We present a search for transient and variable radio sources at 154 MHz with the Murchison Widefield Array 32-tile prototype. 51 images were obtained that cover a field of view of 1430 deg2 centred ...on Hydra A. The observations were obtained over three days in 2010 March and three days in 2011 April and May. The mean cadence of the observations was 26 min and there was additional temporal information on day and year time-scales. We explore the variability of a sample of 105 low-frequency radio sources within the field. Four bright (S > 6 Jy) candidate variable radio sources were identified that displayed low levels of short time-scale variability (26 min). We conclude that this variability is likely caused by simplifications in the calibration strategy or ionospheric effects. On the time-scale of 1 yr we find two sources that show significant variability. We attribute this variability to either refractive scintillation or intrinsic variability. No radio transients were identified and we place an upper limit on the surface density of sources ρ < 7.5 × 10−5 deg−2 with flux densities >5.5 Jy, and characteristic time-scales of both 26 min and 1 yr.
We present phase-resolved, high-resolution (1.3 Å) spectroscopy of the brightest near-synchronous polar BY Cam taken on two different occasions in 1998 and 1999. The first tomographic study of such a ...system reveals line emission spread out over a large velocity range forming a crescent at negative vy velocities in the Doppler maps. In contrast to the majority of synchronous AM Her systems there is only weak indication for the presence of a focused accretion stream. These two facts suggest that the majority of the matter is accreted via an extended curtain. Location and extent of the structure in the Doppler maps can be reproduced with a simple curtain model raised over a wide (~180°) range in azimuth implying that the ballistic stream stretches to a point far behind the white dwarf. In order to reach such small magnetospheric radii mass accretion rates a factor of 10 to 20 in excess of that normally seen in polars would be required. In addition to the curtain emission, the Balmer lines show a narrow emission line component likely originating from the heated side of the secondary star. Its velocity amplitude of 190 $\rm km\,s^{-1}$ together with an illumination model of the secondary star suggests a rather heavy white dwarf of $M_{1}\geq 0.8~M_{\odot}$ and an inclination larger than $i \geq 40$°. Timings of this feature in the present and historical data unequivocally determine the orbital period and have been used to establish a high-precision, long-term ephemeris.
Full text
Available for:
FMFMET, NUK, UL, UM, UPUK
In this paper we explore the possibility that the MWA can be used for the purposes of Space Situational Awareness (SSA). In particular we propose that the MWA can be used as an element of a passive ...radar facility operating in the frequency range 87.5-108 MHz (the commercial FM broadcast band). The imaging capabilities of the MWA can be used to simultaneously detect multiple pieces of space debris, image their positions on the sky as a function of time, and provide tracking data that can be used to determine orbital parameters. We provide a feasibility assessment of this proposal, based on simple calculations and electromagnetic simulations, that shows that the detection of sub-meter size debris should be possible (debris radius of > 0.5 m to ~ 1000 km altitude).
We have analyzed a Chandra HETGS spectrum of the Galactic black hole Cygnus X-1, obtained at a source flux that is approximately twice that commonly observed in its persistent low-intensity, ...spectrally hard state. We find a myriad of absorption lines in the spectrum, including H- and He-like resonance lines from Ne, Na, Mg, and Si. We calculate a flux-weighted mean redshift of 100 km s super(-1) and a flux-weighted mean velocity width of 800 km s super(-1) (FWHM) for lines from these elements. We also detect a number of transitions from Fe XVIII-XXIV and Ni XIX-XX in absorption; however, the identification of these lines is less certain and a greater range of shifts and breadth is measured. Our observation occurred at a binary phase of h 0.76; the lines observed are consistent with absorption in an ionized region of the supergiant O9.7 Iab companion wind. The spectrum is extremely complicated: a rather large range of temperatures is implied, and it is unlikely that a narrow range of ionization parameters can account for the lines observed. Prior Chandra HETGS spectra of Cygnus X-1 were obtained in a similar transition state (at h 0.93) and in the low/hard state (at h 0.84). Considered together, these spectra provide evidence for a companion wind that is focused as it flows onto the black hole primary in this system.
A new generation of low-frequency radio telescopes is seeking to observe the redshifted 21 cm signal from the epoch of reionization (EoR), requiring innovative methods of calibration and imaging to ...overcome the difficulties of wide-field low-frequency radio interferometry. Precise calibration will be required to separate the expected small EoR signal from the strong foreground emission at the frequencies of interest between 80 and 300 MHz. The Moon may be useful as a calibration source for detection of the EoR signature, as it should have a smooth and predictable thermal spectrum across the frequency band of interest. Initial observations of the Moon with the Murchison Widefield Array 32 tile prototype show that the Moon does exhibit a similar trend to that expected for a cool thermally emitting body in the observed frequency range, but that the spectrum is corrupted by reflected radio emission from Earth. In particular, there is an abrupt increase in the observed flux density of the Moon within the internationally recognized frequency modulated (FM) radio band. The observations have implications for future low-frequency surveys and EoR detection experiments that will need to take this reflected emission from the Moon into account. The results also allow us to estimate the equivalent isotropic power emitted by the Earth in the FM band and to determine how bright the Earth might appear at meter wavelengths to an observer beyond our own solar system.
PDF
