New Observations of the Strongly Magnetic O-star NGC 1624-2 Reveal Its Magnetic South Pole

TL;DR

This study presents new spectropolarimetric observations of NGC 1624-2, revealing its magnetic south pole and confirming a longer rotational period, which implies a different magnetic geometry than previously thought.

Contribution

The paper provides the first direct detection of the magnetic south pole in NGC 1624-2, resolving previous ambiguities about its magnetic geometry and rotational period.

Findings

Confirmed the longer rotational period of ~306 days.

Detected a strong negative (south) magnetic polarity.

Suggested both magnetic poles are similarly oriented relative to our line of sight.

Abstract

NCG 1624-2 has the strongest detected magnetic field of all known main-sequence O-type stars. It was originally found that its magnetospheric emission lines followed a $\sim$5 month periodicity, and the existing line-of-sight magnetic measurements were predominantly of strong positive (north) polarity. As such, the field's geometric interpretation has been a mildly tilted (with respect to the rotational axis) dipole, such that only the magnetic north pole is visible during a rotation cycle. However, S. Seadrow et al. (2026) recently reported that new magnetospheric observations no longer phased with the established ephemeris and that the period had to be decreased by a few days. S. Seadrow et al. (2026) also found that existing magnetic measurements did not rule out a period twice as long (306.56 d). This period suggests a different magnetic configuration with a larger dipolar tilt, making both magnetic poles visible over a single rotation. Because previous spectropolarimetric observations did not have sufficient phase coverage to distinguish between the geometries, both were equally viable. In this paper, we present new spectropolarimetric observations obtained specifically to resolve this ambiguity. Our new magnetic measurements have a strong negative (south) polarity, confirming that the rotational period of NGC 1624-2 is indeed nearly twice as long as previously thought. Our measurements show that both poles come within a similar angle to our line of sight and likely have roughly the same local magnetic field strength (with a dipolar strength of 15-20 kG or more depending on the inclination angle).