We have obtained the first detection of spectral absorption lines in one of the high-velocity stars in the vicinity of the Galaxy's central supermassive black hole. Both Brg (2.1661 mm) and He i (2.1126 mm) are seen in absorption in S0-2 with equivalent widths ( and Å ) and an inferred stellar rotational velocity 2.8 ‫ע‬ 0.3 1.7 ‫ע‬ 0.4 ( k ms Ϫ1 ) that are consistent with that of an O8-B0 dwarf, which suggests that it is a massive 220 ‫ע‬ 40 (∼15 M , ) young (less than 10 Myr) main-sequence star. This presents a major challenge to star formation theories, given the strong tidal forces that prevail over all distances reached by S0-2 in its current orbit (130-1900 AU) and the difficulty in migrating this star inward during its lifetime from farther out where tidal forces should no longer preclude star formation. The radial velocity measurements ( km s Ϫ1 ) and our reported Av S p Ϫ510 ‫ע‬ 40 z proper motions for S0-2 strongly constrain its orbit, providing a direct measure of the black hole mass of . The Keplerian orbit parameters have uncertainties that are reduced by a factorof 2-3 compared to previously reported values and include, for the first time, an independent solution for the dynamical center; this location, while consistent with the nominal infrared position of Sgr A*, is localized to a factor of 5 more precisely ‫2ע(‬ mas). Furthermore, the ambiguity in the inclination of the orbit is resolved with the addition of the radial velocity measurement, indicating that the star is behind the black hole at the time of closest approach and counterrevolving against the Galaxy. With further radial velocity measurements in the next few years, the orbit of S0-2 will provide the most robust estimate of the distance to the Galactic center.

Satellite galaxies of the Milky Way are among the most promising targets for dark matter searches in gamma rays. We present a search for dark matter consisting of weakly interacting massive particles, applying a joint likelihood analysis to 10 satellite galaxies with 24 months of data of the Fermi Large Area Telescope. No dark matter signal is detected. Including the uncertainty in the dark matter distribution, robust upper limits are placed on dark matter annihilation cross sections. The 95% confidence level upper limits range from about 10(-26)  cm3  s(-1) at 5 GeV to about 5×10(-23)   cm3  s(-1) at 1 TeV, depending on the dark matter annihilation final state. For the first time, using gamma rays, we are able to rule out models with the most generic cross section (∼3×10(-26)  cm3  s(-1) for a purely s-wave cross section), without assuming additional boost factors.

No abstract

We report the detection of a variable point source, imaged at L'(3.8 µm) with the Keck II 10 m telescope's adaptive optics system, that is coincident to within 18 mas (1 σ) of the Galaxy's central supermassive black hole and the unique radio source Sgr A*. While in 2002 this source (SgrA*-IR) was confused with the stellar source S0-2, in 2003 these two sources are separated by 87 mas allowing the new source's properties to be determined directly. On four separate nights, its observed L' magnitude ranges from 12.2 to 13.8, which corresponds to a dereddened flux density of 4 -17 mJy; no other source in this region shows such large variations in flux density -a factor of 4 over a week and a factor of 2 over 40 min. In addition, it has a K-L' color greater than 2.1, which is at least 1 mag redder than any other source detected at L' in its vicinity. Based on this source's coincidence with the Galaxy's dynamical center, its lack of motion, its variability, and its red color, we conclude that it is associated with the central supermassive black hole. The short timescale for the 3.8 µm flux density variations implies that the emission arises quite close to the black hole, within 5 AU, or 80 R s . We suggest that both the variable 3.8 µm emission and the X-ray flares arise from the same underlying physical process, possibly the acceleration of a small populations of electrons to ultrarelativistic energies. In contrast to the X-ray flares which are only detectable ∼2% of the time, the 3.8 µm emission provides a new, constantly accessible, window into the physical conditions of the plasma in close proximity to the central black hole.

No abstract