Recent work has exploited pulsar survey data to identify temporally isolated, millisecond-duration radio bursts with large dispersion measures (DMs). These bursts have been interpreted as arising ...from a population of extragalactic sources, in which case they would provide unprecedented opportunities for probing the intergalactic medium; they may also be linked to new source classes. Until now, however, all so-called fast radio bursts (FRBs) have been detected with the Parkes radio telescope and its 13-beam receiver, casting some concern about the astrophysical nature of these signals. Here we present FRB 121102, the first FRB discovery from a geographic location other than Parkes. FRB 121102 was found in the Galactic anti-center region in the 1.4 GHz Pulsar Arecibo L-band Feed Array (ALFA) survey with the Arecibo Observatory with a DM = 557.4 + or - 2.0 pc cm super(-3), pulse width of 3.0 + or - 0.5 ms, and no evidence of interstellar scattering. The observed delay of the signal arrival time with frequency agrees precisely with the expectation of dispersion through an ionized medium. Despite its low Galactic latitude (b = 0degrees.2), the burst has three times the maximum Galactic DM expected along this particular line of sight, suggesting an extragalactic origin. A peculiar aspect of the signal is an inverted spectrum; we interpret this as a consequence of being detected in a sidelobe of the ALFA receiver. FRB 121102's brightness, duration, and the inferred event rate are all consistent with the properties of the previously detected Parkes bursts.
UV‐generated excited states of cytosine (C) nucleobases are precursors to mutagenic photoproduct formation. The i‐motif formed from C‐rich sequences is known to exhibit high yields of long‐lived ...excited states following UV absorption. Here the excited states of several i‐motif structures have been characterized following 267 nm laser excitation using time‐resolved infrared spectroscopy (TRIR). All structures possess a long‐lived excited state of ∼300 ps and notably in some cases decays greater than 1 ns are observed. These unusually long‐lived lifetimes are attributed to the interdigitated DNA structure which prevents direct base stacking overlap.
Exciting longevity: UV excitation of the four‐stranded cytosine i‐motif produces excited states lasting into the nanosecond region, suggesting that they are vulnerable to photodamage. A variety of i‐motif forming structures are probed using time‐resolved infrared spectroscopy. Distinctive transient IR features are observed and their possible origins discussed with reference to the unique structure of the i‐motif.
Observations indicate that nearly all galaxies contain supermassive black holes at their centers. When galaxies merge, their component black holes form SMBH binaries (SMBHBs), which emit ...low-frequency gravitational waves (GWs) that can be detected by pulsar timing arrays. We have searched the North American Nanohertz Observatory for Gravitational Waves 11 yr data set for GWs from individual SMBHBs in circular orbits. As we did not find strong evidence for GWs in our data, we placed 95% upper limits on the strength of GWs from such sources. At = 8 nHz, we placed a sky-averaged upper limit of h0 < 7.3(3) × 10−15. We also developed a technique to determine the significance of a particular signal in each pulsar using "dropout" parameters as a way of identifying spurious signals. From these upper limits, we ruled out SMBHBs emitting GWs with = 8 nHz within 120 Mpc for , and within 5.5 Gpc for at our most sensitive sky location. We also determined that there are no SMBHBs with emitting GWs with = 2.8-317.8 nHz in the Virgo Cluster. Finally, we compared our strain upper limits to simulated populations of SMBHBs, based on galaxies in the Two Micron All-Sky Survey and merger rates from the Illustris cosmological simulation project, and found that only 34 out of 75,000 realizations of the local universe contained a detectable source.
ABSTRACT We compute upper limits on the nanohertz-frequency isotropic stochastic gravitational wave background (GWB) using the 9 year data set from the North American Nanohertz Observatory for ...Gravitational Waves (NANOGrav) collaboration. Well-tested Bayesian techniques are used to set upper limits on the dimensionless strain amplitude (at a frequency of 1 yr−1) for a GWB from supermassive black hole binaries of A gw < 1.5 × 10 − 15 . We also parameterize the GWB spectrum with a broken power-law model by placing priors on the strain amplitude derived from simulations of Sesana and McWilliams et al. Using Bayesian model selection we find that the data favor a broken power law to a pure power law with odds ratios of 2.2 and 22 to one for the Sesana and McWilliams prior models, respectively. Using the broken power-law analysis we construct posterior distributions on environmental factors that drive the binary to the GW-driven regime including the stellar mass density for stellar-scattering, mass accretion rate for circumbinary disk interaction, and orbital eccentricity for eccentric binaries, marking the first time that the shape of the GWB spectrum has been used to make astrophysical inferences. Returning to a power-law model, we place stringent limits on the energy density of relic GWs, gw ( f ) h 2 < 4.2 × 10 − 10 . Our limit on the cosmic string GWB, gw ( f ) h 2 < 2.2 × 10 − 10 , translates to a conservative limit on the cosmic string tension with G < 3.3 × 10 − 8 , a factor of four better than the joint Planck and high-l cosmic microwave background data from other experiments.
To analyse the mechanism and kinetics of DNA strand cleavages catalysed by the serine recombinase Tn3 resolvase, we made modified recombination sites with a single-strand nick in one of the two DNA ...strands. Resolvase acting on these sites cleaves the intact strand very rapidly, giving an abnormal half-site product which accumulates. We propose that these reactions mimic second-strand cleavage of an unmodified site. Cleavage occurs in a synapse of two sites, held together by a resolvase tetramer; cleavage at one site stimulates cleavage at the partner site. After cleavage of a nicked-site substrate, the half-site that is not covalently linked to a resolvase subunit dissociates rapidly from the synapse, destabilizing the entire complex. The covalent resolvase-DNA linkages in the natural reaction intermediate thus perform an essential DNA-tethering function. Chemical modifications of a nicked-site substrate at the positions of the scissile phosphodiesters result in abolition or inhibition of resolvase-mediated cleavage and effects on resolvase binding and synapsis, providing insight into the serine recombinase catalytic mechanism and how resolvase interacts with the substrate DNA.
The regularity of pulsar emissions becomes apparent once we reference the pulses' times of arrivals to the inertial rest frame of the solar system. It follows that errors in the determination of ...Earth's position with respect to the solar system barycenter can appear as a time-correlated bias in pulsar-timing residual time series, affecting the searches for low-frequency gravitational waves performed with pulsar-timing arrays. Indeed, recent array data sets yield different gravitational-wave background upper limits and detection statistics when analyzed with different solar system ephemerides. Crucially, the ephemerides do not generally provide usable error representations. In this article, we describe the motivation, construction, and application of a physical model of solar system ephemeris uncertainties, which focuses on the degrees of freedom (Jupiter's orbital elements) most relevant to gravitational-wave searches with pulsar-timing arrays. This model, BayesEphem, was used to derive ephemeris-robust results in NANOGrav's 11 yr stochastic-background search, and it provides a foundation for future searches by NANOGrav and other consortia. The analysis and simulations reported here suggest that ephemeris modeling reduces the gravitational-wave sensitivity of the 11 yr data set and that this degeneracy will vanish with improved ephemerides and with pulsar-timing data sets that extend well beyond a single Jovian orbital period.
Uppermost mantle P wave velocity structure beneath the Baikal rift and southern margin of the Siberian Platform has been investigated by using a grid search method to model Pnl waveforms from two ...moderate earthquakes recorded by station TLY at the southwestern end of Lake Baikal. The results yielded a limited number of successful models which indicate the presence of upper mantle P wave velocities beneath the rift axis and the margin of the platform that are 2–5% lower than expected. The magnitude of the velocity anomalies and their location support the presence of a thermal anomaly that extends laterally beyond the rift proper, possibly created by small‐scale convection or a plume‐like, thermal upwelling.
P wave travel times from regional earthquakes recorded by the Tanzania Broadband Seismic Experiment have been inverted for long wavelength (>100 km) Pn velocity variations beneath Tanzania using a ...generalized inverse algorithm. Pn velocities, on average, are 8.40 to 8.45 km/s beneath the center of the Tanzania Craton, 8.30–8.35 km/s beneath the terminus of the Eastern Branch of the rift system, and 8.35–8.40 km/s beneath the Western Branch. These velocities indicate that there are no broad (>100 km wide) thermal anomalies in the uppermost mantle beneath areas of rifting in Tanzania, and suggest that thermal anomalies present deeper in the mantle have not yet reached the base of the crust.
The Back Cover picture shows that UV excitation of intramolecular i‐motif DNA yields a rich transient infrared spectrum with unusually long excited state lifetimes, extending into the nanosecond ...timescale (i‐motif coordinates from PDB file 1ELN). More information can be found in the Full Paper by P. M. Keane et al. on page 1281 in Issue 9, 2016 (DOI:10.1002/cphc.201501183).
PDF
