The extreme luminosity states of Sagittarius A*

¹ I.Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany e-mail: [sabha;witzel;eckart]@ph1.uni-koeln.de
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³ Instituto de Astrofísica de Andalucía (CSIC), Camino Bajo de Huétor 50, 18008 Granada, Spain e-mail: rainer@iaa.es

Received: 26 August 2009
Accepted: 22 December 2009

Abstract

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.