Multi Unit Spectroscopic Explorer Image Research Articles

Identification of a Helium Donor Star in NGC 247 ULX-1

With Very Large Telescope Multi Unit Spectroscopic Explorer (MUSE) observations, we detected highly variable helium emission lines from the optical counterpart of the supersoft ultraluminous X-ray source (ULX) NGC 247 ULX-1. No Balmer lines can be seen in the source spectrum. This is the first evidence for the presence of a helium donor star in ULXs, consistent with a prediction that helium donor stars may be common in ULXs. The helium lines with an FWHM of about 200 km s−1 are likely produced on the outer accretion disk. Their strong variation implies that the central X-ray source can be significantly obscured to the outer disk. Also, a ring or a double-ring structure is revealed in the MUSE image. It is unknown whether or not it is related to the progenitor of the ULX binary.

Reconstruction of the ground-layer adaptive-optics point spread function for MUSE wide field mode observations

Context. Here we describe a simple, efficient, and most importantly fully operational point-spread-function (PSF)-reconstruction approach for laser-assisted ground layer adaptive optics (GLAO) in the frame of the Multi Unit Spectroscopic Explorer (MUSE) wide field mode. Aims. Based on clear astrophysical requirements derived by the MUSE team and using the functionality of the current ESO Adaptive Optics Facility we aim to develop an operational PSF-reconstruction (PSFR) algorithm and test it both in simulations and using on-sky data. Methods. The PSFR approach is based on a Fourier description of the GLAO correction to which the specific instrumental effects of MUSE wide field mode (pixel size, internal aberrations, etc.) have been added. It was first thoroughly validated with full end-to-end simulations. Sensitivity to the main atmospheric and AO system parameters was analysed and the code was re-optimised to account for the sensitivity found. Finally, the optimised algorithm was tested and commissioned using more than one year of on-sky MUSE data. Results. We demonstrate with an on-sky data analysis that our algorithm meets all the requirements imposed by the MUSE scientists, namely an accuracy better than a few percent on the critical PSF parameters including full width at half maximum and global PSF shape through the kurtosis parameter of a Moffat function. Conclusions. The PSFR algorithm is publicly available and is used routinely to assess the MUSE image quality for each observation. It can be included in any post-processing activity which requires knowledge of the PSF.

Design of an Integral Field Unit for MUSE, and Results from Prototyping

ABSTRACTThe research and development activity on image slicer systems for integral field spectroscopy has increased in recent years, allowing the simultaneous observation on the same detector matrix of two‐dimensional sky maps and the spectral decomposition of light. Nowadays, image slicers lead to possible applications on future instrumentation for ground‐ and space‐based telescopes, covering a spectral domain ranging from UV to mid‐IR wavelengths. Among such projects, MUSE (Multi Unit Spectroscopic Explorer) is an integral field spectrograph proposed by the European Southern Observatory for its second‐generation Very Large Telescope instrumentation. MUSE combines a 1′ × 1′ field of view, with a spectral resolution reaching 3000 and a spatial sampling of 0.″2 provided by an adaptive optics system. MUSE operates in the visible and near‐IR wavelength range (0.465–0.93 μm) and is composed of 24 identical integral field units; each one incorporates an advanced image slicer made up of a combination of mirrors and “minilens” arrays, a spectrograph, and a detector. This paper describes the optical design of the MUSE image slicer, which complies with top‐level requirements at a reasonable cost. The optical manufacturing technique, based on an original method of assembly using molecular adhesion, is presented. Finally, we focus on the optical tests of the prototype both at a system level and at the level of individual optical elements.