Article
Astronomy & Astrophysics
Dominic W. Pesce
Summary: This paper introduces DMC, a model and tool for polarimetric imaging of VLBI data sets that reconstructs full-Stokes emission structure and station-based calibration terms simultaneously. DMC formulates the imaging problem using Hamiltonian Monte Carlo sampling for posterior exploration, providing a natural quantification of uncertainty in image structure and data calibration. Results show DMC accurately recovers image structure and calibration quantities with assessed uncertainties, with the framework being flexible and its implementation continuously developed.
ASTRONOMICAL JOURNAL
(2021)
Article
Physics, Multidisciplinary
Lijing Shao
Summary: The imaging of two supermassive black holes by the Event Horizon Telescope Collaboration confirmed the accuracy of Einstein's general relativity in predicting black hole shadows in highly curved spacetime. This breakthrough provides new evidence in support of his theory.
FRONTIERS OF PHYSICS
(2022)
Article
Astronomy & Astrophysics
Yan Sun, Bo Zhang, Mark J. Reid, Shuangjing Xu, Shiming Wen, Jingdong Zhang, Xingwu Zheng
Summary: In this study, Very Long Baseline Array observations were used to observe 22 GHz H2O and 43 GHz SiO masers toward the Mira variable RR Aql. The absolute stellar position and parallax of RR Aql were estimated, and the results showed a significant tension between radio and optical measurements. The study also determined the K-band zero-point using both very long baseline interferometry (VLBI) and Gaia parallaxes, with statistically consistent results.
ASTROPHYSICAL JOURNAL
(2022)
Article
Chemistry, Analytical
Tae-Suk Bae, Chang-Ki Hong
Summary: A geodetic reference frame is crucial in geoinformation fields, and techniques like VLBI and SLR are used to establish and maintain it. To connect these solutions, researchers have developed an integrated model to precisely determine the invariant point.
Article
Astronomy & Astrophysics
Xu Zhao, Wei-Min Zheng, Li Tong, Mao-Li Ma
Summary: This study proposes a method to simulate VLBI signals and verifies the real-time data-processing capability, aiming to evaluate the dynamic dual-target tracking ability of China's VLBI network in same beam VLBI mode.
PUBLICATIONS OF THE ASTRONOMICAL SOCIETY OF THE PACIFIC
(2022)
Article
Astronomy & Astrophysics
Ilje Cho, Guang-Yao Zhao, Tomohisa Kawashima, Motoki Kino, Kazunori Akiyama, Michael D. Johnson, Sara Issaoun, Kotaro Moriyama, Xiaopeng Cheng, Juan-Carlos Algaba, Taehyun Jung, Bong Won Sohn, Thomas P. Krichbaum, Maciek Wielgus, Kazuhiro Hada, Ru-Sen Lu, Yuzhu Cui, Satoko Sawada-Satoh, Zhiqiang Shen, Jongho Park, Wu Jiang, Hyunwook Ro, Kunwoo Yi, Kiyoaki Wajima, Jee Won Lee, Jeffrey Hodgson, Fumie Tazaki, Mareki Honma, Kotaro Niinuma, Sascha Trippe, Tao An, Yingkang Zhang, Jeong Ae Lee, Se-Jin Oh, Do-Young Byun, Sang-Sung Lee, Jae-Young Kim, Junghwan Oh, Shoko Koyama, Keiichi Asada, Xuezheng Wang, Lang Cui, Yoshiaki Hagiwara, Masanori Nakamura, Mieko Takamura, Tomoya Hirota, Koichiro Sugiyama, Noriyuki Kawaguchi, Hideyuki Kobayashi, Tomoaki Oyama, Yoshinori Yonekura, Jongsoo Kim, Ju-Yeon Hwang, Dong-Kyu Jung, Hyo-Ryoung Kim, Jeong-Sook Kim, Chung-Sik Oh, Duk-Gyoo Roh, Jae-Hwan Yeom, Bo Xia, Weiye Zhong, Bin Li, Rongbing Zhao, Jinqing Wang, Qinghui Liu, Zhong Chen
Summary: In this study, the intrinsic structure of the Galactic Center supermassive black hole Sgr A* was resolved using very long baseline interferometry (VLBI). The major-axis sizes were found to scale with observing wavelength as a power law. These results provide important estimates for the size and compact flux density of the black hole.
ASTROPHYSICAL JOURNAL
(2022)
Article
Astronomy & Astrophysics
K. . E. Gabanyi, S. Belladitta, S. Frey, G. Orosz, L. I. Gurvits, K. Rozgonyi, T. An, H. Cao, Z. Paragi, K. Perger
Summary: AGN are important celestial objects for investigating the early Universe and the growth of supermassive black holes. They have been observed as far as redshift z similar to 7. We conducted high-resolution radio interferometric observations of a candidate blazar at z = 5, which is radio-luminous but X-ray weak. Our observations confirmed its blazar nature with similar radio properties as blazars at lower redshifts.
PUBLICATIONS OF THE ASTRONOMICAL SOCIETY OF AUSTRALIA
(2023)
Article
Astronomy & Astrophysics
Yu-Ching Chen, Xin Liu, Joseph Lazio, Peter Breiding, Sarah Burke-Spolaor, Hsiang-Chih Hwang, Yue Shen, Nadia L. Zakamska
Summary: Dual and off-nucleus active supermassive black holes are common in the universe, and their identification at parsec scales has been a challenge. Using VLBA, we conducted a systematic study and identified 8 candidate dual or off-nucleus quasars.
ASTROPHYSICAL JOURNAL
(2023)
Proceedings Paper
Engineering, Electrical & Electronic
Jade P. Wang, Bryan Bilyeu, Don Boroson, Dave Caplan, Bryan Robinson, Curt Schieler, Michael Johnson, Lindy Blackburn, Sheperd Doeleman, Kari Haworth
Summary: Space-based VLBI imaging improves astronomical radio imaging resolution by increasing baseline distances and eliminating atmospheric-attenuation constraints. However, smaller apertures and sensitivity constraints require high data downlink rates, potentially reaching 256 Gbit/s. Laser communications offer a promising solution for achieving high-rate long-distance downlinks with modest power and aperture demands. We propose a scalable lasercom architecture for enhanced space-based VLBI imaging from geosynchronous orbit (GEO).
FREE-SPACE LASER COMMUNICATIONS XXXV
(2023)
Article
Mathematics
Cosimo Bambi
Summary: Einstein's theory of General Relativity, a pillar of modern physics, has been mainly tested in the weak field regime for decades, but recent years have seen significant progress in testing the theory in the strong field regime with black holes and neutron stars data.
ARABIAN JOURNAL OF MATHEMATICS
(2022)
Article
Astronomy & Astrophysics
Zuobin Zhang, Honghui Liu, Askar B. Abdikamalov, Dimitry Ayzenberg, Cosimo Bambi, Menglei Zhou
Summary: In this study, the spins of black holes were measured using the continuum-fitting and the iron-line methods, and three NuSTAR observations of the black hole binary GRS 1716-249 were analyzed. The analysis confirmed the Kerr nature of the black hole in GRS 1716-249 and provided stringent constraints on possible deviations from the predictions of general relativity.
ASTROPHYSICAL JOURNAL
(2022)
Article
Astronomy & Astrophysics
Shafqat Riaz, Askar B. Abdikamalov, Dimitry Ayzenberg, Cosimo Bambi, Haiyang Wang, Zhibo Yu
Summary: This article investigates the impact of a disk-like corona on the measurement of system properties and testing the Kerr nature of a black hole by simulating observations of a black hole binary. The study finds that while the astrophysical properties of the accretion disk can be well recovered, it is more challenging to constrain the geometric properties of the black hole spacetime.
ASTROPHYSICAL JOURNAL
(2022)
Article
Astronomy & Astrophysics
Ashutosh Tripathi, Askar B. Abdikamalov, Dimitry Ayzenberg, Cosimo Bambi, Victoria Grinberg, Honghui Liu, Menglei Zhou
Summary: In this study, the hypothesis of Kerr black hole was tested with a stellar-mass black hole in GRS 1915+105 by analyzing five disk-dominated RXTE spectra and one reflection-dominated Suzaku spectrum. The combination of the constraints from the continuum-fitting and the iron line methods has the potential to provide more stringent tests of the Kerr metric.
JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS
(2022)
Article
Astronomy & Astrophysics
Honghui Liu, Jiachen Jiang, Zuobin Zhang, Cosimo Bambi, Long Ji, Lingda Kong, Shu Zhang
Summary: The low-mass X-ray binary GX 339-4 underwent a new outburst in 2021, during which the flux rapidly alternated between low and high states. Time-resolved spectral analysis revealed that the flux changes occurred primarily in the hard X-ray band. The variability was attributed to the reflected emission from the inner accretion disc and the power-law continuum from the corona.
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
(2022)
Article
Astronomy & Astrophysics
Honghui Liu, Yuhan Fu, Cosimo Bambi, Jiachen Jiang, Michael L. Parker, Long Ji, Lingda Kong, Liang Zhang, Shu Zhang, Yuexin Zhang
Summary: We analyzed three observations of GRS 1915+105 in 2017 and found strong absorption lines from highly ionized iron in the disk wind outflowing at a velocity of about 1000 km/s. Two of the observations showed significant oscillations in their light curves, corresponding to the kappa state of GRS 1915+105. The variation in the ionization state of the disk wind follows the X-ray continuum on timescales ranging from hundreds of seconds to months, with the wind's radial location consistent with thermal driving. The mass-loss rate due to the outflowing wind is comparable to the mass accretion rate in the inner disk, indicating the important role of the disk wind in the accretion system.
ASTROPHYSICAL JOURNAL
(2022)
Article
Astronomy & Astrophysics
Swarnim Shashank, Shafqat Riaz, Askar B. Abdikamalov, Cosimo Bambi
Summary: X-ray reflection spectroscopy is a leading technique for studying black hole accretion disks, but its accuracy relies on the reflection models used. This study uses a general relativistic magnetohydrodynamic code and ray-tracing techniques to test the capability of current reflection models in recovering the correct input parameters.
ASTROPHYSICAL JOURNAL
(2022)
Article
Astronomy & Astrophysics
Honghui Liu, Cosimo Bambi, Jiachen Jiang, Javier A. Garcia, Long Ji, Lingda Kong, Xiaoqin Ren, Shu Zhang, Shuangnan Zhang
Summary: We analyze the relativistic reflection spectra of GX 339-4 during its 2021 outburst and find strong relativistic reflection signatures indicating a high black hole spin and an intermediate disk inclination angle. The hard-to-soft transition is accompanied by an increasing disk temperature and a softening corona emission, while the inner disk radius remains stable. The corona height, assuming a lamppost geometry, stays close to the black hole across the state transition. Including Comptonization, the scattering fraction parameter decreases during the transition. Our analysis with a reflection model designed for hot accretion disks supports the scenario of corona property variations during the state transition.
ASTROPHYSICAL JOURNAL
(2023)
Article
Astronomy & Astrophysics
Zuobin Zhang, Jiachen Jiang, Honghui Liu, Cosimo Bambi, Christopher S. Reynolds, Andrew C. Fabian, Thomas Dauser, Kristin Madsen, Andrew Young, Luigi Gallo, Zhibo Yu, John Tomsick
Summary: We present the results from a joint XMM-Newton + NuSTAR observation of the Seyfert 1 galaxy ESO 511-G030, showing that the source remained in a low flux and stable state. The broadband spectrum indicates the presence of a power-law continuum, a soft excess below 2 keV, a narrow iron Ka emission, and a high-energy cutoff. Two possible scenarios, a warm and optically thick Comptonizing corona or a relativistic reflection from a high-density inner disk, can explain the observed soft excess. All models require a low temperature for the hot corona.
ASTROPHYSICAL JOURNAL
(2023)
Review
Multidisciplinary Sciences
Cosimo Bambi
Summary: General relativity, one of the pillars of modern physics, has been predominantly tested in the weak-field regime until recently. However, advances in observational facilities have allowed for exploration of the strong-field regime, providing data for gravitational wave tests, black hole imaging, and X-ray tests of general relativity. This manuscript aims to provide a pedagogical review on X-ray tests of general relativity with black holes and compare them with gravitational wave tests and black hole imaging.
Article
Astronomy & Astrophysics
Honghui Liu, Jiachen Jiang, Zuobin Zhang, Cosimo Bambi, Andrew C. Fabian, Javier A. Garcia, Adam Ingram, Erin Kara, James F. Steiner, John A. Tomsick, Dominic J. Walton, Andrew J. Young
Summary: We conducted a high-density relativistic reflection analysis on 21 spectra of six black hole X-ray binaries in the hard state using data from NuSTAR and Swift. Our findings indicate that 76% of the observations in our sample require a disk density higher than the previously assumed 10^15 cm(-3) in reflection analysis. Stellar mass black holes have higher disk densities compared to active galactic nuclei. Our results suggest that the inner disk radius is close to the innermost stable circular orbit in the luminous hard state, and the coronal temperatures are lower than expected, which can be explained by a hybrid plasma model. The disk ionization parameter is overestimated when assuming a fixed density of 10^15 cm(-3), while the inner disk radius remains unaffected.
ASTROPHYSICAL JOURNAL
(2023)
Article
Astronomy & Astrophysics
Q. Yuan, Pankaj Kushwaha, Alok C. Gupta, Ashutosh Tripathi, Paul J. Wiita, M. Zhang, X. Liu, Anne Lahteenmaki, Merja Tornikoski, Joni Tammi, Venkatessh Ramakrishnan, L. Cui, X. Wang, M. F. Gu, Cosimo Bambi, A. E. Volvach
Summary: In this study, correlation and periodicity analyses were performed on the multiband light curves of PKS 1510-089, a flat-spectrum radio quasar. The results show correlations between gamma-ray, optical, and near-infrared emissions with no time lags, while both gamma-ray and optical/near-infrared emissions precede radio radiation. Furthermore, evidence for a quasi-periodic variation of approximately 1540 days was found in the 37 GHz light curve, suggesting periodic behaviors in the unresolved core emission. The color variability analysis reveals that the source follows a redder-when-brighter trend over time, even in the low-flux state.
ASTROPHYSICAL JOURNAL
(2023)
Article
Astronomy & Astrophysics
Swarnim Shashank, Cosimo Bambi
Summary: Gravitational-wave observations of binary black holes were used to test the Kerr hypothesis, and the results showed consistency.
Proceedings Paper
Astronomy & Astrophysics
Phil Uttley, Roland den Hartog, Cosimo Bambi, Didier Barret, Stefano Bianchi, Michal Bursa, Massimo Cappi, Piergiorgio Casella, Webster Cash, Elisa Costantini, Thomas Dauser, Maria Diaz Trigo, Keith Gendreau, Victoria Grinberg, Jan-Willem den Herder, Adam Ingram, Erin Kara, Sera Markoff, Beatriz Mingo, Francesca Panessa, Katja Poppenhaeger, Agata Rozanska, Jiri Svoboda, Ralph Wijers, Richard Willingale, Joern Wilms, Michael Wise
Summary: The proposal suggests developing X-ray interferometry as part of ESA's Voyage 2050 programme for high energy universe exploration with ultra-high spatial resolution. A single spacecraft with a 1 m baseline could achieve a resolution of 100 mu as at 10 angstrom, surpassing the Event Horizon Telescope. Despite challenges, key requirements are already feasible or within reach of development.
SPACE TELESCOPES AND INSTRUMENTATION 2020: ULTRAVIOLET TO GAMMA RAY
(2021)
Article
Astronomy & Astrophysics
Rittick Roy, Askar B. Abdikamalov, Dimitry Ayzenberg, Cosimo Bambi, Shafqat Riaz, Ashutosh Tripathi
Summary: In this paper, a relativistic reflection model is constructed to test the weak-equivalence principle in the strong gravitational field of black holes. The analysis of the reflection features of a black hole binary spectrum helps constrain possible violations of the weak-equivalence principle by massive particles and x-ray photons.