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

Abstract NGC 1624-2 has the strongest detected magnetic field of all known main-sequence O-type stars. Recent work reported that existing magnetospheric emission and longitudinal magnetic field measurements did not rule out a rotation period twice as long as the established period. 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 (306.56 days) 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).