The Radio Source at the Galactic Nucleus

Davies, R. D.; Walsh, D.; Booth, R. S.

doi:10.1093/mnras/177.2.319

Cited by 111 publications

(61 citation statements)

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“…The energy spectrum of accelerated electrons has a power-law index of p = 2.4 in accordance with particle-in-cell simulations of Sadowski et al (2013). The red points are observationally determined values adopted from Davies et al (1976), Falcke & Markoff (2000), Zhao et al (2003), and Marrone et al (2008). The shaded region represents an estimate of the radio flux density of the PWN.…”

Section: Bow-shock Synchrotron Emissionsupporting

confidence: 67%

Nature of the Galactic centre NIR-excess sources

Britzen

Eckart

Shahzamanian

et al. 2017

A&A

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Context. The Dusty S-cluster Object (DSO/G2) orbiting the supermassive black hole (Sgr A*) in the Galactic centre has been monitored in both near-infrared continuum and line emission. There has been a dispute about the character and the compactness of the object: interpreting it as either a gas cloud or a dust-enshrouded star. A recent analysis of polarimetry data in K s -band (2.2 µm) allows us to put further constraints on the geometry of the DSO. Aims. The purpose of this paper is to constrain the nature and the geometry of the DSO. Methods. We compare 3D radiative transfer models of the DSO with the NIR continuum data including polarimetry. In the analysis, we use basic dust continuum radiative transfer theory implemented in the 3D Monte Carlo code Hyperion. Moreover, we implement analytical results of the two-body problem mechanics and the theory of non-thermal processes.Results. We present a composite model of the DSO -a dust-enshrouded star that consists of a stellar source, dusty, optically thick envelope, bipolar cavities, and a bow shock. This scheme can match the NIR total as well as polarized properties of the observed spectral energy distribution (SED). The SED may be also explained in theory by a young pulsar wind nebula that typically exhibits a large linear polarization degree due to magnetospheric synchrotron emission. Conclusions. The analysis of NIR polarimetry data combined with the radiative transfer modelling shows that the DSO is a peculiar source of compact nature in the S cluster (r 0.04 pc). It is most probably a young stellar object embedded in a non-spherical dusty envelope, whose components include optically thick dusty envelope, bipolar cavities, and a bow shock. Alternatively, the continuum emission could be of a non-thermal origin due to the presence of a young neutron star and its wind nebula. Although there has been so far no detection of X-ray and radio counterparts of the DSO, the analysis of the neutron star model shows that young, energetic neutron stars similar to the Crab pulsar could in principle be detected in the S cluster with current NIR facilities and they appear as apparent reddened, near-infrared-excess sources. The searches for pulsars in the NIR bands can thus complement standard radio searches, which can put further constraints on the unexplored pulsar population in the Galactic centre. Both thermal and non-thermal models are in accordance with the observed compactness, total as well polarized continuum emission of the DSO.

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Section: Bow-shock Synchrotron Emissionsupporting

confidence: 67%

Nature of the Galactic centre NIR-excess sources

Britzen

Eckart

Shahzamanian

et al. 2017

A&A

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“…Multiwavelength observations of Sgr A Ã have seen the À2 size dependence expected from strong scattering by variations in the electron density along the propagation path (e.g., Davies et al 1976). Lazio & Cordes (1998b) used the angular broadening of sources seen through the GC region to find a region of ''hyperstrong'' scattering in the central 1 Y2 of the GC.…”

Section: Hyperstrong Scattering Affecting Flux Measurements?mentioning

confidence: 99%

A Wide‐Area VLA Continuum Survey near the Galactic Center at 6 and 20 cm Wavelengths

Law

Yusef‐Zadeh

Cotton

2008

ASTROPHYS J SUPPL S

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We describe the results of a millijansky sensitivity VLA survey of roughly 1 deg 2 near the Galactic center at 6 and 20 cm. Catalogs of compact and filamentary structures are given and compared to previous surveys of the region. Eight of the unusual nonthermal radio filaments are detected in 6 cm polarized emission; three of these are the first such detections, confirming their nonthermal nature. This survey found emission from a filament at (l; b) ¼ (359:1; 0:75), or a projected distance from Sgr A Ã of 200 pc, greatly extending the latitude range observed with such features. There is also new evidence for spatial gradients in the 6/20 cm spectral indices of some filaments, and we discuss models for these gradients. In studying compact sources, the combination of spectral index and polarization information allows us to identify pulsar candidates and compact H ii regions in the survey. There is also some evidence that the flux measurements of compact sources may be affected by electron scattering from the interstellar medium in the central few hundred parsecs of the Galaxy.

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“…Since the broadening of the source structure by scattering follows a λ 2 law (Davies et al 1976; van Langevelde et al 1992;Lo et al 1998;Bower et al 2004) the actual source size φ Sgr A * is given by…”

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confidence: 99%

Jet-lag in Sagittarius A*: what size and timing measurements tell us about the central black hole in the Milky Way

Falcke

Markoff

Bower

2009

A&A

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Context. The black hole at the Galactic Center, Sgr A*, is the prototype of a galactic nucleus at a very low level of activity. Its radio through submm-wave emission is known to come from a region close to the event horizon, however, the source of the emission is still under debate. A successful theory explaining the emission is based on a relativistic jet model scaled down from powerful quasars. Aims. We want to test the predictive power of this established jet model against newly available measurements of wavelengthdependent time lags and the size-wavelength structure in Sgr A*. Methods. Using all available closure amplitude VLBI data from different groups, we again derived the intrinsic wavelength-dependent size of Sgr A*. This allowed us to calculate the expected frequency-dependent time lags of radio flares, assuming a range of in-and outflow velocities. Moreover, we calculated the time lags expected in the previously published pressure-driven jet model. The predicted lags are then compared to radio monitoring observations at 22, 43, and 350 GHz. Results. The combination of time lags and size measurements imply a mildly relativistic outflow with bulk outflow speeds of γβ 0.5−2. The newly measured time lags are reproduced well by the jet model without any major fine tuning. Conclusions. The results further strengthen the case for the cm-to-mm wave radio emission in Sgr A* as coming from a mildly relativistic jet-like outflow. The combination of radio time lag and VLBI closure amplitude measurements is a powerful new tool for assessing the flow speed and direction in Sgr A*. Future VLBI and time lag measurements over a range of wavelengths will reveal more information about Sgr A*, such as the existence of a jet nozzle, and measure the detailed velocity structure of a relativistic jet near its launching point for the first time.

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The Radio Source at the Galactic Nucleus

Cited by 111 publications

References 0 publications

Nature of the Galactic centre NIR-excess sources

Nature of the Galactic centre NIR-excess sources

A Wide‐Area VLA Continuum Survey near the Galactic Center at 6 and 20 cm Wavelengths

Jet-lag in Sagittarius A*: what size and timing measurements tell us about the central black hole in the Milky Way

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