Blind search for 21-cm absorption systems using a new generation of Chinese radio telescopes

Neutral hydrogen clouds are known to exist in the Universe,however their spatial distributions and physical properties are poorly understood.Such missing information can be studied by the new generation of Chinese radio telescopes through a blind search of 21-cm absorption systems.We forecast the capabilities of surveys of 21-cm absorption systems by two representative radio telescopes in China– the Five-hundred-meter Aperture Spherical radio Telescope(FAST) and Tianlai 21-cm cosmology experiment(Tianlai).Facilitated by either the high sensitivity(FAST) or wide field of view(Tianlai)of these telescopes,more than a thousand 21-cm absorption systems can be discovered in a few years,representing orders of magnitude improvement over the cumulative discoveries in the past half a century.     

A VLA search for 2-cm continuum radiation from Comet Halley

We used the VLA² to search for continuum emission from icy grains in a halo around Comet Halley in mid November 1985. We found the 3σ upper limit to the 2-cm flux density from the comet to be 1 × 10⁻⁴ Jy, which is consistent with the detections at 1.3 and 3.5 mm by W.J. Altenhoff et al. (1986, Astron. Astrophys., in press) only if the emission comes from particles which do not radiate efficiently at centimeter wavelengths. These particles could be slightly dirty submillimeter-sized icy grains or small refractory grains.     

A search for narrow band 21-cm wavelength signals from ten nearby stars

A search for narrow band radio signals, in the 21-cm wavelength band, possibly generated by other civilizations, has been made in the direction of ten stars. No such signals were found and upper limits to the power of possible transmissions toward the Earth are given.     

The cross-correlation of the CMB polarization and the 21-cm line fluctuations from cosmic reionization

The cosmic microwave background (CMB) polarization and the 21-cm line fluctuations are powerful probes of cosmological reionization. We study how the cross-correlation between the CMB polarization ( E modes) and the 21-cm line fluctuations can be used to gain further understanding of the reionization history, within the framework of inhomogeneous reionization. Since the E -mode polarization reflects the amplitude of the quadrupole component of the CMB temperature fluctuations, the angular power spectrum of the cross-correlation exhibits oscillations at all multipoles. The first peak of the power spectrum appears at the scale corresponding to the quadrupole at the redshift, which is probed by the 21-cm line fluctuations. The peak reaches its maximum value in redshift when the average ionization fraction of the universe is about half. On the other hand, on small scales, there is a damping that depends on the duration of reionization. Thus, the cross-correlation between the CMB polarization and the 21-cm line fluctuations has the potential to accurately constrain the epoch and the duration of reionization.     

A search for prompt radio emission from GRBs

The Cambridge Low Frequency Synthesis Telescope (CLFST) is being used to search for prompt radio emission from GRBs at 151 MHz. For this search a special observing mode has recently been developed; the antennas of the CLFST are split into seven groups which are spaced about 30° apart. When triggered by the real-time signals from the BACODINE system the antennas are slewed to the GRB position, with data being sampled at 1.5 or 3-s intervals. The antennas slew at about 10° per minute, so that an appreciable fraction of the sky can be accessed on timescales of a few minutes - any prompt radio emission from a GRB might be delayed by this timescale due to propagation effects.     

A search for polarization in radio recombination lines

We have attempted to detect linear polarization in carbon and hydrogen 76 radio recombination lines from the Hii region NGC 2024. The upper limits set by these measurements are briefly discussed.On leave of absence from Cornell University, Ithaca, N.Y., U.S.A.     

A search strategy for finding extraterrestrial radio beacons

The Principle of Anti-Cryptography, together with our knowledge of the structure and properties of the universe, lead to the suggestion that extraterrestrial radio beacons will transmit continuously and omnidirectionally, using binary sense-switched circular polarization modulation. The expected frequency of transmission is the rest frequency of hydrogen, relative to the galactic center. Probable signaling rates are within two orders of magnitude of one second. Methods of implementing the strategy are suggested, from both the receiving and transmitting viewpoints.     

Alternative kind of hydrogen atoms as a possible explanation for the latest puzzling observation of the 21 cm radio line from the early Universe

There is a puzzling astrophysical result concerning the latest observation of the absorption profile of the redshifted radio line 21 cm from the early Universe(as described in Bowman et al.). The amplitude of the profile was more than a factor of two greater than the largest predictions. This could mean that the primordial hydrogen gas was much cooler than expected. Some explanations in the literature suggested a possible cooling of baryons either by unspecified dark matter particles or by some exotic dark matter particles with a charge a million times smaller than the electron charge. Other explanations required an additional radio background. In the present paper, we entertain a possible different explanation for the above puzzling observational result: the explanation is based on the alternative kind of hydrogen atoms(AKHA),whose existence was previously demonstrated theoretically, as well as by the analysis of atomic experiments. Namely, the AKHA are expected to decouple from the cosmic microwave background(CMB) much earlier(in the course of the Universe expansion) than usual hydrogen atoms, so that the AKHA temperature is significantly lower than that of usual hydrogen atoms. This seems to lower the excitation(spin) temperature of the hyperfine doublet(responsible for the 21 cm line) sufficiently enough for explaining the above puzzling observational result. This possible explanation appears to be more specific and natural than the previous possible explanations. Further observational studies of the redshifted 21 cm radio line from the early Universe could help to verify which explanation is the most relevant.     

A search for molecular gas in restarting radio galaxies

Several radio galaxies are known that show radio morphological signatures that are best interpreted as restarting of nuclear activity after a period of quiescence. The conditions surrounding the phenomenon of nuclear recurrence are not understood. In this paper we have attempted to address this question by examining the nuclear fuelling characteristics in a sample of restarting radio galaxies. We have examined the detection rate for molecular gas in a representative sample of nine restarting radio galaxies, for seven of which we present new upper limits to the molecular gas mass derived from CO line observations we made with the IRAM 30-m telescope. We derive a low CO detection rate for the relatively young restarted radio galaxies suggesting that the cessation of the nuclear activity and its subsequent restarting may be a result of instabilities in the fuelling process rather than a case of depletion of fuel followed by a recent fuel acquisition. It appears that abundant molecular gas content at the level of few  10⁸–10⁹ M_⊙  does not necessarily accompany the nuclear restarting phenomenon. For comparison we also discuss the molecular gas properties of five normal giant radio galaxies, three of which we observed using Swedish-ESO Millimetre Telescope (SEST). Despite obvious signs of interactions and nuclear dust discs none of them has been found to host significant quantities of molecular gas.     

A search for radio pulsars around low-mass white dwarfs

Low-mass white dwarfs can be produced either in low-mass X-ray binaries by stable mass transfer to a neutron star, or in a common envelope phase with a heavier white dwarf companion. We have searched eight low-mass white dwarf candidates recently identified in the Sloan Digital Sky Survey for radio pulsations from pulsar companions, using the Green Bank Telescope at 340 MHz. We have found no pulsations down to flux densities of 0.6–0.8 mJy kpc⁻² and conclude that a given low-mass helium-core white dwarf has a probability of  <0.18 ± 0.05  of being in a binary with a radio pulsar.     

A search for intergalactic molecular hydrogen clouds

We report the identification of molecular hydrogen lines in the spectra of 4 high-redshift quasars, OQ172, 4CO5.34, 0528-250 and 1448-232. One absorption system at z = 2.27292 is found in OQ172 and two absorption systems at z = 2.37030 and 2.44478 are found in 4CO5.34.     

A search for solar neutrons near solar maximum

A plastic scintillator counter with anticoincidence screen has been flown in two 4-hour balloon flights, at a floating altitude of 3.2 and 3.5 mb, during day-and night-time respectively. Comparison between the day-and night-time counting-rates gives an upper limit of 10^–2n/cm² sec above 45 MeV for the continuous emission flux.This upper limit is compared with those derived from other experiments in function of the neutron energy spectrum assumed, and also with the expected continuous emission deduced from solar proton fluxes reported in non-flare conditions. It is concluded that with present experimental techniques, non-flare solar neutron emission can be detected only if the proton fluxes observed represent less than 10^–3 of the protons accelerated in the sun.This research has been sponsored in part by the Air Force Cambridge Research Laboratories through the European Office of Aerospace Research, OAR, USAF under contract F 61052-68-C-050.0     

A search for faint giant radio galaxies in the NVSS survey

Giant radio galaxies are the most extended radio sources in the universe having the sizes of groups and clusters of galaxies (about 1 Mpc). Their total number is comparable with the amount of clusters, revealing the Zeldovich-Sunyaev effect, which may lead to a biased estimation of the angular power spectrum for the ZS effect. To assess a possible contribution to the power spectrum, we need to collect the data on all the observed giant radio galaxies. We propose an algorithm for the selection of large sources (sized over 4′) from the NVSS survey catalog, using the parameters of possible components of radio sources, cataloged as compact objects. As a result of the first stage of our work, we have selected 61 new giant radio galaxy candidates, of which 35 have a pronounced morphological type FRII, and 26 galaxies—the FRI type.     

A search for the 53-MHz OH line near G48.4−1.4 using the National MST Radar Facility

We present the results of a search for the ground-state hyperfine transition of the OH radical near 53 MHz using the National Mesosphere–Stratosphere–Troposphere (MST) Radar Facility at Gadanki, India. The observed position was G48.4−1.4 near the Galactic plane. The OH line is not detected. We place a 3σ upper limit for the line flux density at 39 Jy from our observations. We also did not detect recombination lines (RLs) of carbon, which were within the frequency range of our observations. The 3σ upper limit of 20 Jy obtained for the flux density of carbon RLs, along with observations at 34.5 and 327 MHz, are used to constrain the physical properties of the line-forming region. Our upper limit is consistent with the line emission expected from a partially ionized region with electron temperature, density and path lengths in the range 20–300 K, 0.03–0.3 cm⁻³ and 0.1–170 pc, respectively.     

A systematic search for radio pulses associated with gamma-ray bursts

An automated station for the detection of radio bursts at 151 and 408 MHz has been operating at Medicina (Italy) since 1976. Radio data associated with 32 gamma-ray bursts, detected between July 1976 and April 1979, have been analysed. The results will be discussed in the present paper.Paper presented at the Symposium and Workshop on Gamma-Ray Bursts, Toulouse, France, 26–29 November, 1979.     

A search for radio emission from Galactic supersoft X-ray sources

We have made a deep search for radio emission from all the northern hemisphere supersoft X-ray sources using the Very Large Array (VLA) and multi-element radio-linked interferometer network (MERLIN) telescopes, at 5 and 8.4 GHz. Three previously undetected sources, T Pyx, V1974 Cygni and RX J0019.8+2156, were imaged in quiescence using the VLA in order to search for any persistent emission. No radio emission was detected in any of the VLA fields down to a typical 1 σ rms noise of 20 μJy beam⁻¹, however, 17 new point sources were detected in the fields with 5-GHz fluxes between 100 and 1500 μJy, giving an average 100-μJy source density of ∼200 deg⁻², comparable to what was found in the MERLIN Hubble Deep Field survey. The persistent source AG Draconis was observed by MERLIN to provide a confirmation of previous VLA observations and to investigate the source at a higher resolution. The core is resolved at the milliarcsec scale into two components that have a combined flux of ∼1 mJy. It is possible that we are detecting nebulosity, which is becoming resolved out by the higher MERLIN resolution. We have investigated possible causes of radio emission from a wind environment, both directly from the secondary star, and also consequently, of the high X-ray luminosity from the white dwarf. There is an order of magnitude discrepancy between observed and modelled values that can be explained by the uncertainty in fundamental quantities within these systems.     

A search for hard X-ray microflares near solar minimum

Near solar maximum, hard X-ray microflares with peak 20 keV fluxes of 10^–2 (cm² s keV)^–1, more than ten times smaller than for typical flares and subflares, can occur at the rate of about once every five minutes. We report here on a search for hard X-ray microflares made on a long duration balloon flight in February 1987 near solar minimum, at a time when no active regions were on the Sun. No microflares were observed over a total observing time of 16.5 hours spread over three days, implying a statistical upper limit to their rate of occurrence about a factor often lower than observed near solar maximum. Thus hard X-ray microflaring appears to be an active region phenomenon, and apparently not associated with flaring of soft X-ray bright points.     

A search for the tritium hyperfine line from nearby stars

A search for the tritium hyperfine line at 1516 MHz from 108 assorted astronomical objects, with emphasis on 53 nearby stars, was conducted in June 1983. All stars within 20 light-years visible from the 26-m telescope at Hat Creek Radio Observatory were examined using 256 4883-Hz channels. Twelve stars were also examined using 1024 76-Hz channels. The wideband- and narrowband-channel observations achieved sensitivities of 5–14 × 10⁻²⁴ W/m²/channel and 0.7–2 × 10⁻²⁴ W/m²/channel, respectively. No detections were made. The tritium frequency is highly attractive for SETI work because the isotope is cosmically rare and the tritium hyperfine line is centered in the SETI waterhole region of the terrestrial microwave window. In addition to beacon signals, tritium hyperfine emission may occur as a byproduct of extensive nuclear fusion energy production by extraterrestrial civilizations.     

A search for fragmentation debris near Ursa Major Stream stars

Accretion models of planet formation and the early cratering history of the solar system suggest that planet formation is accompanied by a cloud of debris resulting from accumulation and fragmentation. A rough estimate of the infrared luminosities of debris clouds is presented for comparison with measured 10-μm luminosities of young stars. New measurements of 13 F, G, and K main-sequence stars of the Ursa Major Stream, which is thought to be about 2.7 x 10⁸ years old, place constraints on the amount of debris which could be present near these stars.