Photometry of Antares from 1995 to 2025 with SOHO

Antares, a long-period variable supergiant, has been known since antiquity. It is the hearth of the Scorpion constellation. It is red and rivals Mars (Ares in Greek), from which its name is derived.

The study of its light curve is mostly based on Northern hemisphere observations, when the star is low above the Summer Southern horizon.
Its nature as a long-period variable star is therefore not fully known due to the incompleteness of the long-term sample available. Catalogues and manuals of the 20^th century classified Antares as an Irregular Variable.

The photometric observations of Antares are complicated by the absence of nearby stars of comparable luminosity and by heavy airmasses in the line of sight. The AAVSO database reports for Antares only 1/18 observations of the other naked-eye supergiant Betelgeuse, before its 2020 great dimming.

The application of airmass correction to naked-eye observations with distant comparison stars is required for Antares, but it is not yet a common practice among scientists.

We examine the AAVSO V-band data and our AAVSO-SGQ series (2012-2025) of visual observations, complementing thirty years (1995-2025) of SOHO observations made with the coronograph LASCO C3, each year on 2^nd December, to have all stars within 1° of C3 field of view, with vignetting and diffraction acting as systematic effects.

Antares variability measured with SOHO encompasses 0.4 magnitudes and 30 years. V-band data start in 1983 and have large gaps in time and range over 0.9 magnitudes. SGQ series spans over 0.6 magnitudes, with the same personal equation. A Markovian process may affect the visual data, showing an overall brightening in the last decade, partially explainable with a small variability of Spica, used as reference star.

The detection of periods and their modulations is possible by integrating SOHO with other independent measures, and we discuss some preliminary analyses.