MeerKAT

Abstract:

<jats:title>Abstract</jats:title> <jats:p>The optical emission line spectra of X-ray binaries (XRBs) are thought to be produced in an irradiated atmosphere, possibly the base of a wind, located above the outer accretion disc. However, the physical nature of – and physical conditions in – the line-forming region remain poorly understood. Here, we test the idea that the optical spectrum is formed in the transition region between the cool, geometrically thin part of the disc near the mid-plane and a hot, vertically extended atmosphere or outflow produced by X-ray irradiation. We first present a VLT X-Shooter spectrum of XRB MAXI J1820+070 in the soft state associated with its 2018 outburst, which displays a rich set of double-peaked hydrogen and helium recombination lines. Aided by ancillary X-ray spectra and reddening estimates, we then model this spectrum with the Monte Carlo radiative transfer code Python, using a simple biconical disc wind model inspired by radiation-hydrodynamic simulations of irradiation-driven outflows from XRB discs. Such a model can qualitatively reproduce the observed features; nearly all of the optical emission arising from the transonic ‘transition region’ near the base of the wind. In this region, characteristic electron densities are on the order of 1012 − 13 cm−3 , in line with the observed flat Balmer decrement (Hα/Hβ ≈ 1.3). We conclude that strong irradiation can naturally give rise to both the optical line-forming layer in XRB discs and an overlying outflow/atmosphere that produces X-ray absorption lines.</jats:p>

Abstract:

WEAVE, the new wide-field, massively multiplexed spectroscopic survey facility for the William Herschel Telescope, will see first light in late 2022. WEAVE comprises a new 2-degree field-of-view prime-focus corrector system, a nearly 1000-multiplex fibre positioner, 20 individually deployable ‘mini’ integral field units (IFUs), and a single large IFU. These fibre systems feed a dual-beam spectrograph covering the wavelength range 366−959 nm at R ∼ 5000, or two shorter ranges at R ∼ 20 000. After summarising the design and implementation of WEAVE and its data systems, we present the organisation, science drivers and design of a five- to seven-year programme of eight individual surveys to: (i) study our Galaxy’s origins by completing Gaia’s phase-space information, providing metallicities to its limiting magnitude for ∼3 million stars and detailed abundances for ∼1.5 million brighter field and open-cluster stars; (ii) survey ∼0.4 million Galactic-plane OBA stars, young stellar objects and nearby gas to understand the evolution of young stars and their environments; (iii) perform an extensive spectral survey of white dwarfs; (iv) survey ∼400 neutral-hydrogen-selected galaxies with the IFUs; (v) study properties and kinematics of stellar populations and ionised gas in z < 0.5 cluster galaxies; (vi) survey stellar populations and kinematics in ∼25 000 field galaxies at 0.3 ≲ z ≲ 0.7; (vii) study the cosmic evolution of accretion and star formation using >1 million spectra of LOFAR-selected radio sources; (viii) trace structures using intergalactic/circumgalactic gas at z > 2. Finally, we describe the WEAVE Operational Rehearsals using the WEAVE Simulator.