Near infrared flares of Sagittarius A*

et al. 2010

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We discuss mm-wavelength radio, 2.2-11.8 μm NIR and 2-10 keV X-ray light curves of the super massive black hole (SMBH) counterpart of Sagittarius A* (SgrA*) near its lowest and highest observed luminosity states. We investigate the structure and brightness of the central S-star cluster harboring the SMBH to obtain reliable flux density estimates of SgrA* during its low luminosity phases. We then discuss the physical processes responsible for the brightest flare as well as the faintest flare or quiescent emission in the NIR and X-ray domain. To investigate the low state of SgrA* we use three independent methods to remove or strongly suppress the flux density contributions of stars in the central 2 diameter region around SgrA*. The three methods are: a) low-pass filtering the image; b) iterative identification and removal of individual stars; c) automatic point spread function (PSF) subtraction. For the lowest observed flux density state all 3 image reduction methods result in the detection of faint extended emission with a diameter of 0.5 -1.0 and centered on the position of SgrA*. We analyzed two datasets that cover the lowest luminosity states of SgrA* we observed to date. In one case we detect a faint K-band (2.2 μm) source of ∼4 mJy brightness (de-reddened with A K = 2.8) which we identify as SgrA* in its low state. In the other case no source brighter or equal to a de-reddened K-band flux density of ∼2 mJy was detected at that position. As physical emission mechanisms for SgrA* we discuss bremsstrahlung, thermal emission of a hypothetical optically thick disk, synchrotron and synchrotron self-Compton (SSC) emission, and in the case of a bright flare the associated radio response due to adiabatic expansion of the synchrotron radiation emitting source component. The luminosity during the low state can be interpreted as synchrotron emission from a continuous or even spotted accretion disk. For the high luminosity state SSC emission from THz peaked source components can fully account for the flux density variations observed in the NIR and X-ray domain. We conclude that at near-infrared wavelengths the SSC mechanism is responsible for all emission from the lowest to the brightest flare from SgrA*. For the bright flare event of 4 April 2007 that was covered from the radio to the X-ray domain, the SSC model combined with adiabatic expansion can explain the related peak luminosities and different widths of the flare profiles obtained in the NIR and X-ray regime as well as the non detection in the radio domain.

Section: Modeling the April 4 2007 Nir/x-ray Light Curvesmentioning

confidence: 55%

Section: Discussionmentioning

confidence: 99%

The extreme luminosity states of Sagittarius A*

Sabha

Witzel

et al. 2010

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“…Only rapid millimeter to X-ray variability (Baganoff et al 2001;Genzel et al 2003;Ghez et al 2004Mauerhan et al 2005Eckart et al 2004Eckart et al , 2006aEckart et al , 2008Yusef-Zadeh et al 2008) as well as significant polarization features in the near-infrared (Eckart et al 2006b;Zamaninasab et al 2010Zamaninasab et al , 2011 indicate the presence of matter on relativistic orbits close to SgrA*. However, the radius at which the electrons become relativistic and suppress Faraday rotation within a thermal electron gas is unclear.…”

Section: Resultsmentioning

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%

Millimeter to X-ray flares from Sagittarius A*

García-Marín

Vogel

et al. 2012

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146

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.

“…The beam was subsequently restored with a Gaussian of a FWHM near the diffraction limit. These data reduction and deconvolution steps were also taken for polarimetric data (Zamaninasab et al 2010;Witzel et al 2011, in prep.). In these cases, flux densities of observations made with the Wollaston prism were obtained by summing the two orthogonal channels.…”

Section: Introductionmentioning

confidence: 99%

The near-infrared spectral index of Sagittarius A* derived fromKs- andH-band flare statistics

Bremer

Witzel

et al. 2011

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Context. The near-infrared (NIR) counterpart of Sagittarius A* (SgrA*) at the position of the 4 × 10 6 M supermassive black hole at the center of the Milky Way has strongly varying flux densities. The broad-band near-infrared spectral index is an essential parameter to determine the underlying emission mechanism for the observed flare emission. Aims. We present a method to derive the NIR spectral index of SgrA* between the H-and Ks-band from the statistics of the observed flare emission. Our spectral index derivation is therefore based on an unprecedentedly large timebase of about seven years of monitoring the infrared counterpart of SgrA*. Methods. We examined NIR light curves of SgrA* in the H-and Ks-band and established flare number distributions as a function of peak flare flux. We assume that in both bands the same optically thin dominant emission mechanism is at work and produces similar number distributions of flares. We cross-correlated these histograms and determined a statistical expectation value of the H-Ks-band spectral index during the bright phases of SgrA*. Results. With this new method, we can independently confirm that the expectation value of the spectral index for brighter flares is consistent with α = −0.7 (with the flux density (F ν ∝ ν +α )) which is expected for pure synchrotron radiation. We find a tendency for weaker flares to exhibit a steeper spectrum. Conclusions. We conclude that the distribution of spectral indices as a function of Ks-band flux density can successfully be described by an exponential cutoff proportional to exp[−(ν/ν 0 ) 0.5 ] because of synchrotron losses, with ν 0 being a characteristic cutoff frequency. Varying ν 0 between the NIR and sub-mm domain and assuming a sub-mm flux density variation of about one Jansky and optically thin spectral indices of α = −0.7 ± 0.3 explains the observed spectral properties of SgrA* in the NIR.