Origin of the ring ellipticity in the black hole images of M87*

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Table 2.

Measured ellipticity and position angle for EHT 2018 M87^*images.

	Band	`Comrade`	THEMIS	`eht-imaging`	`SMILI`	`DIFMAP`	Average	Band average
τ	Band 3	$0 . 07_{- 0.03}^{+ 0.04}$ $0.07^{+0.04}_{-0.03}$	$0 . 05_{- 0.03}^{+ 0.02}$ $0.05^{+0.02}_{-0.03}$	$0 . 07_{- 0.03}^{+ 0.03}$ $0.07^{+0.03}_{-0.03}$	$0 . 09_{- 0.03}^{+ 0.04}$ $0.09^{+0.04}_{-0.03}$	$0 . 10_{- 0.04}^{+ 0.04}$ $0.10^{+0.04}_{-0.04}$	$0 . 07_{- 0.03}^{+ 0.03}$ $0.07^{+0.03}_{-0.03}$	$0 . 08_{- 0.02}^{+ 0.03}$ $0.08^{+0.03}_{-0.02}$
τ	Band 4	$0 . 09_{- 0.03}^{+ 0.04}$ $0.09^{+0.04}_{-0.03}$	$0 . 09_{- 0.01}^{+ 0.01}$ $0.09^{+0.01}_{-0.01}$	$0 . 06_{- 0.02}^{+ 0.02}$ $0.06^{+0.02}_{-0.02}$	$0 . 07_{- 0.02}^{+ 0.02}$ $0.07^{+0.02}_{-0.02}$	$0 . 05_{- 0.02}^{+ 0.03}$ $0.05^{+0.03}_{-0.02}$	$0 . 08_{- 0.02}^{+ 0.03}$ $0.08^{+0.03}_{-0.02}$	$0 . 08_{- 0.02}^{+ 0.03}$ $0.08^{+0.03}_{-0.02}$

ξ	Band 3	$44 . 1_{- 12.8}^{+ 10.5}$ $44.1^{+10.5}_{-12.8}$	$48 . 3_{- 4.3}^{+ 4.0}$ $48.3^{+4.0}_{-4.3}$	$56 . 0_{- 7.2}^{+ 9.8}$ $56.0^{+9.8}_{-7.2}$	$66 . 6_{- 8.9}^{+ 2.3}$ $66.6^{+2.3}_{-8.9}$	$54 . 2_{- 4.0}^{+ 6.2}$ $54.2^{+6.2}_{-4.0}$	$53 . 5_{- 6.2}^{+ 6.0}$ $53.5^{+6.0}_{-6.2}$	$50 . 1_{- 7.6}^{+ 6.2}$ $50.1^{+6.2}_{-7.6}$
ξ	Band 4	$40 . 0_{- 18.3}^{+ 11.2}$ $40.0^{+11.2}_{-18.3}$	$38 . 9_{- 4.1}^{+ 6.0}$ $38.9^{+6.0}_{-4.1}$	$48 . 4_{- 19.7}^{+ 10.6}$ $48.4^{+10.6}_{-19.7}$	$50 . 4_{- 11.3}^{+ 4.9}$ $50.4^{+4.9}_{-11.3}$	$52 . 4_{- 10.7}^{+ 6.5}$ $52.4^{+6.5}_{-10.7}$	$44 . 0_{- 9.7}^{+ 6.7}$ $44.0^{+6.7}_{-9.7}$	$50 . 1_{- 7.6}^{+ 6.2}$ $50.1^{+6.2}_{-7.6}$

Notes. Ellipticity, τ (in rows labeled τ), and its position angle, ξ in degrees (in rows labeled ξ), measured from the EHT 2018 M87^* images. The position angle ξ is measured east of north. For each method and band, the main value reported is the median, and the error range represents the 1σ uncertainty around the median. The “Average” column provides the weighted median and 1σ error across methods for each band. The “Band Average” column gives the overall weighted median and 1σ error across all methods and both bands for τ and ξ, respectively.

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