Resolving the Jet-Launch Region of the M87 Supermassive Black Hole

New EHT observations on a 3-station array have resolved the base of the relativistic jet in the giant elliptical galaxy M87. When fit with a circular Gaussian model, the 1.3mm VLBI data imply a size (FWHM) of 40 micro arcseconds, corresponding to 5.5 times the Schwarzschild Radius of the M87 black hole (whose mass is estimated to be 6.4x10^9 solar masses).

There is evidence from longer-wavelength observations that this emission comes from the sheath of the prominent jet, tied closely to the inner edge of an unseen accretion disk. The size of the 1.3 mm emission is smaller than the innermost stable circular orbit of an accretion disk around a nonrotating black hole, indicating both that the black hole has spin and that the accretion disk is spinning in a prograde sense relative to the black hole. More detailed theoretical models support these conclusions.

Figure 1 - Measuring the size of the relativistic jet base in M87. The EHT detected M87 using a 1.3mm VLBI array consisting of JCMT and SMA (Hawaii), CARMA (California) and ARO/SMT (Arizona). The data can be fit by a circular Gaussian model with a size of 40 microarcseconds (solid black line). If modeled as a circular Gaussian component and a thin ring corresponding to emission from the last photon orbit of the black hole, the dotted curve is expected (and also fits the data). These data represent the highest angular resolution observations of M87 to date.

Figure 2 - Estimating the spin of the M87 black hole and the orbital direction of the accretion disk. The strong gravity of the M87 black hole magnifies the apparent size of the Innermost Stable Circular Orbit (ISCO) where it is assumed the jet originates. Since the apparent ISCO size is spin-dependent, we can use the EHT size measurement (Figure 1 above) to estimate the black hole spin. Here we show two theoretical models for the ISCO size: the black curve corresponds to a viewing angle down the spin axis, and the red curve for a view in the equatorial plane. Solid lines show prograde (a > 0) orbits, and dashed lines show retrograde (a < 0) orbits. The 1.3-mm VLBI derived size is shown as a horizontal blue line with a cyan band marking the +/- 1-sigma uncertainty. The 3-sigma upper limit on the 1.3-mm VLBI size corresponds to a lower limit on the black hole spin of (a > 0.2). This suggests that the black hole must be spinning and the accretion disk is in a prograde orbit.

Science article abstract