Abstract
Abstract The Event Horizon Telescope (EHT) has recently delivered the first resolved images of M87*, the supermassive black hole in the center of the M87 galaxy. These images were produced using 230 GHz observations performed in 2017 April. Additional observations are required to investigate the persistence of the primary image feature—a ring with azimuthal brightness asymmetry—and to quantify the image variability on event horizon scales. To address this need, we analyze M87* data collected with prototype EHT arrays in 2009, 2011, 2012, and 2013. While these observations do not contain enough information to produce images, they are sufficient to constrain simple geometric models. We develop a modeling approach based on the framework utilized for the 2017 EHT data analysis and validate our procedures using synthetic data. Applying the same approach to the observational data sets, we find the M87* morphology in 2009–2017 to be consistent with a persistent asymmetric ring of ∼40 μas diameter. The position angle of the peak intensity varies in time. In particular, we find a significant difference between the position angle measured in 2013 and 2017. These variations are in broad agreement with predictions of a subset of general relativistic magnetohydrodynamic simulations. We show that quantifying the variability across multiple observational epochs has the potential to constrain the physical properties of the source, such as the accretion state or the black hole spin.
Highlights
The compact radio source in the center of the M87 galaxy, hereafter M87*, has been observed at 1.3 mm wavelength (230 GHz frequency) using very long baseline interferometry (VLBI) since 2009
Though our simple ring models cannot fully reproduce the properties of the abundant and high signal-to-noise data sampled with the 2017 array, they recover diameter and orientation values that are reasonably consistent with those reported in EHTC VI for the 2017 observations, including the counterclockwise shift of the brightness position angle between 2017 April 6 (MODEL1) and 11 (MODEL2)
We note that the underfitting of the data set sampled with the 2017 array results in an artificial narrowing of the parameter posteriors
Summary
The compact radio source in the center of the M87 galaxy, hereafter M87*, has been observed at 1.3 mm wavelength (230 GHz frequency) using very long baseline interferometry (VLBI) since 2009 These observations, performed by early configurations of the Event Horizon Telescope (EHT; Doeleman et al 2009) array, measured the size of the compact emission to be ∼40 μas, with large systematic uncertainties related to the limited baseline coverage (Doeleman et al 2012; Akiyama et al 2015). The addition of new sites and sensitivity improvements leading up to the April 2017 observations yielded the first resolved images of the source (Event Horizon Telescope Collaboration et al 2019a, 2019b, 2019c, 2019d, 2019e, 2019f, hereafter EHTCI–VI) These images revealed an asymmetric ring (a crescent) with a diameter d = 42 ± 3 μas and a position angle of the bright side fB between 150° and 200° east of north (counterclockwise from north/up as seen on the sky; EHTC VI); see the left panel of Figure 1.
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