Unprecedented Near-infrared Brightness and Variability of Sgr A*

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 band, Sgr A* reached flux levels of ∼ 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 µm and 2.1 µm. 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.

A Hidden Friend for the Galactic Center Black Hole, Sgr A*

“…The recent closest approach of S0-2 has been used to test and confirm the prediction of general relativity for the relativistic redshift (e.g., GRAVITY Collaboration et al 2018;Do et al 2019a). This well-studied star provides an opportunity to place limits on the inner orbit's configuration.…”

“…These orbits imply the presence of a 4 million solar masses black hole (e.g., Ghez et al 2000Ghez et al , 2008Gillessen et al 2009;Boehle et al 2016;Gillessen et al 2017;Gravity Collaboration et al 2019), residing in a dense stellar environment, called a nuclear star cluster (e.g., Ghez et al 2003;Gillessen et al 2009;Lu et al 2013). Continued observations have enabled precision measurements of the distance to the galactic center (e.g.. Ghez et al 2003;Boehle et al 2016;Gravity Collaboration et al 2019), new constrains on the fifth force (Hees et al 2017a) and the first gravitational redshift measurements near a SMBH (GRAV-ITY Collaboration et al 2018;Do et al 2019a).…”

A Nonthermal Bomb Explains the Near-infrared Superflare of Sgr A*

“…On May 2019, Do et al (2019) observed an unprecedented NIR flare from Sgr A * -hereafter the "superflare"-with the Keck telescope. The peak flux exceeded the maximum historical value by a factor of two and the light curve (LC) afterwards showed a factor of 75 drop in flux over a 2 hr time span.…”

“…The peak flux exceeded the maximum historical value by a factor of two and the light curve (LC) afterwards showed a factor of 75 drop in flux over a 2 hr time span. Do et al (2019) suggested that an increase in the SMBH accretion ratė M could be responsible for the superflare, possibly due to additional gas deposited by the passage of the G2 object in 2014 or a windy star such as S0-2 in 2018. Nevertheless, Ressler et al (2018) argued that the effect of S0-2 on the RIAF structure should be negligible.…”

Continuation of the X-ray monitoring of Sgr A*: the increase in bright flaring rate confirmed

“…The increase in flaring rate of the most energetic fares must be placed in the context of the recent increase in activity observed from Sgr A* in NIR and X-rays. Do et al (2019) observed the Galactic center with the Very Large Telescope (VLT) and the Keck Telescope in 2019 April and May. They recorded the highest variability ever observed over 20 years of NIR observations (up to a factor of 75 within two hours).…”