Unprecedented variability of Sgr A* in NIR

Do, Tuan; Witzel, Gunther; Gautam, Abhimat K.; Chen, Zhuo; Ghez, Andrea M.; Morris, Mark R.; Becklin, Eric E.; Ciurlo, Anna; Hosek, Matthew, Jr.; Martinez, Gregory D.; Matthews, Keith; Sakai, Shoko; Schödel, Rainer
eprint arXiv:1908.01777


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 band, Sgr A* reached flux levels of mJy, twice the level of the previously observed peak flux from 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 m and 2.1 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 . The probability to observe the flux levels similar to all 4 nights of data in 2019 is less than . 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.