scholarly journals Can we see pulsars around Sgr A⋆? The latest searches with the Effelsberg telescope.

2012 ◽  
Vol 8 (S291) ◽  
pp. 382-384
Author(s):  
R. P. Eatough ◽  
M. Kramer ◽  
B. Klein ◽  
R. Karuppusamy ◽  
D. J. Champion ◽  
...  
Keyword(s):  
Binary Systems ◽  
Compact Objects ◽  
Galactic Centre ◽  
Radio Waves ◽  
Test Bed ◽  
Radio Pulsars ◽  
Massive Black Hole ◽  
Centre Region ◽  
Sgr A ◽  
Temporal Broadening

AbstractRadio pulsars in relativistic binary systems are unique tools to study the curved space-time around massive compact objects. The discovery of a pulsar closely orbiting the super-massive black hole at the centre of our Galaxy, Sgr A⋆, would provide a superb test-bed for gravitational physics. To date, the absence of any radio pulsar discoveries within a few arc minutes of Sgr A⋆ has been explained by one principal factor: extreme scattering of radio waves caused by inhomogeneities in the ionized component of the interstellar medium in the central 100 pc around Sgr A⋆. Scattering, which causes temporal broadening of pulses, can only be mitigated by observing at higher frequencies. Here we describe recent searches of the Galactic centre region performed at a frequency of 18.95 GHz with the Effelsberg radio telescope.

scholarly journals Detection of the Schwarzschild precession in the orbit of the star S2 near the Galactic centre massive black hole

2020 ◽  
Vol 636 ◽  
pp. L5 ◽  
Author(s):  
◽  
R. Abuter ◽  
A. Amorim ◽  
M. Bauböck ◽  
J. P. Berger ◽  
...  
Keyword(s):  
Black Hole ◽  
Reference Frame ◽  
Adaptive Optics ◽  
Galactic Centre ◽  
Massive Black Hole ◽  
Central Mass ◽  
Black Hole Candidate ◽  
Beam Combiner ◽  
Direct Measurements ◽  
Sgr A

The star S2 orbiting the compact radio source Sgr A* is a precision probe of the gravitational field around the closest massive black hole (candidate). Over the last 2.7 decades we have monitored the star’s radial velocity and motion on the sky, mainly with the SINFONI and NACO adaptive optics (AO) instruments on the ESO VLT, and since 2017, with the four-telescope interferometric beam combiner instrument GRAVITY. In this Letter we report the first detection of the General Relativity (GR) Schwarzschild Precession (SP) in S2’s orbit. Owing to its highly elliptical orbit (e = 0.88), S2’s SP is mainly a kink between the pre-and post-pericentre directions of motion ≈±1 year around pericentre passage, relative to the corresponding Kepler orbit. The superb 2017−2019 astrometry of GRAVITY defines the pericentre passage and outgoing direction. The incoming direction is anchored by 118 NACO-AO measurements of S2’s position in the infrared reference frame, with an additional 75 direct measurements of the S2-Sgr A* separation during bright states (“flares”) of Sgr A*. Our 14-parameter model fits for the distance, central mass, the position and motion of the reference frame of the AO astrometry relative to the mass, the six parameters of the orbit, as well as a dimensionless parameter fSP for the SP (fSP = 0 for Newton and 1 for GR). From data up to the end of 2019 we robustly detect the SP of S2, δϕ ≈ 12′ per orbital period. From posterior fitting and MCMC Bayesian analysis with different weighting schemes and bootstrapping we find fSP = 1.10 ± 0.19. The S2 data are fully consistent with GR. Any extended mass inside S2’s orbit cannot exceed ≈0.1% of the central mass. Any compact third mass inside the central arcsecond must be less than about 1000 M⊙.

scholarly journals Possible Evidence for a Binary Massive Black Hole in the Galactic Nucleus

1996 ◽  
Vol 169 ◽  
pp. 285-286
Author(s):  
E.J.A. Meurs
Keyword(s):  
Black Hole ◽  
Emission Line ◽  
Velocity Component ◽  
Orbital Motion ◽  
Galactic Centre ◽  
Line Profiles ◽  
Radial Velocity Component ◽  
Massive Black Hole ◽  
Ir Stars ◽  
Sgr A

The Galactic Centre candidate Sgr A∗ may exhibit a 40 km/s radial velocity component, which is not observed for OH/IR stars around the centre. This could be interpreted as orbital motion of one member of a binary massive black hole. In other galaxies such pairs may be inferred from radio jet precession and emission line profiles.

scholarly journals Acceleration of particles up to PeV energies at the galactic centre

2016 ◽  
Vol 12 (S324) ◽  
pp. 317-321
Author(s):  
Stefano Gabici ◽  
Felix A. Aharonian ◽  
Emmanuel Moulin ◽  
Aion Viana
Keyword(s):  
Black Hole ◽  
High Energy ◽  
Galactic Cosmic Rays ◽  
Galactic Centre ◽  
Acceleration Of Particles ◽  
Supermassive Black Hole ◽  
Centre Region ◽  
Very High Energy ◽  
Sgr A ◽  
Very High

AbstractRecent very high energy observations of the galactic centre region performed by H.E.S.S. revealed the presence of a powerful PeVatron. This is the first of such objects detected, and its most plausible counterpart seems to be associated to Sgr A*, the supermassive black hole in the centre of our galaxy. The implications of this discovery will be discussed, in particular in the context of the problem of the origin of galactic cosmic rays.

scholarly journals X-Ray Observations of the Galactic Centre

1989 ◽  
Vol 136 ◽  
pp. 567-580 ◽  
Author(s):  
G. K. Skinner
Keyword(s):  
Energy Range ◽  
High Energy ◽  
Point Sources ◽  
Galactic Centre ◽  
X Rays ◽  
Diffuse Emission ◽  
Variable Source ◽  
X Ray ◽  
Centre Region ◽  
Sgr A

Observations of the galactic centre region in the photon energy range 2–500 keV are reviewed. Point sources, transients, bursts and a patch of apparently diffuse emission ~1° in extent have all been observed. The relatively detailed information obtained with the Einstein observatory just above the bottom edge of the x-ray window is starting to be supplemented by observations at higher photon energies. Although there is known to be a strong, variable, source of high energy x-rays somewhere in the region there is little reason to associate it with Sgr A West, which is detectable, but relatively weak, in the energy range below 30 keV where detailed measurements have been possible.

scholarly journals Simulating the pericentre passage of the Galactic centre star S2

2018 ◽  
Vol 616 ◽  
pp. L8 ◽  
Author(s):  
M. Schartmann ◽  
A. Burkert ◽  
A. Ballone
Keyword(s):  
Power Law ◽  
Stellar Wind ◽  
Adaptive Mesh Refinement ◽  
Mass Loss Rate ◽  
Galactic Centre ◽  
Massive Black Hole ◽  
X Ray ◽  
Flow Models ◽  
Accretion Flow ◽  
Sgr A

Context. Our knowledge of the density distribution of the accretion flow around Sgr A* – the massive black hole (BH) at our Galactic centre (GC) – relies on two measurements only: one at a distance of a few Schwarzschild radii (Rs) and one at roughly 105 Rs, which are usually bridged by a power law, which is backed by magnetohydrodynamical simulations. The so-called S2 star reached its closest approach to the massive BH at around 1500 Rs in May 2018. It has been proposed that the interaction of its stellar wind with the high-density accretion flow at this distance from Sgr A* will lead to a detectable, month-long X-ray flare. Aims. Our goal is to verify whether or not the S2 star wind can be used as a diagnostic tool to infer the properties of the accretion flow towards Sgr A* at its pericentre (an unprobed distance regime), putting important constraints on BH accretion flow models. Methods. We run a series of three-dimensional adaptive mesh refinement simulations with the help of the RAMSES code which include the realistic treatment of the interaction of S2’s stellar wind with the accretion flow along its orbit and – apart from hydrodynamical and thermodynamical effects – include the tidal interaction with the massive BH. These are post-processed to derive the X-ray emission in the observable 2–10 keV window. Results. No significant excess of X-ray emission from Sgr A* is found for typical accretion flow models. A measurable excess is produced for a significantly increased density of the accretion flow. This can, however, be ruled out for standard power-law accretion flow models as in this case the thermal X-ray emission without the S2 wind interaction would already exceed the observed quiescent luminosity. Only a significant change of the wind parameters (increased mass loss rate and decreased wind velocity) might lead to an (marginally) observable X-ray flaring event. Conclusion. Even the detection of an (month-long) X-ray flare during the pericentre passage of the S2 star would not allow for strict constraints to be put on the accretion flow around Sgr A* due to the degeneracy caused by the dependence on multiple parameters (of the accretion flow model as well as the stellar wind).

The Velocities of 6cm Recombination Line Emission originating near the Galactic Nucleus

1977 ◽  
Vol 3 (2) ◽  
pp. 150-152 ◽  
Author(s):  
F. F. Gardner ◽  
J. B. Whiteoak
Keyword(s):  
Radio Source ◽  
Strong Evidence ◽  
Line Emission ◽  
Hii Regions ◽  
Galactic Centre ◽  
Recombination Line ◽  
The Galaxy ◽  
Centre Region ◽  
Sgr A

Although it is well known that HII regions are present in the innermost regions of the Galaxy their kinematics are still not fully understood. In one study Pauls et al. (1976) surveyed with a beamwidth of 3′ arc the 10 GHz recombination line emission in directions within 15′ arc of the nuclear radio source Sgr A. They found that the emission velocities varied from position to position within the range -50 to + 50 km s-1but appeared to lack any overall pattern. In contrast, we have recently observed the recombination line emission from the galactic centre region with a beamwidth of 4′.5 arc, and find strong evidence of ordered motions near the galactic nucleus.

scholarly journals Radiation from Advection-Dominated Flows

1998 ◽  
Vol 188 ◽  
pp. 417-418
Author(s):  
T. Manmoto
Keyword(s):  
Black Holes ◽  
Compact Objects ◽  
Radiation Process ◽  
Massive Black Hole ◽  
Accretion Flow ◽  
Accretion Flows ◽  
Accretion Process ◽  
Self Consistent ◽  
Consistent Solution ◽  
Sgr A

Advection-dominated accretion flow (hereafter ADAF) is the only self-consistent solution to describe the optically thin accretion flows around compact objects. The main feature of ADAF is that the dynamics of the flow is dominated by accretion process rather than radiation process. As a result of advection domination, the luminosity of ADAFs is very low. Coupled with the existence of the event horizon, ADAF has been successfully applied to the dim accretion black holes such as central core of our Galaxy: Sgr A*. In this issue, we calculate the spectrum radiated from the optically thin ADAFs and show that the observed spectrum of Sgr A* is explained with the accretion massive black hole.

scholarly journals The distribution of stars around the Milky Way’s central black hole

2017 ◽  
Vol 609 ◽  
pp. A28 ◽  
Author(s):  
H. Baumgardt ◽  
P. Amaro-Seoane ◽  
R. Schödel
Keyword(s):  
Black Hole ◽  
Adaptive Optics ◽  
Near Infrared ◽  
Stellar Dynamics ◽  
Galactic Centre ◽  
Test Case ◽  
Central Black Hole ◽  
Massive Black Hole ◽  
Giant Stars ◽  
Sgr A

Context. The distribution of stars around a massive black hole (MBH) has been addressed in stellar dynamics for the last four decades by a number of authors. Because of its proximity, the centre of the Milky Way is the only observational test case where the stellar distribution can be accurately tested. Past observational work indicated that the brightest giants in the Galactic centre (GC) may show a density deficit around the central black hole, not a cusp-like distribution, while we theoretically expect the presence of a stellar cusp. Aims. We here present a solution to this long-standing problem. Methods. We performed direct-summation N-body simulations of star clusters around massive black holes and compared the results of our simulations with new observational data of the GC’s nuclear cluster. Results. We find that after a Hubble time, the distribution of bright stars as well as the diffuse light follow power-law distributions in projection with slopes of Γ ≈ 0.3 in our simulations. This is in excellent agreement with what is seen in star counts and in the distribution of the diffuse stellar light extracted from adaptive-optics (AO) assisted near-infrared observations of the GC. Conclusions. Our simulations also confirm that there exists a missing giant star population within a projected radius of a few arcsec around Sgr A*. Such a depletion of giant stars in the innermost 0.1 pc could be explained by a previously present gaseous disc and collisions, which means that a stellar cusp would also be present at the innermost radii, but in the form of degenerate compact cores.

scholarly journals The Galactic Centre at 408 MHz

1974 ◽  
Vol 60 ◽  
pp. 491-497 ◽  
Author(s):  
A. G. Little
Keyword(s):  
Brightness Temperature ◽  
Radio Telescope ◽  
The Other ◽  
Galactic Centre ◽  
Thermal Spectrum ◽  
Centre Region ◽  
Background Emission ◽  
The Cross ◽  
Sgr A ◽  
Galactic Background

The Molonglo radio telescope has been used to map the galactic centre region with a resolution of 2.9′. Although similar to microwave maps of comparable resolution, there are differences. In particular, the source G1.05-0.1 is not visible at microwaves, and the source G0.1-0.0 is not visible at 408 MHz. The flux values of all the other sources around Sgr A confirm that they are thermal and that the sources G0.7−0.0, G0.5 − 0.0, and G0.2−0.0 are optically thick.A more extensive galactic background survey with the cross shows the existence of a peak in the background emission with a brightness temperature of about 500 K at the galactic centre. This is probably the nonthermal source observed at 85 MHz. The higher-resolution observations of this map show the narrower, hotter region which appears on the microwave map and which surrounds Sgr A. This has a brightness temperature of 2100 K, which cannot be accounted for by a simple thermal spectrum extrapolated from microwaves.

scholarly journals Detection of the gravitational redshift in the orbit of the star S2 near the Galactic centre massive black hole

2018 ◽  
Vol 615 ◽  
pp. L15 ◽  
Author(s):  
◽  
R. Abuter ◽  
A. Amorim ◽  
N. Anugu ◽  
M. Bauböck ◽  
...  
Keyword(s):  
Black Hole ◽  
Adaptive Optics ◽  
Gravitational Redshift ◽  
Galactic Centre ◽  
Order Effects ◽  
Massive Black Hole ◽  
Black Hole Candidate ◽  
Very Large Telescope ◽  
General Relativistic ◽  
Sgr A

The highly elliptical, 16-year-period orbit of the star S2 around the massive black hole candidate Sgr A✻ is a sensitive probe of the gravitational field in the Galactic centre. Near pericentre at 120 AU ≈ 1400 Schwarzschild radii, the star has an orbital speed of ≈7650 km s−1, such that the first-order effects of Special and General Relativity have now become detectable with current capabilities. Over the past 26 years, we have monitored the radial velocity and motion on the sky of S2, mainly with the SINFONI and NACO adaptive optics instruments on the ESO Very Large Telescope, and since 2016 and leading up to the pericentre approach in May 2018, with the four-telescope interferometric beam-combiner instrument GRAVITY. From data up to and including pericentre, we robustly detect the combined gravitational redshift and relativistic transverse Doppler effect for S2 of z = Δλ / λ ≈ 200 km s−1/c with different statistical analysis methods. When parameterising the post-Newtonian contribution from these effects by a factor f , with f = 0 and f = 1 corresponding to the Newtonian and general relativistic limits, respectively, we find from posterior fitting with different weighting schemes f = 0.90 ± 0.09|stat ± 0.15|sys. The S2 data are inconsistent with pure Newtonian dynamics.

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