An independent hybrid imaging of Sgr A* from the data in EHT 2017 observations

TL;DR

This paper challenges the EHT 2017 black hole shadow image of Sgr A*, suggesting it may be an artifact caused by the PSF structure, and presents an independent hybrid imaging analysis indicating a different, more reliable structure.

Contribution

The study provides an independent hybrid imaging method that questions the EHT 2017 shadow interpretation, revealing a different accretion disk structure with better data consistency.

Findings

The EHT2017 shadow diameter matches the PSF sidelobe spacing.

Independent analysis shows an elongated structure consistent with previous observations.

The new image has fewer residuals, indicating higher reliability.

Abstract

We propose that the ring structure found by the Event Horizon Telescope Collaboration (EHTC) as the black hole shadow of Sgr A*is an artifact by the bumpy PSF (Point Spread Function) of the EHT2017. The imaging using sparse u-v data requires detailed scrutiny of the PSF. The estimated shadow diameter (48.7 +- 7 muas) is equal to the spacing between the main beam and the first sidelobe of the PSF (49.09 muas), which immediately suggests a potential problem in the deconvolution of the PSF. We show that the ring image can be derived from non-ring simulated datasets (noise only; point source) with a narrow Field-of-View (FOV) and an assumed self-calibration suggesting the EHT2017's u-v coverage is insufficient for reliable imaging. The EHTC analysis, based on calibrations with assumptions about the source's size and properties, selected the final image by prioritizing appearance rate of the similar structure from a large imaging parameter space over data consistency. Our independent analysis with the conventional hybrid mapping reveals an elongated east-west structure, consistent with previous observations. We believe it to be more reliable than the EHTC image, due to half the residuals in normalized visibility amplitude. The eastern half is brighter, possibly due to a Doppler boost from the rapid rotating disk. We hypothesize our image shows a portion of the accretion disk about 2 to a few Rs away from the black hole, rotating with nearly 60 % of the speed of light viewed from an angle of 40 -45 degrees.