Hot-spot model for accretion disc variability as random process

Pecháček, Tomáš; Karas, V.

doi:10.1051/0004-6361:200809720

Cited by 29 publications

(40 citation statements)

References 37 publications

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“…The overall model - as a sum of all three components -is shown as a thick black solid line. In the framework of previously described multi-spot models or evolving hot-spot models (Eckart et al 2008;Yuan et al 2009; see also Pecháček et al 2008;Zamaninasab et al 2011), the adoption of three components is a reasonable assumption. The offset flux densities above which we attribute the flux density to the flare event are listed in Table 2.…”

Section: An Adiabatic Expansion Model Of the May 2009 Datamentioning

confidence: 99%

“…Several authors (e.g. Schnittman et al 2006;Pecháček et al 2008;Eckart et al 2006b;Zamaninasab et al 2010) have shown that a multi-component hot spot scenario can reproduce the observed flare emission and the overall behavior of the observed SED.…”

Section: Introductionmentioning

confidence: 98%

“…In addition to either spiral arm models (Karas et al 2007;Falanga et al 2007) or jet/jet-base models (Falcke & Markoff 2000Markoff 2005;Markoff et al 2007), recent attempts to explain the emission from the compact region around SgrA* have been based on a hot-spot/disk model (Eckart et al 2006b;Broderick & Loeb 2006), multi-spot models, or even evolving hot-spot models (Eckart et al 2008;Yuan et al 2009; see also Pecháček et al 2008;Zamaninasab et al 2011).…”

Section: Introductionmentioning

confidence: 99%

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Millimeter to X-ray flares from Sagittarius A*

Eckart

García-Marín

Vogel

et al. 2012

A&A

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146

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Context. We report on new simultaneous observations and modeling of the millimeter, near-infrared, and X-ray flare emission of the source Sagittarius A* (SgrA*) associated with the super-massive (4 × 10 6 M ) black hole at the Galactic center. Aims. We study the applicability of the adiabatic synchrotron source expansion model and study physical processes giving rise to the variable emission of SgrA* from the radio to the X-ray domain. Methods. Our observations were carried out on 18 May 2009 using the NACO adaptive optics (AO) instrument at the European Southern Observatory's Very Large Telescope, the ACIS-I instrument aboard the Chandra X-ray Observatory, the LABOCA bolometer at the Atacama Pathfinder EXperiment (APEX), and the CARMA mm telescope array at Cedar Flat, California. Results. The X-ray flare had an excess 2−8 keV luminosity between 6 and 12×10 33 erg s −1 . The observations reveal flaring activity in all wavelength bands that can be modeled as the signal from an adiabatically expanding synchrotron self-Compton (SSC) component.Modeling of the light curves shows that the sub-mm follows the NIR emission with a delay of about three-quarters of an hour with an expansion velocity of about v exp ∼ 0.009c. We find source component sizes of around one Schwarzschild radius, flux densities of a few Janskys, and spectral indices α of about +1 (S (ν) ∝ ν −α ). At the start of the flare, the spectra of the two main components peak just short of 1 THz. To statistically explain the observed variability of the (sub-)mm spectrum of SgrA*, we use a sample of simultaneous NIR/X-ray flare peaks and model the flares using a synchrotron and SSC mechanism. Conclusions. These parameters suggest that either the adiabatically expanding source components 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*. For the bulk of the synchrotron and SSC models, we find synchrotron turnover frequencies in the range of 300−400 GHz. For the pure synchrotron models, this results in densities of relativistic particles of the order of 10 6.5 cm −3 and for the SSC models, the median densities are about one order of magnitude higher. However, to obtain a realistic description of the frequency-dependent variability amplitude of SgrA*, models with higher turnover frequencies and even higher densities are required.

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Section: An Adiabatic Expansion Model Of the May 2009 Datamentioning

confidence: 99%

Section: Introductionmentioning

confidence: 98%

Section: Introductionmentioning

confidence: 99%

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Millimeter to X-ray flares from Sagittarius A*

Eckart

García-Marín

Vogel

et al. 2012

A&A

Self Cite

146

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“…It is not yet clear what kind of process would be the best one to describe the creation and subsequent extinction of the spots. The assumption that spots arise as being statistically independent on each other is a reasonable first approximation, however, it is quite likely that some kind of relationship between the spots will have to be included, for example within the framework of the avalanche mechanism (Pechacek et al 2008). …”

Section: The Ssc Disk Modelmentioning

confidence: 99%

Coordinated multi-wavelength observations of Sgr A*

Eckart

Schödel

Baganoff

et al. 2008

J. Phys.: Conf. Ser.

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Abstract. We report on recent near-infrared (NIR) and X-ray observations of Sagittarius A* (Sgr A*), the electromagnetic manifestation of the ∼4×10 6 M⊙ super-massive black hole (SMBH) at the Galactic Center. The goal of these coordinated multi-wavelength observations is to investigate the variable emission from Sgr A* in order to obtain a better understanding of the underlying physical processes in the accretion flow/outflow. The observations have been carried out using the NACO adaptive optics (AO) instrument at the European Southern Observatory's Very Large Telescope (July 2005, May 2007) and the ACIS-I instrument aboard the Chandra Xray Observatory (July 2005). We report on a polarized NIR flare synchronous to a 8×10 33 erg/s X-ray flare in July 2005, and a further flare in May 2007 that shows the highest sub-flare to flare contrast observed until now. The observations can be interpreted in the framework of a model involving a temporary disk with a short jet. In the disk component flux density variations can be explained due to hot spots on relativistic orbits around the central SMBH. The variations of the sub-structures of the May 2007 flare are interpreted as a variation of the hot spot structure due to differential rotation within the disk.

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“…In our previous paper (Pecháček et al 2008, Paper I) we studied the theory of random processes as a tool for describing the fluctuating signal from accreting sources, such as AGN and Galactic black holes observed in X-rays. These objects exhibit a featureless variability on different timescales, probably originating from an accretion disc surface and its corona.…”

Section: Introductionmentioning

confidence: 99%

Hot-spot model for accretion disc variability as random process

Pecháček

Goosmann

Karas

et al. 2013

A&A

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Context. We study some general properties of accretion disc variability in the context of stationary random processes. In particular, we are interested in mathematical constraints that can be imposed on the functional form of the Fourier power-spectrum density (PSD) that exhibits a multiply broken shape and several local maxima. Aims. We develop a methodology for determining the regions of the model parameter space that can in principle reproduce a PSD shape with a given number and position of local peaks and breaks of the PSD slope. Given the vast space of possible parameters, it is an important requirement that the method is fast in estimating the PSD shape for a given parameter set of the model. Methods. We generated and discuss the theoretical PSD profiles of a shot-noise-type random process with exponentially decaying flares. Then we determined conditions under which one, two, or more breaks or local maxima occur in the PSD. We calculated positions of these features and determined the changing slope of the model PSD. Furthermore, we considered the influence of the modulation by the orbital motion for a variability pattern assumed to result from an orbiting-spot model. Results. We suggest that our general methodology can be useful for describing non-monotonic PSD profiles (such as the trend seen, on different scales, in exemplary cases of the high-mass X-ray binary Cygnus X-1 and the narrow-line Seyfert galaxy Ark 564). We adopt a model where these power spectra are reproduced as a superposition of several Lorentzians with varying amplitudes in the X-ray-band light curve. Our general approach can help in constraining the model parameters and in determining which parts of the parameter space are accessible under various circumstances.

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Hot-spot model for accretion disc variability as random process

Cited by 29 publications

References 37 publications

Millimeter to X-ray flares from Sagittarius A*

Millimeter to X-ray flares from Sagittarius A*

Coordinated multi-wavelength observations of Sgr A*

Hot-spot model for accretion disc variability as random process

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