Brightness temperature constraints from interferometric visibilities

Bruni

et al. 2016

ApJ

138

165

We present the first polarimetric space very long baseline interferometry (VLBI) imaging observations at 22 GHz. BLLacertae was observed in 2013 November 10 with the RadioAstron space VLBI mission, including a ground array of 15 radio telescopes. The instrumental polarization of the space radio telescope is found to be less than 9%, demonstrating the polarimetric imaging capabilities of RadioAstron at 22 GHz. Ground-space fringes were obtained up to a projected baseline distance of 7.9Earth diameters in length, allowing us to image the jet in BLLacertae with a maximum angular resolution of 21 μas, the highest achieved to date. We find evidence for emission upstream of the radio core, which may correspond to a recollimation shock at about 40 μas from the jet apex, in a pattern that includes other recollimation shocks at approximately 100 and 250 μas from the jet apex. Polarized emission is detected in two components within the innermost 0.5 mas from the core, as well as in some knots 3 mas downstream. Faraday rotation analysis, obtained from combining RadioAstron 22 GHz and groundbased 15 and 43 GHz images, shows a gradient in rotation measure and Faraday-corrected polarization vector as a function of position angle with respect to the core, suggesting that the jet in BLLacertae is threaded by a helical magnetic field. The intrinsic de-boosted brightness temperature in the unresolved core exceeds 3 10 12 K, suggesting, at the very least, departure from equipartition of energy between the magnetic field and radiating particles.

Section: Brightness Temperaturementioning

confidence: 74%

Section: Brightness Temperaturementioning

confidence: 99%

PROBING THE INNERMOST REGIONS OF AGN JETS AND THEIR MAGNETIC FIELDS WITH RADIOASTRON. I. IMAGING BL LACERTAE AT 21 μas RESOLUTION

Gómez

Bruni

et al. 2016

ApJ

138

165

“…This corresponds to brightness temperature of 4.0 × 10 13 K in the rest frame of the source. If the brightness temperature is generally determined by the transverse dimension of the flow, as suggested by the analysis of the MOJAVE visibility data (Lobanov 2015), the inner jet of 0642+449 should have a width of ≈0.15 mas to satisfy the limiting brightness temperature estimated above.…”

Section: C1mentioning

confidence: 99%

“…This value is somewhat lower than the one that should be expected at 1.6 GHz, based on the trend log 10 T b [K] = 13.3 − 1.1 log 10 ν[GHz] that can be obtained from the existing measurements at 5, 15, and 86 GHz (Lobanov et al 2000;Kovalev et al 2005;Dodson et al 2008;Lee et al 2008). However, since the morphology of the emitting region can be different from a simple Gaussian shape, the actual brightness temperature of the emission may differ from the modelfit-based estimates, for instance, if the brightness distribution is smooth and the respective brightness temperature is largely determined mostly by the transverse dimension of the flow (Lobanov 2015). To investigate this possibility, the modelfit-based estimates of T b can be compared with estimates obtained directly from visibility amplitudes and their errors, which yields the absolute minimum brightness temperature, T b,min , and an estimate of the limiting brightness temperature, T b,lim that can obtained from the data under the requirements that the structural detail sampled by the given visibility is resolved (Lobanov 2015).…”

Section: Brightness Temperaturementioning

confidence: 99%

RadioAstron space VLBI imaging of polarized radio emission in the high-redshift quasar 0642+449 at 1.6 GHz

Gómez

Bruni

et al. 2015

A&A

Context. Polarization of radio emission in extragalactic jets at a sub-milliarcsecond angular resolution holds important clues for understanding the structure of the magnetic field in the inner regions of the jets and in close vicinity of the supermassive black holes in the centers of active galaxies. Aims. Space VLBI observations provide a unique tool for polarimetric imaging at a sub-milliarcsecond angular resolution and studying the properties of magnetic field in active galactic nuclei on scales of less than 10 4 gravitational radii. Methods. A space VLBI observation of high-redshift quasar TXS 0642+449 (OH 471), made at a wavelength of 18 cm (frequency of 1.6 GHz) as part of the early science programme (ESP) of the RadioAstron mission, is used here to test the polarimetric performance of the orbiting Space Radio Telescope (SRT) employed by the mission, to establish a methodology for making full Stokes polarimetry with space VLBI at 1.6 GHz, and to study the polarized emission in the target object on sub-milliarcsecond scales. Results. Polarization leakage of the SRT at 18 cm is found to be within 9% in amplitude, demonstrating the feasibility of high fidelity polarization imaging with RadioAstron at this wavelength. A polarimetric image of 0642+449 with a resolution of 0.8 mas (signifying an ∼4 times improvement over ground VLBI observations at the same wavelength) is obtained. The image shows a compact corejet structure with low (≈2%) polarization and predominantly transverse magnetic field in the nuclear region. The VLBI data also uncover a complex structure of the nuclear region, with two prominent features possibly corresponding to the jet base and a strong recollimation shock. The maximum brightness temperature at the jet base can be as high as 4 × 10 13 K.

“…If the size of the Gaussian component d is less than d min , given by Equation (3), the latter is used for estimating the lower limit on T b . In addition to this estimate, we also use visibility-based estimates of brightness temperature (Lobanov 2015) and calculate the minimum brightness temperature,…”

Section: Brightness Temperature Estimatesmentioning

confidence: 99%

Global Millimeter VLBI Array Survey of Ultracompact Extragalactic Radio Sources at 86 GHz

Nair

2019 URSI Asia-Pacific Radio Science Conference (AP-RASC)

Krichbaum

et al. 2019

Context. Very long baseline interferometry (VLBI) observations at 86 GHz (wavelength, λ = 3 mm) reach a resolution of about 50 µas, probing the collimation and acceleration regions of relativistic outflows in active galactic nuclei (AGN). The physical conditions in these regions can be studied by performing 86 GHz VLBI surveys of representative samples of compact extragalactic radio sources. Aims. To extend the statistical studies of compact extragalactic jets, a large global 86 GHz VLBI survey of 162 compact radio sources was conducted in 2010-2011 using the Global Millimeter VLBI Array (GMVA). Methods. The survey observations were made in a snapshot mode, with up to five scans per target spread over a range of hour angles in order to optimize the visibility coverage. The survey data attained a typical baseline sensitivity of 0.1 Jy and a typical image sensitivity of 5 mJy/beam, providing successful detections and images for all of the survey targets. For 138 objects, the survey provides the first ever VLBI images made at 86 GHz. Gaussian model fitting of the visibility data was applied to represent the structure of the observed sources and to estimate the flux densities and sizes of distinct emitting regions (components) in their jets. These estimates were used for calculating the brightness temperature (T b ) at the jet base (core) and in one or more moving regions (jet components) downstream from the core. These model-fit-based estimates of T b were compared to the estimates of brightness temperature limits made directly from the visibility data, demonstrating a good agreement between the two methods. Results. The apparent brightness temperature estimates for the jet cores in our sample range from 2.5 × 10 9 K to 1.3 × 10 12 K, with the mean value of 1.8 × 10 11 K. The apparent brightness temperature estimates for the inner jet components in our sample range from 7.0 × 10 7 K to 4.0 × 10 11 K. A simple population model with a single intrinsic value of brightness temperature, T 0 , is applied to reproduce the observed distribution. It yields T 0 = (3.77 +0.10 −0.14 ) × 10 11 K for the jet cores, implying that the inverse Compton losses dominate the emission. In the nearest jet components, T 0 = (1.42 +0.16 −0.19 )× 10 11 K is found, which is slightly higher than the equipartition limit of ∼ 5 × 10 10 K expected for these jet regions. For objects with sufficient structural detail detected, the adiabatic energy losses are shown to dominate the observed changes of brightness temperature along the jet.