ALBERT

Description: SN 2006gy was the most luminous supernova (SN) ever observed at the time of its discovery and the first of the newly defined class of superluminous supernovae (SLSNe). The extraordinary energetics of SN 2006gy and all SLSNe (〉10 51  erg) require either atypically large explosion energies (e.g. pair-instability explosion) or the efficient conversion of kinetic into radiative energy (e.g. shock interaction). The mass-loss characteristics can therefore offer important clues regarding the progenitor system. For the case of SN 2006gy, both a scattered and thermal light echo from circumstellar material (CSM) have been reported at later epochs (day ~800), ruling out the likelihood of a pair-instability event and leading to constraints on the characteristics of the CSM. Owing to the proximity of the SN to the bright host-galaxy nucleus, continued monitoring of the light echo has not been trivial, requiring the high resolution offered by the Hubble Space Telescope (HST) or ground-based adaptive optics (AO). Here, we report detections of SN 2006gy using HST and Keck AO at ~3000 d post-explosion and consider the emission mechanism for the very late-time light curve. While the optical light curve and optical spectral energy distribution are consistent with a continued scattered-light echo, a thermal echo is insufficient to power the K '-band emission by day 3000. Instead, we present evidence for late-time infrared emission from dust that is radiatively heated by CSM interaction within an extremely dense dust shell, and we consider the implications on the CSM characteristics and progenitor system.

Description: Millisecond pulsars are rather weakly magnetized neutron stars which are thought to have been spun up by disc accretion, with magnetic linkage between the star and the disc playing a key role. Their spin history depends sensitively on details of the magnetic field structure, but idealized models from the 1980s and 1990s are still commonly used for calculating the magnetic field components. This paper is the third in a series presenting results from a step-by-step analysis which we are making of the problem, starting with very simple models and then progressively including additional features one at a time, with the aim of gaining new insights into the mechanisms involved. In our first two papers, the magnetic field structure in the disc was calculated for a standard Shakura and Sunyaev model, by solving the magnetic induction equation numerically in the stationary limit within the kinematic approximation; here, we consider a more general velocity field in the disc, including backflow. We find that the profiles of the poloidal and toroidal components of the magnetic field are fairly similar in the two cases but that they can be very different from those in the models mentioned above, giving important consequences for the torque exerted on the central object. In particular, we find that, contrary to what is usually thought, some regions of the disc outwards of the corotation point (rotating more slowly than the neutron star) may nevertheless contribute to spinning up the neutron star on account of the detailed structure of the magnetic field in those parts of disc.

Description: The quiescent state is the dominant accretion mode for black holes on all mass scales. Our knowledge of the X-ray spectrum is limited due to the characteristic low luminosity in this state. Herein, we present an analysis of the sample of dynamically confirmed stellar-mass black holes observed in quiescence in the Chandra/XMM–Newton/Suzaku era resulting in a sample of eight black holes with ~570 ks of observations. In contrast to the majority of active galactic nuclei where observations are limited by contamination from diffuse gas, the stellar-mass systems allow for a clean study of the X-ray spectrum resulting from the accretion flow alone. The data are characterized using simple models. We find a model consisting of a power law or thermal bremsstrahlung to both provide excellent descriptions of the data, where we measure  = 2.06 ± 0.03 and $kT = 5.03^{+0.33}_{-0.31}\,\mathrm{keV}$ , respectively, in the 0.3–10 keV bandpass, at a median luminosity of L x  ~ 5.5 10 –7 L Edd . This result in discussed in the context of our understanding of the accretion flow on to stellar and supermassive black holes at low luminosities.

Description: We present results for models of neutron stars and strange stars constructed using the Hartle–Thorne slow-rotation method with a wide range of equations of state, focusing on the values obtained for the angular momentum J and the quadrupole moment Q , when the gravitational mass M and the rotational frequency are specified. Building on previous work, which showed surprising uniformity in the behaviour of the moment of inertia for neutron-star models constructed with widely different equations of state, we find similar uniformity for the quadrupole moment. These two quantities, together with the mass, are fundamental for determining the vacuum space–time outside neutron stars. We study particularly the dimensionless combination of parameters QM / J 2 (using units for which c  = G  = 1). This quantity goes to 1 in the case of a Kerr-metric black hole and deviations away from 1 then characterize the difference between neutron-star and black hole space–time. It is found that QM / J 2 for both neutron stars and strange stars decreases with increasing mass, for a given equation of state, reaching a value of around 2 (or even less) for maximum-mass models, meaning that their external space–time is then not very far from that of the Kerr metric. If QM / J 2 is plotted against R /2 M (where R is the radius), it is found that the relationship is nearly unique for neutron-star models, independent of the equation of state, while it is significantly different for strange stars. This gives a new way of possibly distinguishing between them.

Description: Periodic pulsations have been found in emission from the ultraluminous X-ray source M82 X-2, strongly suggesting that the emitter is a rotating neutron star rather than a black hole. However, the radiation mechanisms and accretion mode involved have not yet been clearly established. In this paper, we examine the applicability to this object of standard accretion modes for high mass X-ray binaries (HMXBs). We find that spherical wind accretion, which drives OB-type HMXBs, cannot apply here but that there is a natural explanation in terms of an extension of the picture for standard Be-type HMXBs. We show that a neutron star with a moderately strong magnetic field, accreting from a disc-shaped wind emitted by a Be-companion, could be compatible with the observed relation between spin and orbital period. A Roche lobe overflow picture is also possible under certain conditions.

Description: Due to observational challenges, our knowledge of low-level accretion flows around neutron stars is limited. We present NuSTAR , Swift and Chandra observations of the low-mass X-ray binary IGR J17062-6143, which has been persistently accreting at ~=0.1 per cent of the Eddington limit since 2006. Our simultaneous NuSTAR / Swift observations show that the 0.5–79 keV spectrum can be described by a combination of a power law with a photon index of ~= 2, a blackbody with a temperature of kT bb ~= 0.5 keV (presumably arising from the neutron star surface) and disc reflection. Modelling the reflection spectrum suggests that the inner accretion disc was located at R in 100 GM / c 2 (225 km) from the neutron star. The apparent truncation may be due to evaporation of the inner disc into a radiatively-inefficient accretion flow, or due to the pressure of the neutron star magnetic field. Our Chandra gratings data reveal possible narrow emission lines near 1 keV that can be modelled as reflection or collisionally ionized gas, and possible low-energy absorption features that could point to the presence of an outflow. We consider a scenario in which this neutron star has been able to sustain its low accretion rate through magnetic inhibition of the accretion flow, which gives some constraints on its magnetic field strength and spin period. In this configuration, IGR J17062-6143 could exhibit a strong radio jet as well as a (propeller-driven) wind-like outflow.

Description: We present results obtained from the time-resolved X-ray spectral analysis of the Narrow-Line-Seyfert 1 galaxy SWIFT J2127.4+5654 during a ~130 ks XMM–Newton observation. We reveal large spectral variations, especially during the first ~90 ks of the XMM–Newton exposure. The spectral variability can be attributed to a partial eclipse of the X-ray source by an intervening low-ionization/cold absorbing structure (cloud) with column density $N_{\rm {H}} = 2.0^{+0.2}_{-0.3}\times 10^{22}$ cm –2 which gradually covers and then uncovers the X-ray emitting region with covering fraction ranging from zero to ~43 per cent. Our analysis enables us to constrain the size, number density and location of the absorbing cloud with good accuracy. We infer a cloud size (diameter) of D c ≤ 1.5 10 13 cm, corresponding to a density of n c ≥ 1.5 10 9 cm –3 at a distance of R c ≥ 4.3 10 16 cm from the central black hole. All of the inferred quantities concur to identify the absorbing structure with one single cloud associated with the broad line region of SWIFT J2127.4+5654. We are also able to constrain the X-ray emitting region size (diameter) to be D s ≤ 2.3 10 13 cm which, assuming the black hole mass estimated from single-epoch optical spectroscopy (1.5 10 7 M ), translates into D s ≤ 10.5 gravitational radii ( r g ) with larger sizes (in r g ) being associated with smaller black hole masses, and vice versa. We also confirm the presence of a relativistically distorted reflection component off the inner accretion disc giving rise to a broad relativistic Fe K emission line and small soft excess (small because of the high Galactic column density), supporting the measurement of an intermediate black hole spin in SWIFT J2127.4+5654 that was obtained from a previous Suzaku observation.