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Abstract Infrared observations of stellar orbits about Sgr A* probe the mass distribution in the inner parsec of the Galaxy and provide definitive evidence for the existence of a massive black hole. However, the infrared astrometry is relative and is tied to the radio emission from Sgr A* using stellar SiO masers that coincide with infrared-bright stars. To support and improve this two-step astrometry, we present new astrometric observations of 15 stellar SiO masers within 2 pc of Sgr A*. Combined with legacy observations spanning 25.8 yr, we reanalyze the relative offsets of these masers from Sgr A* and measure positions and proper motions that are significantly improved compared to the previously published reference frame. Maser positions are corrected for epoch-specific differential aberration, precession, nutation, and solar gravitational deflection. Omitting the supergiant IRS 7, the mean position uncertainties are 0.46 mas and 0.84 mas in R.A. and decl., and the mean proper motion uncertainties are 0.07 mas yr−1and 0.12 mas yr−1, respectively. At a distance of 8.2 kpc, these correspond to position uncertainties of 3.7 and 6.9 au and proper motion uncertainties of 2.7 and 4.6 km s−1. The reference frame stability, the uncertainty in the variance-weighted mean proper motion of the maser ensemble, is 8μas yr−1(0.30 km s−1) in R.A. and 11μas yr−1(0.44 km s−1) in decl., which represents a 2.3-fold improvement over previous work and a new benchmark for the maser-based reference frame.
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A VLBI Proper Motion Analysis of the Recoiling Supermassive Black Hole Candidate Mrk 1018
Abstract Mrk 1018 is a nearby changing-look active galactic nucleus (AGN) that has oscillated between spectral Type 1.9 and Type 1 over a period of 40 yr. Recently, a recoiling supermassive black hole (rSMBH) scenario has been proposed to explain the spectral and flux variability observed in this AGN. Detections of rSMBHs are important for understanding the processes by which SMBH binaries merge and how rSMBHs influence their galactic environment through feedback mechanisms. However, conclusive identification of any rSMBHs has remained elusive to date. In this paper, we present an analysis of 6.5 yr of multifrequency Very Long Baseline Array monitoring of Mrk 1018. We find that the radio emission is compact down to 2.4 pc, and it displays flux density and spectral variability over the length of our campaign, typical of a flat-spectrum radio core. We observe proper motion in RA of the radio core at −36.4 ± 8.6μas yr−1(4.2σ), or 0.10c± 0.02cat the redshift of Mrk 1018. No significant proper motion is found in DEC (31.3 ± 25.1μas yr−1). We discuss possible physical mechanisms driving the proper motion, including an rSMBH. We conclude that the apparent velocity we observe of the VLBI radio core is too high to reconcile with theoretical predictions of rSMBH velocities and that the proper motion is most likely dominated by an unresolved, outflowing jet component. Future observations may yet reveal the true nature of Mrk 1018. However, our observations are not able to confirm it as a true rSMBH.
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- PAR ID:
- 10431920
- Publisher / Repository:
- DOI PREFIX: 10.3847
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 952
- Issue:
- 1
- ISSN:
- 0004-637X
- Format(s):
- Medium: X Size: Article No. 18
- Size(s):
- Article No. 18
- Sponsoring Org:
- National Science Foundation
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