We report the definite spectroscopic identification of ≃ 40 OB supergiants, giants and main sequence stars in the central parsec of the Galaxy. Detection of their absorption lines have become possible with the high spatial and spectral resolution and sensitivity of the adaptive optics integral field spectrometer SPIFFI/SINFONI on the ESO VLT. Several of these OB stars appear to be helium and nitrogen rich. Almost all of the ≃ 80 massive stars now known in the central parsec (central arcsecond excluded) reside in one of two somewhat thick ( |h|/R ≃ 0.14) rotating disks. These stellar disks have fairly sharp inner edges (R ≃ 1 ′′ ) and surface density profiles that scale as R −2 . We do not detect any OB stars outside the central 0.5 pc. The majority of the stars in the clockwise system appear to be on almost circular orbits, whereas most of those in the 'counter-clockwise' disk appear to be on eccentric orbits. Based on its stellar surface density distribution and dynamics we propose that IRS 13E is an extremely dense cluster (ρ core 3 × 10 8 M ⊙ pc −3 ), which has formed in the counter-clockwise disk. The stellar contents of both systems are remarkably similar, indicating a common age of ≃ 6 ± 2 Myr. The K-band luminosity function of the massive stars suggests a top-heavy mass function and limits the total stellar mass contained in both disks to ≃ 1.5 × 10 4 M ⊙ . Our data strongly favor in situ star formation from dense gas accretion disks for the two stellar disks. This conclusion is very clear for the clockwise disk and highly plausible for the counter-clockwise system.

Ultraluminous X-ray sources (ULX) are off-nuclear point sources in nearby galaxies whose X-ray luminosity exceeds the theoretical maximum for spherical

Magnetars are young and highly magnetized neutron stars which display a wide array of X-ray activity including short bursts, large outbursts, giant flares and quasi-periodic oscillations, often coupled with interesting timing behavior including enhanced spin-down, glitches and anti-glitches. The bulk of this activity is explained by the evolution and decay of an ultrastrong magnetic field, stressing and breaking the neutron star crust, which in turn drives twists of the external magnetosphere and powerful magnetospheric currents. The population of detected magnetars has grown to about 30 objects and shows unambiguous phenomenological connection with very highly magnetized radio pulsars. Recent progress in magnetar theory includes explanation of the hard X-ray component in the magnetar spectrum and development of surface heating models, explaining the sources' remarkable radiative output.Comment: 40 pages, Annual Review of Astronomy and Astrophysics, in pres

We develop a theoretical model that explains the formation of hot coronae around strongly magnetized neutron stars -- magnetars. The starquakes of a magnetar shear its external magnetic field, which becomes non-potential and is threaded by an electric current. Once twisted, the magnetosphere cannot untwist immediately because of its self-induction. The self-induction electric field lifts particles from the stellar surface, accelerates them, and initiates avalanches of pair creation in the magnetosphere. The created plasma corona maintains the electric current demanded by curl(B) and regulates the self-induction e.m.f. by screening. This corona persists in dynamic equilibrium: it is continually lost to the stellar surface on the light-crossing time of 10^{-4} s and replenished with new particles. In essence, the twisted magnetosphere acts as an accelerator that converts the toroidal field energy to particle kinetic energy. Using a direct numerical experiment, we show that the corona self-organizes quickly (on a millisecond timescale) into a quasi-steady state, with voltage ~1 GeV along the magnetic lines. The heating rate of the corona is ~10^{36} erg/s, in agreement with the observed persistent, high-energy output of magnetars. We deduce that a static twist that is suddenly implanted into the magnetosphere will decay on a timescale of 1-10 yrs. The particles accelerated in the corona impact the solid crust, knock out protons, and regulate the column density of the hydrostatic atmosphere of the star. The transition layer between the atmosphere and the corona is the likely source of the observed 100-keV emission from magnetars. The corona emits curvature radiation and can supply the observed IR-optical luminosity. (Abridged)Comment: 70 pages, 14 figures, accepted to Ap

Observations of the galactic center revealed a population of young massive stars within 0.4 pc from Sgr A * -the presumed location of a supermassive black hole. The origin of these stars is a puzzle as their formation in citu should be suppressed by the black hole's tidal field. We find that out of 13 stars whose 3-dimensional velocities have been measured by Genzel et. al. (2000), 10 lie in a thin disk. The half-opening angle of the disk is consistent with zero within the measurement errors, and does not exceed 10 degrees. We propose that a recent burst of star formation has occurred in a dense gaseous disk around Sgr A * . Such a disk is no longer present because, most likely, it has been accreted by the central black hole.The three-dimensional orbit of S2, the young star closest to Sgr A * , has been recently mapped out with high precision. It is inclined to the stellar disk by 75 degrees. We find that the orbit should undergo Lense-Thirring precession with the period of ∼ (5/a) × 10 6 years, where a < 1 is the dimensionless spin of the black hole. Therefore it is possible that originally S2 orbit lay in the disk plane. If so, we can constrain the black hole spin a be greater than 0.2 × (t S2 /5 × 10 6 yr), where t S2 is the age of S2.

Magnetospheres of neutron stars are anchored in the rigid crust and can be twisted by sudden crustal motions ("starquakes"). The twisted magnetosphere does not remain static and gradually untwists, dissipating magnetic energy and producing radiation. The equation describing this evolution is derived, and its solutions are presented. Two distinct regions coexist in untwisting magnetospheres: a potential region where ∇ × B = 0 ("cavity") and a current-carrying bundle of field lines with ∇ × B = 0 ("j-bundle"). The cavity has a sharp boundary, which expands with time and eventually erases all of the twist. In this process, the electric current of the j-bundle is sucked into the star. Observational appearance of the untwisting process is discussed. A hot spot forms at the footprints of the j-bundle. The spot shrinks with time toward the magnetic dipole axis, and its luminosity and temperature gradually decrease. As the j-bundle shrinks, the amplitude of its twist ψ can grow to the maximum possible value ψ max ∼ 1. The strong twist near the dipole axis increases the spindown rate of the star and can generate a broad beam of radio emission. The model explains the puzzling behavior of magnetar XTE J1810-197 -a canonical example of magnetospheric evolution following a starquake. We also discuss implications for other magnetars. The untwisting theory suggests that the nonthermal radiation of magnetars is preferentially generated on a bundle of extended closed field lines near the dipole axis.

Nuclear and Coulomb collisions in gamma‐ray burst (GRB) jets create a hot e± plasma. This collisional heating starts when the jet is still opaque, and extends to the transparent region. The e± plasma radiates its energy. As a result, a large fraction of the jet energy is converted to escaping radiation with a well‐defined spectrum. The process is simulated in detail using the known rates of collisions and accurate calculations of radiative transfer in the expanding jet. The result reproduces the spectra of observed GRBs that typically peak near 1 MeV and extend to much higher energies with a photon index β∼−2.5. This suggests that collisional heating may be the main mechanism for GRB emission.

We calculate the structure of accretion disks around Kerr black holes for accretion ratesṀ = 0.001−10M ⊙ s −1 . Such high-Ṁ disks are plausible candidates for the central engine of gamma-ray bursts. Our disk model is fully relativistic and treats accurately microphysics of the accreting matter: neutrino emissivity, opacity, electron degeneracy, and nuclear composition. The neutrino-cooled disk forms above a critical accretion rateṀ ign that depends on the black hole spin. The disk has the "ignition" radius r ign where neutrino flux rises dramatically, cooling becomes efficient, and the proton-to-nucleon ratio Y e drops. Other characteristic radii are r α where most of α-particles are disintegrated, r ν where the disk becomes ν-opaque, and r tr where neutrinos get trapped and advected into the black hole. We find r α , r ign , r ν , r tr and show their dependence onṀ . We discuss the qualitative picture of accretion and present sample numerical models of the disk structure. All neutrino-cooled disks regulate themselves to a characteristic state such that: (1) electrons are mildly degenerate, (2) Y e ∼ 0.1, and (3) neutrons dominate the pressure in the disk.