Pulsars, transients and relativistic astrophysics

Abstract:

In this paper, the development of the dual mirror Small Size Telescopes (SST) for the Cherenkov Telescope Array (CTA) is reviewed. Up to 70 SST, with a primary mirror diameter of ∼ 4 m, will be produced and installed at the CTA southern site. These will allow investigation of the gamma-ray sky at the highest energies accessible to CTA, in the range from about 1 TeV to 300 TeV. The telescope presented in this contribution is characterized by two major innovations: the use of a dual mirror Schwarzschild-Couder configuration and of an innovative camera using as sensors either multi-anode photomultipliers (MAPM) or silicon photomultipliers (SiPM). The reduced plate-scale of the telescope, achieved with the dual-mirror optics, allows the camera to be compact (∼ 40 cm in diameter), and low-cost. The camera, which has about 2000 pixels of size 6×6 mm2, covers a field of view of ∼ 10◦. The dual mirror telescopes and their cameras are being developed by three consortia, ASTRI (Astrofisica con Specchi a Tecnologia Replicante Italiana, Italy/INAF), GATE (Gamma-ray Telescope Elements, France/Paris Observ.) and CHEC (Compact High Energy Camera, universities in UK, US and Japan) which are merging their efforts in order to finalize an end-to-end design that will be constructed for CTA. A number of prototype structures and cameras are being developed in order to investigate various alternative designs. In this contribution, these designs are presented, along with the technological solutions under study.

Abstract:

We present a 30-day monitoring campaign of the optical counterpart of the bright X-ray transient Swift J1745-26, starting only 19min after the discovery of the source.We observe the system peaking at i~17.6 on day six (MJD 561 92) to then decay at a rate of ~0.04 mag d-1. We show that the optical peak occurs at least 3 d later than the hard X-ray (15-50 keV) flux peak. Our measurements result in an outburst amplitude greater than 4.3 mag, which favours an orbital period ≲21 h and a companion star with a spectral type later than ~A0. Spectroscopic observations taken with the Gran Telescopio de Canarias 10.4 m telescope reveal a broad (full width at half-maximum ~1100 km s-1), double-peaked Ha emission line from which we constrain the radial velocity semi-amplitude of the donor to be K2 > 250 km s-1. The breadth of the line and the observed optical and X-ray fluxes suggest that Swift J1745-26 is a new black hole candidate located closer than ~7 kpc. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

Abstract:

We present Jansky Very Large Array observations of the cold (CO (1→0)) molecular gas in IRAS F10214+4724, a lensed ultraluminous infraRed galaxy (ULIRG) at z = 2.3 with an obscured active nucleus. The galaxy is spatially and spectrally well resolved in the CO (1→0) emission line. The total intensity and velocity maps reveal a reasonably ordered system; however, there is some evidence for minor merger activity. A CO (10) counter-image is detected at the 3σ level. Five of the 42 kms-1 channels (with >5σ detections) are mapped back into the source plane and their total magnification posterior probability distribution functions are sampled. This reveals a roughly linear arrangement, tentatively a rotating disc. We derive a molecular gas mass of Mgas = 1.2 ± 0.2 × 1010 M, assuming a ULIRG LCOto- Mgas conversion ratio of α = 0.8 M (K km s-1 pc2)-1 that agrees well with the derived range of α = 0.3-1.3M (K km s-1 pc2)-1 for separate dynamical mass estimates at assumed inclinations of i = 90°-30°. The lens modelling and CO (1→0) spectrum asymmetry suggest that there may be substantial (factor 2) preferential lensing of certain individual channels; however, the CO (1→0) spatially integrated channel flux uncertainties limit the significance of this result. Based on the AGN and CO (1→0) peak emission positions and the lens model, we predict a distortion of the CO spectral line energy distribution where higher order J lines that may be partially excited by AGN heating will be preferentially lensed owing to their smaller solid angles and closer proximity to the AGN and therefore the cusp of the caustic. Comparison with other lensing inversion results shows that the narrow-line region and AGN radio core in IRAS F10214+4724 are preferentially lensed by a factor of >3 and 11, respectively, relative to the molecular gas emission. This distorts the global continuum emission spectral energy distribution and strongly suggests caution in unsophisticated uses of IRAS F10214+4724 as an archetype high-redshift ULIRG. We explore two large velocity gradient models, incorporating spatial CO (10) and CO (32) information and present tentative evidence for an extended, low-excitation, cold gas component that implies that the total molecular gas mass in IRAS F10214+4724 is a factor of 2 greater than that calculated using spatially unresolved CO observations. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

Abstract:

The black hole candidate, XTE J1752-223, was discovered in 2009 October when it entered an outburst. We obtained radio data from the Australia Telescope Compact Array for the duration of the ~9 month event. The light curves show that the radio emission from the compact jet persisted for the duration of an extended hard state and through the transition to the intermediate state. The flux then rose rapidly by a factor of 10 and the radio source entered a series of at least seven maxima, the first of which was likely to be emission associated with the compact jet. The subsequent six flares were accompanied by variable behaviour in terms of radio spectrum, degree of linear polarization, morphology and associated X-ray behaviour. They were, however, remarkably similar in terms of the estimated minimum power required to launch such an ejection event. We compare the timing of radio peaks with the location of the ejecta, imaged by contemporaneous Very Long Baseline Interferometry experiments. We then discuss the mechanism behind the events, in terms of whether discrete ejections are the most likely description of the behaviour. One ejection, at least, appears to be travelling with apparent superluminal motion. The range of properties, however, suggests that multiple mechanisms may be relevant and that at least some of the emission is coming from shocked interactions amongst the ejecta and between the ejecta and the interstellar medium. We also compare the radio flux density with the X-ray source during the hard state and conclude that XTE J1752-223 is a radio-weak/X-ray-bright outlier on the universal correlation for black hole transient sources. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.