The Herschel view of massive star formation in G035.39-00.33: dense and cold filament of W48 undergoing a mini-starburst

The filament IRDC G035.39-00.33 in the W48 molecular complex is one of the darkest infrared clouds observed by Spitzer. It has been observed by the PACS (70 and 160 mu m) and SPIRE (250, 350, and 500 mu m) cameras of the Herschel Space Observatory as part of the W48 molecular cloud complex in the framework of the HOBYS key programme. The observations reveal a sample of 28 compact sources (deconvolved FWHM sizes <0.3 pc) complete down to similar to 5 M-circle dot in G035.39-00.33 and its surroundings. Among them, 13 compact sources are massive dense cores with masses >20 M-circle dot. The cloud characteristics we derive from the analysis of their spectral energy distributions are masses of 20-50 M-circle dot, sizes of 0.1-0.2 pc, and average densities of 2-20x10(5) cm(-3), which make these massive dense cores excellent candidates to form intermediate-to high-mass stars. Most of the massive dense cores are located inside the G035.39-00.33 ridge and host IR-quiet high-mass protostars. The large number of protostars found in this filament suggests that we are witnessing a mini-burst of star formation with an efficiency of similar to 15% and a rate density of similar to 40 M-circle dot yr(-1) kpc(-2) within similar to 8 pc(2), a large area covering the full ridge. Part of the extended SiO emission observed towards G035.39-00.33 is not associated with obvious protostars and may originate from low-velocity shocks within converging flows, as advocated by previous studies.