Faraday rotation: effect of magnetic field reversals

The standard formula for the rotation measure, RM, which determines the position angle, $\psi={\rm RM}\lambda^2$, due to Faraday rotation, includes contributions only from the portions of the ray path where the natural modes of the plasma are circularly polarized. In small regions of the ray path where the projection of the magnetic field on the ray path reverses sign (called QT regions) the modes are nearly linearly polarized. The neglect of QT regions in estimating RM is not well justified at frequencies below a transition frequency where mode coupling changes from strong to weak. By integrating the polarization transfer equation across a QT region in the latter limit, I estimate the additional contribution $\Delta\psi$ needed to correct this omission. In contrast with a result proposed by \cite{BB10}, $\Delta\psi$ is small and probably unobservable. I identify a new source of circular polarization, due to mode coupling in an asymmetric QT region. I also identify a new circular-polarization-dependent correction to the dispersion measure at low frequencies.Comment: 25 pages 1 figure, accepted for publication in The Astrophysical Journa

Inside the Bondi radius of M87

Chandra X-ray observations of the nearby brightest cluster galaxy M87 resolve the hot gas structure across the Bondi accretion radius of the central supermassive black hole, a measurement possible in only a handful of systems but complicated by the bright nucleus and jet emission. By stacking only short frame-time observations to limit pileup, and after subtracting the nuclear PSF, we analysed the X-ray gas properties within the Bondi radius at 0.12-0.22 kpc (1.5-2.8 arcsec), depending on the black hole mass. Within 2 kpc radius, we detect two significant temperature components, which are consistent with constant values of 2 keV and 0.9 keV down to 0.15 kpc radius. No evidence was found for the expected temperature increase within ~0.25 kpc due to the influence of the SMBH. Within the Bondi radius, the density profile is consistent with $\rho\propto r^{-1}$. The lack of a temperature increase inside the Bondi radius suggests that the hot gas structure is not dictated by the SMBH's potential and, together with the shallow density profile, shows that the classical Bondi rate may not reflect the accretion rate onto the SMBH. If this density profile extends in towards the SMBH, the mass accretion rate onto the SMBH could be at least two orders of magnitude less than the Bondi rate, which agrees with Faraday rotation measurements for M87. We discuss the evidence for outflow from the hot gas and the cold gas disk and for cold feedback, where gas cooling rapidly from the hot atmosphere could feed the cirumnuclear disk and fuel the SMBH. At 0.2 kpc radius, the cooler X-ray temperature component represents ~20% of the total X-ray gas mass and, by losing angular momentum to the hot gas component, could provide a fuel source of cold clouds within the Bondi radius.Comment: 14 pages, 8 figures, accepted by MNRA

The effect of biologging devices on reproduction, growth and survival of adult sea turtles

This is the final version. Available from BMC via the DOI in this record.The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.Background Telemetry and biologging systems, ‘tracking’ hereafter, have been instrumental in meeting the challenges associated with studying the ecology and behaviour of cryptic, wide-ranging marine mega-vertebrates. Over recent decades, globally, sea turtle tracking has increased exponentially, across species and life-stages, despite a paucity of studies investigating the effects of such devices on study animals. Indeed, such studies are key to informing whether data collected are unbiased and, whether derived estimates can be considered typical of the population at large. Methods Here, using a 26-year individual-based monitoring dataset on sympatric green (Chelonia mydas) and loggerhead (Caretta caretta) turtles, we provide the first analysis of the effects of device attachment on reproduction, growth and survival of nesting females. Results We found no significant difference in growth and reproductive correlates between tracked and non-tracked females in the years following device attachment. Similarly, when comparing pre- and post-tracking data, we found no significant difference in the reproductive correlates of tracked females for either species or significant carry-over effects of device attachment on reproductive correlates in green turtles. The latter was not investigated for loggerhead turtles due to small sample size. Finally, we found no significant effects of device attachment on return rates or survival of tracked females for either species. Conclusion While there were no significant detrimental effects of device attachment on adult sea turtles in this region, our study highlights the need for other similar studies elsewhere and the value of long-term individual-based monitoring

Where are all the gravastars? Limits upon the gravastar model from accreting black holes

The gravastar model, which postulates a strongly correlated thin shell of anisotropic matter surrounding a region of anti-de Sitter space, has been proposed as an alternative to black holes. We discuss constraints that present-day observations of well-known black hole candidates place on this model. We focus upon two black hole candidates known to have extraordinarily low luminosities: the supermassive black hole in the Galactic Center, Sagittarius A*, and the stellar-mass black hole, XTE J1118+480. We find that the length scale for modifications of the type discussed in Chapline et al. (2003) must be sub-Planckian.Comment: 11 pages, 4 figure

VLBI observations of the Crab nebula pulsar

Observations were made at meter wave-lengths using very long base-line interferometry techniques. At 196.5 MHz no resolution of the pulsar are observed; all the pulse shapes observed with the interferometers are similar to single dish profiles, and all the power pulsates. At 111.5 MHz besides the pulsing power there is always a steady component, presumably due to interstellar scattering. The pulsar is slightly resolved at 111.5 MHz with an apparent angular diameter of 0.07 sec ? 0.01 sec. A 50 percent linear polarization of the time-averaged power is noted at 196.5 MHz; at 111.5 MHz, 20 percent of the total time-averaged power is polarized, 35 percent of the pulsing power is polarized, and the steady component is unpolarized

The Equation of State of Neutron-Star Matter in Strong Magnetic Fields

We study the effects of very strong magnetic fields on the equation of state (EOS) in multicomponent, interacting matter by developing a covariant description for the inclusion of the anomalous magnetic moments of nucleons. For the description of neutron star matter, we employ a field-theoretical approach which permits the study of several models which differ in their behavior at high density. Effects of Landau quantization in ultra-strong magnetic fields ($B>10^{14}$ Gauss) lead to a reduction in the electron chemical potential and a substantial increase in the proton fraction. We find the generic result for $B>10^{18}$ Gauss that the softening of the EOS caused by Landau quantization is overwhelmed by stiffening due to the incorporation of the anomalous magnetic moments of the nucleons. In addition, the neutrons become completely spin polarized. The inclusion of ultra-strong magnetic fields leads to a dramatic increase in the proton fraction, with consequences for the direct Urca process and neutron star cooling. The magnetization of the matter never appears to become very large, as the value of $|H/B|$ never deviates from unity by more than a few percent. Our findings have implications for the structure of neutron stars in the presence of large frozen-in magnetic fields.Comment: 40 pages, 7 figures, accepted for publication in Ap