Near-infrared Flux Distribution of Sgr A* from 2005–2022: Evidence for an Enhanced Accretion Episode in 2019

Weldon, Grant C.; Do, Tuan; Witzel, Gunther; Ghez, Andrea M.; Gautam, Abhimat K.; Becklin, Eric E.; Morris, Mark R.; Martinez, Gregory D.; Sakai, Shoko; Lu, Jessica R.; Matthews, Keith; Hosek, Matthew W.; Haggard, Zoë

doi:10.3847/2041-8213/acf2f2

Cited by 3 publications

(2 citation statements)

References 52 publications

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“…The consistency between our fitted quiescent emission in 2019 and previous analyses is somewhat shocking. IR variability suggests that Sgr A * was more variable in 2019 than in the past ∼20 yr (Do et al 2019;Weldon et al 2023), which they concluded occurs from enhanced accretion onto the event horizon. As flaring events occur within the accretion flow, one would expect similar outliers in the quiescent submm and radio emission.…”

Section: Quiescent Propertiesmentioning

confidence: 98%

“…To determine if any peaks in the power spectrum are significant, we follow the prescription in Do et al (2009) to compare the measured FFT to a red-noise-dominated power spectrum. We generate 35,000 red-noise light curves (using the stochastic 12 Python package) with a spectral density exponent α = 2.75 (PSD ∝ f −2.75 ; Weldon et al 2023). These red-noise light curves have the same temporal sampling as our light curves.…”

Section: Periodogramsmentioning

confidence: 99%

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Multiwavelength Observations of Sgr A*. II. 2019 July 21 and 26

Michail,

Yusef-Zadeh,

Wardle

et al. 2024

ApJ

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We report on the final two days of a multiwavelength campaign of Sgr A* observing in the radio, submillimeter, infrared (IR), and X-ray bands in 2019 July. Sgr A* was remarkably active, showing multiple flaring events across the electromagnetic spectrum. We detect a transient ∼35 minute periodicity feature in Spitzer light curves on 2019 July 21. Time-delayed emission was detected in Atacama Large Millimeter/submillimeter Array light curves, suggesting a hotspot within the accretion flow on a stable orbit. On the same night, we observe a decreased flux in the submillimeter light curve following an X-ray flare detected by Chandra, and we model the feature with an adiabatically expanding synchrotron hotspot occulting the accretion flow. The event is produced by a plasma 0.55 R S in radius with an electron spectrum p = 2.84. It is threaded by a ∼130 Gauss magnetic field and expands at 0.6% the speed of light. Finally, we reveal an unambiguous flare in the IR, submillimeter, and radio, demonstrating that the variable emission is intrinsically linked. We jointly fit the radio and submillimeter light curves using an adiabatically expanding synchrotron hotspot and find it is produced by a plasma with an electron spectrum p = 0.59, 187 Gauss magnetic field, and radius 0.47 R S that expands at 0.029c. In both cases, the uncertainty in the appropriate lower and upper electron energy bounds may inflate the derived equipartition field strengths by a factor of 2 or more. Our results confirm that both synchrotron- and adiabatic-cooling processes are involved in the variable emission’s evolution at submillimeter and IR wavelengths.

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Section: Quiescent Propertiesmentioning

confidence: 98%

Section: Periodogramsmentioning

confidence: 99%

Multiwavelength Observations of Sgr A*. II. 2019 July 21 and 26

Michail,

Yusef-Zadeh,

Wardle

et al. 2024

ApJ

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HESS J1745−290 spectrum explained by a transition in the diffusion regime of PeV cosmic rays in the Sgr A* accretion flow

Muena,

Riquelme,

Reisenegger

et al. 2024

A&A

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The diffuse TeV gamma-ray emission detected in the inner sim 100 pc of the Galactic center suggests the existence of a central cosmic-ray accelerator reaching sim PeV energies. It is interesting to associate this so-called "PeVatron" with the point source HESS J1745$-$290, whose position is consistent with that of the central supermassive black hole Sgr A*. However, the point source shows a spectral break at a few TeV that is not shown by the diffuse emission, challenging this association. We seek to build an emission model for the point source that is consistent with both emissions being produced by the same population of relativistic protons continuously injected with a power-law spectrum up to sim PeV energies near Sgr A*. In our model, we assume that the point source is produced by hadronic collisions between the cosmic rays and the gas in the accretion flow of Sgr A*. The cosmic-ray density is calculated taking into consideration cosmic-ray transport due to diffusion and advection, while the properties of the gas are obtained from previous numerical simulations of the accretion flow. Our model succeeds in explaining both the point source and the diffuse emission with the same cosmic rays injected in the vicinity of Sgr A*, as long as the coherence length of the magnetic turbulence in the accretion flow is $l_c cm $. The spectral break of the point source appears naturally due to an energy-dependent transition in the way the cosmic rays diffuse within the inner $ 0.1$ pc of the accretion flow (where most of the emission is produced). Our model supports the idea that Sgr A* can be a PeVatron, whose accelerated cosmic rays give rise to both the point source and the diffuse emission. Future TeV telescopes such as CTAO, will be able to test this model.

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General relativistic effects and the near-infrared variability of Sgr A*

von Fellenberg,

Witzel,

Bauboeck

et al. 2024

A&A

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A systematic study, based on the third-moment structure function, of Sgr A*'s variability finds an exponential rise time, $ obs minutes $, and decay time, $ obs minutes $. This symmetry of the flux-density variability is consistent with earlier work, and we interpret it as being caused by the dominance of Doppler boosting, as opposed to gravitational lensing, in Sgr A*'s light curve. A relativistic, semi-physical model of Sgr A* confirms an inclination angle of $i The model also shows that the emission of the intrinsic radiative process can have some asymmetry even though the observed emission does not. The third-moment structure function, which is a measure of the skewness of the light-curve increments, may be a useful summary statistic in other contexts of astronomy because it senses only temporal asymmetry; that is, it averages to zero for any temporally symmetric signal.

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Near-infrared Flux Distribution of Sgr A* from 2005–2022: Evidence for an Enhanced Accretion Episode in 2019

Cited by 3 publications

References 52 publications

Multiwavelength Observations of Sgr A*. II. 2019 July 21 and 26

Multiwavelength Observations of Sgr A*. II. 2019 July 21 and 26

HESS J1745−290 spectrum explained by a transition in the diffusion regime of PeV cosmic rays in the Sgr A* accretion flow

General relativistic effects and the near-infrared variability of Sgr A*

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