Transiting Planetary Debris near the Roche Limit of a White Dwarf on a 4.97$\,$hr Orbit -- and its Vanishing

TL;DR

This paper reports the discovery of irregular, deep transits near the Roche limit of a white dwarf, revealing dynamic debris activity and providing insights into disk evolution and debris composition.

Contribution

It presents the first detailed observation of a white dwarf with transiting debris showing activity cessation and reactivation, advancing understanding of debris evolution near white dwarfs.

Findings

Transits occur every 4.9704 hours near the Roche limit.

Transit depths are over 30% and last 15-40 minutes.

Transit activity can cease and reappear over months.

Abstract

We present the discovery of deep, irregular, periodic transits towards the white dwarf ZTF$\,$J1944$+$4557 using follow-up time-series photometry and spectroscopy from Palomar, Keck, McDonald, Perkins, and Lowell observatories. We find a predominant period of 4.9704$\,$hr, consistent with an orbit near the Roche limit of the white dwarf, with individual dips over 30$\%$ deep and lasting between 15 and 40 minutes. Similar to the first known white dwarf with transiting debris, WD$\,$1145$+$017, the transit events are well-defined with prominent out-of-transit phases where the white dwarf appears unobscured. Spectroscopy concurrent with transit photometry reveals the average Ca$\,$K equivalent width remains constant in and out of transit. The broadening observed in several absorption features cannot be reproduced by synthetic photospheric models, suggesting the presence of circumstellar gas. Simultaneous $g+r$- and $g+i$-band light curves from the CHIMERA instrument reveal no color dependence to the transit depths, requiring transiting dust grains to have sizes $s \gtrsim0.2\,\mu$m. The transit morphologies appear to be constantly changing at a rate faster than the orbital period. Overall transit activity varies in the system, with transit features completely disappearing during the seven months between our 2023 and 2024 observing seasons and then reappearing in 2025$~$March, still repeating at 4.9704$\,$hr. Our observations of the complete cessation and resumption of transit activity provide a novel laboratory for constraining the evolution of disrupted debris and processes like disk exhaustion and replenishment timescales at white dwarfs.