1. |
- Coustenis, A., et al.
(författare)
-
TandEM : Titan and Enceladus mission
- 2009
-
Ingår i: Experimental astronomy. - : Springer Science and Business Media LLC. - 0922-6435 .- 1572-9508. ; 23:3, s. 893-946
-
Tidskriftsartikel (refereegranskat)abstract
- TandEM was proposed as an L-class (large) mission in response to ESA's Cosmic Vision 2015-2025 Call, and accepted for further studies, with the goal of exploring Titan and Enceladus. The mission concept is to perform in situ investigations of two worlds tied together by location and properties, whose remarkable natures have been partly revealed by the ongoing Cassini-Huygens mission. These bodies still hold mysteries requiring a complete exploration using a variety of vehicles and instruments. TandEM is an ambitious mission because its targets are two of the most exciting and challenging bodies in the Solar System. It is designed to build on but exceed the scientific and technological accomplishments of the Cassini-Huygens mission, exploring Titan and Enceladus in ways that are not currently possible (full close-up and in situ coverage over long periods of time). In the current mission architecture, TandEM proposes to deliver two medium-sized spacecraft to the Saturnian system. One spacecraft would be an orbiter with a large host of instruments which would perform several Enceladus flybys and deliver penetrators to its surface before going into a dedicated orbit around Titan alone, while the other spacecraft would carry the Titan in situ investigation components, i.e. a hot-air balloon (MontgolfiSre) and possibly several landing probes to be delivered through the atmosphere.
|
|
2. |
- Gurvits, L. I., et al.
(författare)
-
The science case and challenges of spaceborne sub-millimeter interferometry: the study case of TeraHertz Exploration and Zooming-in for Astrophysics (THEZA)
- 2021
-
Ingår i: Proceedings of the International Astronautical Congress, IAC. - 0074-1795. ; A7
-
Konferensbidrag (refereegranskat)abstract
- Ultra-high angular resolution in astronomy has always been an important vehicle for making fundamental discoveries. Recent results in direct imaging of the vicinity of the super-massive black hole in the nucleus of the radio galaxy M87 by the millimeter VLBI system Event Horizon Telescope (EHT) and various pioneering results of the Space VLBI mission RadioAstron provided new momentum in high angular resolution astrophysics. In both mentioned cases, the angular resolution reached the values of about 10−20 microrcseconds (0.05−0.1 nanoradian). Angular resolution is proportional to the observing wavelength and inversely proportional to the interferometer baseline length. In the case of Earth-based EHT, the highest angular resolution was achieved by combining the shortest possible wavelength of 1.3 mm with the longest possible baselines, comparable to the Earth’s diameter. For RadioAstron, operational wavelengths were in the range from 92 cm down to 1.3 cm, but the baselines were as long as ∼350,000 km. However, these two highlights of radio astronomy, EHT and RadioAstron do not”saturate” the interest to further increase in angular resolution. Quite opposite: the science case for further increase in angular resolution of astrophysical studies becomes even stronger. A natural and, in fact, the only possible way of moving forward is to enhance mm/sub-mm VLBI by extending baselines to extraterrestrial dimensions, i.e. creating a mm/sub-mm Space VLBI system. The inevitable move toward space-borne mm/sub-mm VLBI is a subject of several concept studies. In this presentation we will focus on one of them called TeraHertz Exploration and Zooming-in for Astrophysics (THEZA), prepared in response to the ESA’s call for its next major science program Voyage 2050 (Gurvits et al. 2021). The THEZA rationale is focused at the physics of spacetime in the vicinity of super-massive black holes as the leading science drive. However, it will also open up a sizable new range of hitherto unreachable parameters of observational radio astrophysics and create a multi-disciplinary scientific facility and offer a high degree of synergy with prospective “single dish” space-borne sub-mm astronomy (e.g., Wiedner et al. 2021) and infrared interferometry (e.g., Linz et al. 2021). As an amalgam of several major trends of modern observational astrophysics, THEZA aims at facilitating a breakthrough in high-resolution high image quality astronomical studies.
|
|
3. |
- Ghirlanda, G., et al.
(författare)
-
Compact radio emission indicates a structured jet was produced by a binary neutron star merger
- 2019
-
Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 363:6430, s. 968-971
-
Tidskriftsartikel (refereegranskat)abstract
- The binary neutron star merger event GW170817 was detected through both electromagnetic radiation and gravitational waves. Its afterglow emission may have been produced by either a narrow relativistic jet or an isotropic outflow. High-spatial-resolution measurements of the source size and displacement can discriminate between these scenarios. We present very-long-baseline interferometry observations, performed 207.4 days after the merger by using a global network of 32 radio telescopes. The apparent source size is constrained to be smaller than 2.5 milli-arc seconds at the 90% confidence level. This excludes the isotropic outflow scenario, which would have produced a larger apparent size, indicating that GW170817 produced a structured relativistic jet. Our rate calculations show that at least 10% of neutron star mergers produce such a jet.
|
|
4. |
|
|
5. |
- Litvinov, D. A., et al.
(författare)
-
RadioAstron gravitational redshift experiment: Status update
- 2018
-
Ingår i: 14th Marcel Grossman Meeting On Recent Developments in Theoretical and Experimental General Relativity, Astrophysics and Relativistic Field Theories, Proceedings. - : WORLD SCIENTIFIC. - 9789813226593 ; , s. 3569-3575
-
Konferensbidrag (refereegranskat)abstract
- A test of a cornerstone of general relativity, the gravitational redshift effect, is currently being conducted with the RadioAstron spacecraft, which is on a highly eccentric orbit around Earth. Using ground radio telescopes to record the spacecraft signal, synchronized to its ultra-stable on-board H-maser, we can probe the varying flow of time on board with unprecedented accuracy. The observations performed so far, currently being analyzed, have already allowed us to measure the effect with a relative accuracy of 4 × 10−4. We expect to reach 2.5 × 10−5 with additional observations in 2016, an improvement of almost a magnitude over the 40-year old result of the GP-A mission.
|
|
6. |
- Litvinov, Dmitry, et al.
(författare)
-
Probing the gravitational redshift with an Earth-orbiting satellite
- 2018
-
Ingår i: Physics Letters, Section A: General, Atomic and Solid State Physics. - : Elsevier BV. - 0375-9601. ; 382:33, s. 2192-2198
-
Tidskriftsartikel (refereegranskat)abstract
- We present an approach to testing the gravitational redshift effect using the RadioAstron satellite. The experiment is based on a modification of the Gravity Probe A scheme of nonrelativistic Doppler compensation and benefits from the highly eccentric orbit and ultra-stable atomic hydrogen maser frequency standard of the RadioAstron satellite. Using the presented techniques we expect to reach an accuracy of the gravitational redshift test of order 10−5, a magnitude better than that of Gravity Probe A. Data processing is ongoing, our preliminary results agree with the validity of the Einstein Equivalence Principle.
|
|
7. |
- Stephan, K., et al.
(författare)
-
Regions of interest on Ganymede's and Callisto's surfaces as potential targets for ESA's JUICE mission
- 2021
-
Ingår i: Planetary and Space Science. - : Elsevier. - 0032-0633 .- 1873-5088. ; 208
-
Tidskriftsartikel (refereegranskat)abstract
- The JUpiter Icy moons Explorer (JUICE) will investigate Ganymede's and Callisto's surfaces and subsurfaces from orbit to explore the geologic processes that have shaped and altered their surfaces by impact, tectonics, possible cryovolcanism, space weathering due to micrometeorites, radiation and charged particles as well as explore the structure and properties of the icy crust and liquid shell (Grasset et al., 2013). The best possible synergy of the JUICE instruments is required to answer the major science objective of this mission and to fully exploit the po-tential of the JUICE mission. Therefore, the JUICE team is aiming to define high priority targets on both Gany-mede's and Callisto's surfaces to support the coordination of the planning activities by the individual instrument teams. Based on the science objectives of the JUICE mission and the most recent knowledge of Ganymede's and Callisto's geologic evolution we propose a collection of Regions of Interest (RoIs), which characterize surface features and terrain types representing important traces of geologic processes, from past and/or present cryovolcanic and tectonic activity to space weathering processes, which are crucial to understand the geologic evolution of Ganymede and Callisto. The proposed evaluation of RoIs is based on their scientific importance as well as on the opportunities and conditions to observe them during the currently discussed mission profile.
|
|
8. |
- Duev, Dmitry, et al.
(författare)
-
Planetary Radio Interferometry and Doppler Experiment (PRIDE) technique: A test case of the Mars Express Phobos fly-by
- 2016
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 593:A34
-
Tidskriftsartikel (refereegranskat)abstract
- The closest ever fly-by of the Martian moon Phobos, performed by the European Space Agency's Mars Express spacecraft, gives a unique opportunity to sharpen and test the Planetary Radio Interferometry and Doppler Experiments (PRIDE) technique in the interest of studying planet-satellite systems. Aims. The aim of this work is to demonstrate a technique of providing high precision positional and Doppler measurements of planetary spacecraft using the Mars Express spacecraft. The technique will be used in the framework of Planetary Radio Interferometry and Doppler Experiments in various planetary missions, in particular in fly-by mode. Methods. We advanced a novel approach to spacecraft data processing using the techniques of Doppler and phase-referenced very long baseline interferometry spacecraft tracking. Results. We achieved, on average, mHz precision (30 mu m/s at a 10 s integration time) for radial three-way Doppler estimates and sub-nanoradian precision for lateral position measurements, which in a linear measure (at a distance of 1.4 AU) corresponds to similar to 50 m.
|
|
9. |
- An, T., et al.
(författare)
-
Evolving parsec-scale radio structure in the most distant blazar known
- 2020
-
Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723 .- 2041-1723. ; 11:1
-
Tidskriftsartikel (refereegranskat)abstract
- Blazars are a sub-class of quasars with Doppler boosted jets oriented close to the line of sight, and thus efficient probes of supermassive black hole growth and their environment, especially at high redshifts. Here we report on Very Long Baseline Interferometry observations of a blazar J0906 + 6930 at z = 5.47, which enabled the detection of polarised emission and measurement of jet proper motion at parsec scales. The observations suggest a less powerful jet compared with the general blazar population, including lower proper motion and bulk Lorentz factor. This coupled with a previously inferred high accretion rate indicate a transition from an accretion radiative power to a jet mechanical power based transfer of energy and momentum to the surrounding gas. While alternative scenarios could not be fully ruled out, our results indicate a possibly nascent jet embedded in and interacting with a dense medium resulting in a jet bending.
|
|
10. |
- Frey, S., et al.
(författare)
-
J1026+2542: Proper motion in a blazar jet at z=5.27
- 2014
-
Ingår i: Proceedings of Science. - 1824-8039.
-
Konferensbidrag (refereegranskat)abstract
- The radio-loud AGN J1026+2542 has recently been classified as the second most distant blazar, based on its broad-band spectral energy distribution and X-ray spectrum. The source with a prominent one-sided jet extending to at least ∼20 mas was earlier observed with the VLBA at 5 GHz in January 2006. With our new EVN observation (May 2013), we detected the displacement and directly measured the apparent proper motion of the jet components, for the first time at such a high redshift. The 1.6-GHz EVN observation provided additional information on the extent and the spectral properties of the jet. The VLBI results are consistent with the picture in which J1026+2542 has its jet oriented close to the line of sight, with significant Doppler boosting and a large bulk Lorentz factor. © Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence.
|
|
11. |
- Frey, S., et al.
(författare)
-
The first estimate of radio jet proper motion at z > 5
- 2015
-
Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 446:3, s. 2921-2928
-
Tidskriftsartikel (refereegranskat)abstract
- The extremely high-redshift (z = 5.3) radio source SDSS J102623.61+254259.5 (J1026+2542) is among the most distant and most luminous radio-loud active galactic nuclei (AGN) known to date. Its one-sided radio jet structure on milliarcsecond (mas) and similar to 10-mas scales typical for blazars was first imaged at 5 GHz with very long baseline interferometry (VLBI) in 2006. Here we report on our dual-frequency (1.7 and 5 GHz) imaging observations performed with the European VLBI Network (EVN) in 2013. The prominent jet structure allows us to identify individual components whose apparent displacement can be detected over the time span of 7.33 yr. This is the first time when jet proper motions are directly derived in a blazar at z > 5. The small values of up to similar to 0.1 mas yr(-1) are consistent with what is expected in a relativistic cosmological model if redshift is a measure of distance. The apparent superluminal jet speeds, considered tentative because derived from two epochs only, exceed 10 c for three different components along the jet. Based on modelling its spectral energy distribution, J1026+2542 is known to have its X-ray jet oriented close to the line of sight, with significant Doppler boosting and a large bulk Lorentz factor (Gamma approximate to 13). The new VLBI observations, indicating similar to 2.3 x 10(12) K lower limit to the core brightness temperature, are consistent with this picture. The spectral index in the core region is -0.35.
|
|
12. |
- Gurvits, Leonid I., et al.
(författare)
-
THEZA: TeraHertz Exploration and Zooming-in for Astrophysics
- 2021
-
Ingår i: Experimental Astronomy. - : Springer Science and Business Media LLC. - 0922-6435 .- 1572-9508. ; 51:3, s. 559-594
-
Tidskriftsartikel (refereegranskat)abstract
- This paper presents the ESA Voyage 2050 White Paper for a concept of TeraHertz Exploration and Zooming-in for Astrophysics (THEZA). It addresses the science case and some implementation issues of a space-borne radio interferometric system for ultra-sharp imaging of celestial radio sources at the level of angular resolution down to (sub-) microarcseconds. THEZA focuses at millimetre and sub-millimetre wavelengths (frequencies above similar to 300 GHz), but allows for science operations at longer wavelengths too. The THEZA concept science rationale is focused on the physics of spacetime in the vicinity of supermassive black holes as the leading science driver. The main aim of the concept is to facilitate a major leap by providing researchers with orders of magnitude improvements in the resolution and dynamic range in direct imaging studies of the most exotic objects in the Universe, black holes. The concept will open up a sizeable range of hitherto unreachable parameters of observational astrophysics. It unifies two major lines of development of space-borne radio astronomy of the past decades: Space VLBI (Very Long Base-line Interferometry) and mm- and sub-mm astrophysical studies with "single dish" instruments. It also builds upon the recent success of the Earth-based Event Horizon Telescope (EHT) - the first-ever direct image of a shadow of the super-massive black hole in the centre of the galaxy M87. As an amalgam of these three major areas of modern observational astrophysics, THEZA aims at facilitating a breakthrough in high-resolution high image quality studies in the millimetre and sub-millimetre domain of the electromagnetic spectrum.
|
|
13. |
- Paragi, Z., et al.
(författare)
-
Swift J1644+5734: the EVN view
- 2016
-
Ingår i: Proceedings of the International Astronomical Union. - 1743-9213 .- 1743-9221. ; 12:S324, s. 119-122
-
Tidskriftsartikel (refereegranskat)abstract
- A small fraction of Tidal Disruption Events (TDE) produce relativistic jets, evidenced by their non-thermal X-ray spectra and transient radio emission. Here we present milliarcsecond-resolution imaging results on TDE J1644+5734 with the European VLBI Network (EVN). These provide a strong astrometric constraint on the average apparent jet velocityβapp
|
|
14. |
- Tosi, F., et al.
(författare)
-
Characterization of the Surfaces and Near-Surface Atmospheres of Ganymede, Europa and Callisto by JUICE
- 2024
-
Ingår i: Space Science Reviews. - 0038-6308 .- 1572-9672. ; 220:5
-
Tidskriftsartikel (refereegranskat)abstract
- We present the state of the art on the study of surfaces and tenuous atmospheres of the icy Galilean satellites Ganymede, Europa and Callisto, from past and ongoing space exploration conducted with several spacecraft to recent telescopic observations, and we show how the ESA JUICE mission plans to explore these surfaces and atmospheres in detail with its scientific payload. The surface geology of the moons is the main evidence of their evolution and reflects the internal heating provided by tidal interactions. Surface composition is the result of endogenous and exogenous processes, with the former providing valuable information about the potential composition of shallow subsurface liquid pockets, possibly connected to deeper oceans. Finally, the icy Galilean moons have tenuous atmospheres that arise from charged particle sputtering affecting their surfaces. In the case of Europa, plumes of water vapour have also been reported, whose phenomenology at present is poorly understood and requires future close exploration. In the three main sections of the article, we discuss these topics, highlighting the key scientific objectives and investigations to be achieved by JUICE. Based on a recent predicted trajectory, we also show potential coverage maps and other examples of reference measurements. The scientific discussion and observation planning presented here are the outcome of the JUICE Working Group 2 (WG2): “Surfaces and Near-surface Exospheres of the Satellites, dust and rings”.
|
|
15. |
- Yang, Jun, 1979, et al.
(författare)
-
No apparent superluminal motion in the first-known jetted tidal disruption event Swift J1644+5734
- 2016
-
Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966 .- 1745-3925 .- 1745-3933. ; 462:1, s. L66-L70
-
Tidskriftsartikel (refereegranskat)abstract
- The first-known tidal disruption event (TDE) with strong evidence for a relativistic jet - based on extensive multiwavelength campaigns - is Swift J1644+5734. In order to directly measure the apparent speed of the radio jet, we performed very long baseline interferometry (VLBI) observations with the European VLBI network (EVN) at 5 GHz. Our observing strategy was to identify a very nearby and compact radio source with the real-time e-EVN, and then utilize this source as a stationary astrometry reference point in the later five deep EVN observations. With respect to the in-beam source FIRST J1644+5736, we have achieved a statistical astrometric precision about 12 mu as (68 per cent confidence level) per epoch. This is one of the best phase-referencing measurements available to date. No proper motion has been detected in the Swift J1644+5734 radio ejecta. We conclude that the apparent average ejection speed between 2012.2 and 2015.2 was less than 0.3c with a confidence level of 99 per cent. This tight limit is direct observational evidence for either a very small viewing angle or a strong jet deceleration due to interactions with a dense circum-nuclear medium, in agreement with some recent theoretical studies.
|
|