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We present 2-5 $\mu$m spectroscopic observations of the dust coma of67P/Churyumov-Gerasimenko obtained with the VIRTIS-H instrument onboard Rosettaduring two outbursts that occurred on 2015, 13 September 13.6 h UT and 14September 18.8 h UT at 1.3 AU from the Sun. Scattering and thermal propertiesmeasured before the outburst are in the mean of values measured for moderatelyactive comets. The colour temperature excess (or superheat factor) can beattributed to submicrometre-sized particles composed of absorbing material orto porous fractal-like aggregates such as those collected by the Rosetta insitu dust instruments. The power law index of the dust size distribution is inthe range 2-3. The sudden increase of infrared emission associated to theoutbursts is correlated with a large increase of the colour temperature (from300 K to up to 630 K) and a change of the dust colour at 2-2.5 $\mu$m from redto blue colours, revealing the presence of very small grains ($\leq$ 100 nm) inthe outburst material. In addition, the measured large bolometric albedos($\sim$ 0.7) indicate bright grains in the ejecta, which could either besilicatic grains, implying the thermal degradation of the carbonaceousmaterial, or icy grains. The 3-$\mu$m absorption band from water ice is notdetected in the spectra acquired during the outbursts, whereas signatures oforganic compounds near 3.4 $\mu$m are observed in emission. The H$_2$O2.7-$\mu$m and CO$_2$ 4.3-$\mu$m vibrational bands do not show any enhancementduring the outbursts.

Comet 67P outbursts and quiescent coma at 1.3 AU from the Sun: dust properties from Rosetta/VIRTIS-H observations