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.

2019 Campaign Information

Additional observations of Spitzer/IRAC 4.5 μm plus Chandra 2–8 keV observations have been approved for the summer of 2019. There will be three epochs lasting 16 hours each. Dates and times (UTC) are:
epoch 1startend
Spitzer 2019 Jul 17 23:20:14 Jul 18 15:20
Chandra 2019 day 198 23:13:00 day 199 15:13:00
NuSTAR 2019 day 198 21:40 day 199 16:45
epoch 2
Spitzer 2019 Jul 21 00:20:52 16:21
Chandra 2019 day 202 00:14:00 16:14:00
NuSTAR no coverage (star tracker blockage)
epoch 3
Spitzer 2019 Jul 26 02:00:22 18:00
Chandra 2019 day 207 01:53:01 17:53:01
NuSTAR times for the third epoch should be available next week. We expect them to be close to the other times.

Dates and times above are now final to within a few seconds. NuSTAR will observe roughly 45 minutes on source and then 45 minutes gap because of Earth occultations. Spitzer times are those at the Spitzer observatory. Because of the heliocentric correction, corresponding times at Earth are about 7 minutes earlier, and the Chandra times reflect that. For reference, Sgr A* transits in Chile around 0230 UT and in Hawaii around 0810 UT.

NuSTAR will also observe the first and third epochs above but not the second one. The Submillimeter Array (~300 GHz) will observe all three, and the JVLA (~42 GHz) should observe at least two if the weather cooperates. Other observatories may participate as well. We encourage all observers of Sgr A* in this time frame to coordinate observations to maximize the value of all data sets. If you would like to coordinate with the Spitzer/Chandra observations, please contact us at the address below.

Results of Previous Campaigns

Sgr A* has been observed at 4.5 μm with Spitzer/IRAC on eight visits of ~24 hours each. The last four 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 with publications in preparation.

Times of all Spitzer visits are:
AORKEY Start Time Frame Sets Type
50123264 2013-12-10 03:48:56 92 Map
50123520 2013-12-10 04:20:24 5000 Stare part 1
50123776 2013-12-10 16:04:21 5000 Stare part 2
51040768 2014-06-02 22:32:00 126 Map
51041024 2014-06-02 22:59:37 5000 Stare part 1
51041280 2014-06-03 10:43:22 5000 Stare part 2
51087616 2014-06-17 18:29:35 126 Map
51087872 2014-06-17 18:57:17 5000 Stare part 1
51088128 2014-06-18 06:41:01 5000 Stare part 2
51344128 2014-07-04 13:21:59 126 Map
51344384 2014-07-04 13:49:41 4999 Stare part 1
51344640 2014-07-05 01:33:25 5000 Stare part 2
58115840 2016-07-12 18:04:23 156 Map
58116352 2016-07-12 18:37:45 5142 Stare part 1
58116608 2016-07-13 06:41:14 5142 Stare part 2
58116096 2016-07-18 11:44:02 156 Map
58116864 2016-07-18 12:17:25 5142 Stare part 1
58117120 2016-07-19 00:20:54 5142 Stare part 2
60651008 2017-07-15 22:28:54 156 Map
63303680 2017-07-15 23:02:17 5142 Stare part 1
63303936 2017-07-16 11:05:46 5142 Stare part 2
60651264 2017-07-25 22:39:33 156 Map
63304192 2017-07-25 23:12:57 5142 Stare part 1
63304448 2017-07-26 11:16:26 5141 Stare part 2
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.

General Information

The objectives of the Chandra and Spitzer observations 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*. 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.


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


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: Friday, 12-Jul-2019 10:02:19 EDT