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tokens: be well fit by rigid body rotation at a rate of 0216 ± 0016 yr −1 . Taking into account of SPHERE’s true north uncertainty, the updated motion rate isThis pattern speed points to a driver located at au, or , from the 1.9 M central star (Figure 3 ). Figure 3. Dynamically constrained, assumed circular orbit of a single arm driver. For a 1.9 M central star, the best-fit ±1 σ orbit has a radius of ( au). Our best-fit measurement of the companion-driven spiral pattern speed is consistent with the R18 measurement within 3 σ , 18 18 Only 3 σ uncertainties are well-constrained in R18 . while our derived uncertainty is ∼40 times smaller, thanks to the use of the same instrument and the maps that are the least biased by postprocessing methods. In numerical simulations, an approximately five Jupiter mass arm driver located at ∼09 has been proposed by Baruteau et al. ( 2019 ), which is within 2 σ from our best-fit companion location assuming a 1.9 M central star, or 1 σ assuming a 1.56 M central star. 4. Discussion 4.1. Direct Imaging Constraints We obtain the direct imaging constraints on the mass of the putative planet orbiting MWC 758 with hot-start evolutionary models (i.e., Sonora, Bobcat; M. Marley et al. 2020, in preparation) using 6.73 h of Keck/NIRC2 -band archival observations: 2025 s on 2015 October 24 (Program ID: C220N2, PI: E. Serabyn, Reggiani et al. 2018 ), 11,160 s on 2016 February 12 (Program ID: U131N2, PI: E. Chiang), 3200 s on 2016 October 24 (Program ID: C221N2, PI: G. Ruane, Reggiani et al. 2018 ), and 7830 s on 2017 February 2 (Program ID: U072, PI: E. Chiang). Following the method described in Ruffio et al. ( 2018 ) and taking into account of the orbital period uncertainty for the driver, we obtain an apparent L -band magnitude of 18 at 99.9% confidence level (i.e., a 3 σ equivalent upper limit). Adopting an age of 10.9 Myr (Garufi et al. 2018 ), this corresponds to a mass of 5 M Jupiter . Using the contrast curve of Keck/NIRC2 in the M s -band (Mawet et al. 2019 ) and assuming Gaussian noise, we expect that a 5 M Jupiter planetary driver, whose mass has been predicted by Dong et al. ( 2015b ) and Baruteau et al. ( 2019 ) with the semimajor axis updated in this study, can be detected at 5 σ level if it is a hot-start planet using four half-nights of NIRC2 M s -band high contrast imaging observations (or at 3.5 σ with the Spiegel Burrows 2012 cold-start model using the same observation). 4.2. Eccentric Driver If the driver has a non-zero eccentricity e , its semimajor axis cannot be uniquely determined from an assessment of the instantaneous angular frequency due to the unknown orbital phase. Giant planets of several Jupiter masses interacting with a gaseous disk are expected to have their eccentricities quickly damped to below the disk aspect ratio, here about 20% (Dunhill et al. 2013 ; Duffell Chiang 2015 ). Simulations have also shown that density waves excited by planets with e ≳ 0.2 develop wiggles and bifurcations, as the