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Abstract:

In response to major changes in the mass accretion rate within the inner accretion flow, black hole binary transients undergo dramatic evolution in their X-ray timing and spectral behaviour during outbursts. In recent years a paradigm has arisen in which 'soft' X-ray states are associated with an inner disc radius at, or very close to, the innermost stable circular orbit (ISCO) around the black hole, while in 'hard' X-ray states the inner edge of the disc is further from the black hole. Models of advective flows suggest that as the X-ray luminosity drops in hard states, the inner disc progressively recedes, from a few gravitational radii (Rg) at the ISCO, to hundreds of Rg. Recent observations which show broad iron line detections and estimates of the disc component strength suggest that a non-recessed disc could still be present in bright hard states. In this study we present a comprehensive analysis of the spectral components associated with the inner disc, utilizing bright states data from X-ray missions with sensitive low-energy responses (e.g. Swift, SAX), including re-analyses of previously published results. A key component of the study is to fully estimate systematic uncertainties associated with such spectral fits. In particular we investigate in detail the effect on the measured disc flux and radius of having a hydrogen column density that is fixed or free to vary. We conclude that at X-ray luminosities above ∼0.01 of the Eddington limit, systematic uncertainties only allow us to constrain the disc to be ≲10Rg from spectral fits. There is, however, clear evidence that at X-ray luminosities between 10^-2 and 10^-3 of the Eddington rate, the disc does begin to recede. We include measurements of disc radii in two quiescent black hole binaries at bolometric luminosities of <10^-7 Eddington, and present the inferred evolution of disc luminosity, temperature, inner radius and accretion rate/efficiency across the entire range of bolometric luminosities 10^-8-1 Eddington. We compare our results with theoretical models, and note that the implied rate of disc recession with luminosity is consistent with recent empirical results on the X-ray timing behaviour of black holes of all masses. © 2009 RAS.

Abstract:

We report on Hobby-Eberly Telescope and Palomar 5 m spectroscopy of recently identified γ-ray blazars in the Fermi LAT Bright Source List. These data provide identifications for 10 newly discovered γ-ray flat spectrum radio quasars and six new BL Lacertae (BL Lac) objects plus improved spectroscopy for six additional BL Lac objects. We substantially improve the identification completeness of the bright LAT blazars and give new redshifts and z constraints, new estimates of the black hole masses, and new measurements of the optical spectral energy distribution. © 2009 The American Astronomical Society. All rights reserved.

Abstract:

We have detected significant rotation measure (RM) variations for nine bright pulsars, as a function of pulse longitude. An additional sample of 10 pulsars showed a rather constant RM with phase, yet a small degree of RM fluctuation is visible in at least three of those cases. In all cases, we have found that the rotation of the polarization position angle across our 1.4 GHz observing band is consistent with the λ² law of interstellar Faraday rotation. We provide for the first time convincing evidence that RM variations across the pulse are largely due to interstellar scattering, although we cannot exclude that magnetospheric Faraday rotation may still have a minor contribution; alternative explanations of this phenomenon, like erroneous de-dispersion and the presence of non-orthogonal polarization modes, are excluded. If the observed, phase-resolved RM variations are common amongst pulsars, then many of the previously measured pulsar RMs may be in error by as much as a few tens of rad m^-2. © 2009 RAS.

Abstract:

We present comprehensive photometric and spectroscopic observations of the faint transient SN 2008S discovered in the nearby galaxy NGC 6946. SN 2008S exhibited slow photometric evolution and almost no spectral variability during the first nine months, implying a long photon diffusion time and a high-density circumstellar medium. Its bolometric luminosity (≃10⁴¹ erg s^-1 at peak) is low with respect to most core-collapse supernovae but is comparable to the faintest Type II-P events. Our quasi-bolometric light curve extends to 300 d and shows a tail phase decay rate consistent with that of ⁵⁶Co. We propose that this is evidence for an explosion and formation of ⁵⁶Ni (0.0014 ± 0.0003 M⊙). Spectra of SN 2008S show intense emission lines of Hα, [Ca ii] doublet and Ca ii near-infrared (NIR) triplet, all without obvious P-Cygni absorption troughs. The large mid-infrared (MIR) flux detected shortly after explosion can be explained by a light echo from pre-existing dust. The late NIR flux excess is plausibly due to a combination of warm newly formed ejecta dust together with shock-heated dust in the circumstellar environment. We reassess the progenitor object detected previously in Spitzer archive images, supplementing this discussion with a model of the MIR spectral energy distribution. This supports the idea of a dusty, optically thick shell around SN 2008S with an inner radius of nearly 90 au and outer radius of 450 au, and an inferred heating source of 3000 K. The luminosity of the central star is L ≃ 10^4.6 L ⊙. All the nearby progenitor dust was likely evaporated in the explosion leaving only the much older dust lying further out in the circumstellar environment. The combination of our long-term multiwavelength monitoring data and the evidence from the progenitor analysis leads us to support the scenario of a weak electron-capture supernova explosion in a super-asymptotic giant branch progenitor star (of initial mass 6-8 M ⊙) embedded within a thick circumstellar gaseous envelope. We suggest that all of main properties of the electron-capture SN phenomenon are observed in SN 2008S and future observations may allow a definitive answer. © 2009 RAS.

Abstract:

LOFAR, the Low Frequency Array, is a new radio telescope under construction in the Netherlands, designed to operate between 30 and 240 MHz. The Transients Key Project is one of the four Key Science Projects which comprise the core LOFAR science case. The remit of the Transients Key Project is to study variable and transient radio sources detected by LOFAR, on timescales from milliseconds to years. This will be achieved via both regular snapshot monitoring of historical and newly-discovered radio variables and, most radically, the development of a `Radio Sky Monitor' which will survey a large fraction of the northern sky on a daily basis.