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Multiple long and variable transits caused by dust from possiblydisintegrating asteroids were detected in light curves of WD 1145+017. Wepresent time-resolved spectroscopic observations of this target with QUCAM CCDsmounted in the Intermediate dispersion Spectrograph and Imaging System at the4.2-m William Herschel Telescope in two different spectral arms: the blue armcovering 3800-4025 {\AA} and the red arm covering 7000-7430 {\AA}. Whencomparing individual transits in both arms, our observations show with 20{\sigma} significance an evident colour difference between the in- andout-of-transit data of the order of 0.05-0.1 mag, where transits are deeper inthe red arm. We also show with > 6 {\sigma} significance that spectral lines inthe blue arm are shallower during transits than out-of-transit. For thecircumstellar lines it also appears that during transits the reduction inabsorption is larger on the red side of the spectral profiles. Our resultsconfirm previous findings showing the u'-band excess and a decrease in lineabsorption during transits. Both can be explained by an opaque body blocking afraction of the gas disc causing the absorption, implying that the absorbinggas is between the white dwarf and the transiting objects. Our results alsodemonstrate the capability of EMCCDs to perform high-quality time resolvedspectroscopy of relatively faint targets.

Time resolved spectroscopy of dust and gas from extrasolar planetesimals orbiting WD 1145+017