| 1. |
- Akiyama, Kazunori, et al.
(författare)
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First Sagittarius A* Event Horizon Telescope Results. II. EHT and Multiwavelength Observations, Data Processing, and Calibration
- 2022
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Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8213 .- 2041-8205. ; 930:2
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Tidskriftsartikel (refereegranskat)abstract
- We present Event Horizon Telescope (EHT) 1.3 mm measurements of the radio source located at the position of the supermassive black hole Sagittarius A* (Sgr A*), collected during the 2017 April 5-11 campaign. The observations were carried out with eight facilities at six locations across the globe. Novel calibration methods are employed to account for Sgr A*'s flux variability. The majority of the 1.3 mm emission arises from horizon scales, where intrinsic structural source variability is detected on timescales of minutes to hours. The effects of interstellar scattering on the image and its variability are found to be subdominant to intrinsic source structure. The calibrated visibility amplitudes, particularly the locations of the visibility minima, are broadly consistent with a blurred ring with a diameter of similar to 50 mu as, as determined in later works in this series. Contemporaneous multiwavelength monitoring of Sgr A* was performed at 22, 43, and 86 GHz and at near-infrared and X-ray wavelengths. Several X-ray flares from Sgr A* are detected by Chandra, one at low significance jointly with Swift on 2017 April 7 and the other at higher significance jointly with NuSTAR on 2017 April 11. The brighter April 11 flare is not observed simultaneously by the EHT but is followed by a significant increase in millimeter flux variability immediately after the X-ray outburst, indicating a likely connection in the emission physics near the event horizon. We compare Sgr A*'s broadband flux during the EHT campaign to its historical spectral energy distribution and find that both the quiescent emission and flare emission are consistent with its long-term behavior.
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| 2. |
- Tuccari, Gino, et al.
(författare)
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DBBC3 Towards the BRAND EVN Receiver
- 2019
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Ingår i: Proceedings of the 24th European VLBI Group for Geodesy and Astrometry Working Meeting. ; , s. 27-30
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Konferensbidrag (refereegranskat)abstract
- The DBBC3 is a flexible VLBI backend and environment that supports a wide range of observational needs via a suite of FPGA firmware types. The hardware can sample up to eight 4 GHz-wide baseband signals and convert to digital streams over multiple 10GE links on fibre. The development team has an ongoing development programme that has enhanced existing modes and introduced new desired modes as user requirements evolve. Three dierent firmware types for observing have been implemented which will be briefly summarised: Direct Sampling Conversion (DSC), arbitrary selection of bands (OCT), Digital Down Conversion (DDC). These modes cover all the requirements of astronomical, VGOS and legacy geodetic VLBI of the present, but also of the near future. At the same time the DBBC3 is an important platform for additional new modes to be implemented for the BRAND receiver. This paper describes the use of the DBBC3 for the receiver development, pointing out which element in the current DBBC3 structure will be part of the BRAND receiver in order to simplify its introduction into the existing VLBI environment at telescopes with a DBBC3 backend.
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| 3. |
- Tuccari, Gino, et al.
(författare)
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New Observing Modes for the DBBC3
- 2019
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Ingår i: International VLBI Service for Geodesy and Astrometry 2018 General Meeting Proceedings “Global Geodesy and the Role of VGOS – Fundamental to Sustainable Development” (NASA/CP–2019-219039). ; NASA/CP-2019-219039, s. 47-49
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Konferensbidrag (refereegranskat)abstract
- The DBBC3 was further enhanced by introducing new modes. Three different firmwares have recently been implemented for observing: Direct Sampling Conversion (DSC), arbitrary selection of bands (OCT), and Digital Down Conversion (DDC). These modes cover all the requirements of astronomical, VGOS, and legacy geodetic VLBI for the time being and the immediate future. In addition, the DBBC3 offers unsurpassed compatibility to the relatively large number of other existing VLBI backends. A number of test observations were performed in the last months to achieve the best performance of the VGOS modes, and similar tests are planned for the EVN network. At the same time the DBBC3 is an important platform for additional new modes to be implemented for the BRAND receiver. Several DBBC3 systems are deployed in the field and more are under construction, with the number of 4-GHz bands ranging from two up to eight with resulting output data rates from 32 Gbps to 128 Gbps.
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