J/ApJ/863/15 Spitzer, VLT and Keck NIR light curves of SgrA* (Witzel+, 2018)
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 G., Martinez G., Hora J., Willner S.P., Morris M.R., Gammie C.,
Becklin E.E., Ashby M.L.N., Baganoff F., Carey S., Do T., Fazio G.G.,
Ghez A., Glaccum W.J., Haggard D., Herrero-Illana R., Ingalls J.,
Narayan R., Smith H.A.
<Astrophys. J., 863, 15 (2018)>
=2018ApJ...863...15W 2018ApJ...863...15W
ADC_Keywords: Galactic center; Photometry, infrared
Keywords: accretion, accretion disks ; black hole physics ; Galaxy: center ;
methods: statistical ; radiation mechanisms: non-thermal
Abstract:
Sagittarius A* (Sgr A*) is the variable radio, near-infrared (NIR),
and X-ray source associated with accretion onto the Galactic center
black hole. We present an analysis of the most comprehensive NIR
variability data set of Sgr A* to date: eight 24 hr epochs of
continuous monitoring of Sgr A* at 4.5µm with the IRAC instrument
on the Spitzer Space Telescope, 93 epochs of 2.18µm data from Naos
Conica at the Very Large Telescope, and 30 epochs of 2.12µm data
from the NIRC2 camera at the Keck Observatory, in total 94929
measurements. A new approximate Bayesian computation method for
fitting the first-order structure function extracts information beyond
current fast Fourier transformation (FFT) methods of power spectral
density (PSD) estimation. With a combined fit of the data of all three
observatories, the characteristic coherence timescale of Sgr A* is
τb=243-57+82min (90% credible interval). The PSD has no
detectable features on timescales down to 8.5min (95% credible level),
which is the ISCO orbital frequency for a dimensionless spin parameter
a=0.92. One light curve measured simultaneously at 2.12 and 4.5µm
during a low flux-density phase gave a spectral index αs=1.6±0.1
(Fν∝ν-αs_^). This value implies that the Sgr A*
NIR color becomes bluer during higher flux-density phases. The
probability densities of flux densities of the combined data sets are
best fit by log-normal distributions. Based on these distributions,
the Sgr A* spectral energy distribution is consistent with synchrotron
radiation from a non-thermal electron population from below 20GHz
through the NIR.
Description:
All observations (between 2013 Dec 10 and 2017 Jul 26) in this Spitzer
Space Telescope program (Program IDs 10060, 12034, and 13027) used
IRAC subarray mode, which reads a 32x32-pixel region of the IRAC 4.5um
detector array 10 times per second.
Each of eight Spitzer observing epochs used the same basic observing
procedure. This comprised an initial peakup from a reference star to
place Sgr A* at the center of pixel (16,16), making a small map,
during which time the telescope temperature settled down, a second
peakup, a staring mode observation lasting ∼12hr, a third peakup, and
a second stare.
As a test of our method, we also extracted and modeled the output of a
reference pixel in the same way as for pixel (16,16). The reference
pixel was at an image location with a significant gradient and not on
a local maximum, similar to pixel (16,16) but far enough away from it
that the pixel will not see the variability from Sgr A*.
See Section 2.1 for further details.
The VLT data (previously reported by Witzel+ 2012ApJS..203...18W 2012ApJS..203...18W) were
taken with the adaptive optics camera Naos Conica (NaCo) in Ks-band
(2.18um). The NaCo images have 68mas resolution and integration times
of 30-40s. Data were taken between 2003 June 13 and 2010 June 16.
The Keck data were obtained with the NIRC2 camera (PI: Keith Matthews)
in the K'-band (2.12um). Images have 53mas resolution and a fixed
integration time of 28s. The data set contains 3157 images between
2004 July 16 and 2013 July 19.
Objects:
---------------------------------------------------
RA (ICRS) DE Designation(s)
---------------------------------------------------
17 45 40.04 -29 00 28.2 Sgr A* = NAME Sgr A*
---------------------------------------------------
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table3.dat 46 94928 Sgr A* light curve data
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See also:
J/AJ/127/3399 : Sgr A* variability at cm wavelengths (Herrnstein+, 2004)
J/ApJ/698/895 : Variations in QSOs optical flux (Kelly+, 2009)
J/A+A/511/A18 : NIR observations of stars near SgrA* (Schoedel+, 2010)
J/ApJ/725/331 : Astrometry in the Galactic Center (Yelda+, 2010)
J/A+A/534/A117 : NIR polarimetry of sources near SgrA* (Buchholz+, 2011)
J/ApJ/727/L36 : 1.3mm VLBI observations of Sagittarius A* (Fish+, 2011)
J/ApJ/764/154 : Stellar populations in the central 0.5pc. I. (Do+, 2013)
J/MNRAS/428/2731 : SgrA* emission at 7mm (Beaklini+, 2013)
J/A+A/557/A82 : NIR polarimetry of Galactic center sources (Buchholz+, 2013)
J/ApJ/793/120 : Spitzer/IRAC Sgr A* light curve data (Hora+, 2014)
J/MNRAS/454/1525 : XMM-Newton and Chandra monitoring of Sgr A* (Ponti+, 2015)
J/ApJ/802/69 : VLA, ALMA and SMA monitoring of Sgr A* (Bower+, 2015)
J/ApJ/820/90 : 4yr 1.3mm VLBI observations of SgrA* with EHT (Fish+, 2016)
J/ApJ/830/17 : Gal. Center secondary IR astrometric standards (Boehle+, 2016)
J/A+A/589/A116 : Multiwavelength study of Sgr A* (Mossoux+, 2016)
J/A+A/601/A80 : LABOCA 345GHz observations of Sgr A* (Subroweit+, 2017)
J/ApJ/859/60 : Sgr A* 1.3mm VLBI obs. with the EHT in 2013 (Lu+, 2018)
J/A+A/618/A35 : ALMA images of the CND and SgrA* (Goicoechea+, 2018)
J/A+A/618/L10 : SgrA* orbital motions with GRAVITY (GRAVITY Col., 2018)
J/A+A/621/A119 : SgrA* and NRAS530 86GHz images (Brinkerink+, 2019)
Byte-by-byte Description of file: table3.dat
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Bytes Format Units Label Explanations
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1- 12 A12 --- Obs Observatory/Instrument used in the observation (1)
14- 26 F13.7 d HMJD [52803.1/57961] Heliocentric modified Julian Date
28- 36 F9.6 Jy Flux [-0.004/0.01] Excess flux density at position
of Sgr A* (2)
38- 46 F9.6 Jy RefFlux [-0.005/0.02]? Excess flux density of
reference pixel (2)
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Note (1): 8 epochs of Spitzer data at 4.5um, 93 epochs of VLT data at 2.18um,
and 30 epochs of Keck data at 2.12um.
Note (2): As observed, uncorrected for interstellar extinction.
See also the "Description" section above.
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History:
From electronic version of the journal
(End) Prepared by [AAS], Emmanuelle Perret [CDS] 02-Aug-2019