THE TWO STATES OF Sgr A* IN THE NEAR-INFRARED: BRIGHT EPISODIC FLARES ON TOP OF LOW-LEVEL CONTINUOUS VARIABILITY

In this paper, we examine properties of the variable source Sgr A* in the near-infrared (NIR) using a very extensive Ks-band data set from NACO/VLT observations taken in 2004–2009. We investigate the variability of Sgr A* with two different photometric methods and analyze its flux distribution. We find that Sgr A* is continuously emitting and continuously variable in the near-infrared, with some variability occurring on timescales as long as weeks. The flux distribution can be described by a lognormal distribution at low intrinsic fluxes (≲5 mJy, dereddened with AKs = 2.5). The lognormal distribution has a median flux of ≈1.1 mJy, but above 5 mJy the flux distribution is significantly flatter (high flux events are more common) than expected for the extrapolation of the lognormal distribution to high fluxes. We make a general identification of the low-level emission above 5 mJy as flaring emission and of the low-level emission as the quiescent state. We also report here the brightest Ks-band flare ever observed (from 2008 August 5) which reached an intrinsic Ks-band flux of 27.5 mJy (mKs = 13.5). This flare was a factor 27 increase over the median flux of Sgr A*, close to double the brightness of the star S2, and 40% brighter than the next brightest flare ever observed from Sgr A*.

[1]  Prateek Sharma,et al.  TIME-DEPENDENT MODELS OF FLARES FROM SAGITTARIUS A* , 2010, 1005.0389.

[2]  Bonn,et al.  The extreme luminosity states of Sagittarius A , 2010, 1001.1351.

[3]  F. Yuan,et al.  ELECTRON HEATING AND ACCELERATION BY MAGNETIC RECONNECTION IN HOT ACCRETION FLOWS , 2009, 0911.4560.

[4]  Robert Mann,et al.  Astronomical Data Analysis Software and Systems XXI , 2012 .

[5]  K. Mužić,et al.  Peering through the veil: near-infrared photometry and extinction for the Galactic nuclear star cluster , 2009, 0912.1273.

[6]  Reinhard Genzel,et al.  What is limiting near-infrared astrometry in the Galactic Centre? , 2009, 0909.2592.

[7]  S. Markoff.  From Multiwavelength to Mass Scaling: Accretion and Ejection in Microquasars and AGN , 2009, 0909.2574.

[8]  E. Agol,et al.  MILLIMETER FLARES AND VLBI VISIBILITIES FROM RELATIVISTIC SIMULATIONS OF MAGNETIZED ACCRETION ONTO THE GALACTIC CENTER BLACK HOLE , 2009, 0909.0267.

[9]  Pasadena,et al.  HIGH ANGULAR RESOLUTION INTEGRAL-FIELD SPECTROSCOPY OF THE GALAXY'S NUCLEAR CLUSTER: A MISSING STELLAR CUSP? , 2009, 0908.0311.

[10]  H. Bushouse,et al.  SIMULTANEOUS MULTI-WAVELENGTH OBSERVATIONS OF Sgr A* DURING 2007 APRIL 1–11 , 2009, 0907.3786.

[11]  T. Paumard,et al.  EVIDENCE FOR X-RAY SYNCHROTRON EMISSION FROM SIMULTANEOUS MID-INFRARED TO X-RAY OBSERVATIONS OF A STRONG Sgr A* FLARE , 2009, 0903.3416.

[12]  A. Eckart,et al.  A POWER-LAW BREAK IN THE NEAR-INFRARED POWER SPECTRUM OF THE GALACTIC CENTER BLACK HOLE , 2009, 0902.2475.

[13]  R. Genzel,et al.  MONITORING STELLAR ORBITS AROUND THE MASSIVE BLACK HOLE IN THE GALACTIC CENTER , 2008, 0810.4674.

[14]  Jessica R. Lu,et al.  A NEAR-INFRARED VARIABILITY STUDY OF THE GALACTIC BLACK HOLE: A RED NOISE SOURCE WITH NO DETECTED PERIODICITY , 2008, 0810.0446.

[15]  K. Mužić,et al.  UvA-DARE ( Digital Academic Repository ) Simultaneous NIR / sub-mm observation of flare emission from Sagittarius A * , 2008 .

[16]  A. Eckart,et al.  A 600 Minute Near-Infrared Light Curve of Sagittarius A* , 2008, 0809.2580.

[17]  Jessica R. Lu,et al.  Measuring Distance and Properties of the Milky Way’s Central Supermassive Black Hole with Stellar Orbits , 2008, 0808.2870.

[18]  A. Eckart,et al.  UvA-DARE ( Digital Academic Repository ) Polarized NIR and X-ray flares from Sagittarius A * , 2008 .

[19]  W. Duschl,et al.  On the orientation of the Sagittarius A* system (Research Note) , 2007, 0708.2637.

[20]  Aya Kubota,et al.  Modelling the behaviour of accretion flows in X-ray binaries , 2007, 0708.0148.

[21]  Jessica R. Lu,et al.  A Constant Spectral Index for Sagittarius A* during Infrared/X-Ray Intensity Variations , 2007, 0706.1782.

[22]  Geoffrey C. Bower,et al.  The possibility of detecting Sagittarius A* at 8.6$\,\mu$m from sensitive imaging of the Galactic center , 2007 .

[23]  P. Uttley,et al.  Active galactic nuclei as scaled-up Galactic black holes , 2006, Nature.

[24]  T. Paumard,et al.  A polarized infrared flare from Sagittarius A* and the signatures of orbiting plasma hotspots , 2006, astro-ph/0611737.

[25]  C. Fryer,et al.  MHD SIMULATIONS OF ACCRETION ONTO Sgr A*: QUIESCENT FLUCTUATIONS, OUTBURSTS, AND QUASIPERIODICITY , 2006, astro-ph/0611269.

[26]  K-band polarimetry of an Sgr A* flare with a clear sub-flare structure , 2006, astro-ph/0610147.

[27]  S. Trippe,et al.  Polarimetry of near-infrared flares from Sagittarius A* , 2006, astro-ph/0610103.

[28]  T. Paumard,et al.  Variations in the Spectral Slope of Sagittarius A* during a Near-Infrared Flare , 2005, astro-ph/0511302.

[29]  S. Shapiro,et al.  A Multiwavelength Study of Sgr A*: The Role of Near-IR Flares in Production of X-Ray, Soft γ-Ray, and Submillimeter Emission , 2005, astro-ph/0510787.

[30]  Norbert Hubin,et al.  SINFONI in the Galactic Center: Young Stars and Infrared Flares in the Central Light-Month , 2005 .

[31]  University of Leicester,et al.  Non‐linear X‐ray variability in X‐ray binaries and active galaxies , 2005 .

[32]  E. Quataert,et al.  Synchrotron Radiation from Radiatively Inefficient Accretion Flow Simulations: Applications to Sagittarius A* , 2004, astro-ph/0411627.

[33]  M. F. Radioastronomie,et al.  The Proper Motion of Sagittarius A*. II. The Mass of Sagittarius A* , 2004, astro-ph/0408107.

[34]  A. Eckart,et al.  First simultaneous NIR/X-ray detection of a flare from Sgr A , 2004, astro-ph/0403577.

[35]  Caltech,et al.  Variable Infrared Emission from the Supermassive Black Hole at the Center of the Milky Way , 2003, astro-ph/0309076.

[36]  H. Falcke,et al.  A scheme to unify low-power accreting black holes Jet-dominated accretion flows and the radio/X-ray correlation , 2003, astro-ph/0305335.

[37]  A. Loeb.  Direct feeding of the black hole at the Galactic Centre with radial gas streams from close‐in stellar winds , 2003, astro-ph/0311512.

[38]  D. Rouan,et al.  Near-infrared flares from accreting gas around the supermassive black hole at the Galactic Centre , 2003, Nature.

[39]  Ramesh Narayan,et al.  Nonthermal Electrons in Radiatively Inefficient Accretion Flow Models of Sagittarius A* , 2003, astro-ph/0304125.

[40]  E. Quataert.  Radiatively Inefficient Accretion Flow Models of Sgr A , 2003, astro-ph/0304099.

[41]  Anne-Marie Lagrange,et al.  NAOS-CONICA first on sky results in a variety of observing modes , 2003, SPIE Astronomical Telescopes + Instrumentation.

[42]  David Mouillet,et al.  NAOS, the first AO system of the VLT: on-sky performance , 2003, SPIE Astronomical Telescopes + Instrumentation.

[43]  K. Menten,et al.  A star in a 15.2-year orbit around the supermassive black hole at the centre of the Milky Way , 2002, Nature.

[44]  P. Wizinowich,et al.  Limits on the Short-Term Variability of Sagittarius A* in the Near-Infrared , 2002, astro-ph/0208239.

[45]  W. N. Brandt,et al.  Rapid X-ray flaring from the direction of the supermassive black hole at the Galactic Centre , 2001, Nature.

[46]  Laird M. Close,et al.  Analysis of isoplanatic high resolution stellar fields by the StarFinder code , 2000 .

[47]  A. Cox,et al.  Allen's astrophysical quantities , 2000 .

[48]  R. Narayan,et al.  Hybrid Thermal-Nonthermal Synchrotron Emission from Hot Accretion Flows , 2000, astro-ph/0004195.

[49]  John Scott Drilling,et al.  in Allen''''s Astrophysical Quantities , 2000 .

[50]  Y. Lyubarskii,et al.  Flicker noise in accretion discs , 1997 .

[51]  R. L. Brown,et al.  Intense sub-arcsecond structure in the galactic center , 1974 .