16 research outputs found
Multimode electromagnetically-induced transparency on a single atomic line
We experimentally investigate electromagnetically-induced transparency (EIT)
created on an inhomogeneously broadened 5S_1/2-5P_1/2 transition in rubidium
vapor using a control field of a complex temporal shape. A comb-shaped
transparency spectrum enhances the delay-bandwidth product and the light
storage capacity for a matched probe pulse by a factor of about 50 compared to
a single EIT line [D. D. Yavuz, Phys. Rev. A 75, 031801 (2007)]. If the
temporal mode of the control field is slowly changed while the probe is
propagating through the EIT medium, the probe will adiabatically follow,
providing a means to perform frequency conversion and optical routing
Faraday Rotation of Extended Emission as a Probe of the Large-Scale Galactic Magnetic Field
The Galactic magnetic field is an integral constituent of the interstellar
medium (ISM), and knowledge of its structure is crucial to understanding
Galactic dynamics. The Rotation Measures (RM) of extragalactic (EG) sources
have been the basis of comprehensive Galactic magnetic field models. Polarised
extended emission (XE) is also seen along lines of sight through the Galactic
disk, and also displays the effects of Faraday rotation. Our aim is to
investigate and understand the relationship between EG and XE RMs near the
Galactic plane, and to determine how the XE RMs, a hitherto unused resource,
can be used as a probe of the large-scale Galactic magnetic field. We used
polarisation data from the Canadian Galactic Plane Survey (CGPS), observed near
1420 MHz with the Dominion Radio Astrophysical Observatory (DRAO) Synthesis
Telescope. We calculated RMs from a linear fit to the polarisation angles as a
function of wavelength squared in four frequency channels, for both the EG
sources and the XE. Across the CGPS area, , the RMs of the XE closely track the RMs of the EG
sources, with XE RMs about half the value of EG-source RMs. The exceptions are
places where large local HII complexes heavily depolarise more distant
emission. We conclude that there is valuable information in the XE RM dataset.
The factor of 2 between the two types of RM values is close to that expected
from a Burn slab model of the ISM. This result indicates that, at least in the
outer Galaxy, the EG and XE sources are likely probing similar depths, and that
the Faraday rotating medium and the synchrotron emitting medium have similar
variation with galactocentric distance.Comment: Accepted to Galaxies, March 22, 201
Characterization of the John A. Galt telescope for radio holography with CHIME
The Canadian Hydrogen Intensity Mapping Experiment (CHIME) will measure the
21 cm emission of astrophysical neutral hydrogen to probe large scale structure
at redshifts z=0.8-2.5. However, detecting the 21 cm signal beneath
substantially brighter foregrounds remains a key challenge. Due to the high
dynamic range between 21 cm and foreground emission, an exquisite calibration
of instrument systematics, notably the telescope beam, is required to
successfully filter out the foregrounds. One technique being used to achieve a
high fidelity measurement of the CHIME beam is radio holography, wherein
signals from each of CHIME's analog inputs are correlated with the signal from
a co-located reference antenna, the 26 m John A. Galt telescope, as the 26 m
Galt telescope tracks a bright point source transiting over CHIME. In this work
we present an analysis of several of the Galt telescope's properties. We employ
driftscan measurements of several bright sources, along with background
estimates derived from the 408 MHz Haslam map, to estimate the Galt system
temperature. To determine the Galt telescope's beam shape, we perform and
analyze a raster scan of the bright radio source Cassiopeia A. Finally, we use
early holographic measurements to measure the Galt telescope's geometry with
respect to CHIME for the holographic analysis of the CHIME and Galt
interferometric data set
The Global Magneto-Ionic Medium Survey (GMIMS): The brightest polarized region in the Southern sky at 75cm and its implications for Radio Loop II
Using the Global Magneto-Ionic Medium Survey (GMIMS) Low-Band South (LBS)
southern sky polarization survey, covering 300 to 480 MHz at 81 arcmin
resolution, we reveal the brightest region in the Southern polarized sky at
these frequencies. The region, G150-50, covers nearly 20deg, near
(l,b)~(150 deg,-50 deg). Using GMIMS-LBS and complementary data at higher
frequencies (~0.6--30 GHz), we apply Faraday tomography and Stokes QU-fitting
techniques. We find that the magnetic field associated with G150-50 is both
coherent and primarily in the plane of the sky, and indications that the region
is associated with Radio Loop II. The Faraday depth spectra across G150-50 are
broad and contain a large-scale spatial gradient. We model the magnetic field
in the region as an expanding shell, and we can reproduce both the observed
Faraday rotation and the synchrotron emission in the GMIMS-LBS band. Using
QU-fitting, we find that the Faraday spectra are produced by several Faraday
dispersive sources along the line-of-sight. Alternatively, polarization horizon
effects that we cannot model are adding complexity to the high-frequency
polarized spectra. The magnetic field structure of Loop II dominates a large
fraction of the sky, and studies of the large-scale polarized sky will need to
account for this object. Studies of G150-50 with high angular resolution could
mitigate polarization horizon effects, and clarify the nature of G150-50.Comment: 25 pages, 14 figures. Accepted for publication in MNRA
A Detection of Cosmological 21 cm Emission from CHIME in Cross-correlation with eBOSS Measurements of the Lyman- Forest
We report the detection of 21 cm emission at an average redshift in the cross-correlation of data from the Canadian Hydrogen Intensity
Mapping Experiment (CHIME) with measurements of the Lyman- forest from
eBOSS. Data collected by CHIME over 88 days in the ~MHz frequency band
() are formed into maps of the sky and high-pass delay filtered
to suppress the foreground power, corresponding to removing cosmological scales
with at the average redshift.
Line-of-sight spectra to the eBOSS background quasar locations are extracted
from the CHIME maps and combined with the Lyman- forest flux
transmission spectra to estimate the 21 cm-Lyman- cross-correlation
function. Fitting a simulation-derived template function to this measurement
results in a detection significance. The coherent accumulation of the
signal through cross-correlation is sufficient to enable a detection despite
excess variance from foreground residuals times brighter than the
expected thermal noise level in the correlation function. These results are the
highest-redshift measurement of \tcm emission to date, and set the stage for
future 21 cm intensity mapping analyses at
Sub-second periodicity in a fast radio burst
Fast radio bursts (FRBs) are millisecond-duration flashes of radio waves that
are visible at distances of billions of light-years. The nature of their
progenitors and their emission mechanism remain open astrophysical questions.
Here we report the detection of the multi-component FRB 20191221A and the
identification of a periodic separation of 216.8(1) ms between its components
with a significance of 6.5 sigmas. The long (~3 s) duration and nine or more
components forming the pulse profile make this source an outlier in the FRB
population. Such short periodicity provides strong evidence for a neutron-star
origin of the event. Moreover, our detection favours emission arising from the
neutron-star magnetosphere, as opposed to emission regions located further away
from the star, as predicted by some models.Comment: Updated to conform to the accepted versio
Faraday Tomography with CHIME: The “Tadpole” Feature G137+7
A direct consequence of Faraday rotation is that the polarized radio sky does not resemble the total intensity sky at long wavelengths. We analyze G137+7, which is undetectable in total intensity but appears as a depolarization feature. We use the first polarization maps from the Canadian Hydrogen Intensity Mapping Experiment. Our 400–729 MHz bandwidth and angular resolution, – , allow us to use Faraday synthesis to analyze the polarization structure. In polarized intensity and polarization angle maps, we find a tail extending 10° from the head and designate the combined object, the tadpole. Similar polarization angles, distinct from the background, indicate that the head and tail are physically associated. The head appears as a depolarized ring in single channels, but wideband observations show that it is a Faraday rotation feature. Our investigations of H I and Hα find no connections to the tadpole. The tail suggests motion of either the gas or an ionizing star through the interstellar medium; the B2(e) star HD 20336 is a candidate. While the head features a coherent, ∼ ‑8 rad m‑2 Faraday depth, Faraday synthesis also identifies multiple components in both the head and tail. We verify the locations of the components in the spectra using QU fitting. Our results show that approximately octave-bandwidth Faraday rotation observations at ∼600 MHz are sensitive to low-density ionized or partially ionized gas, which is undetectable in other tracers
Deciphering Galactic Magnetism from Polarisation Structures in the Northern Sky
The Milky Way is permeated by magnetic fields ranging from stellar scales up to coherence lengths comparable to the Galactic spiral arms. Determining the Galactic magnetic field (GMF) morphology over a range of spatial scales is necessary in understanding the Galaxy’s formation and evolution, and the role of magnetism in its interactions with other Galactic constituents. While ambient magnetic fields do not radiate, they can affect radiation, the results of which are observable in polarisation via Faraday rotation. The aim of my thesis is to explore three-dimensional GMF properties using new observational techniques. I present my analysis of two complementary radio polarisation datasets: the Synthesis Telescope (ST) data from the Canadian Galactic Plane Survey (CGPS) and the Global Magneto-Ionic Medium Survey (GMIMS) single-antenna data. While the CGPS ST data provide high angular resolution, the broad frequency coverage of GMIMS allows for detection of Faraday complexity along the lines of sight. I found that over large regions in the Galactic disk, the extragalactic (EG) and extended emission (XE) rotation measures (RMs) of the ST trace similar patterns with Galactic longitude, indicative of the two observables probing similar volumes through the disk magnetic field. Modelling reveals basic GMF configurations describing the observed ratio of approximately two between the EG and XE RMs. Examining latitudinal variation in both the EG and XE RMs, I found further evidence of the diagonal orientation of the large-scale magnetic field reversal in the inner Galaxy, in agreement with an earlier study of mine. In GMIMS, I found overall large-scale agreement with previous observations of EG RMs at high latitudes. At low latitudes, differences between the GMIMS and the ST RMs are likely the result of beam depolarisation. I observed Faraday complexity along the line of sight towards a depolarising HII (ionised hydrogen) region, which I show to be useful in probing the surrounding GMF. My experiments in spatial filtering of a combined GMIMS and ST dataset revealed that the large-scale field reversal is obscured in GMIMS by an apparent foreground structure of significant angular size
The Magnetic Field in the Galactic Disk from Extended Emission Rotation Measures
I present my analysis of the extended emission (XE) polarisation data from the Canadian Galactic Plane Survey. I used this dataset to determine Rotation Measures (RMs) with the aim of studying the Galactic magnetic field (GMF) structure. I have shown that RMs of the XE do contain useful information, despite predicted difficulties with this kind of dataset. I have found evidence that the large-scale field reversal between the local arm and the Sagittarius arm is not strictly perpendicular to the Galactic plane as previously thought, but has some component within the Galactic disk. This geometry lends credibility to a model of the GMF similar to the Parker spiral Solar magnetic field, an idea proposed in 1982 but rejected shortly thereafter on the grounds of incompatibility with observations available at the time. I have created a Galactic scale version of this model of the magnetic field, constrained by the XE RMs