Magnetic field measurements and their uncertainties: the FORS1 legacy

Abstract

Context. During the last decade, the FORS1 instrument of the ESO Very Large Telescope has been extensively used to study stellar magnetism. A number of interesting discoveries of magnetic fields in several classes of stars have been announced, many of which obtained at a ∼3σ level; some of the discoveries are confirmed by measurements obtained with other instruments, some are not. Aims. We investigate the reasons for the discrepancies between the results obtained with FORS1 and those obtained with other instruments. Methods. Using the ESO FORS pipeline, we have developed a semi-automatic procedure for magnetic field determination. We have applied this procedure to the full content of circular spectropolarimetric measurements of the FORS1 archive (except for most of the observations obtained in multi-object spectropolarimetric mode). We have devised and applied a number of consistency checks to our field determinations, and we have compared our results to those previously published in the literature. Results. We find that for high signal-to-noise ratio measurements, photon noise does not account for the full error bars. We discuss how field measurements depend on the specific algorithm adopted for data reduction, and we show that very small instrument flexures, negligible in most of the instrument applications, may be responsible for some spurious field detections in the null profiles. Finally, we find that we are unable to reproduce some results previously published in the literature. Consequently, we do not confirm some important discoveries of magnetic fields obtained with FORS1 and reported in previous publications. Conclusions. Our revised field measurements show that there is no contradiction between the results obtained with the low-resolution spectropolarimeter FORS1 and those obtained with high-resolution spectropolarimeters. FORS1 is an instrument capable of performing reliable magnetic field measurements, provided that the various sources of uncertainties are properly taken into account.

THE FIRST SPECTROPOLARIMETRIC MONITORING OF THE PECULIAR O4 Ief SUPERGIANT ζ PUPPIS

“…The feasibility of longitudinal magnetic field measurements in massive stars using low-resolution spectropolarimetric observations was demonstrated by previous studies of O-and B-type stars (e.g., Hubrig et al 2006Hubrig et al , 2009Hubrig et al , 2008Hubrig et al , 2011cHubrig et al , 2013Hubrig et al , 2015b. A number of discrepancies in the published measurement accuracies using FORS 1 data were reported by Bagnulo et al (2012), who used the ESO FORS 1 pipeline to reduce the FORS 1 archive. Some of the reasons for higher error bars in the study of Bagnulo et al (2012) are discussed by Hubrig et al (2016b).…”

“…A number of discrepancies in the published measurement accuracies using FORS 1 data were reported by Bagnulo et al (2012), who used the ESO FORS 1 pipeline to reduce the FORS 1 archive. Some of the reasons for higher error bars in the study of Bagnulo et al (2012) are discussed by Hubrig et al (2016b). Furthermore, we have carried out Monte Carlo bootstrapping tests.…”

First constraints on the magnetic field strength in extra-Galactic stars: FORS2 observations of Of?p stars in the Magellanic Clouds

“…However, as extensively discussed by Bagnulo et al (2012) and Bagnulo et al (2013), it is not possible to assign the classical statistical significance to the error bars formally derived from photon noise (that in this case would lead to the conclusion that a magnetic field has been detected with a very high confidence). Bagnulo et al (2012) have argued that because of tiny instrument instabilities, as well as the uncertainty introduced by the choices made during data reduction, a field measurement with FORS1/2 may be considered to be a firm detection only when it corresponds to at least 5 σ confidence (but note that 5 σ is a general guideline and not a number derived from rigorous theoretical considerations). We should note that we also detected a null field at the 2 σ level in the same spectrum, which might further weaken the credibility of our B z detection at 2.8 σ level.…”

“…FORS2 is a multipurpose instrument capable of imaging and low resolution spectroscopy, and equipped with polarimetric optics (a retarder waveplate and a Wollaston prism). For the measurement of the magnetic field we have used the technique described by Bagnulo et al (2002) and Bagnulo et al (2012). The five extra-Galactic Of?p stars were observed in multi-object mode, following the procedure already adopted by Bagnulo et al (2006) for a survey of magnetic stars in open clusters, and by Nazé et al (2012) for a magnetic survey of bright X-ray emitters.…”

“…As a quality check, field measurements were also estimated from the null profiles (see Bagnulo et al 2012, for an extensive discussion on the use of null profiles for quality check).…”