Correlation between Flux and Spectral Index during Flares in Sagittarius A*

Bittner, Jonathan M.; Liu, Siming; Fryer, Christopher L.; Petrosian, V.

doi:10.1086/516743

Cited by 9 publications

(18 citation statements)

References 32 publications

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“…We also present the results of short observations made on 2006 July 16 using the VLA and CSO, as well as the cross correlation of flare emission at 7 and 13 mm observed with the VLA on 2005 February 10 and 2006 February 10. Finally, fitting a simple adiabatically expanding plasma model gives reasonable matches to the light curve at two or more frequencies placing strong constraints on the physical parameters and size of the expanding plasma that are consistent with the parameters inferred for the accretion flow around Sgr A Ã and for the emission regions responsible for the NIR flares ( Liu et al 2006a( Liu et al , 2006bBittner et al 2007). …”

Section: Introductionsupporting

confidence: 67%

“…The transient population of accelerated electrons may be produced via reconnection, acceleration in weak shocks (e.g., Yuan et al 2003) or by heating by plasma waves (Liu et al 2006a(Liu et al , 2006b) driven by instabilities in the accretion flow or dissipation of magnetic turbulence. Detailed modeling of the latter process implies that the regions are compact suggesting that the flaring may be used to study the detailed evolution of the accelerated electron population in response to heating and radiative mechanisms (Bittner et al 2007).…”

Section: Modeling Of Submillimeter and Radio Flaresmentioning

confidence: 99%

“…These studies imply that the NIR is synchrotron emission while the X-rays are most likely produced by synchrotron-self-Comption (SSC) or inverse Compton emission ). Modeling of the processes responsible for heating and cooling the transient population of electrons responsible for the NIR and X-ray flares implies that it is produced in localized regions in the inner part of the accretion flow ( Liu et al 2006a( Liu et al , 2006bBittner et al 2007). …”

Section: Introductionmentioning

confidence: 99%

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SimultaneousChandra, CSO, and VLA Observations of Sgr A*: The Nature of Flaring Activity

Yusef‐Zadeh

Wardle

Heinke

et al. 2008

ApJ

117

161

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Sgr AÃ , the massive black hole at the center of the Galaxy, varies in radio through X-ray emission on hourly timescales. The flare activity is thought to arise from the innermost region of an accretion flow onto Sgr A Ã . We present simultaneous light curves of Sgr A Ã in radio, submillimeter and X-rays that show a possible time delay of 110 AE 17 minutes between X-ray and 850 m suggesting that the submillimeter flare emission is optically thick. At radio wavelengths, we detect time lags of 20:4 AE 6:8, 30 AE 12, and 20 AE 6 minutes between the flare peaks observed at 13 and 7 mm (22 and 43 GHz) in three different epochs using the VLA. Linear polarization of 1% AE 0:2% and 0:7 AE 0:1% is detected at 7 and 13 mm, respectively, when averaged over the entire observation on 2006 July 17. A simple model of a bubble of synchrotron-emitting electrons cooling via adiabatic expansion can explain the time delay between various wavelengths, the asymmetric shape of the light curves, and the observed polarization of the flare emission at 43 and 22 GHz. The derived physical quantities that characterize the emission give an expansion speed of v exp $ 0:003Y0:1 c, magnetic field of B $ 10Y70 G, and particle spectral index p $ 1Y2. These parameters suggest that the associated plasma cannot escape from Sgr A Ã unless it has a large bulk motion.

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Section: Introductionsupporting

confidence: 67%

Section: Modeling Of Submillimeter and Radio Flaresmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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SimultaneousChandra, CSO, and VLA Observations of Sgr A*: The Nature of Flaring Activity

Yusef‐Zadeh

Wardle

Heinke

et al. 2008

ApJ

117

161

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“…These components also require steeper spectral indices as expected -if there is a correlation between NIR flare strength and spectral index (Ghez et al 2005;Gillessen et al 2006;Krabbe et al 2006;Hornstein et al 2006, but see Hornstein et al 2007) as is also discussed in Eckart et al (2006a) and Bittner et al (2007). Larger sizes and steeper spectra for these source components are required in models B1 and B2 to match the early section of the 43 GHz VLA data and to fulfill the flux density measures and limits in the NIR and X-ray domain.…”

Section: The Magnetic Fieldmentioning

confidence: 70%

Modeling mm- to X-ray flare emission from Sagittarius A*

Eckart

Baganoff

Morris

et al. 2009

A&A

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Context. We report on new modeling results based on the mm-to X-ray emission of the SgrA* counterpart associated with the massive ∼4 × 10 6 M black hole at the Galactic Center. Aims. We investigate the physical processes responsible for the variable emission from SgrA*. Methods. Our modeling is based on simultaneous observations carried out on 07 July, 2004, using the NACO adaptive optics (AO) instrument at the European Southern Observatory's Very Large Telescope and the ACIS-I instrument aboard the Chandra X-ray Observatory as well as the Submillimeter Array SMA on Mauna Kea, Hawaii, and the Very Large Array in New Mexico. Results. The observations revealed several flare events in all wavelength domains. Here we show that the flare emission can be described with a combination of a synchrotron self-Compton (SSC) model followed by an adiabatic expansion of the source components. The SSC emission at NIR and X-ray wavelengths involves up-scattered sub-millimeter photons from a compact source component. At the start of the flare, spectra of these components peak at frequencies between several 100 GHz and 2 THz. The adiabatic expansion then accounts for the variable emission observed at sub-mm/mm wavelengths. The derived physical quantities that describe the flare emission give a blob expansion speed of v exp ∼ 0.005 c, magnetic field of B around 60 G or less and spectral indices of α = 0.8 to 1.4, corresponding to a particle spectral index p ∼ 2.6 to 3.8. Conclusions. A combined SSC and adiabatic expansion model can fully account for the observed flare flux densities and delay times covering the spectral range from the X-ray to the mm-radio domain. The derived model parameters suggest that the adiabatic expansion takes place in source components that have a bulk motion larger than v exp or the expanding material contributes to a corona or disk, confined to the immediate surroundings of SgrA*.

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“…In the damping phase, there are strong couplings between the charged background particles and turbulent motion, and the background particles are energized. This process is a major mechanism for heating the solar flares and other astrophysical plasmas Bittner et al 2007). On the other hand, the process also significantly damps the plasma turbulence and affects the dispersion relation.…”

Section: Appendix B Damping and Hot Plasma Dispersion Relationmentioning

confidence: 99%

Cascade and Damping of Alfvén-Cyclotron Fluctuations: Application to Solar Wind Turbulence

Jiang

Liu

Petrosian

2009

ApJ

Self Cite

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It is well recognized that the presence of magnetic fields will lead to anisotropic energy cascade and dissipation of astrophysical turbulence. With the diffusion approximation and linear dissipation rates, we study the cascade and damping of Alfvén-cyclotron fluctuations in solar plasmas numerically for two diagonal diffusion tensors, one (isotropic) with identical components for the parallel and perpendicular directions (with respect to the magnetic field) and one with different components (nonisotropic). It is found that for the isotropic case the steady-state turbulence spectra are nearly isotropic in the inertial range and can be fitted by a single power-law function with a spectral index of −3/2, similar to the Iroshnikov-Kraichnan phenomenology, while for the nonisotropic case the spectra vary greatly with the direction of propagation. The energy fluxes in both cases are much higher in the perpendicular direction than in the parallel direction due to the angular dependence (or inhomogeneity) of the components. In addition, beyond the MHD regime the kinetic effects make the spectrum softer at higher wavenumbers. In the dissipation range the turbulence spectrum cuts off at the wavenumber, where the damping rate becomes comparable to the cascade rate, and the cutoff wavenumber changes with the wave propagation direction. The angle-averaged turbulence spectrum of the isotropic model resembles a broken power law, which cuts off at the maximum of the cutoff wavenumbers or the 4 He cyclotron frequency. Taking into account the Doppler effects, the model naturally reproduces the broken power-law turbulence spectra observed in the solar wind and predicts that a higher break frequency always comes along with a softer dissipation range spectrum that may be caused by the increase of the turbulence intensity, the reciprocal of the plasma β p , and/or the angle between the solar wind velocity and the mean magnetic field. These predictions can be tested by detailed comparisons with more accurate observations.

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Correlation between Flux and Spectral Index during Flares in Sagittarius A*

Cited by 9 publications

References 32 publications

SimultaneousChandra, CSO, and VLA Observations of Sgr A*: The Nature of Flaring Activity

SimultaneousChandra, CSO, and VLA Observations of Sgr A*: The Nature of Flaring Activity

Modeling mm- to X-ray flare emission from Sagittarius A*

Cascade and Damping of Alfvén-Cyclotron Fluctuations: Application to Solar Wind Turbulence

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