High-cadence spectroscopy of M-dwarfs. I. Analysis of systematic effects in HARPS-N line profile measurements on the bright binary GJ 725A+B (1604.05312v1)

Abstract: Understanding the sources of instrumental systematic noise is a must to improve the design of future spectrographs. In this study, we alternated observations of the well-suited pair of M-stars GJ 725A+B to delve into the sub-night HARPS-N response. Besides the possible presence of a low-mass planet orbiting GJ 725B, our observations reveal changes in the spectral energy distribution (SED) correlated with measurements of the width of the instrumental line profile and, to a lower degree, with the Doppler measurements. To study the origin of these effects, we searched for correlations among several quantities defined and measured on the spectra and on the acquisition images. We find that the changes in apparent SED are very likely related to flux losses at the fibre input. Further tests indicate that such flux losses do not seriously affect the shape of the instrumental point spread function of HARPS-N, but identify an inefficient fitting of the continuum as the most likely source of the systematic variability observed in the FWHM. This index, accounting for the HARPS-N cross-correlation profiles width, is often used to decorrelate Doppler time-series. We show that the Doppler measurement obtained by a parametric least-squares fitting of the spectrum accounting for continuum variability is insensitive to changes in the slope of the SED, suggesting that forward modeling techniques to measure moments of the line profile are the optimal way to achieve higher accuracy. Remaining residual variability at ~1 m/s suggests that for M-stars Doppler surveys the current noise floor still has an instrumental origin.

Analysis of Systematic Effects in HARPS-N Line Profile Measurements

The paper "High-cadence spectroscopy of M-dwarfs. I. Analysis of systematic effects in HARPS-N line profile measurements on the bright binary GJ~725A+B" investigates the sources of instrumental systematic noise in spectrograph data by conducting high-cadence observations of the M-dwarf binary GJ~725A+B. This paper aims to enhance the design and data processing of future spectrographs by identifying and exploring factors affecting spectral energy distribution (SED) and line profile measurements within the HARPS-N spectrograph.

The research focused on isolating the systematic effects influencing the measurements of radial velocity (RV) and full width at half maximum (FWHM) in spectroscopic data from HARPS-N. By alternating observations between the nearly identical components of the binary system, the authors identified significant, correlated systematic variability on both stars. This variability was primarily linked to the SED shift and was systematically correlated with atmospheric parameters such as airmass. The authors propose that these correlations arise due to atmospheric differential effects not fully corrected by the HARPS-N's atmospheric dispersion corrector (ADC), including atmospheric extinction and chromatic seeing.

Through comprehensive analyses, strong correlations were found between the SED changes, as quantified by the newly defined chromatic index (K), and FWHM values. This correlation was deemed to have an algorithmic origin within the data reduction pipeline rather than representing physical changes in the stellar line profiles. Additional evidence revealed limitations in the pipeline's ability to normalize the spectral continuum across variations in SED, reinforcing the conclusion that the systematic error is primarily software-related, not instrumental.

The authors devised empirical methods to decorrelate these systematic effects, substantially mitigating their impact on the FWHM and RV measurements. For instance, the derived correction factors significantly reduced the variability in FWHM, thereby improving its reliability as an activity indicator over high-cadence timescales. Despite these corrections, residual RV variability remains, suggesting that further refinements to the instrumental and algorithmic components of HARPS-N data acquisition and processing are needed.

Regarding future applications, this paper underscores the necessity of improving ADC capabilities and algorithmic corrections to effectively leverage high-precision instruments, especially for detecting small exoplanets and characterizing stellar activity in M-dwarfs. The findings also propose the GJ~725A+B system as a potential benchmark for the calibration and validation of future high-precision instruments.

Finally, a signal consistent with a low-mass companion orbiting GJ~725B is detected, characterized by a tentative 2.7-day periodicity. The authors call for further observations to confirm this potential planet, highlighting the importance of such systematic studies in enhancing the fidelity of exoplanet detection and characterization efforts utilizing radial velocity measurements through instruments like HARPS-N.