FAR INFRARED VARIABILITY OF SAGITTARIUS A*: 25.5 hr OF MONITORING WITH HERSCHEL*

Stone, Jordan; Marrone, Daniel P.; Dowell, C. D.; Schulz, B.; Heinke, C. O.; Yusef‐Zadeh, F.

doi:10.3847/0004-637x/825/1/32

Cited by 26 publications

(20 citation statements)

References 52 publications

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“…First, we consider the shape of the submm to NIR total intensity spectrum (e.g., Falcke et al 1998;Bower et al 2015;Stone et al 2016;von Fellenberg et al 2018;Bower et al 2019;Schödel et al 2011;Dodds-Eden et al 2011;Witzel et al 2018) and the rms variability fraction as a function Quantitatively, we require a submm spectral index between 230 and 690 GHz of −0.35−0.25 (Marrone 2006;Bower et al 2019) and an upper limit to the median NIR flux density of < 1.4 mJy (Dodds-Eden et al 2011;Witzel et al 2018, GRAVITY collaboration 2020, and a 230 GHz total flux density rms of 20 − 40% for a model to be considered viable.…”

Section: Spectrum and Variabilitymentioning

confidence: 99%

“…The most extensively studied low-luminosity accretion flow is that onto the Galactic centre massive black hole candidate Sagittarius A* (Sgr A*). Detected in the radio (Balick & Brown 1974), Sgr A* shows an inverted spectrum up to a peak at submillimeter (submm) wavelengths (Serabyn et al 1997;Falcke et al 1998;Stone et al 2016;von Fellenberg et al 2018;Bower et al 2019). The bolometric luminosity is 5×10 35 erg s −1 (Bower et al 2019), roughly 9 orders of magnitude sub-Eddington for the mass of 4 × 10 6 M measured from the orbit of the star S2 (Ghez et al 2008;Gillessen et al 2009Gillessen et al , 2017Gravity Collaboration et al 2018a jason.dexter@colorado.edu † ajimenez@mpe.mpg.de Do et al 2019).…”

Section: Introductionmentioning

confidence: 98%

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A parameter survey of Sgr A* radiative models from GRMHD simulations with self-consistent electron heating

Dexter

Jiménez-Rosales

Ressler

et al. 2020

Monthly Notices of the Royal Astronomical Society

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The Galactic center black hole candidate Sgr A* is the best target for studies of lowluminosity accretion physics, including with near-infrared and submillimeter wavelength long baseline interferometry experiments. Here we compare images and spectra generated from a parameter survey of general relativistic MHD simulations to a set of radio to near-infrared observations of Sgr A*. Our models span the limits of weak and strong magnetization and use a range of sub-grid prescriptions for electron heating. We find two classes of scenarios can explain the broad shape of the submillimeter spectral peak and the highly variable near-infrared flaring emission. Weakly magnetized "disk-jet" models where most of the emission is produced near the jet wall, consistent with past work, as well as strongly magnetized (magnetically arrested disk) models where hot electrons are present everywhere. Disk-jet models are strongly depolarized at submillimeter wavelengths as a result of strong Faraday rotation, inconsistent with observations of Sgr A*. We instead favor the strongly magnetized models, which provide a good description of the median and highly variable linear polarization signal. The same models can also explain the observed mean Faraday rotation measure and potentially the polarization signals seen recently in Sgr A* near-infrared flares.

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Section: Spectrum and Variabilitymentioning

confidence: 99%

Section: Introductionmentioning

confidence: 98%

A parameter survey of Sgr A* radiative models from GRMHD simulations with self-consistent electron heating

Dexter

Jiménez-Rosales

Ressler

et al. 2020

Monthly Notices of the Royal Astronomical Society

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“…The black points (upper-lower limits) in Fig. 1 show a compilation of measurements of Sgr A ⋆ 's quiescent emission from radio to mid-IR (values are taken from Falcke et al 1998;Markoff et al 2001;An et al 2005;Marrone et al 2006;Schödel et al 2007;Dodds-Eden et al 2009;Bower et al 2015;Brinkerink et al 2015;Liu et al 2016;Stone et al 2016) as well as the radiatively inefficient accretion flow model proposed by Yuan et al (2003). The bulk of Sgr A ⋆ 's steady radiation is emitted at sub-mm frequencies, forming the so called "sub-mm bump" (dot-dashed line in Fig.…”

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confidence: 99%

A powerful flare from Sgr A* confirms the synchrotron nature of the X-ray emission

Ponti

George

Scaringi

et al. 2017

Monthly Notices of the Royal Astronomical Society

119

142

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We present the first fully simultaneous fits to the NIR and X-ray spectral slope (and its evolution) during a very bright flare from Sgr A ⋆ , the supermassive black hole at the Milky Way's center. Our study arises from ambitious multi-wavelength monitoring campaigns with XMMNewton, NuSTAR and SINFONI. The average multi-wavelength spectrum is well reproduced by a broken power-law with Γ N IR = 1.7 ± 0.1 and Γ X = 2.27 ± 0.12. The difference in spectral slopes (∆Γ = 0.57 ± 0.09) strongly supports synchrotron emission with a cooling break. The flare starts first in the NIR with a flat and bright NIR spectrum, while X-ray radiation is detected only after about 10 3 s, when a very steep X-ray spectrum (∆Γ = 1.8 ± 0.4) is observed. These measurements are consistent with synchrotron emission with a cooling break and they suggest that the high energy cut-off in the electron distribution (γ max ) induces an initial cut-off in the optical-UV band that evolves slowly into the X-ray band. The temporal and spectral evolution observed in all bright X-ray flares are also in line with a slow evolution of γ max . We also observe hints for a variation of the cooling break that might be induced by an evolution of the magnetic field (from B ∼ 30 ± 8 G to B ∼ 4.8 ± 1.7 G at the X-ray peak). Such drop of the magnetic field at the flare peak would be expected if the acceleration mechanism is tapping energy from the magnetic field, such as in magnetic reconnection. We conclude that synchrotron emission with a cooling break is a viable process for Sgr A ⋆ 's flaring emission.

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“…SED of Sgr A*: the radio and sub-mm data are fromFalcke et al (1998),Bower et al (2015Bower et al ( , 2019,Brinkerink et al (2015),Liu et al (2016). The far infrared data is fromStone et al (2016) andvon Fellenberg et al (2018). The NIR M-band data is the median flux density inferred from the lognormal model ofWitzel et al (2018).…”

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confidence: 99%

The flux distribution of Sgr A*

Abuter

Amorim

Bauböck

et al. 2020

A&A

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The Galactic center black hole Sagittarius A* is a variable near-infrared (NIR) source that exhibits bright flux excursions called flares. When flux from Sgr A* is detected, the light curve has been shown to exhibit red noise characteristics and the distribution of flux densities is non-linear, non-Gaussian, and skewed to higher flux densities. However, the low-flux density turnover of the flux distribution is below the sensitivity of current single-aperture telescopes. For this reason, the median NIR flux has only been inferred indirectly from model fitting, but it has not been directly measured. In order to explore the lowest flux ranges, to measure the median flux density, and to test if the previously proposed flux distributions fit the data, we use the unprecedented resolution of the GRAVITY instrument at the VLTI. We obtain light curves using interferometric model fitting and coherent flux measurements. Our light curves are unconfused, overcoming the confusion limit of previous photometric studies. We analyze the light curves using standard statistical methods and obtain the flux distribution. We find that the flux distribution of Sgr A* turns over at a median flux density of (1.1 ± 0.3) mJy. We measure the percentiles of the flux distribution and use them to constrain the NIR K-band spectral energy distribution. Furthermore, we find that the flux distribution is intrinsically right-skewed to higher flux density in log space. Flux densities below 0.1 mJy are hardly ever observed. In consequence, a single powerlaw or lognormal distribution does not suffice to describe the observed flux distribution in its entirety. However, if one takes into account a power law component at high flux densities, a lognormal distribution can describe the lower end of the observed flux distribution. We confirm the rms–flux relation for Sgr A* and find it to be linear for all flux densities in our observation. We conclude that Sgr A* has two states: the bulk of the emission is generated in a lognormal process with a well-defined median flux density and this quiescent emission is supplemented by sporadic flares that create the observed power law extension of the flux distribution.

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FAR INFRARED VARIABILITY OF SAGITTARIUS A: 25.5 hr OF MONITORING WITH HERSCHEL

Cited by 26 publications

References 52 publications

A parameter survey of Sgr A* radiative models from GRMHD simulations with self-consistent electron heating

A parameter survey of Sgr A* radiative models from GRMHD simulations with self-consistent electron heating

A powerful flare from Sgr A* confirms the synchrotron nature of the X-ray emission

The flux distribution of Sgr A*

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