1,927 research outputs found
Repeatability of the seasonal variations of ozone near the mesopause from Observations of the 11.072-GHz line
VLBA Observations of G5.89–0.39: OH Masers and Magnetic Field Structure
We present VLBA observations of 1667 MHz OH maser emission from the massive star formation region G5.89-0.39. The observations were phase-referenced, allowing the absolute positions of the masers to be obtained. The 1667 MHz masers have radial velocities that span ~50 km s^(-1) but show little evidence of tracing the bipolar molecular outflow, as has been claimed in previous studies. We identify 23 Zeeman pairs through comparison of masers in left and right circular polarization. Magnetic field strengths range from -2 to +2 mG, and an ordered reversal in magnetic field direction is observed toward the southern region of the UC H II region. We suggest that the velocity and magnetic field structure of the 1667 MHz masers can be explained in the context of a model in which the masers arise in a neutral shell just outside a rapidly expanding ionized shell
Zooming towards the Event Horizon - mm-VLBI today and tomorrow
Global VLBI imaging at millimeter and sub-millimeter wavelength overcomes the
opacity barrier of synchrotron self-absorption in AGN and opens the direct view
into sub-pc scale regions not accessible before. Since AGN variability is more
pronounced at short millimeter wavelength, mm-VLBI can reveal structural
changes in very early stages after outbursts. When combined with observations
at longer wavelength, global 3mm and 1mm VLBI adds very detailed information.
This helps to determine fundamental physical properties at the jet base, and in
the vicinity of super-massive black holes at the center of AGN. Here we present
new results from multi-frequency mm-VLBI imaging of OJ287 during a major
outburst. We also report on a successful 1.3mm VLBI experiment with the APEX
telescope in Chile. This observation sets a new record in angular resolution.
It also opens the path towards future mm-VLBI with ALMA, which aims at the
mapping of the black hole event horizon in nearby galaxies, and the study of
the roots of jets in AGN.Comment: 6 pages, to appear in 11th European VLBI Network Symposium, ed. P.
Charlot et al., Bordeaux (France), October 9-12, 201
Observations of the 6 Centimeter Lines of OH in Evolved (OH/IR) Stars
Recent observational and theoretical advances have called into question
traditional OH maser pumping models in evolved (OH/IR) stars. The detection of
excited-state OH lines would provide additional constraints to discriminate
amongst these theoretical models. In this Letter, we report on VLA observations
of the 4750 MHz and 4765 MHz lines of OH toward 45 sources, mostly evolved
stars. We detect 4765 MHz emission in the star forming regions Mon R2 and LDN
1084, but we do not detect excited-state emission in any evolved stars. The
flux density and velocity of the 4765 MHz detection in Mon R2 suggests that a
new flaring event has begun.Comment: 4 pages, to appear in ApJ
Demonstrating the Principles of Aperture Synthesis with TableTop Laboratory Exercises
Many undergraduate radio astronomy courses are unable to give a detailed
treatment of aperture synthesis due to time constraints and limited math
backgrounds of students. We have taken a laboratory-based approach to teaching
radio interferometry using a set of college-level, table-top exercises. These
are performed with the Very Small Radio Telescope (VSRT), an interferometer
developed at the Haystack Observatory using satellite TV electronics as
detectors and compact fluorescent light bulbs as microwave signal sources. The
hands-on experience provided by the VSRT in these labs allows students to gain
a conceptual understanding of radio interferometry and aperture synthesis
without the rigorous mathematical background traditionally required.
The data are quickly and easily processed using a user-friendly data analysis
Java package, VSRTI\_Plotter.jar. This software can also be used in the absence
of the equipment as an interactive computer activity to demonstrate an
interferometer's responses to assorted surface brightness distributions. The
students also gain some familiarity with Fourier transforms and an appreciation
for the Fourier relations in interferometry using another Java package, the
Tool for Interactive Fourier Transforms (TIFT). We have successfully used these
tools in multiple offerings of our radio astronomy course at Union CollegeComment: 10 pages, ISE 2 A "International Symposium on Education in Astronomy
and Astrobiology
The effects of social service contact on teenagers in England
Objective: This study investigated outcomes of social service contact during teenage years.
Method: Secondary analysis was conducted of the Longitudinal Survey of Young People in England (N = 15,770), using data on reported contact with social services resulting from teenagers’ behavior. Outcomes considered were educational achievement and aspiration, mental health, and locus of control. Inverse-probability-weighted regression adjustment was used to estimate the effect of social service contact.
Results: There was no significant difference between those who received social service contact and those who did not for mental health outcome or aspiration to apply to university. Those with contact had lower odds of achieving good exam results or of being confident in university acceptance if sought. Results for locus of control were mixed.
Conclusions: Attention is needed to the role of social services in supporting the education of young people in difficulty. Further research is needed on the outcomes of social services contact
Probing the Parsec-scale Accretion Flow of 3C 84 with Millimeter Polarimetry
We report the discovery of Faraday rotation toward radio source 3C 84, the
active galactic nucleus in NGC1275 at the core of the Perseus Cluster. The
rotation measure (RM), determined from polarization observations at wavelengths
of 1.3 and 0.9 mm, is (8.7 +/- 2.3) x 10^5 radians/m^2, among the largest ever
measured. The RM remained relatively constant over a 2 year period even as the
intrinsic polarization position angle wrapped through a span of 300 degrees.
The Faraday rotation is likely to originate either in the boundary layer of the
radio jet from the nucleus, or in the accretion flow onto the central black
hole. The accretion flow probably is disk-like rather than spherical on scales
of less than a parsec, otherwise the RM would be even larger.Comment: 7 pages, 3 figures, accepted for publication in Ap
Radiative Models of Sagittarius A* and M87 from Relativistic MHD Simulations
Ongoing millimeter VLBI observations with the Event Horizon Telescope allow
unprecedented study of the innermost portion of black hole accretion flows.
Interpreting the observations requires relativistic, time-dependent physical
modeling. We discuss the comparison of radiative transfer calculations from
general relativistic MHD simulations of Sagittarius A* and M87 with current and
future mm-VLBI observations. This comparison allows estimates of the viewing
geometry and physical conditions of the Sgr A* accretion flow. The viewing
geometry for M87 is already constrained from observations of its large-scale
jet, but, unlike Sgr A*, there is no consensus for its millimeter emission
geometry or electron population. Despite this uncertainty, as long as the
emission region is compact, robust predictions for the size of its jet
launching region can be made. For both sources, the black hole shadow may be
detected with future observations including ALMA and/or the LMT, which would
constitute the first direct evidence for a black hole event horizon.Comment: 8 pages, 2 figures, submitted to the proceedings of AHAR 2011: The
Central Kiloparse
Excited-state OH Mainline Masers in AU Geminorum and NML Cygni
Excited-state OH maser emission has previously been reported in the
circumstellar envelopes of only two evolved stars: the Mira star AU Geminorum
and the hypergiant NML Cygni. We present Very Large Array (VLA) observations of
the 1665, 1667, and excited-state 4750 MHz mainline OH transitions in AU Gem
and Expanded Very Large Array (EVLA) observations of the excited-state 6030 and
6035 MHz OH mainline transitions in NML Cyg. We detect masers in both mainline
transitions in AU Gem but no excited-state emission in either star. We conclude
that the excited-state OH emission in AU Gem is either a transient phenomenon
(such as for NML Cyg outlined below), or possibly an artifact in the data, and
that the excited state OH emission in NML Cyg was generated by an episode of
enhanced shock between the stellar mass-loss and an outflow of the Cyg OB2
association. With these single exceptions, it therefore appears that
excited-state OH emission indeed should not be predicted nor observable in
evolved stars as part of their normal structure or evolution.Comment: ApJ Letter, accepted, 4 pages, 2 figure
Radiometer Calibration Using Colocated GPS Radio Occultation Measurements
We present a new high-fidelity method of calibrating a cross-track scanning microwave radiometer using Global Positioning System (GPS) radio occultation (GPSRO) measurements. The radiometer and GPSRO receiver periodically observe the same volume of atmosphere near the Earth's limb, and these overlapping measurements are used to calibrate the radiometer. Performance analyses show that absolute calibration accuracy better than 0.25 K is achievable for temperature sounding channels in the 50-60-GHz band for a total-power radiometer using a weakly coupled noise diode for frequent calibration and proximal GPSRO measurements for infrequent (approximately daily) calibration. The method requires GPSRO penetration depth only down to the stratosphere, thus permitting the use of a relatively small GPS antenna. Furthermore, only coarse spacecraft angular knowledge (approximately one degree rms) is required for the technique, as more precise angular knowledge can be retrieved directly from the combined radiometer and GPSRO data, assuming that the radiometer angular sampling is uniform. These features make the technique particularly well suited for implementation on a low-cost CubeSat hosting both radiometer and GPSRO receiver systems on the same spacecraft. We describe a validation platform for this calibration method, the Microwave Radiometer Technology Acceleration (MiRaTA) CubeSat, currently in development for the National Aeronautics and Space Administration (NASA) Earth Science Technology Office. MiRaTA will fly a multiband radiometer and the Compact TEC/Atmosphere GPS Sensor in 2015
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