2015/06/25 by Takafumi Haga, Akihiro Doi, Yasuhiro Murata +3 · 25 citations
Physics and Astronomy · #Accretion (finance) #Active galactic nucleus #Astronomy #Astrophysical Phenomena and Observations #Astrophysics #Astrophysics and Cosmic Phenomena #Core (optical fiber) #Galaxies: Formation, Evolution, Phenomena #Galaxy #Optics #Physics #Position angle #RADIUS #Spectral index #Spectral line #Very Long Baseline Array #astro-ph.GA #astro-ph.HE
paper · pdf · doi:10.1088/0004-637x/807/1/15
published in The Astrophysical Journal 807(1), 15 (IOP Publishing) · The paper is already published as Haga et al. 2015, ApJ, 807, 15
openalex publication_date 2015/06/25 · arxiv created 2015/10/02 · arxiv updated 2015/10/14 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/06
We report multifrequency phase-referenced observations of the nearby radio galaxy NGC 4261, which has prominent two-sided jets, using the Very Long Baseline Array at 1.4–43 GHz. We measured radio core positions showing observing frequency dependences (known as "core shift") in both approaching jets and counterjets. The limit of the core position as the frequency approaches infinity, which suggests a jet base, is separated by 82 ± 16 μ as upstream in projection, corresponding to (310 ± 60) R s ( R s : Schwarzschild radius) as a deprojected distance, from the 43 GHz core in the approaching jet. In addition, the innermost component at the counterjet side appeared to approach the same position at infinity of the frequency, indicating that cores on both sides are approaching the same position, suggesting a spatial coincidence with the central engine. Applying a phase-referencing technique, we also obtained spectral index maps, which indicate that emission from the counterjet is affected by free–free absorption (FFA). The result of the core shift profile on the counterjet also requires FFA because the core positions at 5–15 GHz cannot be explained by a simple core shift model based on synchrotron self-absorption (SSA). Our result is apparently consistent with the SSA core shift with an additional disk-like absorber over the counterjet side. Core shift and opacity profiles at the counterjet side suggest a two-component accretion: a radiatively inefficient accretion flow at the inner region and a truncated thin disk in the outer region. We proposed a possible solution about density and temperature profiles in the outer disk on the basis of the radio observation.