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We present Space-VLBI RadioAstron observations at 1.6 GHz and 4.8 GHz of the flat spectrum radio quasar 3C 273, with detections on baselines up to 4.5 and 3.3 Earth Diameters, respectively. Achieving the best angular resolution at 1.6 GHz to date, we have imaged limb-brightening in the jet, not previously detected in this source. In contrast, at 4.8 GHz, we detected emission from a central stream of plasma, with a spatial distribution complementary to the limb-brightened emission, indicating an origin in the spine of the jet. While a stratification across the jet width in the flow density, internal energy, magnetic field, or bulk flow velocity are usually invoked to explain the limb-brightening, the different jet structure detected at the two frequencies probably requires a stratification in the emitting electron energy distribution. Future dedicated numerical simulations will allow the determination of which combination of physical parameters are needed to reproduce the spine-sheath structure observed by Space-VLBI with RadioAstron in 3C 273. © ESO 2021. ; JLG and AF acknowledge financial support from the Spanish Ministerio de Economia y Competitividad (grants AYA2016-80889-P, PID2019-108995GB-C21), the Consejeria de Economia, Conocimiento, Empresas y Universidad of the Junta de Andalucia (grant P18-FR-1769), the Consejo Superior de Investigaciones Cientificas (grant 2019AEP112), and the State Agency for Research of the Spanish MCIU through the Center of Excellence Severo Ochoa award for the Instituto de Astrofisica de Andalucia (SEV-2017-0709). APL, YYK, and ABP were supported by the Russian Science Foundation (project 20-62-46021). TS was partly supported by the Academy of Finland projects 274477 and 315721. MP acknowledges the support by the Spanish Ministerio de Ciencia e Innovacion (MICINN) under grant PID2019-105510GB-C31. MP and JMM acknowledge financial support from the Spanish Ministry of Science through Grants PID2019-107427GB-C33 and AYA2016-77237-C3-3-P, and from the Generalitat Valenciana through grant PROMETEU/2019/071. JMA was supported by the German Research Foundation grant HE5937/2-2. LIG acknowledges support by the CSIRO Distinguished Visitor Programme. The RadioAstron project is led by the Astro Space Center of the Lebedev Physical Institute of the Russian Academy of Sciences and the Lavochkin Scientific and Production Association under a contract with the Roscosmos State Corporation, in collaboration with partner organizations in Russia and other countries. This publication has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No. 730562 [RadioNet]. ; Peer reviewed

Funding Information: We thank the anonymous referee for useful comments. JLG and AF acknowledge financial support from the Spanish Ministerio de Economia y Competitividad (grants AYA2016-80889-P, PID2019-108995GBC21), the Consejeria de Economia, Conocimiento, Empresas y Universidad of the Junta de Andalucia (grant P18-FR-1769), the Consejo Superior de Investigaciones Cientificas (grant 2019AEP112), and the State Agency for Research of the Spanish MCIU through the Center of Excellence Severo Ochoa award for the Instituto de Astrofisica de Andalucia (SEV-2017-0709). APL, YYK, and ABP were supported by the Russian Science Foundation (project 20-62-46021). TS was partly supported by the Academy of Finland projects 274477 and 315721. MP acknowledges the support by the Spanish Ministerio de Ciencia e Innovacion (MICINN) under grant PID2019-105510GB-C31. MP and JMM acknowledge financial support from the Spanish Ministry of Science through Grants PID2019-107427GB-C33 and AYA2016-77237-C3-3-P, and from the Generalitat Valenciana through grant PROMETEU/2019/071. JMA was supported by the German Research Foundation grant HE5937/2-2. LIG acknowledges support by the CSIRO Distinguished Visitor Programme. Funding Information: Acknowledgements. We thank the anonymous referee for useful comments. JLG and AF acknowledge financial support from the Spanish Ministerio de Economía y Competitividad (grants AYA2016-80889-P, PID2019-108995GB-C21), the Consejería de Economía, Conocimiento, Empresas y Universidad of the Junta de Andalucía (grant P18-FR-1769), the Consejo Superior de Investi-gaciones Científicas (grant 2019AEP112), and the State Agency for Research of the Spanish MCIU through the Center of Excellence Severo Ochoa award for the Instituto de Astrofísica de Andalucía (SEV-2017-0709). APL, YYK, and ABP were supported by the Russian Science Foundation (project 20-62-46021). TS was partly supported by the Academy of Finland projects 274477 and 315721. MP acknowledges the support by the Spanish Ministerio de Ciencia e Innovación (MICINN) under grant PID2019-105510GB-C31. MP and JMM acknowledge financial support from the Spanish Ministry of Science through Grants PID2019-107427GB-C33 and AYA2016-77237-C3-3-P, and from the Generali-tat Valenciana through grant PROMETEU/2019/071. JMA was supported by the German Research Foundation grant HE5937/2-2. LIG acknowledges support by the CSIRO Distinguished Visitor Programme. The RadioAstron project is led by the Astro Space Center of the Lebedev Physical Institute of the Russian Academy of Sciences and the Lavochkin Scientific and Production Association under a contract with the Roscosmos State Corporation, in collaboration with partner organizations in Russia and other countries. This publication has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No. 730562 [RadioNet]. This paper includes data observed with the 100-m Effelsberg radio-telescope, which is operated by the Max-Planck-Institut für Radioastronomie in Bonn (Germany). The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. The European VLBI Network is a joint facility of independent European, African, Asian, and North American radio astronomy institutes. The Long Baseline Array is part of the Australia Telescope National Facility which is funded by the Australian Government for operation as a National Facility managed by CSIRO. This research made use of Python (http://www.python.org), Numpy (van der Walt et al. 2011), Pandas (McKinney 2010), and Matplotlib (Hunter 2007). We also made use of Astropy Publisher Copyright: © ESO 2021. ; We present Space-VLBI RadioAstron observations at 1.6 GHz and 4.8 GHz of the flat spectrum radio quasar 3C 273, with detections on baselines up to 4.5 and 3.3 Earth Diameters, respectively. Achieving the best angular resolution at 1.6 GHz to date, we have imaged limb-brightening in the jet, not previously detected in this source. In contrast, at 4.8 GHz, we detected emission from a central stream of plasma, with a spatial distribution complementary to the limb-brightened emission, indicating an origin in the spine of the jet. While a stratification across the jet width in the flow density, internal energy, magnetic field, or bulk flow velocity are usually invoked to explain the limb-brightening, the different jet structure detected at the two frequencies probably requires a stratification in the emitting electron energy distribution. Future dedicated numerical simulations will allow the determination of which combination of physical parameters are needed to reproduce the spine-sheath structure observed by Space-VLBI with RadioAstron in 3C 273. ; Peer reviewed