562 research outputs found
Alternative Adaptive Filter Structures for Improved Radio Frequency Interference Cancellation in Radio Astronomy
In radio astronomy, reference signals from auxiliary antennas that receive
only the radio frequency interference (RFI) can be modified to model the RFI
environment at the astronomy receivers. The RFI can then be canceled from the
astronomy signal paths. However, astronomers typically only require signal
statistics. If the RFI statistics are changing slowly, the cancellation can be
applied to the signal correlations at a much lower rate than is required for
standard adaptive filters. In this paper we describe five canceler setups;
precorrelation and postcorrelation cancelers that use one or two reference
signals in different ways. The theoretical residual RFI and added noise levels
are examined and are demonstrated using microwave television RFI at the
Australia Telescope Compact Array. The RFI is attenuated to below the system
noise, a reduction of at least 20 dB. While dual-reference cancelers add more
reference noise than single-reference cancelers, this noise is zero-mean and
only adds to the system noise, decreasing the sensitivity. The residual RFI
that remains in the output of single-reference cancelers (but not
dual-reference cancelers) sets a nonzero noise floor that does not act like
random system noise and may limit the achievable sensitivity. Thus,
dual-reference cancelers often result in superior cancellation. Dual-reference
precorrelation cancelers require a double-canceler setup to be useful and to
give equivalent results to dual-reference postcorrelation cancelers.Comment: 11 pages created using emulateap
Polarization observations in a low synchrotron emission field at 1.4 GHz
We present the first observation of the diffuse polarized synchrotron
radiation of a patch () in the BOOMERanG field,
one of the areas with the lowest CMB foreground emission. The work has been
carried out with the Australia Telescope Compact Array at 1.4 GHz with 3.4
arcmin resolution and sensitivity of mJy beam. The mean
polarized signal has been found to be mK, nearly one order of magnitude below than in the Galactic
Plane.
Extrapolations to frequencies of interest for cosmological investigations
suggest that polarized synchrotron foreground noise should allow the detection
of the CMB Polarization --mode already at 32 GHz and make us confident that,
at 90 GHz, it is accessible with no relevant foreground contamination. Last but
not least, even the --mode detection for is not ruled out in
such a low emission patch.Comment: Uses emulateapj.sty, onecolfloat.sty, 5 pages 4 fig., accepted for
publication in ApJ
The variable radio counterpart and possible large-scale jet of the new Z-source XTE J1701-462
We report radio observations, made with the Australia Telescope Compact
Array, of the X-ray transient XTE J1701-462. This system has been classified as
a new `Z' source, displaying characteristic patterns of behaviour probably
associated with accretion onto a low magnetic field neutron star at close to
the Eddington limit. The radio counterpart is highly variable, and was detected
in six of sixteen observations over the period 2006 January -- April. The
coupling of radio emission to X-ray state, despite limited sampling, appears to
be similar to that of other `Z' sources, in that there is no radio emission on
the flaring branch. The mean radio and X-ray luminosities are consistent with
the other Z sources for a distance of 5--15 kpc. The radio spectrum is
unusually flat, or even inverted, in contrast to the related sources, Sco X-1
and Cir X-1, which usually display an optically thin radio spectrum. Deep
wide-field observations indicate an extended structure three arcminutes to the
south which is aligned with the X-ray binary. This seems to represent a
significant overdensity of radio sources for the field and so, although a
background source remains a strong possibility, we consider it plausible that
this is a large-scale jet associated with XTE J1701-462.Comment: Accepted for publication as a Letter in MNRA
Unveiling the nature of IGR J17177-3656 with X-ray, NIR and Radio observations
We report on the first broad-band (1-200 keV) simultaneous Chandra-INTEGRAL
observations of the recently discovered hard X-ray transient IGR J17177-3656
that took place on 2011, March 22, about two weeks after the source discovery.
The source had an average absorbed 1-200 keV flux of about 8x10^(-10) erg
cm^(-2) s^(-1). We extracted a precise X-ray position of IGR J17177-3656, RA=17
17 42.62, DEC= -36 56 04.5 (90% uncertainty of 0.6"). We also report Swift,
near infrared and quasi simultaneous radio follow-up observations. With the
multi-wavelength information at hand, we propose IGR J17177-3656 is a low-mass
X-ray binary, seen at high inclination, probably hosting a black hole.Comment: 8 pages, 8 figures, accepted for publication in Ap
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