ALBERT

Description: We have used pointed RXTE data to examine the long-term X-ray light curves of six transient black hole X-ray binaries during their decay from outburst to quiescence. In most cases, there is a period of exponential decay as the source approaches the soft- to hard-state transition, and another period of exponential decay following this transition as the source decays in the hard state. The e-folding times change around the time of the state transition, from typically 12 d at the end of the soft state to 7 d at the beginning of the hard state. This factor ~2 change in the decay time-scale is expected if there is a change from radiatively efficient emission in the soft state to radiatively inefficient emission in the hard state, overlying an exponential decay in the mass accretion rate. This adds support to the idea that the X-ray emitting region is governed by radiatively inefficient accretion (such as an advection-dominated or jet-dominated accretion flow) during the fading hard state.

Description: The magnetar model has been proposed to explain the apparent energy injection in the X-ray light curves of short gamma-ray bursts (SGRBs), but its implications across the full broad-band spectrum are not well explored. We investigate the broad-band modelling of four SGRBs with evidence for energy injection in their X-ray light curves, applying a physically motivated model in which a newly formed magnetar injects energy into a forward shock as it loses angular momentum along open field lines. By performing an order of magnitude search for the underlying physical parameters in the blast wave, we constrain the characteristic break frequencies of the synchrotron spectrum against their manifestations in the available multiwavelength observations for each burst. The application of the magnetar energy injection profile restricts the successful matches to a limited family of models that are self-consistent within the magnetic dipole spin-down framework. We produce synthetic light curves that describe how the radio signatures of these SGRBs ought to have looked given the restrictions imposed by the available data, and discuss the detectability of these signatures with present-day and near-future radio telescopes. Our results show that both the Atacama Large Millimeter Array (ALMA) and the upgraded Very Large Array are now sensitive enough to detect the radio signature within two weeks of trigger in most SGRBs, assuming our sample is representative of the population as a whole. We also find that the upcoming Square Kilometre Array will be sensitive to depths greater than those of our lower limit predictions.

Description: We present calculations of auroral radio powers of magnetized hot Jupiters orbiting Sun-like stars, computed using global magnetohydrodynamic (MHD) modelling of the magnetospheric and ionospheric convection arising from the interaction between the magnetosphere and the stellar wind. Exoplanetary auroral radio powers are traditionally estimated using empirical or analytically derived relations, such as the radiometric Bode’s law (RBL), which relates radio power to the magnetic or kinetic energy dissipated in the stellar wind–planet interaction. Such methods risk an oversimplification of the magnetospheric electrodynamics giving rise to radio emission. As the next step towards a self-consistent picture, we model the stellar wind–magnetosphere–ionosphere coupling currents using a 3D MHD model. We compute electron-cyclotron maser instability-driven emission from the calculated ionospheric field-aligned current density. We show that the auroral radio power is highly sensitive to interplanetary magnetic field (IMF) strength, and that the emission is saturated for plausible hot Jupiter Pedersen conductances, indicating that radio power may be largely independent of ionospheric conductance. We estimate peak radio powers of 1014 W from a planet exposed to an IMF strength of 103 nT, implying flux densities at a distance of 15 pc from Earth potentially detectable with current and future radio telescopes. We also find a relation between radio power and planetary orbital distance that is broadly consistent with results from previous analytic models of magnetosphere–ionosphere coupling at hot Jupiters, and indicates that the RBL likely overestimates the radio powers by up to two orders of magnitude in the hot Jupiter regime.

Description: Extended emission (EE) is a high-energy, early time rebrightening sometimes seen in the light curves of short gamma-ray bursts (GRBs). We present the first contiguous fits to the EE tail and the later X-ray plateau seen by the Swift mission, unified within a single model. Our central engine is a magnetar surrounded by a fall-back accretion disc, formed by either the merger of two compact objects or the accretion-induced collapse of a white dwarf. During the EE phase, material is accelerated to super-Keplarian velocities and ejected from the system by the rapidly rotating ( P 1–10 ms) and very strong (10 15 G) magnetic field in a process known as magnetic propellering. The X-ray plateau is modelled as magnetic dipole spin-down emission. We first explore the range of GRB phenomena that the propeller could potentially reproduce, using a series of template light curves to devise a classification scheme based on phenomology. We then obtain fits to the light curves of nine GRBs with EE, simultaneously fitting both the propeller and the magnetic dipole spin-down and finding typical disc masses of a few 10 –3 M to a few 10 –2 M . This is done for ballistic, viscous disc and exponential accretion rates. We find that the conversion efficiency from kinetic energy to EM emission for propellered material needs to be 10 per cent and that the best-fitting results come from an exponential accretion profile.

Description: We investigate the influence of high-mass X-ray binaries (HMXBs) on their high-redshift environments. Using a one-dimensional radiative transfer code, we predict the ionization and temperature profiles surrounding a coeval stellar population, composed of main-sequence stars and HMXBs, at various times after its formation. We consider both uniform density surroundings, and a cluster embedded in a 10 8 M Navarro–Frenk–White (NFW) halo. HMXBs in a constant density environment produce negligible enhanced ionization because of their high-energy spectral energy distributions and short lifetimes. In this case, HMXBs only marginally contribute to the local heating rate. For NFW profiles, radiation from main-sequence stars cannot prevent the initially ionized volume from recombining since it is unable to penetrate the high-density galactic core. However, HMXB photons stall recombinations behind the front, keeping it partially ionized for longer. The increased electron density in these partially ionized regions promotes further cooling, resulting in lower intergalactic medium (IGM) temperatures. In the context of this starburst model, we have shown that HMXBs do not make a major contribution to reionization or IGM heating. However, X-ray escape fractions are high in both density profile cases. Continuous star formation may result in the build up of X-rays over time, reducing the ionization time-scale and potentially leading to low level ionization of the distant IGM.

Description: Extended emission gamma-ray bursts are a subset of the ‘short’ class of burst which exhibit an early time rebrightening of gamma emission in their light curves. This extended emission arises just after the initial emission spike, and can persist for up to hundreds of seconds after trigger. When their light curves are overlaid, our sample of 14 extended emission bursts show a remarkable uniformity in their evolution, strongly suggesting a common central engine powering the emission. One potential central engine capable of this is a highly magnetized, rapidly rotating neutron star, known as a magnetar. Magnetars can be formed by two compact objects coalescing, a scenario which is one of the leading progenitor models for short bursts in general. Assuming that a magnetar is formed, we gain a value for the magnetic field and late-time spin period for nine of the extended emission bursts by fitting the magnetic dipole spin-down model of Zhang and Mészáros. Assuming that the magnetic field is constant, and the observed energy release during extended emission is entirely due to the spin-down of this magnetar, we then derive the spin period at birth for the sample. We find that all birth spin periods are in good agreement with those predicted for a newly born magnetar.

Description: By comparing the orbital period distributions of black hole and neutron star low-mass X-ray binaries (LMXBs) in the Ritter–Kolb catalogue we show that there is statistical evidence for a dearth of black hole systems at short orbital periods ( P orb 〈 4 h). This could either be due to a true divergence in orbital period distributions of these two types of system, or to black hole LMXBs being preferentially hidden from view at short orbital periods. We explore the latter possibility, by investigating whether black hole LMXBs could be concealed by a switch to radiatively inefficient accretion at low luminosities. The peak luminosity and the duration of X-ray binary outbursts are related to the disc radius and, hence, the orbital period. At short periods, where the peak outburst luminosity drops close to the threshold for radiatively inefficient accretion, black hole LMXBs have lower outburst luminosities, shorter outburst durations and lower X-ray duty cycles than comparable neutron star systems. These factors can combine to severely reduce the detection probability of short period black hole LMXBs relative to those containing neutron stars. We estimate the outburst properties and orbital period distribution of black hole LMXBs using two models of the transition to radiatively inefficient accretion: an instantaneous drop in accretion efficiency () to zero, at a fraction ( f ) of the Eddington luminosity ( L Edd ) and a power-law efficiency decrease, $\eta \propto \dot{M}^n$ , for L 〈 f L Edd . We show that a population of black hole LMXBs at short orbital periods can only be hidden by a sharp drop in efficiency, either instantaneous or for n 3. This could be achieved by a genuine drop in luminosity or through abrupt spectral changes that shift the accretion power out of a given X-ray band.

Description: RS Ophiuchi (RS Oph) is a symbiotic variable and a recurrent nova (RN). We have monitored it with the Nordic Optical Telescope and obtained 30 high-resolution ( R = 46 000) optical spectra over one orbital cycle during quiescence. To our knowledge, this is the best-sampled high-resolution spectroscopic data set of RS Oph over one orbital period. We do not detect any direct signatures of an accretion disc such as double peaked emission lines, but many line profiles are complex consisting of superimposed emission and absorption components. We measure the spin of the red giant and conclude that it is tidally locked to the binary orbit. We observe Na  i absorption features, probably arising from the circumbinary medium, that has been shaped by previous RN outbursts. We do not detect any intrinsic polarization in the optical wavelengths.