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The 2007 outburst of the X-ray binary XTE J1856+053

On 28 February 2007 a new outburst of the previously known transient source XTE J1856+053 was detected with RXTE/ASM. We present here the results of an XMM-Newton (0.5-10.0 keV) Target of Opportunity observation performed on 14 March 2007, aimed at constraining the mass of the compact object in this X-ray binary and determining its main properties. The EPIC-pn camera was used in Timing mode and its spectrum fit together with the RGS data. IR observations with GROND at the 2.2 m telescope in La Silla provide further information on the system. The X-ray light curve shows that both the 1996 and the 2007 outbursts had two peaks. The X-ray spectrum is well fit with a thermal accretion disk model, with kT=0.75+/-0.01 keV and foreground absorption N_H=4.5(+/-0.1)E22 cm**-2. The low disk temperature favours a black-hole as accreting object, with an estimated mass in the range 1.3-4.2 M_sun. From the IR upper limits we argue that XTE J1856+053 is a low mass X-ray binary. We estimate the orbital period of the system to be between 3 and 12 hours.