Results of INTEGRAL TOO observation of the transient X-ray burster XTE J1810-189

We report results of the INTEGRAL Target of Opportunity observations of the transient X-ray burster XTE J1810-189. The observations were performed on April 3--6, 2008, soon after the discovery of the source and near the peak of its outburst. That time the source had a flux of about 50 mCrab and exhibited a hard Comptonized X-ray spectrum extending well above 100 keV. Being approximated by a power law with an exponetial cut-off in the broad 3--100 keV energy band it gave the average photon index $\Gamma\simeq 1.6$ and $kT_{cutoff}\simeq 67$ keV. We found only slight indications for changes in the index during the observation ($\Gamma$ first steady decreased from $\sim 2.0$ to $\sim1.3$ and then increased back to $\sim 2.0$). However the $N_{\rmn H}$ value measured by absorption in the low energy part of the spectrum changed drastically and very irregularly (from $\sim 4\times 10^{22}$ till $\sim 100\times 10^{22}$ cm$^{-2}$). There were 10 type I X-ray bursts detected from the source during these TOO observations. Assuming that the Eddington luminosity was reached during the burst with the highest peak flux we get an upper estimate for a distance to the source $D=6.4\pm0.6$ kpc. From the X-ray burst parameters we conclude that this LMXB harboures an evolved star.