We have produced a new software package for the simulation of pulsar populations, PsrPopPy, based on the Psrpop package. The codebase has been re-written in Python (save for some external libraries, ...which remain in their native Fortran), utilizing the object-oriented features of the language, and improving the modularity of the code. Pre-written scripts are provided for running the simulations in 'standard' modes of operation, but the code is flexible enough to support the writing of personalised scripts. The modular structure also makes the addition of experimental features (such as new models for period or luminosity distributions) more straightforward than with the previous code. We also discuss potential additions to the modelling capabilities of the software. Finally, we demonstrate some potential applications of the code; first, using results of surveys at different observing frequencies, we find pulsar spectral indices are best fitted by a normal distribution with mean −1.4 and standard deviation 1.0. Secondly, we model pulsar spin evolution to calculate the best fit for a relationship between a pulsar's luminosity and spin parameters. We used the code to replicate the analysis of Faucher-Giguère & Kaspi, and have subsequently optimized their power-law dependence of radio luminosity, L, with period, P, and period derivative, Ṗ. We find that the underlying population is best described by L ∝ P
−1.39±0.09
Ṗ
0.48±0.04 and is very similar to that found for γ-ray pulsars by Perera et al. Using this relationship, we generate a model population and examine the age-luminosity relation for the entire pulsar population, which may be measurable after future large-scale surveys with the Square Kilometre Array.
We have embarked on a survey for pulsars and fast transients using the 13-beam multibeam receiver on the Parkes Radio Telescope. Installation of a digital backend allows us to record 400 MHz of ...bandwidth for each beam, split into 1024 channels and sampled every 64 μs. Limits of the receiver package restrict us to a 340 MHz observing band centred at 1352 MHz. The factor of 8 improvement in frequency resolution over previous multibeam surveys allows us to probe deeper into the Galactic plane for short-duration signals such as the pulses from millisecond pulsars. We plan to survey the entire southern sky in 42 641 pointings, split into low, mid and high Galactic latitude regions, with integration times of 4200, 540 and 270 s, respectively. Simulations suggest that we will discover 400 pulsars, of which 75 will be millisecond pulsars. With ∼30 per cent of the mid-latitude survey complete, we have redetected 223 previously known pulsars and discovered 27 pulsars, five of which are millisecond pulsars. The newly discovered millisecond pulsars tend to have larger dispersion measures than those discovered in previous surveys, as expected from the improved time and frequency resolution of our instrument.
The detection of five new fast radio bursts (FRBs) found in the 1.4-GHz High Time Resolution Universe high-latitude survey at Parkes, is presented. The rate implied is 7
$^{+5}_{-3}\times 10^3$
...(95 per cent) FRBs sky−1 d−1 above a fluence of 0.13 Jy ms for an FRB of 0.128 ms duration to 1.5 Jy ms for 16 ms duration. One of these FRBs has a two-component profile, in which each component is similar to the known population of single component FRBs and the two components are separated by 2.4 ± 0.4 ms. All the FRB components appear to be unresolved following deconvolution with a scattering tail and accounting for intrachannel smearing. The two-component burst, FRB 121002, also has the highest dispersion measure (1629 pc cm−3) of any FRB to-date. Many of the proposed models to explain FRBs use a single high-energy event involving compact objects (such as neutron-star mergers) and therefore cannot easily explain a two-component FRB. Models that are based on extreme versions of flaring, pulsing, or orbital events, however, could produce multiple component profiles. The compatibility of these models and the FRB rate implied by these detections is discussed.
Submicron particles were collected on board the NOAA R/V Ronald H. Brown during the VAMOS Ocean‐Cloud‐Atmosphere‐Land Study Regional Experiment (VOCALS‐REx) in the southeast Pacific marine boundary ...layer in October and November 2008. The aerosol in this region was characterized by low numbers of particles (150–700 cm−3) that were dominated by sulfate ions at concentrations of 0.9 ± 0.7 μg m−3 and organic mass at 0.6 ± 0.4 μg m−3, with no measurable nitrate and low ammonium ion concentrations. Measurements of submicron organic aerosol functional groups and trace elements show that continental outflow of anthropogenic emissions is the dominant source of organic mass (OM) to the southeast Pacific with an additional, smaller contribution of organic mass from primary marine sources. This continental source is supported by a correlation between OM and radon. Saturated aliphatic C‐CH (alkane) composed 41 ± 27% of OM. Carboxylic acid COOH (32 ± 23% of OM) was observed in single particles internally mixed with ketonic carbonyl, carbonate, and potassium. Organosulfate COSO3 (4 ± 8% of OM) was observed only during the periods of highest organic and sulfate concentrations and lowest ammonium concentrations, consistent with a sulfuric acid epoxide hydrolysis for proposed surrogate compounds (e.g., isoprene oxidation products) or reactive glyoxal uptake mechanisms from laboratory studies. This correlation suggests that in high‐sulfate, low‐ammonium conditions, the formation of organosulfate compounds in the atmosphere contributes a significant fraction of aerosol OM (up to 13% in continental air masses). Organic hydroxyl C‐OH composed 20 ± 12% of OM and up to 50% of remote marine OM and was inversely correlated with radon indicating a marine source. A two‐factor solution of positive matrix factorization (PMF) analysis resulted in one factor dominated by organic hydroxyl (>70% by mass) and one factor dominated by saturated aliphatic C‐CH (alkane) and carboxylic acid (together, 90% by mass), identified as the marine and combustion factors, respectively. Measurements of particle concentrations in the study region compared with concentrations estimated from MODIS aerosol optical depth indicate that continental outflow results in MBL particle concentrations elevated up to 2 times the background level (less than 300 cm−3) away from shore and up to 10 times the background level at the coast. The presence of both coastal fossil fuel combustion and marine sources of oxygenated organic aerosol results in little change in the oxygenated fraction and oxygen to carbon ratio (O/C) along the outflow of the region's dominant organic particle source.
Discoveries of rotating radio transients and fast radio bursts (FRBs) in pulsar surveys suggest that more of such transient sources await discovery in archival data sets. Here we report on a ...single-pulse search for dispersed radio bursts over a wide range of Galactic latitudes (|b| < 60°) in data previously searched for periodic sources by Burgay et al. We re-detected 20 of the 42 pulsars reported by Burgay et al. and one rotating radio transient reported by Burke-Spolaor. No FRBs were discovered in this survey. Taking into account this result, and other recent surveys at Parkes, we corrected for detection sensitivities based on the search software used in the analyses and the different back-ends used in these surveys and find that the all-sky FRB event rate for sources with a fluence above 4.0 Jy ms at 1.4 GHz to be
${\cal R} = 4.4^{+5.2}_{-3.1} \times 10^3$
FRBs d−1 sky−1, where the uncertainties represent a 99 per cent confidence interval. While this rate is lower than inferred from previous studies, as we demonstrate, this combined event rate is consistent with the results of all systematic FRB searches at Parkes to date and does not require the need to postulate a dearth of FRBs at intermediate latitudes.
Thermobia domestica belongs to an ancient group of insects and has a remarkable ability to digest crystalline cellulose without microbial assistance. By investigating the digestive proteome of ...Thermobia, we have identified over 20 members of an uncharacterized family of lytic polysaccharide monooxygenases (LPMOs). We show that this LPMO family spans across several clades of the Tree of Life, is of ancient origin, and was recruited by early arthropods with possible roles in remodeling endogenous chitin scaffolds during development and metamorphosis. Based on our in-depth characterization of Thermobia's LPMOs, we propose that diversification of these enzymes toward cellulose digestion might have endowed ancestral insects with an effective biochemical apparatus for biomass degradation, allowing the early colonization of land during the Paleozoic Era. The vital role of LPMOs in modern agricultural pests and disease vectors offers new opportunities to help tackle global challenges in food security and the control of infectious diseases.
The pulsar spectral index distribution Bates, S. D.; Lorimer, D. R.; Verbiest, J. P. W.
Monthly Notices of the Royal Astronomical Society,
05/2013, Letnik:
431, Številka:
2
Journal Article
Recenzirano
Odprti dostop
The flux-density spectra of radio pulsars are known to be steep and, to first order, described by a power-law relationship of the form S
ν ∝ να, where S
ν is the flux density at some frequency ν and ...α is the spectral index. Although measurements of α have been made over the years for several hundred pulsars, a study of the intrinsic distribution of pulsar spectra has not been carried out. From the result of pulsar surveys carried out at three different radio frequencies, we use population synthesis techniques and a likelihood analysis to deduce what underlying spectral index distribution is required to replicate the results of these surveys. We find that in general the results of the surveys can be modelled by a Gaussian distribution of spectral indices with a mean of −1.4 and unit standard deviation. We also consider the impact of the so-called gigahertz-peaked spectrum pulsars proposed by Kijak et al. The fraction of peaked-spectrum sources in the population with any significant turnover at low frequencies appears to be at most 10 per cent. We demonstrate that high-frequency (>2 GHz) surveys preferentially select flatter spectrum pulsars and the converse is true for lower frequency (<1 GHz) surveys. This implies that any correlations between α and other pulsar parameters (for example age or magnetic field) need to carefully account for selection biases in pulsar surveys. We also expect that many known pulsars which have been detected at high frequencies will have shallow, or positive, spectral indices. The majority of pulsars do not have recorded flux density measurements over a wide frequency range, making it impossible to constrain their spectral shapes. We also suggest that such measurements would allow an improved description of any populations of pulsars with 'non-standard' spectra. Further refinements to this picture will soon be possible from the results of surveys with the Green Bank Telescope and LOFAR.
Reliable characterization of particles freshly emitted from the ocean surface requires a sampling method that is able to isolate those particles and prevent them from interacting with ambient gases ...and particles. Here we report measurements of particles directly emitted from the ocean using a newly developed in situ particle generator (Sea Sweep). The Sea Sweep was deployed alongside R/V Atlantis off the coast of California during May of 2010. Bubbles were generated 0.75 m below the ocean surface with stainless steel frits and swept into a hood/vacuum hose to feed a suite of aerosol instrumentation on board the ship. The number size distribution of the directly emitted, nascent particles had a dominant mode at 55–60 nm (dry diameter) and secondary modes at 30–40 nm and 200–300 nm. The nascent aerosol was not volatile at 230°C and was not enriched in SO4=, Ca++, K+, or Mg++above that found in surface seawater. The organic component of the nascent aerosol (7% of the dry submicrometer mass) volatilized at a temperature between 230 and 600°C. The submicrometer organic aerosol characterized by mass spectrometry was dominated by non‐oxygenated hydrocarbons. The nascent aerosol at 50, 100, and 145 nm dry diameter behaved hygroscopically like an internal mixture of sea salt with a small organic component. The CCN/CN activation ratio for 60 nm Sea Sweep particles was near 1 for all supersaturations of 0.3 and higher indicating that all of the particles took up water and grew to cloud drop size. The nascent organic aerosol mass fraction did not increase in regions of higher surface seawater chlorophyll but did show a positive correlation with seawater dimethylsulfide (DMS).
Key Points
The ocean is a source of sub 100nm particles to the atmosphere
Hygroscopically the particles behave like an internal mixture of sea salt/organic
Organic mass fraction did not correlate with chlorophyll
ABSTRACT To understand the nature of supernovae and neutron star (NS) formation, as well as binary stellar evolution and their interactions, it is important to probe the distribution of NS masses. ...Until now, all double NS (DNS) systems have been measured as having a mass ratio close to unity (q ≥ 0.91). Here, we report the measurement of the individual masses of the 4.07-day binary pulsar J0453+1559 from measurements of the rate of advance of periastron and Shapiro delay: the mass of the pulsar is Mp = 1.559 0.005 M and that of its companion is M ; q = 0.75. If this companion is also an NS, as indicated by the orbital eccentricity of the system (e = 0.11), then its mass is the smallest precisely measured for any such object. The pulsar has a spin period of 45.7 ms and a spin period derivative of = (1.8616 0.0007)×10−19 s s−1; from these, we derive a characteristic age of ∼ 4.1×109 years and a magnetic field of ∼ 2.9×109 G, i.e., this pulsar was mildly recycled by the accretion of matter from the progenitor of the companion star. This suggests that it was formed with (very approximately) its current mass. Thus, NSs form with a wide range of masses, which is important for understanding their formation in supernovae. It is also important for the search for gravitational waves released during an NS-NS merger: it is now evident that we should not assume that all DNS systems are symmetric.
We report on the discovery of four millisecond pulsars (MSPs) in the High Time Resolution Universe (HTRU) pulsar survey being conducted at the Parkes 64 m radio telescope. All four MSPs are in binary ...systems and are likely to have white dwarf companions. In addition, we present updated timing solutions for 12 previously published HTRU MSPs, revealing new observational parameters such as five proper motion measurements and significant temporal dispersion measure variations in PSR J1017−7156. We discuss the case of PSR J1801−3210, which shows no significant period derivative Ṗ after four years of timing data. Our best-fitting solution shows a Ṗ of the order of 10−23, an extremely small number compared to that of a typical MSP. However, it is likely that the pulsar lies beyond the Galactic Centre, and an unremarkable intrinsic Ṗ is reduced to close to zero by the Galactic potential acceleration. Furthermore, we highlight the potential to employ PSR J1801−3210 in the strong equivalence principle test due to its wide and circular orbit. In a broader comparison with the known MSP population, we suggest a correlation between higher mass functions and the presence of eclipses in 'very low mass binary pulsars', implying that eclipses are observed in systems with high orbital inclinations. We also suggest that the distribution of the total mass of binary systems is inversely related to the Galactic height distribution. Finally, we report on the first detection of PSRs J1543−5149 and J1811−2404 as gamma-ray pulsars.
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