A determination of the mass of Sagittarius A* from its radio spectral and source size measurements

Agol

2000

Self Cite

1,023

917

In recent years, evidence for the existence of an ultracompact concentration of dark mass associated with the radio source Sagittarius A* in the Galactic center has become very strong. However, unambiguous proof that this object is indeed a black hole is still lacking. A defining characteristic of a black hole is the event horizon. To a distant observer, the event horizon casts a relatively large "shadow" with an apparent diameter of ∼10 gravitational radii that is due to the bending of light by the black hole, and this shadow is nearly independent of the black hole spin or orientation. The predicted size (∼30 mas) of this shadow for Sgr A* approaches the resolution of current radio interferometers. If the black hole is maximally spinning and viewed edge-on, then the shadow will be offset by ∼8 mas from the center of mass and will be slightly flattened on one side. Taking into account the scatter broadening of the image in the interstellar medium and the finite achievable telescope resolution, we show that the shadow of Sgr A* may be observable with very long baseline interferometry at submillimeter wavelengths, assuming that the accretion flow is optically thin in this region of the spectrum. Hence, there exists a realistic expectation of imaging the event horizon of a black hole within the next few years.

Section: ϫ3mentioning

confidence: 99%

Viewing the Shadow of the Black Hole at the Galactic Center

Falcke

Agol

2000

Self Cite

1,023

917

“…The sharp cutoff at the lowa S ∝ n a ∼ 0.19-0.34 n frequency end (∼0.5 GHz) appears to be due to free-free absorption in the ionized gas along the line of sight (Davies, Walsh, & Booth 1976). In the millimeter region, a spectral bump (Zylka, Mezger, & Lesch 1992;Zylka et al 1995) has been confirmed by simultaneous 20 cm to 1 mm radio observations (Falcke et al 1998), suggesting that a distinct emission component surrounds the event horizon, since the highest frequencies appear to correspond to the smallest spatial scales (Melia, Jokipii, & Narayanan 1992;Melia 1992Melia , 1994Narayan, Yi, & Mahadevan 1995;Falcke et al 1998;. The spectrum turns over in the submillimeter range.…”

mentioning

confidence: 93%

New Constraints on the Nature of Radio Emission in Sagittarius A*

Liu

2001

Self Cite

The millimeter to submillimeter spectrum of Sagittarius A* at the Galactic center, as well as its polarization characteristics, are consistent with the inner ∼10 Schwarzschild radii of a tight Keplerian emitting region of hot, magnetized, orbiting gas. This plasma may also be the source (through self-Comptonization) of the X-rays detected by Chandra. It has long been suspected that the circularization region between the quasi-spherical infall at large radii, and this inner zone, is responsible for producing the rest of Sgr A*'s spectrum. In this Letter, we report the results of a detailed study of this region, with several important conclusions that will be highly relevant to upcoming coordinated multiwavelength observations. First, the combination of existing centimeter and X-ray data preclude the possibility of producing the observed strong 1.36 GHz radio flux via thermal synchrotron within a bounded flow. If Sgr A*'s radio spectrum is produced by accreting gas, it appears that a nonthermal particle distribution is a necessity. This may not be surprising, given that the energy associated with the radial motion is probably dissipated by shocks before the gas circularizes, which can produce the required power-law distribution. Second, if this is the correct picture for how Sgr A*'s spectrum is produced, it appears that the Chandra-detected X-rays may originate either from self-Comptonization in the inner Keplerian region or from optically thin nonthermal synchrotron emission in the much larger circularization zone, extending up to 500 Schwarzschild radii or more. This is a question that should be answered by upcoming broadband observations, since the millimeter bump and X-rays are strongly correlated in the former case, whereas the X-rays are strongly correlated to the centimeter radio flux in the latter. In addition, X-rays produced in the circularized gas could show periodic or quasi-periodic variations but not those produced via nonthermal synchrotron emission much farther out.

“…Other sce-11 n p 2 # 10 narios, such as the jet model (Falcke & Markoff 2000) and the advection-dominated accretion flow disk model (Narayan, Yi, & Mahadevan 1995), also require that the medium goes transparent across this range of frequencies. The fortunate aspect about this transition is that it happens to occur at about the same set of frequencies where the scatter broadening in the interstellar medium decreases below the intrinsic source size (see also Melia, Jokipii, & Narayanan 1992). showed that taking this effect into account, as well as the finite achievable telescope resolution, the shadow of Sgr A* should be observable with very long baseline interferometry (VLBI) below ∼1 mm; it should be quite distinctive at 0.6 mm.…”

Section: Introductionmentioning

confidence: 99%

Polarimetric Imaging of the Massive Black Hole at the Galactic Center

Bromley

Liu

2001

Self Cite

119

115

The radio source Sagittarius A* in the Galactic center emits a polarized spectrum at millimeter and submillimeter wavelengths that is strongly suggestive of relativistic disk accretion onto a massive black hole. We use the wellconstrained mass of Sgr A* and a magnetohydrodynamic model of the accretion flow to match both the total flux and polarization from this object. Our results demonstrate explicitly that the shift in the position angle of the polarization vector, seen at wavelengths near the peak of the millimeter to submillimeter emission from this source, is a signal of relativistic accretion flow in a strong gravitational field. We provide maps of the polarized emission to illustrate how the images of polarized intensity from the vicinity of the black hole would appear in upcoming observations with very long baseline radio interferometers (VLBIs). Our results suggest that near-term VLBI observations will be able to directly image the polarized Keplerian portion of the flow near the horizon of the black hole.