Pulsars, transients and relativistic astrophysics

Abstract:

LOFT (Large area Observatory For x-ray Timing) is an innovative mission submitted in response to the Cosmic Vision "Call for a Medium-size mission opportunity for a launch in 2022" recently issued by ESA. LOFT is an ideal candidate for the next generation of (extremely) large experiments for X-ray timing dedicated to the study of the physics of compact objects and to the understanding of the behavior of matter in strong gravitational fields. Recent developments in the field of large area monolithic silicon detectors allowed us to reach an effective area ∼12 m2 (15 m2 goal), more than a order of magnitude larger that RXTE/PCA, in the energy range 2-30 keV (1-40 keV goal). This Large Area Detector (LAD) will have both high timing resolution (<10 μs, 5 μs goal) and good spectral capabilities (<260 eV, <180 eV goal). A Wide Field Monitor (WFM), sensitive in the ∼1-50 keV energy range, will observe simultaneously more than a quarter of the sky in order to both discover and localize transient events and study their long term evolution. © Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence.

Abstract:

Recent studies of different X-ray binaries (XRBs) have shown a clear correlation between the radio and X-ray emission. We present evidence of a close relationship found between the radio and X-ray emission at different epochs for GRS 1915+105, using observations from the Ryle Telescope and Rossi X-ray Timing Explorer satellite. The strongest correlation was found during the hard state (also known as the "plateau" state), where a steady AU-scale jet is known to exist. Both the radio and X-ray emission were found to decay from the start of most plateau states, with the radio emission decaying faster. An empirical relationship of was then fitted to data taken only during the plateau state, resulting in a power-law index of ξ ~ 1.7 ± 0.3, which is significantly higher than in other black hole XRBs in a similar state. An advection-flow model was then fitted to this relationship and compared to the universal XRB relationship as described by Gallo et al. (2003, MNRAS, 344, 60). We conclude that either (I) the accretion disk in this source is radiatively efficient, even during the continuous outflow of a compact jet, which could also suggest a universal turn-over from radiatively inefficient to efficient for all stellar-mass black holes at a critical mass accretion rate (M c≈1018.5 g/s); or (II) the X-rays in the plateau state are dominated by emission from the base of the jet and not the accretion disk (e.g. via inverse Compton scattering from the outflow). © 2010 ESO.

Abstract:

We have detected an Hα bow shock nebula around PSR J1741-2054, a pulsar discovered through its GeV γ-ray pulsations. The pulsar is only ∼1'.5 behind the leading edge of the shock. Optical spectroscopy shows that the nebula is non-radiative, dominated by Balmer emission. The Hα images and spectra suggest that the pulsar wind momentum is equatorially concentrated and implies a pulsar space velocity ≈150kms-1, directed 15° ± 10° out of the plane of the sky. The complex Hα profile indicates that different portions of the post-shock flow dominate line emission as gas moves along the nebula and provide an opportunity to study the structure of this unusual slow non-radiative shock under a variety of conditions. CXO ACIS observations reveal an X-ray pulsar wind nebula within this nebula, with a compact ∼2.5 equatorial structure and a trail extending several arcminutes behind. Together these data support a close (≤0.5 kpc) distance, a spin geometry viewed edge-on, and highly efficient γ-ray production for this unusual, energetic pulsar. © 2010. The American Astronomical Society. All rights reserved. Printed in the U.S.A.

Abstract:

A simple variational calculation is presented showing that a uniformly rotating barotropic fluid in an external potential attains a true energy minimum if and only if the rotation profile is everywhere subsonic. If regions of supersonic rotation are present, fluid variations exist that could take the sytem to states of lower energy. In any given system, these states may or may not be dynamically accessible, but their existence is important. It means that extending the degrees of freedom available to the fluid (say by weak magnetic fields) may open a path to fluid instabilities. Whether astrophysical gaseous nebula tend toward states of uniform rotation or toward more Keplerian core-disk systems appears to be largely a matter of whether the rotation profile is transonic or not. The suggestion is made that the length scale associated with coherent molecular cloud cores is related to the requirement that the cores be stable and rotate subsonically.