A repeating fast radio burst source localized to a nearby spiral galaxy.

Fast radio bursts (FRBs) are brief, bright, extragalactic radio flashes1,2. Their physical origin remains unknown, but dozens of possible models have been postulated3. Some FRB sources exhibit repeat bursts4-7. Although over a hundred FRB sources have been discovered8, only four have been localized and associated with a host galaxy9-12, and just one of these four is known to emit repeating FRBs9. The properties of the host galaxies, and the local environments of FRBs, could provide important clues about their physical origins. The first known repeating FRB, however, was localized to a low-metallicity, irregular dwarf galaxy, and the apparently non-repeating sources were localized to higher-metallicity, massive elliptical or star-forming galaxies, suggesting that perhaps the repeating and apparently non-repeating sources could have distinct physical origins. Here we report the precise localization of a second repeating FRB source6, FRB 180916.J0158+65, to a star-forming region in a nearby (redshift 0.0337 ± 0.0002) massive spiral galaxy, whose properties and proximity distinguish it from all known hosts. The lack of both a comparably luminous persistent radio counterpart and a high Faraday rotation measure6 further distinguish the local environment of FRB 180916.J0158+65 from that of the single previously localized repeating FRB source, FRB 121102. This suggests that repeating FRBs may have a wide range of luminosities, and originate from diverse host galaxies and local environments.

A direct localization of a fast radio burst and its host.

Fast radio bursts are astronomical radio flashes of unknown physical nature with durations of milliseconds. Their dispersive arrival times suggest an extragalactic origin and imply radio luminosities that are orders of magnitude larger than those of all known short-duration radio transients. So far all fast radio bursts have been detected with large single-dish telescopes with arcminute localizations, and attempts to identify their counterparts (source or host galaxy) have relied on the contemporaneous variability of field sources or the presence of peculiar field stars or galaxies. These attempts have not resulted in an unambiguous association with a host or multi-wavelength counterpart. Here we report the subarcsecond localization of the fast radio burst FRB 121102, the only known repeating burst source, using high-time-resolution radio interferometric observations that directly image the bursts. Our precise localization reveals that FRB 121102 originates within 100 milliarcseconds of a faint 180-microJansky persistent radio source with a continuum spectrum that is consistent with non-thermal emission, and a faint (twenty-fifth magnitude) optical counterpart. The flux density of the persistent radio source varies by around ten per cent on day timescales, and very long baseline radio interferometry yields an angular size of less than 1.7 milliarcseconds. Our observations are inconsistent with the fast radio burst having a Galactic origin or its source being located within a prominent star-forming galaxy. Instead, the source appears to be co-located with a low-luminosity active galactic nucleus or a previously unknown type of extragalactic source. Localization and identification of a host or counterpart has been essential to understanding the origins and physics of other kinds of transient events, including gamma-ray bursts and tidal disruption events. However, if other fast radio bursts have similarly faint radio and optical counterparts, our findings imply that direct subarcsecond localizations may be the only way to provide reliable associations.

Hydrogen and carbon isotope fractionation during aerobic biodegradation of benzene.

The main aim of the study was to evaluate hydrogen and carbon isotope fractionation during biodegradation of benzene as a possible tool to trace the process in contaminated environments. Aerobic biodegradation of benzene by two bacterial isolates, Acinetobacter sp. and Burkholderia sp., was accompanied by significant hydrogen and carbon isotope fractionation with hydrogen isotope enrichment factors of -12.8 +/- 0.7 per thousand and -11.2 +/- 1.8 per thousand, respectively, and average carbon isotope enrichment factors of -1.46 +/- 0.06 per thousand and -3.53 +/- 0.26 per thousand, respectively. Inorganic carbon produced by Acinetobacter sp. was depleted in 13C by 3.6-6.2 per thousand as compared to the initial delta13C of benzene, while the produced biomass was enriched in 13C by 3.8 per thousand. The secondary aim was to determine isotope ratios of benzenes from different manufacturers with regard to the use of isotopes for source differentiation. While two of the four analyzed benzenes had similar delta13C values, each of them had a distinct delta2H-delta13C pair and delta2H values spread over a range of 66.5 per thousand. Thus, combined analyses of hydrogen and carbon isotopes may be a more promising approach to trace sources and/or biodegradation of benzene than measuring carbon isotopes only.

Monitoring biodegradation of methyl tert-butyl ether (MTBE) using compound-specific carbon isotope analysis.

Methyl tert-butyl ether (MTBE), the most common gasoline oxygenate, is frequently detected in surface water and groundwater. The aim of this study was to evaluate the potential of compound-specific isotope analysis to assess in situ biodegradation of MTBE in groundwater. For that purpose, the effect of relevant physical and biological processes on carbon isotope ratios of MTBE was evaluated in laboratory studies. Carbon isotope fractionation during organic phase/gas-phase partitioning (0.50 +/- 0.15@1000), aqueous phase/gas-phase partitioning (0.17 +/- 0.05@1000), and organic phase/aqueous-phase partitioning (0.18 +/- 0.24@1000) was small in comparison to carbon isotope fractionation measured during biodegradation of MTBE in microcosms based on aquifer sediments of the Borden site. In experiments with MTBE as the only substrate and a cometabolic experiment with 3-methypentane as primary substrate, MTBE became enriched in 13C by 5.1 to 6.9@1000 after 95 to 97% degradation. For both experiments, similar isotopic enrichment factors were obtained (-1.52 +/- 0.06 to -1.97 +/- 0.05@1000). Biodegradation of TBA, which accumulated transiently in the cometabolic microcosms, was also accompanied by carbon isotope fractionation, with an isotopic enrichment factor of -4.21 +/- 0.07@1000. This study suggests that carbon isotope analysis is a potential tool to trace in situ biodegradation of MTBE and TBA and thus to better understand the fate of these contaminants in the environment.

Mercury radar imaging: evidence for polar ice.

The first unambiguous full-disk radar mapping of Mercury at 3.5-centimeter wavelength, with the Goldstone 70-meter antenna transmitting and 26 antennas of the Very Large Array receiving, has provided evidence for the presence of polar ice. The radar experiments, conducted on 8 and 23 August 1991, were designed to image the half of Mercury not photographed by Mariner 10. The orbital geometry allowed viewing beyond the north pole of Mercury; a highly reflective region was clearly visible on the north pole during both experiments. This polar region has areas in which the circular polarization ratio (pt) was 1.0 to 1.4; values < approximately 0.1 are typical for terrestrial planets. Such high values of have hitherto been observed in radar observations only from icy regions of Mars and icy outer planet satellites.

Radar images of Mars.

Full disk images of Mars have been obtained with the use of the Very Large Array (VLA) to map the radar reflected flux density. The transmitter system was the 70-m antenna of the Deep Space Network at Goldstone, California. The surface of Mars was illuminated with continuous wave radiation at a wavelength of 3,5 cm. The reflected energy was mapped in individual 12-minute snapshots with the VLA in its largest configuration; fringe spacings as small as 67 km were obtained. The images reveal near-surface features including a region in the Tharsis volcano area, over 2000 km in east-west extent, that displayed no echo to the very low level of the radar system noise. The feature, called Stealth, is interpreted as a deposit of dust or ash with a density less than about 0.5 gram per cubic centimeter and free of rocks larger than 1 cm across. The deposit must be several meters thick and may be much deeper. The strongest reflecting geological feature was the south polar ice cap, which was reduced in size to the residual south polar ice cap at the season of observation. The cap image is interpreted as arising from nearly pure CO(2) or H(2)O ice with a small amount of martian dust (less than 2 percent by volume) and a depth greater than 2 to 5 m. Only one anomalous reflecting feature was identified outside of the Tharsis region, although the Elysium region was poorly sampled in this experiment and the north pole was not visible from Earth.

Cyclic AMP-dependent induction of serotonin N-acetyltransferase activity in photoreceptor-enriched chick retinal cell cultures: characterization and inhibition by dopamine.

The activity of serotonin N-acetyltransferase (NAT), a key regulatory enzyme in the melatonin biosynthetic pathway, was examined in low-density monolayer cultures of chick embryo retinal cells prepared with three levels of photoreceptor enrichment. In cultures prepared from embryonic day 8 retinas (E8), photoreceptors represented approximately 30% of the total cell population, whereas in those prepared from embryonic day 6 retinas (E6), approximately 70% of the cells were photoreceptors. In E8 retinas treated with kainic acid to destroy neurons (E8K), the relative content of photoreceptors was increased to approximately 50%. NAT activity was detectable in the cultures under all conditions studied, and was markedly increased by drugs that increase intracellular cyclic AMP levels and cyclic AMP-dependent protein kinase activity: 8-bromocyclic AMP, forskolin, and 3-isobutyl-1-methylxanthine (IBMX). Consistent with the hypothesis that NAT is localized in photoreceptors, the effects of the stimulatory treatments were significantly greater in E6 and E8K cultures than in E8 cultures. The stimulation of NAT activity in E6 cultures was inhibited by actinomycin D and cycloheximide, suggesting the involvement of RNA and protein synthesis. Dopamine inhibited the induction of NAT activity by forskolin and IBMX, but not that elicited by 8-bromocyclic AMP. The dopamine-mediated suppression of activity was significantly inhibited by pertussis toxin and by spiperone and sulpiride, both D2-dopamine receptor antagonists, but not by SCH 23390, a D1-dopamine receptor blocker, or antagonists of alpha-adrenergic, beta-adrenergic, or serotonergic receptors. Because the inhibitory effect of dopamine on E6 and E8K cultures was at least as great as that on E8 cultures, the results suggest that dopamine acts on D2-like receptors on photoreceptors. The receptors appear to be coupled to adenylate cyclase through an inhibitory GTP-binding protein and to mediate inhibition of cyclic AMP synthesis and consequent induction of NAT activity.

Radar reflectivity of titan.

The present understanding of the atmosphere and surface conditions on Saturn's largest moon, Titan, including the stability of methane, and an application of thermodynamics leads to a strong prediction of liquid hydrocarbons in an ethane-methane mixture on the surface. Such a surface would have nearly unique microwave reflection properties due to the low dielectric constant. Attempts were made to obtain reflections at a wavelength of 3.5 centimeters by means of a 70-meter antenna in California as the transmitter and the Very Large Array in New Mexico as the receiving instrument. Statistically significant echoes were obtained that show Titan is not covered with a deep, global ocean of ethane, as previously thought. The experiment yielded radar cross sections normalized by the Titan disk of 0.38 +/- 0.15, 0.78 +/- 0.15, and 0.25 +/- 0.15 on three consecutive nights during which the sub-Earth longitude on Titan moved 50 degrees. The result for the combined data for the entire experiment is 0.35 +/- 0.08. The cross sections are very high, most consistent with those of the Galilean satellites; no evidence of the putative liquid ethane was seen in the reflection data. A global ocean as shallow as about 200 meters would have exhibited reflectivities smaller by an order of magnitude, and below the detection limit of the experiment. The measured emissivity at similar wavelengths of about 0.9 is somewhat inconsistent with the high reflectivity.

K(+)-evoked depolarization induces serotonin N-acetyltransferase activity in photoreceptor-enriched retinal cell cultures. Involvement of calcium influx through l-type channels.

The roles of membrane depolarization and calcium influx in the regulation of retinal serotonin N-acetyltransferase (NAT) activity were investigated in low-density monolayer cultures of chick retinal cells, in which photoreceptors represented approximately 70% of the total cell population. NAT activity expressed by the cells in these cultures was markedly increased by elevating the concentration of extracellular K(+). Activity increased rapidly during the first 6 h of exposure to K(+), and remained elevated for at least 30 h. Chelation of calcium in the culture medium abolished the K(+)-evoked increase in NAT activity. Antagonists of voltage-sensitive calcium channels, nifedipine, methoxyverapamil (D600), Mn(2+), Mg(2+), and Cd(2+) inhibited the K(+)-evoked increase of NAT activity. Bay K 8644, a dihydropyridine calcium channel agonist, increased NAT activity when added alone and potentiated the K(+)-evoked increase of activity. The effect of Bay K 8644 was antagonized by nifedipine. Addition of nifedipine 18 h after addition of K(+), when NAT activity is elevated, caused activity to decrease to basal levels. These studies indicate that the increase of retinal NAT activity induced by K(+)-depolarization is mediated by a calcium-dependent process that involves sustained Ca(2+) influx through L-type voltage-sensitive Ca(2+)-channels.