0 Assigned ID: 1101 ----- Attempting Submission 1 (Fri Aug 16 15:11:04 GMT 2019) ----- 0 ----- Attempting Submission 1 (Wed Aug 21 01:34:12 GMT 2019) ----- Monitoring the Unprecendented Near-Infrared Variability of Sgr A* Sgr A*, the electromagnetic counterpart to the supermassive black hole at the Galactic center offers us a unique view into the physics black hole accretion and the growth of black holes. This summer, near-infrared observations of Sgr A* show unprecedented brightness and variability. This is increased activity is compared to over 20 years of historical near-infrared observations, which have until 2019 shown consistent variability characteristics. Unusual activity has also been reported by X-ray observatories with large outbursts. The current activity may be indicating a change Sgr A*'s accretion state or a change in the accretion flow. However, a physical explanation is lacking because basic questions about this activity is unknown, including: is this elevated activity a brief change, or is it a precursor to even larger events? With ground-based near-infrared observations soon unable to observe Sgr A*, we propose a monitoring program to observe Sgr A* during the remaining observable HST period in 2019 for Sgr A* (Sept. & Oct.) with WFC3-IR. This will provide us critical data on the nature of the current Sgr A* activity and a unique opportunity to study supermassive black hole accretion. The main science goal of this proposal is to monitor the supermassive black hole, Sgr A*, at the Galactic center to assess whether it is increasing in activity and characterize its variability. The most important part of this program is cadence of the monitoring. We request monitoring of the source from early-September to late-October, when the HST observing window for the Galactic center closes. To measure Sgr A* variability, we request observations using WFC3-IR and the F153M filter. We request observations using SAMP-SEQ=STEP50, NSAMP=12 for individual exposure times of 349.233 s. This exposure time is a balance between observing short time-scale variations in Sgr A* and minimizing the overhead from reading out the detector. We request an ORIENT=90 deg to maximize the available number of dates for scheduling. This ORIENT will also match WCF3-IR Galactic center data from 2010,2011,and 2012. This will increase the legacy value of this data for astrometry and variability studies. Our team consists of experts in high-angular resolution near-infrared observations of the Galactic center both from the ground and with HST. Dr. Do, Lu and Ghez have had many successful HST programs to understand stellar populations in extreme environments and perform precise astrometry measurements. Dr. Hosek and Lu are experts on HST reduction and crowded field photometry and astrometry. Dr. Do, Witzel, Morris, and Becklin are experts on the study of the variability of Sgr A* at multiple wavelengths and with both ground and space based observatories. Our team has published over a dozen papers on Sgr A*. Massive Black Holes And Their Host Galaxies Galaxies and the IGM 0 false SUPPORTED STELLAR CLUSTER NUCLEUS NO This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 J2000 WFC3/IR Imaging No No F153M POS-TARG DITHER 0.42 LINE false false 27 11.0 SPIRAL 2 false /Users/tdo/Google Drive/proposals/hst_cycle26_ddt_sgra/phase2/OP-cache/sgra_ddt-v01.xml The scheduling data was generated by CASM 27.2. CASM was run in Phase II mode. 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