Imaging the Radio Photospheres of Mira Variables

We have used the VLA at 43 GHz to image the radio continuum emission from o Ceti, R Leo, and W Hya and to precisely locate their SiO maser emission with respect to the star. The radio continuum emission region for all three stars has a diameter close to 5.6 AU. These diameters are similar to those measured at infrared wavelengths in bands containing strong molecular opacity and about twice those measured in line-free regions of the infrared spectrum. Thus, the radio photosphere and the infrared molecular layer appear to be coextensive. The 43 GHz continuum emission is consistent with temperatures near 1600 K and opacity from H-minus free-free interactions. While the continuum image of o Ceti appears nearly circular, both R Leo and W Hya display significant elongations. The SiO masers for all three stars show partial rings with diameters close to 8 AU.Comment: 14 pages; 3 figure

Near-Field Radio Holography of Large Reflector Antennas

We summarise the mathematical foundation of the holographic method of measuring the reflector profile of an antenna or radio telescope. In particular, we treat the case, where the signal source is located at a finite distance from the antenna under test, necessitating the inclusion of the so-called Fresnel field terms in the radiation integrals. We assume a ``full phase'' system with reference receiver to provide the reference phase. We describe in some detail the hardware and software implementation of the system used for the holographic measurement of the 12m ALMA prototype submillimeter antennas. We include a description of the practicalities of a measurement and surface setting. The results for both the VertexRSI and AEC (Alcatel-EIE-Consortium) prototype ALMA antennas are presented.Comment: 14 pages, 14 figures, to appear in IEEE Antennas and Propagation Magazine, Vol. 49, No. 5, October 2007. Version 2 includes nice mug-shots of the author

Radio Emission Signatures in the Crab Pulsar

Our high time resolution observations of individual pulses from the Crab pulsar show that both the time and frequency signatures of the interpulse are distinctly different from those of the main pulse. Main pulses can occasionally be resolved into short-lived, relatively narrow-band nanoshots. We believe these nanoshots are produced by soliton collapse in strong plasma turbulence. Interpulses at centimeter wavelengths are very different. Their dynamic spectrum contains regular, microsecond-long emission bands. We have detected these bands, proportionately spaced in frequency, from 4.5 to 10.5 GHz. The bands cannot easily be explained by any current theory of pulsar radio emission; we speculate on possible new models.Comment: 26 pages, 10 figures, to appear in Ap

The late time radio emission from SN 1993J at meter wavelengths

We present the investigations of SN 1993J using low frequency observations with the Giant Meterwave Radio Telescope. We analyze the light curves of SN 1993J at 1420, 610, 325 and 243 MHz during $7.5-10$ years since explosion.The supernova has become optically thin early on in the 1420 MHz and 610 MHz bands while it has only recently entered the optically thin phase in the 325 MHz band. The radio light curve in the 235 MHz band is more or less flat. This indicates that the supernova is undergoing a transition from an optically thick to optically thin limit in this frequency band. In addition, we analyze the SN radio spectra at five epochs on day 3000, 3200, 3266, 3460 and 3730 since explosion. Day 3200 spectrum shows a synchrotron cooling break. SN 1993J is the only young supernova for which the magnetic field and the size of the radio emitting region are determined through unrelated methods. Thus the mechanism that controls the evolution of the radio spectra can be identified. We suggest that at all epochs, the synchrotron self absorption mechanism is primarily responsible for the turn-over in the spectra. Light curve models based on free free absorption in homogeneous or inhomogeneous media at high frequencies overpredict the flux densities at low frequencies. The discrepancy is increasingly larger at lower and lower frequencies. We suggest that an extra opacity, sensitively dependent on frequency, is likely to account for the difference at lower frequencies. The evolution of the magnetic field (determined from synchrotron self absorption turn-over) is roughly consistent with $B \propto t^{-1}$. Radio spectral index in the optically thin part evolves from $\alpha \sim 0.8-1.0$ at few tens of days to $\sim 0.6$ in about 10 years.Comment: 37 pages, 9 figures in LaTex; scheduled for ApJ 10 September 2004, v612 issue; send comments to: [email protected]

Radioactivity and Electron Acceleration in Supernova Remnants

We argue that the decays of radioactive nuclei related to $^{44}$Ti and $^{56}$Ni ejected during supernova explosions can provide a vast pool of mildly relativistic positrons and electrons which are further accelerated to ultrarelativistic energies by reverse and forward shocks. This interesting link between two independent processes - the radioactivity and the particle acceleration - can be a clue for solution of the well known theoretical problem of electron injection in supernova remnants. In the case of the brightest radio source Cas A, we demonstrate that the radioactivity can supply adequate number of energetic electrons and positrons for interpretation of observational data provided that they are stochastically pre-accelerated in the upstream regions of the forward and reverse shocks.Comment: 6 pages, 1 figure, revised version accepted to Phys.Rev.

Cicrumnuclear Supernova Remnants and HII Regions in NGC 253

Archival VLA data has been used to produce arcsecond-resolution 6- and 20-cm images of the region surrounding the nuclear 200-pc (~15") starburst in NGC 253. Twenty-two discrete sources stronger than 0.4 mJy have been detected within ~2 kpc (~3') of the galaxy nucleus; almost all these sources must be associated with the galaxy. None of the radio sources coincides with a detected X-ray binary, so they appear to be due to supernova remnants and H II regions. The region outside the central starburst has a derived radio supernova rate of <~0.1/yr, and may account for at least 20% of the recent star formation in NGC 253. Most of the newly identified sources have steep, nonthermal radio spectra, but several relatively strong thermal sources also exist, containing the equivalent of tens of O5 stars. These stars are spread over tens of parsecs, and are embedded in regions having average ionized gas densities of 20-200/cm^3, much lower than in the most active nuclear star-forming regions in NGC 253 or in the super star clusters seen in other galaxies. The strongest region of thermal emission coincides with a highly reddened area seen at near-infrared wavelengths, possibly containing optically obscured H II regions.Comment: 17 pages, 3 postscript figures, AASTeX format, in press for Astronomical Journal, July 200

Water Maser Emission from the Active Nucleus in M51

22 GHz water vapor `kilomaser' emission is reported from the central region of the Whirlpool galaxy M 51 (NGC 5194). The red-shifted spectral features (Vlsr ~ 560 km/s), flaring during most of the year 2000, originate from a spatially unresolved maser spot of size < 30 mas (< 1.5 pc), displaced by < 250 mas from the nucleus. The data provide the first direct evidence for the association of an H2O kilomaser with an active galactic nucleus (AGN). In early 2001, blue-shifted maser emission (Vlsr ~ 435 km/s) was also detected. Red- and blue-shifted features bracket the systemic velocity asymmetrically. Within the standard model of a rotating Keplerian torus, this may either suggest the presence of a highly eccentric circumnuclear cloud or red- and blue-shifted `high velocity' emission from a radially extended torus. Most consistent with the measured H2O position is, however, an association of the red-shifted H2O emission with the northern part of the bipolar radio jet. In this scenario, the (weaker) northern jet is receding while the blue-shifted H2O emission is associated with the approaching southern jet.Comment: 9 pages, 1 figur

Photometric Variability and Astrometric Stability of the Radio Continuum Nucleus in the Seyfert Galaxy NGC 5548

The NRAO VLA and VLBA were used from 1988 November to 1998 June to monitor the radio continuum counterpart to the optical broad line region (BLR) in the Seyfert galaxy NGC 5548. Photometric and astrometric observations were obtained at 21 epochs. The radio nucleus appeared resolved, so comparisons were limited to observations spanning 10-60 days and 3-4 yr, and acquired at matched resolutions of 1210 mas = 640 pc (9 VLA observations), 500 mas = 260 pc (9 VLA observations), or 2.3 mas = 1.2 pc (3 VLBA observations). The nucleus is photometrically variable at 8.4 GHz by $33\pm5$% and $52\pm5$% between VLA observations separated by 41 days and 4.1 yr, respectively. The 41-day changes are milder ($19\pm5$%) at 4.9 GHz and exhibit an inverted spectrum ($\alpha \sim +0.3\pm0.1$, $S\propto \nu ^{+\alpha}$) from 4.9 to 8.4 GHz. The nucleus is astrometrically stable at 8.4 GHz, to an accuracy of 28 mas = 15 pc between VLA observations separated by 4.1 yr and to an accuracy of 1.8 mas = 0.95 pc between VLBA observations separated by 3.1 yr. Such photometric variability and astrometric stability is consistent with a black hole being the ultimate energy source for the BLR, but is problematic for star cluster models that treat the BLR as a compact supernova remnant and, for NGC 5548, require a new supernova event every 1.7 yr within an effective radius $r_e =$ 42 mas = 22 pc. A deep image at 8.4 GHz with resolution 660 mas = 350 pc was obtained by adding data from quiescent VLA observations. This image shows faint bipolar lobes straddling the radio nucleus and spanning 12 arcsec = 6.4 kpc. These synchrotron-emitting lobes could be driven by twin jets or a bipolar wind from the Seyfert 1 nucleus.Comment: with 9 figures, to appear in the Astrophysical Journal, 2000 March 10, volume 53

Biology of Consciousness

The Dynamic Core and Global Workspace hypotheses were independently put forward to provide mechanistic and biologically plausible accounts of how brains generate conscious mental content. The Dynamic Core proposes that reentrant neural activity in the thalamocortical system gives rise to conscious experience. Global Workspace reconciles the limited capacity of momentary conscious content with the vast repertoire of long-term memory. In this paper we show the close relationship between the two hypotheses. This relationship allows for a strictly biological account of phenomenal experience and subjectivity that is consistent with mounting experimental evidence. We examine the constraints on causal analyses of consciousness and suggest that there is now sufficient evidence to consider the design and construction of a conscious artifact

A Wide Field, Low Frequency Radio Survey of the Field of M31: I. Construction and Statistical Analysis of the Source Catalog

We present here the results of a 325 MHz radio survey of M31, conducted with the A-configuration of the Very Large Array. The survey covered an area of 7.6 deg$^2$, and a total of 405 radio sources between \la6\arcsec and 170\arcsec in extent were mapped with a resolution of 6\arcsec and a 1$\sigma$ sensitivity of $\sim$0.6 \mjyb. For each source, its morphological class, major axis $\theta_M$, minor axis $\theta_m$, position angle $\theta_{PA}$, peak flux $I$, integrated flux density $S$, spectral index $\alpha$ and spectral curvature parameter $\phi$ were calculated. A comparison of the flux and radial distribution -- both in the plane of the sky and in the plane of M31 -- of these sources with those of the XMM--LSS and WENSS radio surveys revealed that a vast majority of sources detected are background radio galaxies. As a result of this analysis, we expect that only a few sources are intrinsic to M31. These sources are identified and discussed in an accompanying paper.Comment: 29 pages with 4 tables and 10 figures (JPEGs), accepted for publication in ApJS. Full-resolution images available on reques