Pulsar braking indices revisited

Using the standard equation for the slowdown of a neutron star, we derive a formula for the braking index via integration rather than the conventional differentiation. The new formula negates the need to measure the second time derivative of the rotation frequency, ν¨ . We show that the method gives similar braking indices for PSR B1509−58 and the Crab pulsar to those already in the literature. We point out that our method is useful for obtaining the braking indices of moderate-aged pulsars without the need for long, phase-connected timing solutions. We applied the method to 20 pulsars and discuss the implications of the results. We find that virtually all the derived braking indices are dominated by the effects of (unseen) glitches, the recovery from which corrupts the value of ν˙ . However, any real, large, positive braking index has implications for magnetic field decay and offers support to recent models of pulsar evolution.     

Optical spectroscopy of GX 339−4 during the high–soft and low–hard states – I

We carried out spectroscopic observations of the candidate black hole binary GX 339−4 during its low–hard and high–soft X-ray states. We have found that the spectrum is dominated by emission lines of neutral elements with asymmetric, round-topped profiles in the low–hard state. In the high–soft state, however, the emission lines from both neutral and ionized elements have unambiguously resolved double-peaked profiles. The detection of double-peaked emission lines in the high–soft state, with a larger peak separation for higher ionization lines, indicates the presence of an irradiatively heated accretion disc. The round-topped lines in the low–hard state are probably caused by a dense matter outflow from an inflated non-Keplerian accretion disc. Our data do not show velocity modulations of the line centres caused by the orbital motion of the compact object, neither do the line basewidths show substantial variations in each observational epoch. There are no detectable absorption lines from the companion star. All these features are consistent with those of a system with a low-mass companion star and low orbital inclination.     

The representation of tropical upper tropospheric water in EC Earth V2

Tropical upper tropospheric humidity, clouds, and ice water content, as well as outgoing longwave radiation (OLR), are evaluated in the climate model EC Earth with the aid of satellite retrievals. The Atmospheric Infrared Sounder and Microwave Limb Sounder together provide good coverage of relative humidity. EC Earth’s relative humidity is in fair agreement with these observations. CloudSat and CALIPSO data are combined to provide cloud fractions estimates throughout the altitude region considered (500–100?hPa). EC Earth is found to overestimate the degree of cloud cover above 200?hPa and underestimate it below. Precipitating and non-precipitating EC Earth ice definitions are combined to form a complete ice water content. EC Earth’s ice water content is below the uncertainty range of CloudSat above 250?hPa, but can be twice as high as CloudSat’s estimate in the melting layer. CERES data show that the model underestimates the impact of clouds on OLR, on average with about 9?W?m^?2. Regionally, EC Earth’s outgoing longwave radiation can be ～20?W?m^?2 higher than the observation. A comparison to ERA-Interim provides further perspectives on the model’s performance. Limitations of the satellite observations are emphasised and their uncertainties are, throughout, considered in the analysis. Evaluating multiple model variables in parallel is a more ambitious approach than is customary.     

An empirical model for the beams of radio pulsars

Motivated by recent results on the location of the radio emission in pulsar magnetospheres, we have developed a model which can account for the large diversity found in the average profile shapes of pulsars. At the centre of our model lies the idea that radio emission at a particular frequency arises from a wide range of altitudes above the surface of the star, and that it is confined to a region close to the last open field lines. We assert that the radial height range over which emission occurs is responsible for the complex average pulse shapes rather than the transverse (longitudinal) range proposed in most current models. By implementing an abrupt change in the height range to discriminate between young, short-period, highly energetic pulsars and their older counterparts, we obtain the observed transition between the simple and complex average pulse profiles observed in each group respectively. Monte Carlo simulations are used to demonstrate the match of our model to real observations.     

Phase equilibria and melt productivity in the pelitic system: implications for the origin of peraluminous granitoids and aluminous granulites

Peraluminous granitoid magmas are a characteristic product of ultrametamorphism leading to anatexis of aluminous metasedimentary rocks in the continental crust. The mechanisms and characteristic length-scales over which these magmas can be mobilized depend strongly on their melt fraction, because of their high viscosities. Thus, it is of fundamental importance to understand the controls exerted by pressure, temperature and bulk composition of the source material on melt productivity. We have studied experimentally the vapour-absent melting behaviour of a natural metapelitic rock and our results differ greatly from those of previous experimental and theoretical investigations of melt productivity from metamorphic rocks. Under H₂O-undersaturated conditions, bulk composition of the source material is the overriding factor controlling melt fraction at temperatures on the order of 850–900° C. Granitoid melts formed in this temperature interval by the peritectic dehydration-melting reaction: $$\begin{gathered} Biotite + plagioclase + aluminosilicate + quartz \hfill \\ = melt + garnet \hfill \\ \end{gathered} $$ have a restricted compositional range. As a consequence, melt fractions will be maximized from protoliths whose modes coincide with the stoichiometry of the melting reaction. This “optimum mode” (approximately 38% biotite, 32% quartz, 22% plagioclase and 8% aluminosilicate) reflects the fact that generation of low-temperature granitoid liquids requires both fusible quartzo-feldspathic components and H₂O (from hydrous minerals). Metapelitic rocks rich in mica and aluminosilicate and poor in plagioclase contain an excess of refractory material (Al₂O₃, FeO, MgO) with low solubility in low-temperature silicic melts, and will therefore be poor magma sources. Melt fraction varies inversely with pressure in the range 7–13 kbar, but the effect is not strong: the decrease (at constant temperature) over this pressure range is of at most 15 vol% (absolute). The liquids produced in our experiments are silicarich (68–73 wt% SiO₂), strongly peraluminous (2–5 wt% normative corundum) and very felsic (MgO+FeO^* +TiO₂ less than 3 wt%, even at temperatures above 1000° C). The last observation suggests that peraluminous granitoids with more than 10% mafic minerals (biotite, cordierite, garnet) contain some entrained restite. Furthermore, because liquids are also remarkably constant in composition, we believe that restite separation is more important than fractional crystallization in controlling the variability within and among peraluminous granitoids. We present liquidus phase diagrams that allow us to follow the phase relationships of melting of silica-and alumina-saturated rocks at pressures corresponding to the mid- to deep-continental crust. Garnet, aluminosilicate, quartz and ilmenite are the predominant restitic phases at temperatures of about 900° C, but Ti-rich biotite or calcic plagioclase can also be present, depending on the bulk composition of the protolith. At temperatures above 950–1050° C (depending on the pressure) the restitic assemblage is: hercynitic spinel+ilmenite+quartz±aluminosilicate. Our results therefore support the concept that aluminous granulites (garnet-spinel-plagioclase-aluminosilicate-quartz) can be the refractory residuum of anatectic events.     

Anhydrous PT phase relations of an Aleutian high-MgO basalt: an investigation of the role of olivine-liquid reaction in the generation of arc high-alumina basalts

We report results of anhydrous 1 atm and piston-cylinder experiments on ID16, an Aleutian high-magnesia basalt (HMB), designed to investigate potential petrogenetic links between arc high-alumina basalts (HABs) and less common HMBs. ID16 is multiply saturated with a plagioclase/spinel iherzolite mineral assemblage (olivine, plagioclase, clinopyroxene, orthopyroxene, spinel) immediately beneath the 12 kbar liquidus. Derivative liquids produced at high temperatures in the 10–20 kbar melting interval of ID16 have compositions resembling those published of many moderate-CaO HABs, although lower-temperature liquids are poorer in CaO and richer in alkalies than are typical HABs. Isomolar pseudoternary projections and numerical mass-balance modeling suggest that derivative melts of ID16 enter into a complex reaction relationship with olivine at 10 kbar and 1,200° C–1,150° C. We sought to test such a mechanism to explain the lack of liquidus olivine in anhydrous experiments on mafic high-alumina basalts such as SSS. 1.4 (Johnston 1986). These derivative liquids, however, do not resemble typical arc high-alumina basalts, suggesting that olivine-liquid reaction does not account for Johnston's (1986) observations. Instead, we suggest that olivine can be brought onto the liquidus of such compositions only through the involvement of H₂O, which will affect the influence of bulk CaO, MgO, and Al₂O₃ contents on the identity of HAB liquidus phases (olivine or plagioclase) at pressures less than 12 kbar.