72 research outputs found
Coherent Receiver Arrays for Astronomy and Remote Sensing
Monolithic Millimeter-wave Integrated Circuits (MMICs) provide a level of integration that makes possible
the construction of large focal plane arrays of radio-frequency detectors—effectively the first “Radio
Cameras”—and these will revolutionize radio-frequency observations with single dishes, interferometers,
spectrometers, and spacecraft over the next two decades. The key technological advances have been
made at the Jet Propulsion Laboratory (JPL) in collaboration with the Northrop Grumman Corporation
(NGC). Although dramatic progress has been made in the last decade in several important areas, including
(i) packaging that enables large coherent detector arrays, (ii) extending the performance of amplifiers
to much higher frequencies, and (iii) reducing room-temperature noise at high frequencies, funding to
develop MMIC performance at cryo-temperatures and at frequencies below 150GHz has dropped nearly
to zero over the last five years. This has severely hampered the advance of the field. Moreover, because
of the high visibility of < 150GHz cryogenic detectors in astrophysics and cosmology, lack of progress in
this area has probably had a disproportionate impact on perceptions of the potential of coherent detectors
in general.
One of the prime objectives of the Keck Institute for Space Studies (KISS) is to select crucial areas of
technological development in their embryonic stages, when relatively modest funding can have a highly
significant impact by catalyzing collaborations between key institutions world-wide, supporting in-depth
studies of the current state and potential of emerging technologies, and prototyping development of key
components—all potentially leading to strong agency follow-on funding.
The KISS large program “Coherent Instrumentation for Cosmic Microwave Background Observations”
was initiated in order to investigate the scientific potential and technical feasibility of these “Radio
Cameras.” This opens up the possibility of bringing support to this embryonic area of detector development
at a critical phase during which KISS can catalyze and launch a coherent, coordinated, worldwide
effort on the development of MMIC Arrays. A number of key questions, regarding (i) the importance and
breadth of the scientific drivers, (ii) realistic limits on sensitivity, (iii) the potential of miniaturization into
receiver “modules,” and (iv) digital signal processing, needed to be studied carefully before embarking on
a major MMIC Array development effort led by Caltech/JPL/NGC and supported by KISS, in the hope
of attracting adequate subsequent government funding. For this purpose a large study was undertaken
under the sponsorship and aegis of KISS. The study began with a workshop in Pasadena on “MMIC
Array Receivers and Spectrographs” (July 21–25, 2008)1, immediately after an international conference
“CMB Component Separation and the Physics of Foregrounds” (July 14–18, 2008)2 that was organized in
conjunction with the MMIC workshop. There was then an eight-month study period, culminating in a
final “MMIC 2Workshop” (March 23–27, 2009).3 These workshops were very well attended, and brought
together the major international groups and scientists in the field of coherent radio-frequency detector
arrays. A notable aspect of the workshops is that they were well attended by young scientists—there
are many graduate students and post-doctoral fellows coming into this area. The two workshops focused
both on detailed discussions of key areas of interest and on the writing of this report. They were
conducted in a spirit of full and impartial scrutiny of the pros and cons of MMICs, in order to make an
objective assessment of their potential. It serves no useful purpose to pursue lines of technology development
based on unrealistic and over-optimistic projections. This is crucially important for KISS, Caltech,
and JPL which can only have real impact if they deliver on the promise of the technologies they develop.
A broad range of opinions was evident at the start of the first workshop, but in the end a strong consensus
was achieved on the most important questions that had emerged. This report reflects the workshop
deliberations and that consensus.
The key scientific drivers for the development of the MMIC technology are: (i) large angular-scale Bmode
polarization observations of the cosmic microwave background—here MMICs are one of two key
technologies under development at JPL, both of which are primary detectors on the recently-launched
Planck mission; (ii) large-field spectroscopic surveys of the Galaxy and nearby galaxies at high spectral
resolution, and of galaxy clusters at low resolution; (iii) wide-field imaging via deployment as focal plane
arrays on interferometers; (iv) remote sensing of the atmosphere and Earth; and (v) wide-field imaging in
planetary missions. These science drivers are discussed in the report.
The most important single outcome of the workshops, and a sine qua non of this whole program,
is that consensus was reached that it should be possible to reduce the noise of individual HEMTs or
MMICs operating at cryogenic temperatures to less than three times the quantum limit at frequencies up
to 150 GHz, by working closely with a foundry (in this case NGC) and providing rapid feedback on the
performance of the devices they are fabricating, thus enabling tests of the effects of small changes in the
design of these transistors. This kind of partnership has been very successful in the past, but can now be
focused more intensively on cryogenic performance by carrying out tests of MMIC wafers, including tests
on a cryogenic probe station. It was felt that a properly outfitted university laboratory dedicated to this
testing and optimization would be an important element in this program, which would include MMIC
designs, wafer runs, and a wide variety of tests of MMIC performance at cryogenic temperatures.
This Study identified eight primary areas of technology development, including the one singled out
above, which must be actively pursued in order to exploit the full potential of MMIC Arrays in a timely
fashion:
1. Reduce the noise levels of individual transistors and MMICs to three times the quantum limit or
lower at cryogenic temperatures at frequencies up to 150 GHz.
2. Integrate high-performing MMICs into the building blocks of large arrays without loss of performance.
Currently factors of two in both noise and bandwidth are lost at this step.
3. Develop high performance, low mass, inexpensive feed arrays.
4. Develop robust interconnects and wiring that allow easy fabrication and integration of large arrays.
5. Develop mass production techniques suitable for arrays of differing sizes.
6. Reduce mass and power. (Requirements will differ widely with application. In the realm of planetary
instruments, this is often the most important single requirement.)
7. Develop planar orthomode transducers with low crosstalk and broad bandwidth.
8. Develop high power and high efficiency MMIC amplifiers for LO chains, etc.
Another important outcome of the two workshops was that a number of new collaborations were
forged between leading groups worldwide with the object of focusing on the development of MMIC
arrays
Polarization Observations with the Cosmic Background Imager
We describe polarization observations of the CMBR with the Cosmic Background Imager, a 13 element interferometer which operates in the 26-36 GHz band from Llano de Chajnantour in northern Chile. The array consists of 90-cm Cassegrain antennas mounted on a steerable platform which can be rotated about the optical axis to facilitate polarization observations. The CBI employs single mode circularly polarized receivers which sample multipoles from â„“~400
to â„“~4250. The instrumental polarization of the CBI was calibrated with 3C279, a bright polarized point source
which was monitored with the VLA
Optical Spectroscopy of Bright Fermi LAT Blazars
We report on HET and Palomar 5 m spectroscopy of recently identified
-ray blazars in the {\it Fermi} LAT Bright Source List. These data
provide identifications for 10 newly discovered -ray flat spectrum
radio quasars (FSRQ) and six new BL Lacs plus improved spectroscopy for six
additional BL Lacs. We substantially improve the identification completeness of
the bright LAT blazars and give new redshifts and constraints, new
estimates of the black hole masses and new measurements of the optical SED.Comment: 8 pages, 5 figures, 2 tables. Accepted for publication in Ap
Coherent Receiver Arrays for Astronomy and Remote Sensing
Monolithic Millimeter-wave Integrated Circuits (MMICs) provide a level of integration that makes possible
the construction of large focal plane arrays of radio-frequency detectors—effectively the first “Radio
Cameras”—and these will revolutionize radio-frequency observations with single dishes, interferometers,
spectrometers, and spacecraft over the next two decades. The key technological advances have been
made at the Jet Propulsion Laboratory (JPL) in collaboration with the Northrop Grumman Corporation
(NGC). Although dramatic progress has been made in the last decade in several important areas, including
(i) packaging that enables large coherent detector arrays, (ii) extending the performance of amplifiers
to much higher frequencies, and (iii) reducing room-temperature noise at high frequencies, funding to
develop MMIC performance at cryo-temperatures and at frequencies below 150GHz has dropped nearly
to zero over the last five years. This has severely hampered the advance of the field. Moreover, because
of the high visibility of < 150GHz cryogenic detectors in astrophysics and cosmology, lack of progress in
this area has probably had a disproportionate impact on perceptions of the potential of coherent detectors
in general.
One of the prime objectives of the Keck Institute for Space Studies (KISS) is to select crucial areas of
technological development in their embryonic stages, when relatively modest funding can have a highly
significant impact by catalyzing collaborations between key institutions world-wide, supporting in-depth
studies of the current state and potential of emerging technologies, and prototyping development of key
components—all potentially leading to strong agency follow-on funding.
The KISS large program “Coherent Instrumentation for Cosmic Microwave Background Observations”
was initiated in order to investigate the scientific potential and technical feasibility of these “Radio
Cameras.” This opens up the possibility of bringing support to this embryonic area of detector development
at a critical phase during which KISS can catalyze and launch a coherent, coordinated, worldwide
effort on the development of MMIC Arrays. A number of key questions, regarding (i) the importance and
breadth of the scientific drivers, (ii) realistic limits on sensitivity, (iii) the potential of miniaturization into
receiver “modules,” and (iv) digital signal processing, needed to be studied carefully before embarking on
a major MMIC Array development effort led by Caltech/JPL/NGC and supported by KISS, in the hope
of attracting adequate subsequent government funding. For this purpose a large study was undertaken
under the sponsorship and aegis of KISS. The study began with a workshop in Pasadena on “MMIC
Array Receivers and Spectrographs” (July 21–25, 2008)1, immediately after an international conference
“CMB Component Separation and the Physics of Foregrounds” (July 14–18, 2008)2 that was organized in
conjunction with the MMIC workshop. There was then an eight-month study period, culminating in a
final “MMIC 2Workshop” (March 23–27, 2009).3 These workshops were very well attended, and brought
together the major international groups and scientists in the field of coherent radio-frequency detector
arrays. A notable aspect of the workshops is that they were well attended by young scientists—there
are many graduate students and post-doctoral fellows coming into this area. The two workshops focused
both on detailed discussions of key areas of interest and on the writing of this report. They were
conducted in a spirit of full and impartial scrutiny of the pros and cons of MMICs, in order to make an
objective assessment of their potential. It serves no useful purpose to pursue lines of technology development
based on unrealistic and over-optimistic projections. This is crucially important for KISS, Caltech,
and JPL which can only have real impact if they deliver on the promise of the technologies they develop.
A broad range of opinions was evident at the start of the first workshop, but in the end a strong consensus
was achieved on the most important questions that had emerged. This report reflects the workshop
deliberations and that consensus.
The key scientific drivers for the development of the MMIC technology are: (i) large angular-scale Bmode
polarization observations of the cosmic microwave background—here MMICs are one of two key
technologies under development at JPL, both of which are primary detectors on the recently-launched
Planck mission; (ii) large-field spectroscopic surveys of the Galaxy and nearby galaxies at high spectral
resolution, and of galaxy clusters at low resolution; (iii) wide-field imaging via deployment as focal plane
arrays on interferometers; (iv) remote sensing of the atmosphere and Earth; and (v) wide-field imaging in
planetary missions. These science drivers are discussed in the report.
The most important single outcome of the workshops, and a sine qua non of this whole program,
is that consensus was reached that it should be possible to reduce the noise of individual HEMTs or
MMICs operating at cryogenic temperatures to less than three times the quantum limit at frequencies up
to 150 GHz, by working closely with a foundry (in this case NGC) and providing rapid feedback on the
performance of the devices they are fabricating, thus enabling tests of the effects of small changes in the
design of these transistors. This kind of partnership has been very successful in the past, but can now be
focused more intensively on cryogenic performance by carrying out tests of MMIC wafers, including tests
on a cryogenic probe station. It was felt that a properly outfitted university laboratory dedicated to this
testing and optimization would be an important element in this program, which would include MMIC
designs, wafer runs, and a wide variety of tests of MMIC performance at cryogenic temperatures.
This Study identified eight primary areas of technology development, including the one singled out
above, which must be actively pursued in order to exploit the full potential of MMIC Arrays in a timely
fashion:
1. Reduce the noise levels of individual transistors and MMICs to three times the quantum limit or
lower at cryogenic temperatures at frequencies up to 150 GHz.
2. Integrate high-performing MMICs into the building blocks of large arrays without loss of performance.
Currently factors of two in both noise and bandwidth are lost at this step.
3. Develop high performance, low mass, inexpensive feed arrays.
4. Develop robust interconnects and wiring that allow easy fabrication and integration of large arrays.
5. Develop mass production techniques suitable for arrays of differing sizes.
6. Reduce mass and power. (Requirements will differ widely with application. In the realm of planetary
instruments, this is often the most important single requirement.)
7. Develop planar orthomode transducers with low crosstalk and broad bandwidth.
8. Develop high power and high efficiency MMIC amplifiers for LO chains, etc.
Another important outcome of the two workshops was that a number of new collaborations were
forged between leading groups worldwide with the object of focusing on the development of MMIC
arrays
Demonstration of magnetic field tomography with starlight polarization towards a diffuse sightline of the ISM
The availability of large datasets with stellar distance and polarization
information will enable a tomographic reconstruction of the
(plane-of-the-sky-projected) interstellar magnetic field in the near future. We
demonstrate the feasibility of such a decomposition within a small region of
the diffuse ISM. We combine measurements of starlight (R-band) linear
polarization obtained using the RoboPol polarimeter with stellar distances from
the second Gaia data release. The stellar sample is brighter than 17 mag in the
R band and reaches out to several kpc from the Sun. HI emission spectra reveal
the existence of two distinct clouds along the line of sight. We decompose the
line-of-sight-integrated stellar polarizations to obtain the mean polarization
properties of the two clouds. The two clouds exhibit significant differences in
terms of column density and polarization properties. Their mean
plane-of-the-sky magnetic field orientation differs by 60 degrees. We show how
our tomographic decomposition can be used to constrain our estimates of the
polarizing efficiency of the clouds as well as the frequency dependence of the
polarization angle of polarized dust emission. We also demonstrate a new method
to constrain cloud distances based on this decomposition. Our results represent
a preview of the wealth of information that can be obtained from a tomographic
map of the ISM magnetic field.Comment: 25 pages, 14 figures, published in ApJ, data appear in journa
Filling in the Gaps in the 4.85 GHz Sky
We describe a 4.85 GHz survey of bright, flat-spectrum radio sources
conducted with the Effelsberg 100 m telescope in an attempt to improve the
completeness of existing surveys, such as CRATES. We report the results of
these observations and of follow-up 8.4 GHz observations with the VLA of a
subset of the sample. We comment on the connection to the WMAP point source
catalog and on the survey's effectiveness at supplementing the CRATES sky
coverage.Comment: 13 pages, 3 figures, 2 tables. Accepted for publication in the
Astronomical Journal. Tables available in electronic form:
http://astro.stanford.edu/gaps
The Extreme Behavior of the Radio-loud Narrow-line Seyfert 1 Galaxy J0849+5108
Simultaneous radio, optical (both photometry and polarimetry), X-ray, and Îł-ray observations of the radio-loud narrow-line Seyfert 1 (RL-NLSy1) galaxy J0849+5108 are presented. A massive three-magnitude optical flare across five nights in 2013 April is detected, along with associated flux increases in the Îł-ray, infrared, and radio regimes; no comparable event was detected in the X-rays, though this may be due to poor coverage. A spectral energy distribution (SED) for the object using quasi-simultaneous data centered on the optical flare is compared to the previously published SEDs for the object by D'Ammando et al. The flare event coincided with a high degree of optical polarization. High amplitude optical microvariability is clearly detected, and is found to be of comparable amplitude when the object is observed in both faint and bright states. The object is also seen to undergo rapid shifts in polarization in both degree and electric vector position angle within a single night. J0849+5108 appears to show even more extreme variability than that previously reported for the similar object J0948+0022. These observations appear to support the growing claim that some RL-NLSy1 galaxies constitute a sub-class of blazar-like active galactic nuclei
CGRaBS: An All-Sky Survey of Gamma-Ray Blazar Candidates
We describe a uniform all-sky survey of bright blazars, selected primarily by
their flat radio spectra, that is designed to provide a large catalog of likely
gamma-ray AGN. The defined sample has 1625 targets with radio and X-ray
properties similar to those of the EGRET blazars, spread uniformly across the
|b| > 10 deg sky. We also report progress toward optical characterization of
the sample; of objects with known R < 23, 85% have been classified and 81% have
measured redshifts. One goal of this program is to focus attention on the most
interesting (e.g., high redshift, high luminosity, ...) sources for intensive
multiwavelength study during the observations by the Large Area Telescope (LAT)
on GLAST.Comment: 18 pages, 6 figures, 1 machine-readable table available at
http://astro.stanford.edu/CGRaBS/ ; accepted for publication in ApJ
An Exceptional Radio Flare in Markarian 421
In September 2012, the high-synchrotron-peaked (HSP) blazar Markarian 421
underwent a rapid wideband radio flare, reaching nearly twice the brightest
level observed in the centimeter band in over three decades of monitoring. In
response to this event we carried out a five epoch centimeter- to
millimeter-band multifrequency Very Long Baseline Array (VLBA) campaign to
investigate the aftermath of this emission event. Rapid radio variations are
unprecedented in this object and are surprising in an HSP BL Lac object. In
this flare, the 15 GHz flux density increased with an exponential doubling time
of about 9 days, then faded to its prior level at a similar rate. This is
comparable with the fastest large-amplitude centimeter-band radio variability
observed in any blazar. Similar flux density increases were detected up to
millimeter bands. This radio flare followed about two months after a similarly
unprecedented GeV gamma-ray flare (reaching a daily E>100 MeV flux of (1.2 +/-
0.7)x10^(-6) ph cm^(-2) s^(-1)) reported by the Fermi Large Area Telescope
(LAT) collaboration, with a simultaneous tentative TeV detection by ARGO-YBJ. A
cross-correlation analysis of long-term 15 GHz and LAT gamma-ray light curves
finds a statistically significant correlation with the radio lagging ~40 days
behind, suggesting that the gamma-ray emission originates upstream of the radio
emission. Preliminary results from our VLBA observations show brightening in
the unresolved core region and no evidence for apparent superluminal motions or
substantial flux variations downstream.Comment: 5 pages, 8 figures. Contributed talk at the meeting "The Innermost
Regions of Relativistic Jets and Their Magnetic Fields", Granada, Spain.
Updated to correct author list and reference
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