分类： 天文学 >> 天文学 提交时间： 2023-02-19
Razieh Emami
Paul Tiede
Sheperd S. Doeleman
Freek Roelofs
Maciek Wielgus
Lindy Blackburn
Matthew Liska
Koushik Chatterjee
Bart Ripperda
Antonio Fuentes
Avery Broderick
Lars Hernquist
Charles Alcock
Ramesh Narayan
Randall Smith
Grant Tremblay
Angelo Ricarte
He Sun
Richard Anantua
Yuri Y. Kovalev
摘要： We propose to trace the dynamical motion of a shearing hot spot near the SgrA* source through a dynamical image reconstruction algorithm, StarWarps. Such a hot spot may form as the exhaust of magnetic reconnection in a current sheet near the black hole horizon. A hot spot that is ejected from the current sheet into an orbit in the accretion disk may shear and diffuse due to instabilities at its boundary during its orbit, resulting in a distinct signature. We subdivide the motion to two distinct phases; the first phase refers to the appearance of the hot spot modelled as a bright blob, followed by a subsequent shearing phase simulated as a stretched ellipse. We employ different observational arrays, including EHT(2017,2022) and the next generation event horizon telescope (ngEHTp1, ngEHT) arrays, in which few new additional sites are added to the observational array. We make dynamical image reconstructions for each of these arrays. Subsequently, we infer the hot spot phase in the first phase followed by the axes ratio and the ellipse area in the second phase. We focus on the direct observability of the orbiting hot spot in the sub-mm wavelength. Our analysis demonstrates that newly added dishes may easily trace the first phase as well as part of the second phase, before the flux is reduced substantially. The algorithm used in this work can be extended to any other types of the dynamical motion. Consequently, we conclude that the ngEHT is a key to directly observe the dynamical motions near variable sources, such as SgrA*.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Guang-Yao Zhao
Jose L. Gomez
Antonio Fuentes
Thomas P. Krichbaum
E. Traianou
Rocco Lico
Ilje Cho
Eduardo Ros
S. Komossa
Kazunori Akiyama
Keiichi Asada
Lindy Blackburn
Silke Britzen
Gabriele Bruni
Geoffrey Crew
Rohan Dahale
Lankeswar Dey
Roman Gold
Achamveedu Gopakumar
Sara Issaoun
摘要： We present the first very-long-baseline interferometric (VLBI) observations of the blazar OJ287 carried out jointly with the Global Millimeter VLBI Array (GMVA) and the phased Atacama Large Millimeter/submillimeter Array (ALMA) at 3.5 mm on April 2, 2017. Participation of phased-ALMA not only has improved the GMVA north-south resolution by a factor of ~3, but also has enabled fringe detection with signal-to-noise ratios up to 300 at baselines longer than 2 G{\lambda}. The high sensitivity has motivated us to image the data with the newly developed regularized maximum likelihood imaging methods, revealing the innermost jet structure with unprecedentedly high angular resolution. Our images reveal a compact and twisted jet extending along the northwest direction with two bends within the inner 200 {\mu}as that resembles a precessing jet in projection. The component at the southeastern end shows a compact morphology and high brightness temperature, and is identified as the VLBI core. An extended jet feature that lies at ~200 {\mu}as northwest of the core shows a conical shape in both total and linearly polarized intensity, and a bimodal distribution of the linear polarization electric vector position angle. We discuss the nature of this feature by comparing our observations with models and simulations of oblique and recollimation shocks with various magnetic field configurations. Our high-fidelity images also enabled us to search for possible jet features from the secondary supermassive black hole (SMBH) and test the SMBH binary hypothesis proposed for this source.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Svetlana Jorstad
Maciek Wielgus
Rocco Lico
Sara Issaoun
Avery E. Broderick
Dominic W. Pesce
Jun Liu
Guang-Yao Zhao
Thomas P. Krichbaum
Lindy Blackburn
Chi-Kwan Chan
Michael Janssen
Venkatessh Ramakrishnan
Kazunori Akiyama
Antxon Alberdi
Juan Carlos Algaba
Katherine L. Bouman
Ilje Cho
Antonio Fuentes
Jose L. Gomez
摘要： We report on the observations of the quasar NRAO 530 with the Event Horizon Telescope (EHT) on 2017 April 5-7, when NRAO 530 was used as a calibrator for the EHT observations of Sagittarius A*. At z=0.902 this is the most distant object imaged by the EHT so far. We reconstruct the first images of the source at 230 GHz, at an unprecedented angular resolution of $\sim$ 20 $\mu$as, both in total intensity and in linear polarization. We do not detect source variability, allowing us to represent the whole data set with static images. The images reveal a bright feature located on the southern end of the jet, which we associate with the core. The feature is linearly polarized, with a fractional polarization of $\sim$5-8% and has a sub-structure consisting of two components. Their observed brightness temperature suggests that the energy density of the jet is dominated by the magnetic field. The jet extends over 60 $\mu$as along a position angle PA$\sim -$28$^\circ$. It includes two features with orthogonal directions of polarization (electric vector position angle, EVPA), parallel and perpendicular to the jet axis, consistent with a helical structure of the magnetic field in the jet. The outermost feature has a particularly high degree of linear polarization, suggestive of a nearly uniform magnetic field. Future EHT observations will probe the variability of the jet structure on ${\mu}$as scales, while simultaneous multi-wavelength monitoring will provide insight into the high energy emission origin.
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