An MMIC Low-Noise Amplifier Design Technique Varonen, Mikko; Reeves, Rodrigo; Kangaslahti, Pekka ...
IEEE transactions on microwave theory and techniques,
03/2016, Letnik:
64, Številka:
3
Journal Article
Recenzirano
In this paper we discuss the design of low-noise amplifiers (LNAs) for both cryogenic and room-temperature operation in general and take the stability and linearity of the amplifiers into special ...consideration. Oscillations that can occur within a multi-finger transistor are studied and verified with simulations and measurements. To overcome the stability problem related to the multi-finger transistor design approach a parallel two-finger unit transistor monolithic microwave integrated circuit LNA design technique, which enables the design of wideband and high-linearity LNAs with very stable, predictable, and repeatable operation, is proposed. The feasibility of the proposed design technique is proved by demonstrating a three-stage LNA packaged in a WR10 waveguide housing and fabricated using a 35-nm InP HEMT technology that achieves more than a 20-dB gain from 75 to 116 GHz and 26-33-K noise temperature from 85 to 116 GHz when cryogenically cooled to 27 K.
We report the discovery of a rare new form of long-term radio variability in the light curves of active galaxies (AG)-symmetric achromatic variability (SAV)-a pair of opposed and strongly skewed ...peaks in the radio flux density observed over a broad frequency range. We propose that SAV arises through gravitational milli-lensing when relativistically moving features in AG jets move through gravitational lensing caustics created by subhalo condensates or black holes located within intervening galaxies. The lower end of this mass range has been inaccessible with previous gravitational lensing techniques. This new interpretation of some AG variability can easily be tested and if it passes these tests, will enable a new and powerful probe of cosmological matter distribution on these intermediate-mass scales, as well as provide, for the first time, micro-arcsecond resolution of the nuclei of AG-a factor of 30-100 greater resolution than is possible with ground-based millimeter very-long-baseline interferometry.
We compare the variability properties of very high energy gamma-ray emitting BL Lac objects in the optical and radio bands. We use variability information to distinguish multiple emission components ...in the jet, to be used as a guidance for spectral energy distribution modelling. Our sample includes 32 objects in the northern sky for which we have data for at least two years in both bands. We use optical R-band data from the Tuorla blazar monitoring program and 15 GHz radio data from the Owens Valley Radio Observatory blazar monitoring program. We estimate the variability amplitudes using the intrinsic modulation index, and study the time-domain connection by cross-correlating the optical and radio light curves assuming power law power spectral density. Our sample objects are in general more variable in the optical than radio. We find correlated flares in about half of the objects, and correlated long-term trends in more than 40% of the objects. In these objects we estimate that at least 10−50% of the optical emission originates in the same emission region as the radio, while the other half is due to faster variations not seen in the radio. This implies that simple single-zone spectral energy distribution models are not adequate for many of these objects.
ABSTRACT
Variability of a galaxy’s core radio source can be a significant consequence of active galactic nucleus accretion. However, this variability has not been well studied, particularly at high ...radio frequencies. As such, we report on a campaign monitoring the high radio frequency variability of 20 nearby, cool-core brightest cluster galaxies. From our representative sample, we show that most vary significantly on time-scales of approximately 1 yr and longer. Our highest cadence observations are at 15 GHz and are from the Owens Valley Radio Observatory. They have a median time interval of 7 d and mostly span between 8 and 13 yr. We apply a range of variability detection techniques to the sources’ light curves to analyse changes on week to decade long time-scales. Most notably, at least half of the sources show 20 per cent peak to trough variability on 3 yr time-scales, while at least a third vary by 60 per cent on 6 yr time-scales. Significant variability, which is important to studies of the Sunyaev–Zel’dovich Effect in the radio/sub-mm, is therefore a common feature of these sources. We also show how the variability relates to spectral properties at frequencies of up to 353 GHz using data from the Korean VLBI network, the NIKA2 instrument of the IRAM 30-m telescope, and the SCUBA-2 instrument of the James Clerk Maxwell Telescope.
SMILE: Search for MIlli-LEnses Casadio, C; Blinov, D; Readhead, A C S ...
Monthly notices of the Royal Astronomical Society. Letters,
10/2021, Letnik:
507, Številka:
1
Journal Article
Recenzirano
Odprti dostop
ABSTRACT
Dark matter (DM) haloes with masses below ∼108 M⊙, which would help to discriminate between DM models, may be detected through their gravitational effect on distant sources. The same applies ...to primordial black holes, considered as an alternative scenario to DM particle models. However, there is still no evidence for the existence of such objects. With the aim of finding compact objects in the mass range of ∼106–109 M⊙, we search for strong gravitational lenses on milliarcsec scales (<150 mas). For our search, we used the Astrogeo very long baseline interferometry (VLBI) fits image data base – the largest publicly available data base, containing multifrequency VLBI data of 13 828 individual sources. We used the citizen science approach to visually inspect all sources in all available frequencies in search for images with multiple compact components on mas scales. At the final stage, sources were excluded based on the surface brightness preservation criterion. We obtained a sample of 40 sources that passed all steps and therefore are judged to be mas lens candidates. These sources are currently followed up with ongoing European VLBI network observations at 5 and 22 GHz. Based on spectral index measurements, we suggest that two of our candidates have a higher probability to be associated with gravitational lenses.
Context.
Next-generation wide-field optical polarimeters such as Wide-Area Linear Optical Polarimeters (WALOPs) have a field of view (FoV) of tens of arcminutes. Wide-field polarimetric flat sources ...are essential to the efficient and accurate calibration of these instruments. However, no established wide-field polarimetric standard or flat sources exist at present.
Aims.
This study tests the feasibility of using the polarized sky patches of the size of around 10 × 10 arcminutes
2
, at a distance of up to 20° from the Moon, on bright-Moon nights as a wide-field linear polarimetric flat source.
Methods.
We observed 19 patches of the sky adjacent to the bright-Moon with the RoboPol instrument in the SDSS-
r
broadband filter. These patches were observed on five nights within two days of the full-Moon across two RoboPol observing seasons.
Results.
We find that for 18 of the 19 patches, the uniformity in the measured normalized Stokes parameters
q
and
u
is within 0.2%, with 12 patches exhibiting uniformity within 0.07% or better for both
q
and
u
simultaneously, making them reliable and stable wide-field linear polarization flats.
Conclusions.
We demonstrate that the sky on bright-Moon nights is an excellent wide-field linear polarization flat source. Various combinations of the normalized Stokes parameters
q
and
u
can be obtained by choosing suitable locations of the sky patch with respect to the Moon.
Abstract
We report Fermi Large Area Telescope (LAT) observations of the radio-loud active galactic nucleus SBS 0846+513 (z = 0.5835), optically classified as a narrow-line Seyfert 1 galaxy, together ...with new and archival radio-to-X-ray data. The source was not active at γ-ray energies during the first two years of Fermi operation. A significant increase in activity was observed during 2010 October-2011 August. In particular, a strong γ-ray flare was observed in 2011 June reaching an isotropic γ-ray luminosity (0.1-300 GeV) of 1.0 × 1048 erg s−1, comparable to that of the brightest flat spectrum radio quasars, and showing spectral evolution in γ rays. An apparent superluminal velocity of (8.2 ± 1.5)c in the jet was inferred from 2011 to 2012 Very Long Baseline Array (VLBA) images, suggesting the presence of a highly relativistic jet.
Both the power released by this object during the flaring activity and the apparent superluminal velocity are strong indications of the presence of a relativistic jet as powerful as those of blazars. In addition, variability and spectral properties in radio and γ-ray bands indicate blazar-like behaviour, suggesting that, except for some distinct optical characteristics, SBS 0846+513 could be considered as a young blazar at the low end of the blazar's black hole mass distribution.
ABSTRACT
The Andromeda galaxy (M 31) is our closest neighbouring spiral galaxy, making it an ideal target for studying the physics of the interstellar medium in a galaxy very similar to our own. ...Using new observations of M 31 at 4.76 GHz by the C-Band All-Sky Survey (C-BASS), and all available radio data at 1° resolution, we produce the integrated spectrum and put new constraints on the synchrotron spectral index and anomalous microwave emission (AME) from M 31. We use aperture photometry and spectral modelling to fit for the integrated spectrum of M 31, and subtract a comprehensive model of nearby background radio sources. The AME in M 31 is detected at 3σ significance with a peak near 30 GHz and flux density 0.27 ± 0.09 Jy. The synchrotron spectral index of M 31 is flatter than our own Galaxy at α =−0.66 ± 0.03 with no strong evidence of spectral curvature. The emissivity of AME averaged over the total emission from M 31 is lower than typical AME sources in our Galaxy, implying that AME is not uniformly distributed throughout M 31 and instead is likely confined to sub-regions – this will need to be confirmed using future higher resolution observations around 20–30 GHz.
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