Context. The solar convection manifests as granulation and intergranulation at the solar surface. In the photosphere, convective motions induce differential Doppler shifts to spectral lines. The observed convective blueshift varies across the solar disk. Aims. We focus on the impact of solar convection on the atmosphere and aim to resolve its velocity stratification in the photosphere. Methods. We performed high-resolution spectroscopic observations of the solar spectrum in the 6302 Å range with the Laser Absolute Reference Spectrograph at the Vacuum Tower Telescope. A laser frequency comb enabled the calibration of the spectra to an absolute wavelength scale with an accuracy of 1 m s −1 . We systematically scanned the quiet Sun from the disk center to the limb at ten selected heliocentric positions. The analysis included 99 time sequences of up to 20 min in length. By means of ephemeris and reference corrections, we translated wavelength shifts into absolute line-of-sight velocities. A bisector analysis on the line profiles yielded the shapes and convective shifts of seven photospheric lines. Results. At the disk center, the bisector profiles of the iron lines feature a pronounced C-shape with maximum convective blueshifts of up to −450 m s −1 in the spectral line wings. Toward the solar limb, the bisectors change into a "\"-shape with a saturation in the line core at a redshift of +100 m s −1 . The center-to-limb variation of the line core velocities shows a slight increase in blueshift when departing the disk center for larger heliocentric angles. This increase in blueshift is more pronounced for the magnetically less active meridian than for the equator. Toward the solar limb, the blueshift decreases and can turn into a redshift. In general, weaker lines exhibit stronger blueshifts. Conclusions. Best spectroscopic measurements enabled the accurate determination of absolute convective shifts in the solar photosphere. We convolved the results to lower spectral resolution to permit a comparison with observations from other instruments.

Context. Granular convective motions reach into the lower solar atmosphere, typically causing photospheric spectral lines to exhibit a differential line shift. This Doppler shift to shorter wavelengths is commonly known as convective blueshift. Aims. Spectroscopic high-accuracy measurements provide us with a refined determination of the absolute convective blueshift and its atmospheric distribution from disk center to the solar limb. Methods. We performed systematic observations of the quiet Sun with the Laser Absolute Reference Spectrograph (LARS) at the German Vacuum Tower Telescope. The solar disk was scanned along the meridian and the equator, from the disk center toward the limb. The solar spectrum around 6173 Å was calibrated with a laser frequency comb on an absolute wavelength scale with an accuracy of a few meters per second. We applied a bisector analysis on the spectral lines to reveal the changes of convective blueshift and line asymmetry at different heliocentric positions. Results. Being a signature for convective motions, the bisector curve of Fe i 6173.3 Å describes a "C"-shape at disk center. When approaching the solar limb, the bisector transforms into a "\"-shape. The analysis of the time-and bisector-averaged line shifts yields three distinct results. Firstly, the center-to-limb variation of Doppler velocities measured with LARS reveals a significant discrepancy (up to 200 m s −1 ) to the full-disk Dopplergrams of the Helioseismic and Magnetic Imager (HMI). Secondly, we obtained a significant decrease of convective blueshift toward the solar limb. Thirdly, the line-of-sight effect of solar activity, including p-mode oscillations and supergranular flows, leads to a scatter of up to ±100 m s −1 at intermediate heliocentric positions. Conclusions. The accurate observation of the absolute convective blueshift with LARS allows the identification of systematic discrepancy with Doppler velocities measured by HMI. The center-to-limb variation of HMI suffers from an additional blueshift for µ < 0.9 that is incompatible with our results. LARS measurements can be taken as a reference for the correction of systematic errors in the synoptic HMI Dopplergrams.Article published by EDP Sciences A34, page 1 of 8 A&A 622, A34 (2019)

It has been recently claimed by two different groups that the spectral modulation observed in gamma rays from Galactic pulsars and supernova remnants can be due to conversion of photons into ultra-light axion-like-particles (ALPs) in large-scale Galactic magnetic fields. While we show the required best-fit photon-ALP coupling, gaγ  ∼ 2 × 10-10 GeV-1, to be consistent with constraints from observations of photon-ALPs mixing in vacuum, this is in conflict with other bounds, specifically from the CAST solar axion limit, from the helium-burning lifetime in globular clusters, and from the non-observations of gamma rays in coincidence with SN 1987A. In order to reconcile these different results, we propose that environmental effects in matter would suppress the ALP production in dense astrophysical plasma, allowing to relax previous bounds and make them compatible with photon-ALP conversions in the low-density Galactic medium. If this explanation is correct, the claimed ALP signal would be on the reach of next-generations laboratory experiments such as ALPS II.