1 National Space Institute, Technical University of Denmark2 Astrophysics, National Space Institute, Technical University of Denmark3 University of Geneva4 University of Southampton5 European Space Astronomy Centre and European Space Agency6 National Institute for Astrophysics7 Commisariat Energie Atomique8 Stanford University9 SRON10 Florida Gulf Coast University11 University of Geneva12 University of Southampton13 Stanford University
During the INTEGRAL observations in direction of IGR J17544-2619 and the Galactic bulge performed from 2012-09-16T01:15 to 2012-09-17 05:34 UTC, a bright new transient is detected at coordinates RA=266.29,Dec=-26.40 with a 0.6 arcmin, 90% c.l. confinement radius. The location of the INTEGRAL source is consistent with the newly discovered transient Swift J174510.8-262411 (GCN #13774, GCN #13775, ATEL #4380). The average broad band 5-500 keV JEM-X (61.6 ks) plus IBIS/ISGRI (63.9 ks) spectrum can be well fit by a power-law with exponential cut-off at high energy: Γ=1.29±0.04, Ecut=(122±10) keV (χ2=0.9 for 19 d.o.f., 1% systematic uncertainty). The 5-100 keV flux is (8.3±0.3)×10-9 erg/s/cm2. No significant features appear in the 3-35 keV JEM-X light curve (time bins 2 s-10 min). The source presents a monotonically increasing flux in all bands: the continuous coverage by IBIS/ISGRI shows that the 20-40 keV flux at the beginning of the observation was 63±4 mCrab and reached 617±26 mCrab at its end. By comparing the corresponding rise in the harder 40-80 keV band from 78±5 to 805±40 mCrab and performing a linear fit to the hardness ratio, we found significant evidence (~8σ c.l.) of a spectral softening with time. On the base of the rapid flux increase, the high value of the energy cut-off and the absence of thermonuclear bursts, we argue that Swift J174510.8-262411 might be a new black-hole transient. Further observations at all wavelengths are encouraged to unveil the nature of this source. We thank the INTEGRAL Galactic bulge monitoring group for their collaboration.