The discovery of hypervelocity binary stars (HVBs) in the Galactic halo would provide definite evidence of the existence of a massive black hole companion to Sgr A*. Here we use an hybrid approach to compute the rate of ejection and the total number of HVBs produced by a hypothetical intermediate-mass black hole (IMBH, M_2<10^5\msun) orbiting Sgr A*. Depending on the mass of M_2 and on the properties of binary stars in the central parsec of the Milky Way, we show that the number of undisrupted HVBs expected to be expelled from the Galactic Center before binary black hole coalescence ranges from zero to a few dozens at most. Therefore, the non-detection of stellar binaries in a complete survey of hypervelocity stars would not rule out the occurrence of an IMBH-Sgr A* in-spiralling event within the last few 10^8 years.
Hypervelocity stars (HVSs) are a natural consequence of the presence of a massive nuclear black hole (Sgr A*) in the Galactic Center. Here we use the Brown et al. sample of unbound and bound HVSs together with numerical simulations of the propagation of HVSs in the Milky Way halo to constrain three plausible ejection mechanisms: 1) the scattering of stars bound to Sgr A* by an inspiraling intermediate-mass black hole (IMBH); 2) the disruption of stellar binaries in the tidal field of Sgr A*; and 3) the two-body scattering of stars off a cluster of stellar-mass black holes orbiting Sgr A*. We c…
A summary of the current knowledge on hypervelocity stars (HVSs). HVSs are fascinating because their properties are linked to Sgr A* and the stellar environment of the Galactic Center. Observing the distribution of HVSs can address: 1) the nature of the black hole ejection mechanism, 2) the in-fall history of stars onto Sgr A*, 3) the types of stars orbiting Sgr A*, and 4) a unique measurement the shape of the Galaxy's dark matter potential. The challenge to observers is to find new HVSs and strengthen the connection between HVSs and the Galactic Center.
The hypervelocity star SDSS J090745.0+024507 in the halo of the Milky Way galaxy (Brown et al. 2005) most likely originated from the breakup of a binary star system by the central black hole, SgrA* (Hills 1988). We examine the fate of former binary companions to similar hypervelocity stars (HVSs) by simulating 600 different binary orbits around SgrA* with a direct N-body integration code. For some orbital parameters, the binary breakup process leads to HVSs with ejection velocities that are almost an order of magnitude larger than the velocity observed for SDSS J090745.0+024507. The former com…
Stars within 0.1 pc of the supermassive black hole Sgr A* at the Galactic centre are expected to encounter a cluster of stellar-mass black holes (BHs) that have segregated to that region. Some of these stars will scatter off an orbiting BH and be kicked out of the Galactic centre with velocities up to ~2000 km/s. We calculate the resulting ejection rate of hypervelocity stars (HVSs) by this process under a variety of assumptions, and find it to be comparable to the tidal disruption rate of binary stars by Sgr A*, first discussed by Hills (1988). Under some conditions, this novel process is suf…
Recent surveys have identified seven hypervelocity stars (HVSs) in the halo of the Milky Way. Most of these stars may have originated from the breakup of binary star systems by the nuclear black hole SgrA*. In some instances, the breakup of the binary may lead to a collision between its member stars. We examine the dynamical properties of these collisions by simulating thousands of different binary orbits around SgrA* with a direct N-body integration code. For some orbital parameters, the two stars collide with an impact velocity lower than their escape velocity and may therefore coalesce. It …
We study three processes that eject hypervelocity (>10^3 km/s) stars from the Galactic center: (i) close encounters of two single stars; (ii) tidal breakup of binary stars by the central black hole, as originally proposed by Hills; and (iii) three-body interactions between a star and a binary black hole (BBH). Mechanism (i) expels hypervelocity stars to the solar radius at a negligible rate, ~10^{-11}/yr. Mechanism (ii) expels hypervelocity stars at a rate ~ 10^{-5}(η/0.1)/yr, where ηis the fraction of stars in binaries with semimajor axis a_b<~0.3 AU. For solar-mass stars, the corresponding n…
Here we report on recent near-infrared observations of the Sgr A* counterpart associated with the super-massive ~ 4x10^6 M_sun black hole at the Galactic Center. We find that the May 2007 flare shows the highest sub-flare contrast observed until now, as well as evidence for variations in the profile of consecutive sub-flares. We modeled the flare profile variations according to the elongation and change of the shape of a spot due to differential rotation within the accretion disk.
We report the results of a mm-wave molecular line survey of the nearby (D ~ 70 pc), 12 Myr-old system V4046 Sgr -- a tight (9 R_sun separation), short-period (2.42 day) binary with nearly equal component masses of ~0.9 M_sun -- conducted with the 30 m telescope of the Institut de Radio Astronomie Millimetrique (IRAM). We detected rotational transitions of 12CO 13CO, HCN, CN, and HCO+. The double-peaked CO line profiles of V4046 Sgr are well fit by a model invoking a Keplerian disk with outer radius of ~250 AU that is viewed at an inclination i = 35 degrees. We infer minimum disk gas and dust m…
The star S0-2, which is orbiting Sgr A* with a 15-yr period, almost certainly did not form in situ. We propose that it was injected into this close orbit by the tidal disruption of a massive-star binary, whose primary was more massive than S0-2 and at least 60 Msun. From numerical integrations we find that 1-2% of incoming binaries with closest approach equal to 130 AU leave the secondary in an orbit with eccentricity within 0.01 of that of SO-2. If additional stars are found orbiting Sgr A* with relatively short periods, they could be used to probe the formation of massive stars in the Galact…
Hypervelocity stars (HVSs) discovered in the Milky Way (MW) halo are thought to be ejected from near the massive black hole (MBH) at the galactic centre. In this paper we investigate the spatial and velocity distributions of the HVSs which are expected to be similarly produced in the Andromeda galaxy (M31). We consider three different HVS production mechanisms: (i) the disruption of stellar binaries by the galactocentric MBH; (ii) the ejection of stars by an in-spiraling intermediate mass black hole; and (iii) the scattering of stars off a cluster of stellar-mass black holes orbiting around th…
Context: Recent near-infrared polarization measurements of SgrA* show that its emission is significantly polarized during flares and consists of a non- or weakly polarized main flare with highly polarized sub-flares. The flare activity suggests a quasi-periodicity of ~20 minutes in agreement with previous observations. Aims: By simultaneous fitting of the lightcurve fluctuations and the time-variable polarization angle, we address the question of whether these changes are consistent with a simple hot spot/ring model, in which the interplay of relativistic effects plays the major role, or wheth…
We present radio continuum observations of the spiral-shaped ionized feature (Sgr A West) within the inner pc of the Galactic center at three epochs spanning 1986 to 1995. The VLA A-configuration was used at $λ$2cm (resolution of 0\dasec1$\times$0\dasec2). We detect proper motions of a number of features in the Northern and Eastern Arms of Sgr A West including the ionized gas associated with IRS 13 with V(RA)= 113 \pm 10, V(Dec)=150 \pm15 km/s, IRS 2 with V(RA)= 122 \pm 11, V(Dec)=24 \pm 34 km/s and the Norther Arm V(RA)= 126 \pm 30, V(Dec)=--207 \pm 58 km/s. We also report the detection of fe…
Recent measurements, of the velocities of stars near the center of the Milky Way have provided the strongest evidence for the presence of a supermassive black hole in a galaxy, but the observational uncertainties poorly constrain many of the properties of the black hole. Determining the accelerations of stars in their orbits around the center provides much more precise information about the position and mass of the black hole. Here we report measurements of the accelerations for three stars located ~0.005 pc from the central radio source Sgr A*; these accelerations are comparable to those expe…
It is believed that young massive stars orbiting Sgr A* in two stellar discs on scales of 0.1-0.2 parsecs were formed either farther out in the Galaxy and then quickly migrated inward, or in situ in a massive self-gravitating disc. Comparing N-body evolution of stellar orbits with observational constraints, we set upper limits on the masses of the two stellar systems. These masses turn out to be few times lower than the expected total stellar mass estimated from the observed young high-mass stellar population and the standard galactic IMF. If these stars were formed in situ, in a massive self-…
The supermassive black hole at the Galactic center harbors a bound cluster of massive stars that should leave neutron-star remnants. Extrapolating from the available data, we estimate that 100-1000 radio pulsars may presently orbit Sgr A* with periods of <~100 yr. Optimistically, 1-10 of the most luminous of these pulsars may be detectable with current telescopes in periodicity searches at frequencies near 10 GHz, where the effects of interstellar scattering are alleviated. Long-term timing observations of such a pulsar would clearly reveal its Keplerian motion, and possibly show the effects o…
Measurements of stellar velocities and variable X-ray emission near the centre of the Milky Way have provided the strongest evidence so far that the dark mass concentrations seen in many galactic nuclei are likely supermassive black holes, but have not yet excluded several alternative configurations. Here we report ten years of high resolution astrometric imaging that allow us to trace two thirds of the orbit of the star currently closest to the compact radio source and massive black hole candidate SgrA*. In particular, we have observed both peri- and apocentre passages. Our observations show …
The recent identification of one or two sub-parsec disks of young, massive stars orbiting the ~4e6 solar mass black hole Sgr A* has prompted an "in-situ" scenario for star formation in disks of gas formed from a cloud captured from the Galactic center environment. To date there has been no explanation given for the low angular momentum of the disks relative to clouds passing close to the center. Here we show that the partial accretion of extended Galactic center clouds, such as the 50 km/s giant molecular cloud, that temporarily engulf Sgr A* during their passage through the central region of …
The results of photometric CCD monitoring and spectral observations of the black hole binary and microquasar V4641 Sgr in the quiet state are presented. The ellipsoidal light curve with large amplitude of 0.36^m in R band suggests the influence of a massive object orbiting around a normal B9 star. In the spectra taken with the 6-m telescope one hour before black hole inferior conjunction, an absorption component in the red wing of Halpha line is visible. It is formed by gaseous stream moving in the direction to the normal star. That suggests the grazing conjunction in this system. Maximum velo…
Sub milli-arcsecond astrometry and imaging of the black hole Sgr A* at the Galactic centre may become possible in the near future at infrared and sub-millimetre wavelengths. Motivated by observations of short-term infrared and X-ray variability of Sgr A*, in a previous paper we computed the expected images and light curves, including polarization, associated with an compact emission region orbiting the central black hole. We extend this work, using a more realistic hot-spot model and including the effects of opacity in the underlying accretion flow. We find that at infrared wavelengths the qua…
We develop a model of SGR in which a supernova leaves planets orbiting a neutron star in intersecting eccentric orbits. These planets will collide in $\sim 10^4$ years if their orbits are coplanar. Some fragments of debris lose their angular momentum in the collision and fall onto the neutron star, producing a SGR. The initial accretion of matter left by the collision with essentially no angular momentum may produce a superburst like that of March 5, 1979, while debris fragments which later lose their angular momentum produce an irregular pattern of smaller bursts.
We present deep CCD photometry of a field in the newly discovered dwarf spheroidal galaxy in Sagittarius (hereafter Sgr), and of a nearby control field. These data (totaling ~3.5 hrs exposure time in V and I) were used to produce color-magnitude (CM) diagrams reaching I ~ 22.3 in both fields. After statistically removing the field-stars from the CM diagram we find that Sgr is dominated by a moderately old (age ~ 10 Gyr) population, significantly younger than a typical globular-cluster population. There is some evidence for a weak intermediate-age component or alternatively a population of blue…
Holz & Wheeler (\cite{hw}) have recently proposed that a Schwarzschild black hole may act as a retro-lens which, if illuminated by a powerful light source, deflects light ray paths to large bending angles and a series of luminous arcs (or rings in the case of aligned objects) centered on the black hole may form. Obviously, the most convenient geometry to get retro-lensing images would be that of a very bright star close to a massive black hole, say the putative $\sim 4\times 10^6$ M$_{\odot}$ black hole at the galactic center. Recent observations of the galactic center region in the $K$-band h…
One of the main astrophysical processes leading to strong emission of gravitational waves to be detected by the future space-borne interferometer LISA is the capture of a compact star by a black hole with a mass of a few million solar masses in the center of a galaxy. In previous studies, main sequence stars were thought not to contribute because they suffer from early tidal disruption. Here we show that, according to our simulations of the stellar dynamics of the Sgr A* cluster, there must be one to a few low-mass main sequence stars sufficiently bound to the central Galactic black hole to be…
Multiwavelength monitoring observations of Sagittarius A* exhibit variability on timescales of minutes to hours, indicating emission regions localized near the event horizon. (Sub)Millimeter-wavelength VLBI is uniquely suited to probe the environment of the assumed black hole on these scales. We consider a range of orbiting hot-spot and accretion-disk models and find that periodicity in Sgr A* flares is detectable using closure quantities. Our methods are applicable to any model producing source structure changes near the black hole, including jets and magnetohydrodynamic disk instabilities, a…
The history of star formation in the strong gravitational potential of the Galactic center has been of much interest, recently. We propose that the sub-parsec-scale disk of massive stars orbiting the massive black hole at the Galactic center can be interpreted in terms of partial accretion of extended Galactic center clouds, such as the 50 \kms molecular cloud, as these clouds envelop Sgr A* on their passage through the inner Galactic center. The loss of angular momentum of the captured cloud material by self-interaction subsequent to gravitationally focusing by Sgr A* naturally creates a comp…
The polarized mm/sub-mm radiation from Sgr A* is apparently produced by a Keplerian structure whose peak emission occurs within several Schwarzschild radii (r_S=2GM/c^2) of the black hole. The Chandra X-ray counterpart, if confirmed, is presumably the self-Comptonized component from this region. In this paper, we suggest that sub-mm timing observations could yield a signal corresponding to the period P_0 of the marginally stable orbit, and therefore point directly to the black hole's spin a. Sgr A*'s mass is now known to be (2.6\pm 0.2)\times 10^6 M_\odot (an unusually accurate value for super…
A ring of dense molecular gas extending 2-7 pc orbits the supermassive black hole Sgr A* at the center of our Galaxy. Using the Green Bank Telescope, we detected water maser lines and both narrow (0.35 km/s) and broad (30 - 50 km/s) methanol emission from the molecular ring. Two of the strongest methanol lines at 44 GHz are confirmed as masers by interferometric observations. These class I methanol masers are collisionally excited and are signatures of early phases of massive star formation in the disk of the Galaxy, suggesting that star formation in the molecular ring is in its early phase. C…
The existence of a massive black hole in the center of the Milky Way, coinciding with the radio source Sgr A*, is being established on more and more solid ground. In principle, this black hole, acting as a gravitational lens, is able to bend the light emitted by stars moving within its neighborhood, eventually generating secondary images. Extending a previous analysis of the gravitational lensing phenomenology to a new set of 28 stars, whose orbits have been well determined by recent observations, we have calculated all the properties of their secondary images, including time and magnitude of …
Recent measurements of stellar orbits provide compelling evidence that the compact radio source Sagittarius A* at the Galactic Centre is a 3.6-million-solar-mass black hole. Sgr A* is remarkably faint in all wavebands other than the radio region, however, which challenges current theories of matter accretion and radiation surrounding black holes. The black hole's rotation rate is not known, and therefore neither is the structure of space-time around it.Here we report high-resolution infrared observations of Sgr A* that reveal 'quiescent' emission and several flares. The infrared emission origi…
We develop a formalism for studying the dynamics of massive black hole binaries embedded in gravitationally-bound stellar cusps, and study the binary orbital decay by three-body interactions, the impact of stellar slingshots on the density profile of the inner cusp, and the properties of the ejected hypervelocity stars (HVSs). We find that the scattering of bound stars shrinks the binary orbit and increases its eccentricity more effectively than that of unbound ambient stars. Binaries with initial eccentricities e>0.3 and/or unequal-mass companions (M_2/M_1<0.1) can decay by three-body interac…
