IRAC - Infrared Array Camera for the Spitzer Space Telescope Spitzer Space Telescope

Spitzer/IRAC and Chandra Observations of Sgr A*

The Galactic center supermassive black hole is by far the closest example for studying the mechanisms of accretion onto such objects. Sgr A* is the fluctuating source of electromagnetic radiation derived from the accretion flow or perhaps a related jet. The variable radiation has been detected at radio, submillimeter (submm), near infrared (NIR), and X-ray wavelengths. Recent numerical general relativistic magneto-hydrodynamic (GRMHD) flare models across the electromagnetic spectrum indicate that variability may be connected to a tilted inner disk or to the black hole gravitationally lensing a bright spot in the accretion flow. Different models predict different amplitudes of flaring, and observations can therefore distinguish between strong-mean- magnetization accretion and weak magnetization. Disentangling the power source and emission mechanisms of the flares is a central challenge to our understanding of the Sgr A* accretion flow.

In order to (1) probe the accretion physics of Sgr A* on event-horizon scales via multi-wavelength monitoring and (2) detect any effect of the object G2 on Sgr A*, in 2013 our group initiated a monitoring campaign with the IRAC camera on the Spitzer Space Telescope. Starting in 2016, we were able to obtain simultaneous X-ray observations with Chandra. 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.

Campaign Information

Sgr A* was observed at 4.5 μm with Spitzer/IRAC on eight visits of ~24 hours each and 3 visits of 16 hours each. The last seven of these visits have complete simultaneous X-ray observations with Chandra. There is also some simultaneous coverage with ALMA and the Submillimeter Array and with other telescopes. Publications so far are listed below.

Times of all Spitzer/IRAC visits are:
AORKEY Start Time Frame Sets Type Full Chandra coverage?
50123264 2013-12-10 03:48:56 92 Map N
50123520 2013-12-10 04:20:24 5000 Stare part 1 N
50123776 2013-12-10 16:04:21 5000 Stare part 2 N
51040768 2014-06-02 22:32:00 126 Map N
51041024 2014-06-02 22:59:37 5000 Stare part 1 N
51041280 2014-06-03 10:43:22 5000 Stare part 2 N
51087616 2014-06-17 18:29:35 126 Map N
51087872 2014-06-17 18:57:17 5000 Stare part 1 N
51088128 2014-06-18 06:41:01 5000 Stare part 2 N
51344128 2014-07-04 13:21:59 126 Map N
51344384 2014-07-04 13:49:41 4999 Stare part 1 N
51344640 2014-07-05 01:33:25 5000 Stare part 2 N
58115840 2016-07-12 18:04:23 156 Map N
58116352 2016-07-12 18:37:45 5142 Stare part 1 Y
58116608 2016-07-13 06:41:14 5142 Stare part 2 Y
58116096 2016-07-18 11:44:02 156 Map Y
58116864 2016-07-18 12:17:25 5142 Stare part 1 Y
58117120 2016-07-19 00:20:54 5142 Stare part 2 Y
60651008 2017-07-15 22:28:54 156 Map Y
63303680 2017-07-15 23:02:17 5142 Stare part 1 Y
63303936 2017-07-16 11:05:46 5142 Stare part 2 Y
60651264 2017-07-25 22:39:33 156 Map Y
63304192 2017-07-25 23:12:57 5142 Stare part 1 Y
63304448 2017-07-26 11:16:26 5142 Stare part 2 Y
65898240 2019-07-17 22:48:08 156 Map Y
69965312 2019-07-17 23:21:30 6856 Stare part 1 Y
69965568 2019-07-18 07:25:01 6856 Stare part 2 Y
65898496 2019-07-20 23:47:56 156 Map Y
69965824 2019-07-21 00:21:18 6856 Stare part 1 Y
69966080 2019-07-21 08:24:49 6856 Stare part 2 Y
70086400 2019-07-26 01:28:00 156 Map Y
69966336 2019-07-26 02:01:22 6856 Stare part 1 Y
69966592 2019-07-26 10:04:53 6856 Stare part 2 Y
Note — start times are UTC at the Observatory. Equivalent heliocentric start times at Earth are a few minutes earlier.

Results and data are reported in the publications listed below. Major results of the infrared observations, including most existing ground-based near infrared data, are:

  1. The variability power spectral density (PSD) can be represented as a broken power law with the break at 245 (+88,−61) minutes.
  2. There is no PSD break on timescales of 9–100 minutes, implying a black hole spin parameter >0.9.
  3. The near infrared spectral index is ≈0.6 for observed flux densities above ≈0.3 mJy but redder at lower flux densities.
  4. The dim-phase spectral energy distribution of Sgr A* from radio to near infrared wavelengths is consistent with synchrotron radiation from a non-thermal electron population.

Access to Data

The original Spitzer data in the above table can be downloaded from the Spitzer Heritage Archive. Lightcurves from the 2013-2014 epochs that were published in Hora et al. 2014 can be downloaded from VizieR, and a text file with the data is available here. The reduced Spitzer and other IR data from 2017 and prior years can be downloaded from the Witzel et al. (2018) paper in MRT-format or accessed on VizieR. The Chandra X-ray data can be downloaded from the Chandra X-ray center. The Boyce et al. (2019) paper provides a table that gives the information on the associated Chandra and Spitzer observations from the 2017 and prior epoch observations. SMA and Spitzer/IRAC light curves from the Witzel et al. (2021) paper in MRT form are here.


For more information, contact:
Giovanni Fazio, Joe Hora, or Steve Willner.


Multi-wavelength Variability of Sagittarius A* in July 2019 Boyce, H. et al. 2022, ApJ, in press
Constraining particle acceleration in Sgr A* with simultaneous GRAVITY, Spitzer, NuSTAR, and Chandra observations R. Abuter et al. 2021, A&A, 654, A22
ADS Abstract
Rapid Variability of Sgr A* across the Electromagnetic Spectrum Witzel et al. 2021 ApJ 917, 73
light curves
Simultaneous X-ray and Infrared Observations of Sagittarius A*'s Variability Boyce et al. 2019, ApJ, 871, 161
ADS Abstract
Variability Timescale and Spectral Index of Sgr A* in the Near Infrared: Approximate Bayesian Computation Analysis of the Variability of the Closest Supermassive Black Hole Witzel et al. 2018, ApJ, 863, 15
ADS Abstract
Multiwavelength Light Curves of Two Remarkable Sagittarius A* Flares Fazio et al. 2018, ApJ 864, 58
ADS Abstract
Spitzer/IRAC Observations of the Variability of Sgr A* and the Object G2 at 4.5 μm Hora et al. 2014, ApJ 793, 120
ADS Abstract

Last updated: Monday, 28-Mar-2022 13:35:11 EDT