaring activity of the Milky Way's Galactic Center black hole in 2019
• New Evidence for a Flux-independent Spectral Index of Sgr A* in the Near-infrared
• Candidate Young Stellar Objects in the S-cluster: The Kinematic Analysis of a Sub-population of the Low-mass G-objects close to Sgr A*
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• Monitoring the Size and Flux Density of Sgr A* during the Active State in 2019 with East Asian VLBI Network
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• Tidal phenomena in the Galactic Center: The curious case of X7
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