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Galactic Center

Since 1992 we have observed the central parsec of the galactic centerin K-Band. We mainly use our Speckle-CameraSHARPI on the NTT located at La Silla, Chile, and NAOS/CONICA at the VLT on Paranal, which are operated by the EuropeanSouthern Observatory.

• whether there is a massive black hole
• when and where the stars were born
• the nature of these stars
• the dynamics in this region

So far, we have shown that at least two periods of star formation have occured. There are three sorts of stars in the galactic center:

• Stars with He I - emission
• Stars which show CO - absorption
• Embedded stars with featureless spectra

The supermassive black hole in the centre of the Milky Way wasdiscovered as a bright non-thermal radio source in the 1970s andtermed Sagittarius A* (Sgr A*). Potential X-ray radiation by Sgr A*was detected with the X-ray observatory Rosat in the 1990s. A reliableidentification of X-rays from Sgr A* was finally possible with the newX-ray satellites Chandra and XMM, with their high spatial resolutionand sensitivity. The radio emission of SgrA* only varies slowly ontime scales of several days to a few hundred days and generally withan amplitude <10%. However, in the X-ray regime, SgrA* was found toexhibit two different states. On the one hand, in the quiescent state,weak X-ray emission appears to come from a slightly extended areaaround the black hole that appears to be evidence of hot accreting gasin the environment of SgrA*. On the other hand, SgrA* shows X-rayflares with a period of about one per day. During these flares, theemission rises by factors up to 100 during several tens of minutes anda distinctive point source becomes visible at the location of SgrA*.The short rise-and-decay times of the flares suggest that theradiation must origin from a region within less than 10 Schwarzschildradii of a 3.6 million solar mass black hole.

Near-infrared high-resolution observations of the galactic centre (GC)became possible since the beginning of the 1990s. Since then, the GCstellar was regularly monitored by high-resolution NIRimaging. However, in spite of all efforts, no unambiguous NIRcounterpart of SgrA* could be detected up to 2003. On the 9th of May,during routine observations of the GC star cluster at 1.7 microns withthe CONICA/NAOS adaptive optics imager/near infrared camera at the ESOVLT, we witnessed a powerful flare at the location of the black hole.Within a few minutes, the flux of a faint source increased by a factorof 5-6 and fainted again after about 30 min. The flare was found tohave happened within a few milli-arcseconds of the position of SgrA*. The short rise-and-decay times told us that the source of theflare was located within less than 10 Schwarzschild radii of the blackhole.

Fig 1:Detection of NIR emission from Sgr A*. The images show raw AOimages (60 s total exposure time) of an area 1''x1''around Sgr A*, observed on May 9, 2003 UT. The left image was takenat the beginning of the observations, the right image about 40 minlater. The flaring source is easily detected in the right image. Itsposition is offset -1.4±3.0 mas in R.A. and -0.2±3.0 inDec. from the dynamical position of Sgr A* as it was determinedfrom the orbit of the star S2. The star S2 is marked by a cross, theposition of Sgr A* is indicated by a white circle.

During subsequent observations in 2003, we could observe more flaresfrom Sgr A* and also quiescent emission from a source at thislocation. With hindsight, we could also detect a flaring source inolder, longer wavelength data from 2002. At the moment, we haveobserved a total of four flares in the H, K and L-bands (1.7, 2.2 and3.8 microns). The flares were observed at four different epochs withina few milli-arcseconds of the location of SgrA*, which makes it highlyprobable that they are indeed associated with matter in the immediateenvironment of the black hole, which is also reflected in the veryshort rise-and-fall time scales of the light curves. Independently,flaring and variability of SgrA* in the L-band was also observed atthe Keck telescope by researchers from the University of California,LA, in June 2003.

Fig 2:Light curves of the Sgr A* NIR flares in 2002 and 2003, observed withNACO/VLT. The L'-band flare on August 30, 2002, was only partiallycovered by observations. Gaps in the time series of the H-band flareon May 10, 2003, and of the K_S-band flare on June 15, 2003, aredue to sky observations and instrument failure, respectively. Forcomparison, the emission of the steady emission of the star S1 nearSgr A* is shown in all the plots (light grey data points). Arrows inthe plots of the two K_S-band flares indicate substructure peaksof the flares. Both K_S-band flares show very similarquasi-periodicity, although the second flare was observed more than24 h after the first one and must thus have been an unrelatedevent. The upper right panel shows the normalised power spectrum ofthe two K_S-band flares. Both of them show a significant peak ata frequency corresponding to time scales of 16.8±2.0 min. Inboth cases, the power spectrum of S1 does not show such afrequency.

The quiescent and flaring NIR emission from SgrA* fills an importantgap in our knowledge of the spectrum of this source and will allow toconstrain the existing models on how the radiation is produced. Whilethe quiescent emission appears to be largely consistent with an originin the high-energy tail of a synchrotron spectrum, the mechanism ofthe NIR flares is uncertain. Although the NIR flares were observed atdifferent epochs, they might hint at a blue colour of the flares whichwould be a challenge to current theories. Simultaneous,Multi-wavelength NIR and X-ray observations of the GC are planned forthe next year. The chances are high that these observations willprovide the required data to constrain the models and to establish (orexclude) a relation between the X-ray and NIR variability.

Fig 3:Spectral energy distribution of theemission from Sgr A*. This plot shows the extinction and absorptioncorrected luminosities.All error bars are ±1 sigma and include statistical andsystematic errors. Black triangles denote the radio spectrum ofSgr A*. Open grey circles mark variousinfrared upper limits from the literature. The threeX-ray data ranges are (from bottom to top) the quiescent state asdetermined with Chandra (black; Baganoff et al., 2003), the autumn2000 Chandra flare (red; Baganoff et al., 2000), and the autumn 2002flare observed by XMM (light blue; Porquet et al., 2003). Open redsquares with crosses mark the de-reddened peak emission (minusquiescent emission) of the four NIR flares. Open blue circles mark the de-reddened H,K_S, and L' luminosities of the quiescent state,derived from the local background subtracted flux density of thepoint source at the position at Sgr A*, thus eliminating thecontribution from extended, diffuse light due to the stellar cusparound Sgr A*.

The two K-band flares observed on the 15th and 16th of June 2003 arethe flares that were completely covered by observations. Although theyhappened more than 24 hours apart and thus appear to be unrelatedevents, they both show a striking quasi-periodicity of the flare with aperiod of about 17 min. Of all possible periodic processes near ablack hole (acoustic modes of a thin disk, Lense-Thirring precession,precession of orbital nodes, orbital motion), the period of mattercircling the black hole near the last stable orbit is the shortestone. The observed period of 17 min is so short, however, that the onlyreasonable explanation is that the oscillations are produced byDoppler boosting of hot gas near the last stable orbit of a spinning(Kerr) black hole. The spin of the black hole will allow for a laststable orbit closer to the event horizon and thus with a shorterorbital frequency. From the observed 17 min period we estimate thatthe supermassive black hole Sgr A* has a spin that is half as big asthe maximum possible spin of such an object.

Additional observations of flares and their quasi-periodicity will beneeded in order to confirm this result. Should the quasi-periodicityindeed be an intrinsic feature of the flares then this will mean thatthe era of black hole physics has begun with the properties of blackholes accessible to direct measurements!

© Infrared and Submillimeter Astronomy Group at MPE