The white dwarf WDJ165335.21-100116.33 is the cooled core of a star, about the size of Earth but with the mass of the Sun. On its surface, there is an unusual hot spot that appears and disappears. Astronomers long puzzled over why the star's brightness changes every 80 hours. New observations with the VLT telescope using the FORS2 instrument have revealed that the star has a weak magnetic field that strengthens and weakens over time.
The star's magnetic field acts like an invisible hand, compressing the gas in its atmosphere. When the field is stronger, the gas heats up and glows more brightly: this is how the hot spot is born. At the same time, the star's overall brightness drops because the spot blocks some of the light. It's like how a cloud's shadow makes the day darker, while the cloud itself glows from within.
The scientists compared this star with other similar white dwarfs. They found that in all of them, brightness and glow intensity change in opposite phases, like a seesaw: when one parameter rises, the other falls. This means that all such stars operate by the same mechanism — a magnetic spot on the surface creates a hot region above the star that shines like a tiny sun.
Now the star has been officially reclassified: it is no longer simply 'non-magnetic' but belongs to the class of weakly magnetic white dwarfs. This is the first time the magnetic field of such a star has been measured directly. This suggests that magnetic fields may be present in many more white dwarfs than previously thought.
as written by the authors arXiv:2603.12048
The small DAHe and DAe spectral classes comprise isolated, hydrogen-dominated atmosphere white dwarfs that exhibit variable photometric flux and Balmer line emission. These mysterious systems offer unique insight into the complex interplay between magnetic fields, stellar rotation and atmospheric activity in single white dwarfs. DAHe stars have detectable magnetic fields through Zeeman-split spectral lines, whereas DAe stars lack such splitting. We report the first discovery and characterisation of magnetism in the DAe white dwarf WDJ165335.21-100116.33 with new time-resolved spectropolarimetry from FORS2. We detect a weak but variable longitudinal magnetic field with values $$\langle B_z \rangle > -9.2 \pm 2.4$$ kG and $$\langle B_z \rangle < -2.2 \pm 1.0$$ kG. Independent ZTF and ATLAS photometry reveal a consistent period of P = 80.3070 $$\pm$$ 0.0007 h. Time-resolved optical spectroscopy obtained with six ground-based instruments demonstrates strong modulation in the strength of the H$$\alpha$$ and H$$\beta$$ Balmer line emission with P = 80.2922 $$\pm$$ 0.0108 h. The photometric flux and Balmer emission strength vary in antiphase, with the strongest magnetic detections coinciding with phases of low photometric flux and strong line emission. These characteristics support the theory that a magnetically active, temperature-inverted spot/region is producing an optically thin chromospheric emission region. Comparison with other DAe and DAHe white dwarfs reveals all systems have a strikingly similar antiphase phenomenology, reinforcing the theory that they are subject to a unified physical mechanism. With the detection of a weak magnetic field, we reclassify WDJ165335.21-100116.33 as a low-field DAHe white dwarf.
The magnetic field of this star is thousands of times weaker than that of a typical refrigerator magnet, yet it is enough to heat gas to tens of thousands of degrees.