We present a description of the Prototype All-Sky Imager (PASI), a backend correlator and imager of the first station of the Long Wavelength Array (LWA1). PASI cross-correlates a live stream of 260 dual-polarization dipole antennas of the LWA1, creat
es all-sky images, and uploads them to the LWA-TV website in near real-time. PASI has recorded over 13,000 hours of all-sky images at frequencies between 10 and 88 MHz creating opportunities for new research and discoveries. We also report rate density and pulse energy density limits on transients at 38, 52, and 74 MHz, for pulse widths of 5 s. We limit transients at those frequencies with pulse energy densities of $>2.7times 10^{-23}$, $>1.1times 10^{-23}$, and $>2.8times 10^{-23}$ J m$^{-2}$ Hz$^{-1}$ to have rate densities $<1.2times10^{-4}$, $<5.6times10^{-4}$, and $<7.2times10^{-4}$ yr$^{-1}$ deg$^{-2}$
We present the findings from the Prototype All-Sky Imager (PASI), a backend correlator of the first station of the Long Wavelength Array (LWA1), which has recorded over 11,000 hours of all-sky images at frequencies between 25 and 75 MHz. In a search
of this data for radio transients, we have found 49 long (10s of seconds) duration transients. Ten of these transients correlate both spatially and temporally with large meteors (fireballs), and their signatures suggest that fireballs emit a previously undiscovered low frequency, non-thermal pulse. This emission provides a new probe into the physics of meteors and identifies a new form of naturally occurring radio transient foreground.
As a backend to the first station of the Long Wavelength Array (LWA1) the Prototype All Sky Imager (PASI) has been imaging the sky $>$ -26$^{circ}$ declination during 34 Gamma Ray Bursts (GRBs) between January 2012 and May 2013. Using this data we we
re able to put the most stringent limits to date on prompt low frequency emission from GRBs. While our limits depend on the zenith angle of the observed GRB, we estimate a 1$sigma$ RMS sensitivity of 68, 65 and 70 Jy for 5 second integrations at 37.9, 52.0, and 74.0 MHz at zenith. These limits are relevant for pulses $geq$ 5 s and are limited by dispersion smearing. For pulses of length 5 s we are limited to dispersion measures ($DM$s) $leq$ 220, 570, and 1,600 pc cm$^{-3}$ for the frequencies above. For pulses lasting longer than 5s, the $DM$ limits increase linearly with the duration of the pulse. We also report two interesting transients, which are, as of yet, of unknown origin, and are not coincident with any known GRBs. For general transients, we give rate density limits of $leq$ $7.5times10^{-3}$, $2.9times10^{-2}$, and $1.4times10^{-2}$ yr$^{-1}$ deg$^{-2}$ with pulse energy densities $>1.3times 10^{-22}$, $1.1times 10^{-22}$, and $1.4times 10^{-22}$ J m$^{-2}$ Hz$^{-1}$ and pulse widths of 5 s at the frequencies given above.
The parsec-scale radio properties of blazars detected by the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope have been investigated using observations with the Very Long Baseline Array (VLBA). Comparisons between LAT and non-L
AT detected samples were made using contemporaneous data. In total, 232 sources were used in the LAT-detected sample. This very large, radio flux-limited sample of active galactic nuclei (AGN) provides insights into the mechanism that produces strong gamma-ray emission. It has been found that LAT-detected BL Lac objects are very similar to the non-LAT BL Lac objects in most properties, although LAT BL Lac objects may have longer jets. The LAT flat spectrum radio quasars (FSRQs) are significantly different from non-LAT FSRQs and are likely extreme members of the FSRQ population. Contemporaneous observations showed a strong correlation, whereas no correlation is found using archival radio data. Most of the differences between the LAT and non-LAT populations are related to the cores of the sources, indicating that the gamma-ray emission may originate near the base of the jets (i.e., within a few pc of the central engine). There is some indication that LAT-detected sources may have larger jet opening angles than the non-LAT sources. Strong core polarization is significantly more common among the LAT sources, suggesting that gamma-ray emission is related to strong, uniform magnetic fields at the base of the jets of the blazars. Observations of sources in two epochs indicate that core fractional polarization was higher when the objects were detected by the LAT. Included in our sample are several non-blazar AGN such as 3C84, M82, and NGC 6251.
We report on a multi-frequency, multi-epoch campaign of Very Long Baseline Interferometry observations of the radio galaxy 1946+708 using the VLBA and a Global VLBI array. From these high-resolution observations we deduce the kinematic age of the rad
io source to be $sim$4000 years, comparable with the ages of other Compact Symmetric Objects (CSOs). Ejections of pairs of jet components appears to take place on time scales of 10 years and these components in the jet travel outward at intrinsic velocities between 0.6 and 0.9 c. From the constraint that jet components cannot have intrinsic velocities faster than light, we derive H_0 > 57 km s^-1 Mpc^-1 from the fastest pair of components launched from the core. We provide strong evidence for the ejection of a new pair of components in ~1997. From the trajectories of the jet components we deduce that the jet is most likely to be helically confined, rather than purely ballistic in nature.
We examine the core of the X-ray bright galaxy cluster 2A 0335+096 using deep Chandra X-ray imaging and spatially-resolved spectroscopy, and include new radio observations. The set of around eight X-ray bright blobs in the core of the cluster, appear
ing like eggs in a birds nest, contains multiphase gas from ~0.5 to 2 keV. The morphology of the coolest X-ray emitting gas at 0.5 keV temperature is similar to the Halpha emitting nebula known in this cluster, which surrounds the central galaxy. XMM-Newton grating spectra confirm the presence of material at these temperatures, showing excellent agreement with Chandra emission measures. On scales of 80 to 250 kpc there is a low temperature, high metallicity, swirl of intracluster medium as seen in other clusters. In the core we find evidence for a further three X-ray cavities, in addition to the two previously discovered. Enhancements in 1.5 GHz radio emission are correlated with the X-ray cavities. The total 4PV enthalpy associated with the cavities is around 5x10^59 erg. This energy would be enough to heat the cooling region for ~5x10^7 yr. We find a maximum pressure discontinuity of 26 per cent (2 sigma) across the surface brightness edge to the south-west of the cluster core. This corresponds to an upper limit on the Mach number of the cool core with respect to its surroundings of 0.55.
We present results from deep Chandra and XMM-Newton observations of the relaxed X-ray luminous galaxy cluster Abell 2204. We detect metallicity inhomogeneities in the intracluster medium on a variety of distance scales, from a ~12 kpc enhancement con
taining a few times 10^7 Msun of iron in the centre, to a region at 400 kpc radius with an excess of a few times 10^9 Msun. Subtracting an average surface brightness profile from the X-ray image yields two surface brightness depressions to the north and south of the cluster. Their morphology is similar to the cavities observed in cluster cores, but they have radii of 240 kpc and 160 kpc and have a total enthalpy of 2x10^62 erg. If they are fossil radio bubbles, their buoyancy timescales imply a total mechanical heating power of 5x10^46 erg/s, the largest such bubble heating power known. More likely, they result from the accumulation of many past bubbles. Energetically this is more feasible, as the enthalpy of these regions could combat X-ray cooling in this cluster to 500 kpc radius for around 2 Gyr. The core of the cluster also contains five to seven ~4 kpc radius surface brightness depressions that are not associated with the observed radio emission. If they are bubbles generated by the nucleus, they are too small to balance cooling in the core by an order of magnitude. However if the radio axis is close to the line of sight, projection effects may mask more normal bubbles. Using RGS spectra we detect a FeXVII line. Spectral fitting reveals temperatures down to ~0.7 keV; the cluster therefore shows a range in X-ray temperature of at least a factor of 15. The quantity of low temperature gas is consistent with a mass deposition rate of 65 Msun/yr.
SN 2001em is a peculiar supernova, originally classified as Type Ib/c. About two years after the SN it was detected in the radio, showing a rising radio flux with an optically thin spectral slope, and it also displayed a large X-ray luminosity (~10^{
41} erg/s). Thus it was suspected to harbor a decelerating (by then, mildly) relativistic jet pointing away from us. About 3 years after its discovery the optical spectrum of SN 2001em showed a broad H-alpha line, and it was therefore reclassified as Type IIn. Here we constrain its proper motion and expansion velocity by analyzing four epochs of VLBI observations, extending out to 5.4 years after the SN. The supernova is still unresolved 5.4 years after the explosion. For the proper motion we obtain (23,000 +/- 30,000) km/s while our 2-sigma upper limit on the expansion velocity is 6000 km/s. These limits are somewhat tighter than those derived by Bietenholz & Bartel, and confirm their conclusion that late time emission from SN 2001em, a few years after the explosion, is not driven by a relativistic jet. VLA observations of the radio flux density, at 8.46 GHz, show a decay as t^{-1.23 +/- 0.40} starting ~2.7 years after the SN. Collectively, the observations suggest interaction of the SN ejecta with a very dense circumstellar medium, though the implied opacity constraints still present a challenge.
Context: This is the first of a series of papers presenting VLBI observations of the 293 Caltech-Jodrell Bank Flat-Spectrum (hereafter CJF) sources and their analysis. Aims: One of the major goals of the CJF is to make a statistical study of the appa
rent velocities of the sources. Methods: We have conducted global VLBI and VLBA observations at 5 GHz since 1990, accumulating thirteen separate observing campaigns. Results: We present here an overview of the observations, give details of the data reduction and present the source parameters resulting from a model-fitting procedure. For every source at every observing epoch, an image is shown, built up by restoring the model-fitted components, convolved with the clean beam, into the residual image, which was made by Fourier transforming the visibility data after first subtracting the model-fitted components in the uv-plane. Overplotted we show symbols to represent the model components. Conclusions: We have produced VLBI images of all but 5 of the 293 sources in the complete CJF sample at several epochs and investigated the kinematics of 266 AGN.
