Diagnosing the Black Hole Accretion Physics of Sgr A*
Subject Category: ACTIVE GALAXIES AND QUASARS
Proposal Number: 18*
Title: Diagnosing the Black Hole Accretion Physics of Sgr A*
PI Name: Giovanni Fazio
The Galactic center offers the closest opportunity for studying accretion onto supermassive
black holes. The fluctuating source, Sgr A*, is detected across the electromagnetic spectrum and
may originate in the accretion flow or jet. Recent general relativistic magneto-hydrodynamic
(GRMHD) models indicate that variability can be produced by a tilted inner disk, gravitational
lensing of bright spots in the disk by the hole, or particle acceleration in reconnection events.
These models produce different flare characteristics, and in particular better characterization of
flares may enable us to distinguish between strong and weakly magnetized disks. Disentangling
the power source and emission mechanisms of the flares is a central challenge to our
understanding of the Sgr A* accretion flow. Following our successful observations of the
variability of Sgr A* with IRAC in 2013 and 2014, we propose simultaneous IRAC (4.5 micron)
and Chandra (2-10 keV) observations to (1) probe the accretion physics of Sgr A* on
event-horizon scales and (2) detect any effect of the object G2 on Sgr A*. Specifically, we
propose six additional epochs of observation, each of 24 uninterrupted hours; four in 2017 July
and two in 2018 July. In this proposal we request two 24-hour (86.4 ks) Chandra periods, and
are requesting another four through the Chandra TAC to have simultaneous X-ray observations
in each of the six Spitzer epochs. Independent of this proposal we will also request NuSTAR
(3-79 keV), SMA/ALMA/APEX (0.8 mm), and Keck/Magellan NIR (2.2 micron) observations
during the IRAC/Chandra epochs. Only such long-duration, continuous, multi-wavelength
observations can achieve a comprehensive view of the dominant emission process(es) and
quantify the physical properties near the event horizon. Theoretical models are increasing in
physical sophistication, and our study will provide essential constraints for the next generation
of models.
