On The Nature of the Compact Dark Mass at the Galactic Center

TL;DR

The paper investigates whether Sgr A* could harbor a physical surface or must possess an event horizon. By treating a hypothetical surface as a thermally emitting body and using near-infrared flux limits along with GR-lensed surface area, it derives an upper bound on the accretion rate, $\dot M_{max}$, and compares it to the rate needed to power the observed luminosity; the result strongly disfavors a surface, suggesting an event horizon. Submillimeter VLBI imaging of a radiatively inefficient accretion flow shows that surface radii $R$ near or beyond the photon orbit produce detectable silhouettes, potentially constraining $R$ to $\lesssim 3M$ when combined with the NIR limits. The study concludes that, under reasonable assumptions, Sgr A* must have an event horizon, and future high-resolution imaging could further tighten the bounds on ultra-compact alternatives.

Abstract

We consider a model in which Sgr A*, the 3.5x10^6 M_sun supermassive black hole candidate at the Galactic Center, is a compact object with a surface. Given the very low quiescent luminosity of Sgr A* in the near infrared, the existence of a hard surface, even in the limit in which the radius approaches the horizon, places severe constraints upon the steady mass accretion rate in the source, requiring dM/dt < 10^-12 M_sun/yr. This limit is well below the minimum accretion rate needed to power the observed submillimeter luminosity of Sgr A*. We thus argue that Sgr A* does not have a surface, i.e., it must have an event horizon. The argument could be made more restrictive by an order of magnitude with microarcsecond resolution imaging, e.g., with submillimeter VLBI.

Figures (2)

• Figure 2: The four curves show upper limits on the mass accretion rate of Sgr A* as a function of the surface radius $R$, derived from the observed quiescent fluxes at $1.6$, $2.1$, $3.8$ and $4.8\,\mu{\rm m}$ listed in Table \ref{['flux_limits']}. The surface is assumed to radiate as a blackbody. For reference, the photon orbit $R=3M$ is shown by the vertical dashed line. The cross-hatched area at the top corresponds to typical mass accretion rates in RIAF models of Sgr A*, and the horizontal dashed line represents the minimum accretion rate needed to power the bolometric luminosity of Sgr A* (see § 1).
• Figure 3: $350\,{\rm GHz}$ images of a RIAF model of Sgr A* (the $a=0$ model in Brod-Loeb:05b) in which the gas accretes onto a compact thermally emitting surface in a Schwarzschild spacetime. The panels correspond to different assumed radii of Sgr A*, given in the upper left corner of each image, ranging from $1\%$ larger than the horizon to $6M$. For comparison, an image of the same accretion flow onto a black hole is shown in the upper-left panel. All the images correspond to a viewing angle $45^\circ$ above the orbital plane. The brightness scale is normalized separately in each image, ranging from maximum intensity to zero (black). The white circles show the size of the Schwarzschild horizon $2M$. Note that the thermally emitting surface is practically invisible. This is because of its very low temperature ($\sim10^4$ K) compared to the brightness temperature of the relativistic accreting gas ($>10^{10}$ K).