# Once in a blue moon: detection of 'bluing' during debris transits in the white dwarf WD1145+017

## TL;DR

This study reports the first detection of 'bluing' during transits of WD1145+017, revealing that circumstellar gas absorption decreases during transits, indicating a shared line-of-sight and a common disc structure.

## Contribution

It provides the first multi-band photometric detection of color change during transits, linking gas absorption variability to transiting debris in a white dwarf system.

## Key findings

- Bluing observed during transits, with up to -0.05 mag in u'-r' color.
- Reduced circumstellar absorption during transits.
- Gas, debris, and dust likely part of the same disc structure.

## Abstract

The first transiting planetesimal orbiting a white dwarf was recently detected in K2 data of WD1145+017 and has been followed up intensively. The multiple, long, and variable transits suggest the transiting objects are dust clouds, probably produced by a disintegrating asteroid. In addition, the system contains circumstellar gas, evident by broad absorption lines, mostly in the u'-band, and a dust disc, indicated by an infrared excess. Here we present the first detection of a change in colour of WD1145+017 during transits, using simultaneous multi-band fast-photometry ULTRACAM measurements over the u'g'r'i'-bands. The observations reveal what appears to be 'bluing' during transits; transits are deeper in the redder bands, with a u'-r' colour difference of up to ~-0.05 mag. We explore various possible explanations for the bluing. 'Spectral' photometry obtained by integrating over bandpasses in the spectroscopic data in- and out-of-transit, compared to the photometric data, shows that the observed colour difference is most likely the result of reduced circumstellar absorption in the spectrum during transits. This indicates that the transiting objects and the gas share the same line-of-sight, and that the gas covers the white dwarf only partially, as would be expected if the gas, the transiting debris, and the dust emitting the infrared excess, are part of the same general disc structure (although possibly at different radii). In addition, we present the results of a week-long monitoring campaign of the system.