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