scholarly journals MOJAVE. XIX. Brightness Temperatures and Intrinsic Properties of Blazar Jets

2021 ◽  
Vol 923 (1) ◽  
pp. 67
Author(s):  
D. C. Homan ◽  
M. H. Cohen ◽  
T. Hovatta ◽  
K. I. Kellermann ◽  
Y. Y. Kovalev ◽  
...  
Keyword(s):  
Brightness Temperature ◽  
Temperature Data ◽  
Line Of Sight ◽  
Field Energy ◽  
Intrinsic Properties ◽  
Brightness Temperatures ◽  
Magnetic Field Energy ◽  
Primary Difference ◽  
Doppler Factor ◽  
Bl Lacs

Abstract We present multiepoch, parsec-scale core brightness temperature observations of 447 active galactic nucleus (AGN) jets from the MOJAVE and 2 cm Survey programs at 15 GHz from 1994 to 2019. The brightness temperature of each jet over time is characterized by its median value and variability. We find that the range of median brightness temperatures for AGN jets in our sample is much larger than the variations within individual jets, consistent with Doppler boosting being the primary difference between the brightness temperatures of jets in their median state. We combine the observed median brightness temperatures with apparent jet speed measurements to find the typical intrinsic Gaussian brightness temperature of 4.1( ± 0.6) × 1010 K, suggesting that jet cores are at or below equipartition between particle and magnetic field energy in their median state. We use this value to derive estimates for the Doppler factor for every source in our sample. For the 309 jets with both apparent speed and brightness temperature data, we estimate their Lorentz factors and viewing angles to the line of sight. Within the BL Lac optical class, we find that high-synchrotron-peaked BL Lacs have smaller Doppler factors, lower Lorentz factors, and larger angles to the line of sight than intermediate and low-synchrotron-peaked BL Lacs. We confirm that AGN jets with larger Doppler factors measured in their parsec-scale radio cores are more likely to be detected in γ rays, and we find a strong correlation between γ-ray luminosity and Doppler factor for the detected sources.

scholarly journals Outflow Angles and Bulk Lorentz Factors for Different Categories of AGN

1997 ◽  
Vol 163 ◽  
pp. 695-696 ◽  
Author(s):  
Erick. J. Guerra ◽  
Ruth A. Daly
Keyword(s):  
Lorentz Factor ◽  
Radio Galaxies ◽  
Line Of Sight ◽  
Radio Sources ◽  
Bl Lacertae Objects ◽  
Bl Lacertae ◽  
High Polarization ◽  
Inverse Compton ◽  
Doppler Factor ◽  
Bl Lacs

AbstractRelativistic outflows from AGN can be parameterized by θ, the angle subtended by the direction of the outflow and the line of sight to the observer, and γ, the bulk Lorentz factor of the outflow. The Doppler factor, δ, and the apparent speed in the plane of the sky, βapp, are combinations of θ and γ. The Doppler factor can be estimated using either the equipartition Doppler factor, δeq (Readhead 1994), or the inverse Compton Doppler factor, δIC. These Doppler factor estimates are combined with observed βapp to solve for θ and γ for different categories of AGN.Ghisellini et al. (1993) compute δIC for 105 compact radio sources, and Güijosa & Daly (1996) compute δeq for the same sample. Daly, Guerra, & Güijosa (1996) estimate θ and γ for the 43 sources that have βapp listed by Vermeulen & Cohen (1994) and δeq computed by Güijosa & Daly (1996).Solutions and errors for θ and γ are presented in Figures 1 and 2 using δeq and δIC respectively. Guerra & Daly (1996) discuss these estimates and errors in greater detail. These AGN fall into the following categories: BL Lacertae objects (BL Lacs), core-dominated high-polarization quasars (CDHPQ), core-dominated low-polarization quasars (CDLPQ), core-dominated quasars with no polarization information (CDQ(NPI)), lobe-dominated quasars (LDQ), and radio galaxies (RG).

scholarly journals Radio variability of blazars

2010 ◽  
Vol 6 (S275) ◽  
pp. 164-167
Author(s):  
J. H. Fan ◽  
W. Xu ◽  
J. Pan ◽  
Y. H. Yuan
Keyword(s):  
Brightness Temperature ◽  
Data Base ◽  
Preliminary Data ◽  
Light Curves ◽  
Spectrum Radio ◽  
Variability Index ◽  
Doppler Factor ◽  
Bl Lacs

AbstractIn this work, we present the analysis results using UMRAO preliminary data base. We used the light curves 1) to get the shortest timescales and then to get the brightness temperature so that we can estimate the Doppler factors; 2) to investigate the periodicity and discuss the variability index. We also used the data base to discuss the polarization properties of blazars. We found that the periodicity distribution in BL Lacs and that in the flat spectrum radio quasars should be from the same distribution. The Doppler factor in FSRQs is higher than that in BL. The polarization in BLs are higher than that in the flat spectrum radio quasars

scholarly journals Characterization of a hoar-development episode using SSM/I brightness temperatures in the vicinity of the GISP2 site, Greenland

1993 ◽  
Vol 17 ◽  
pp. 183-188 ◽  
Author(s):  
Christopher A. Shuman ◽  
Richard B. Alley ◽  
Sridhar Anandakrishnan
Keyword(s):  
Brightness Temperature ◽  
Surface Density ◽  
Temperature Data ◽  
Gradual Decline ◽  
Near Surface ◽  
Microwave Brightness Temperature ◽  
Brightness Temperatures ◽  
Microwave Imager ◽  
High Winds

Formation of a surface-hoar/depth-hoar complex at the GISP2 site in central Greenland was correlated with large changes in Special Sensor Microwave/Imager (SSM/I) brightness-temperature data. Pass-averaged SSM/I brightness-temperature data over a 1/2° latitude by 1° longitude cell for the 19 and 37 GHz, vertically (V) and horizontally (Η) polarized bands were manipulated to yield differential (V-Η) trends which clearly show a gradual decline as the hoar formation caused a progressively rougher surface with progressively lower density. The hoar episode ended as snowfall, and high winds buried and destroyed the surface-hoar layer and caused rapid V-Η increases in ≈ 1 day. Comparison of the different trends with changes in the field-monitored variables and theoretical values suggest that the V-Η trends are sensitive primarily to changes in surface roughness, and secondarily to near-surface density changes. Consistent expression of trends in microwave brightness temperature over 35 adjacent study cells indicates that this technique may provide a remote-sensing signature capable of defining the timing and spatial extent of surface- and depth-hoar formation in central Greenland.

scholarly journals Characterization of a hoar-development episode using SSM/I brightness temperatures in the vicinity of the GISP2 site, Greenland

1993 ◽  
Vol 17 ◽  
pp. 183-188 ◽  
Author(s):  
Christopher A. Shuman ◽  
Richard B. Alley ◽  
Sridhar Anandakrishnan
Keyword(s):  
Brightness Temperature ◽  
Surface Density ◽  
Temperature Data ◽  
Gradual Decline ◽  
Near Surface ◽  
Microwave Brightness Temperature ◽  
Brightness Temperatures ◽  
Microwave Imager ◽  
High Winds

Formation of a surface-hoar/depth-hoar complex at the GISP2 site in central Greenland was correlated with large changes in Special Sensor Microwave/Imager (SSM/I) brightness-temperature data. Pass-averaged SSM/I brightness-temperature data over a 1/2° latitude by 1° longitude cell for the 19 and 37 GHz, vertically (V) and horizontally (Η) polarized bands were manipulated to yield differential (V-Η) trends which clearly show a gradual decline as the hoar formation caused a progressively rougher surface with progressively lower density. The hoar episode ended as snowfall, and high winds buried and destroyed the surface-hoar layer and caused rapid V-Η increases in ≈ 1 day. Comparison of the different trends with changes in the field-monitored variables and theoretical values suggest that the V-Η trends are sensitive primarily to changes in surface roughness, and secondarily to near-surface density changes. Consistent expression of trends in microwave brightness temperature over 35 adjacent study cells indicates that this technique may provide a remote-sensing signature capable of defining the timing and spatial extent of surface- and depth-hoar formation in central Greenland.

INFLUENCE OF REGULARITY OF SATELLITE MICROWAVE RADIOMETER MEASUREMENTS ON THE ACCURACY OF BRIGHTNESS TEMPERATURE REPRODUCTION IN AREAS OF THE TROPICAL CYCLONES

2021 ◽  
pp. 78-85
Author(s):  
А. G. Grankov ◽  
◽  
А. А. Milshin ◽  
Keyword(s):  
Brightness Temperature ◽  
Tropical Cyclones ◽  
Microwave Radiometer ◽  
Measurement Data ◽  
Hurricane Wilma ◽  
Brightness Temperatures ◽  
Atmosphere System ◽  
Ocean Atmosphere ◽  
The Caribbean ◽  
Satellite Microwave

An accuracy of reproduction of daily variations in the ocean–atmosphere system brightness temperature in the areas of development and movement of tropical hurricanes in the Caribbean Sea and Gulf of Mexico is analyzed. The analysis is based on the data of single and group satellite microwave radiometer measurements. The results are obtained using archival measurement data of SSM/I radiometers from the F11, F13, F14, and F15 DMSP satellites during the period of existence of tropical hurricanes Bret and Wilma. An example is given to demonstrate the use of daily brightness temperatures obtained from DMSP satellites for monitoring the development and propagation of hurricane Wilma.

scholarly journals Exploring the bulk of the BL Lacertae object population

2018 ◽  
Vol 618 ◽  
pp. A175
Author(s):  
F. D’Ammando ◽  
M. Giroletti ◽  
S. Rainó
Keyword(s):  
Radio Emission ◽  
Test Statistic ◽  
Large Area ◽  
Bl Lacertae ◽  
The Third ◽  
Long Baseline ◽  
Photon Index ◽  
Multi Wavelength ◽  
Doppler Factor ◽  
Bl Lacs

Aims. We are studying an unbiased sample of 42 nearby (z < 0.2) BL Lacertae objects with a multi-wavelength approach. The results of Very Long Baseline Interferometry observations were presented in the first paper of this series. In this paper, we study the γ-ray properties of the sample. Methods. We analyse data collected by the Fermi-Large Area Telescope (LAT) during its first 8.5 yr of operation in the energy range 0.1−300 GeV. Results. We reveal 23 sources with a test statistic greater than 25 (corresponding to ~4.6-σ) out of 42, with 3 sources not detected in the third LAT active galactic nucleus (AGN) catalogue, and fluxes between 3.5 × 10−10 and 7.4 × 10−8 ph cm−2 s−1. The majority of the sources have hard spectra (Γ ≤ 2), with only four having values in the range 2.1–2.4. The three newly detected sources have fluxes in the range between 0.54 × 10−9 and 1.35 × 10−9 ph cm−2 s−1 and photon index 1.7–1.9. Among the 23 LAT-detected sources, 19 are included in the third catalogue of hard Fermi-LAT sources, with a spectrum that connects relatively smoothly from 0.1 GeV to 2 TeV. LAT-detected BL Lacs are more luminous on parsec scales with respect to non-LAT-detected sources and have larger core dominance according to the unified models. Conclusions. The LAT-detected BL Lacs seem to be composed of a bulk of “classical” sources dominated by Doppler boosting and characterised by compact and bright radio emission as well as hard γ-ray spectra. Moreover, we have identified a possible population of low-luminosity BL Lacs not detected by LAT, lacking a VLBI core, and with a small Doppler factor. Furthermore, three LAT-detected sources show non-classical properties for γ-ray emitting BL Lacs (no evidence of relativistic jet, low Doppler factor in radio images, relatively low core dominance) and three other sources, while showing radio emission on parsec scales, are not detected in γ rays so far.

scholarly journals Comparison of SMMR and SSM/I passive microwave data collected over Antarctica

1993 ◽  
Vol 17 ◽  
pp. 131-136 ◽  
Author(s):  
Kenneth C. Jezek ◽  
Carolyn J. Merry ◽  
Don J. Cavalieri
Keyword(s):  
Brightness Temperature ◽  
Microwave Radiometer ◽  
Correlated Data ◽  
Data Sets ◽  
Microwave Brightness Temperature ◽  
Brightness Temperatures ◽  
Scatter Plots ◽  
Microwave Imager ◽  
Highly Correlated ◽  
The Antarctic

Spaceborne data are becoming sufficiently extensive spatially and sufficiently lengthy over time to provide important gauges of global change. There is a potentially long record of microwave brightness temperature from NASA's Scanning Multichannel Microwave Radiometer (SMMR), followed by the Navy's Special Sensor Microwave Imager (SSM/I). Thus it is natural to combine data from successive satellite programs into a single, long record. To do this, we compare brightness temperature data collected during the brief overlap period (7 July-20 August 1987) of SMMR and SSM/I. Only data collected over the Antarctic ice sheet are used to limit spatial and temporal complications associated with the open ocean and sea ice. Linear regressions are computed from scatter plots of complementary pairs of channels from each sensor revealing highly correlated data sets, supporting the argument that there are important relative calibration differences between the two instruments. The calibration scheme was applied to a set of average monthly brightness temperatures for a sector of East Antarctica.

scholarly journals Combined SMAP–SMOS thin sea ice thickness retrieval

2019 ◽  
Vol 13 (2) ◽  
pp. 675-691 ◽  
Author(s):  
Cătălin Paţilea ◽  
Georg Heygster ◽  
Marcus Huntemann ◽  
Gunnar Spreen
Keyword(s):  
Soil Moisture ◽  
Sea Ice ◽  
Brightness Temperature ◽  
Incidence Angle ◽  
Data Availability ◽  
Ice Thickness ◽  
Polar Regions ◽  
Sea Ice Thickness ◽  
Brightness Temperatures ◽  
Spatial Coverage

Abstract. The spaceborne passive microwave sensors Soil Moisture Ocean Salinity (SMOS) and Soil Moisture Active Passive (SMAP) provide brightness temperature data in the L band (1.4 GHz). At this low frequency the atmosphere is close to transparent and in polar regions the thickness of thin sea ice can be derived. SMOS measurements cover a large incidence angle range, whereas SMAP observes at a fixed 40∘ incidence angle. By using brightness temperatures at a fixed incidence angle obtained directly (SMAP), or through interpolation (SMOS), thin sea ice thickness retrieval is more consistent as the incidence angle effects do not have to be taken into account. Here we transfer a retrieval algorithm for the thickness of thin sea ice (up to 50 cm) from SMOS data at 40 to 50∘ incidence angle to the fixed incidence angle of SMAP. The SMOS brightness temperatures (TBs) at a given incidence angle are estimated using empirical fit functions. SMAP TBs are calibrated to SMOS to provide a merged SMOS–SMAP sea ice thickness product. The new merged SMOS–SMAP thin ice thickness product was improved upon in several ways compared to previous thin ice thickness retrievals. (i) The combined product provides a better temporal and spatial coverage of the polar regions due to the usage of two sensors. (ii) The radio frequency interference (RFI) filtering method was improved, which results in higher data availability over both ocean and sea ice areas. (iii) For the intercalibration between SMOS and SMAP brightness temperatures the root mean square difference (RMSD) was reduced by 30 % relative to a prior attempt. (iv) The algorithm presented here allows also for separate retrieval from any of the two sensors, which makes the ice thickness dataset more resistant against failure of one of the sensors. A new way to estimate the uncertainty of ice thickness retrieval was implemented, which is based on the brightness temperature sensitivities.

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