1 National Space Institute, Technical University of Denmark2 Astrophysics, National Space Institute, Technical University of Denmark3 University of California at Berkeley4 University of Geneva5 European Space Astronomy Centre and European Space Agency6 Commisariat Energie Atomique7 U. Paris8 Stanford University9 U. Erlangen, Bamberg10 PSU11 MPA IKI12 National Institute for Astrophysics13 NASA Goddard Space Flight Center14 University of California at Berkeley15 University of Geneva16 Stanford University17 NASA Goddard Space Flight Center
The black hole candidate Swift J1910.2-0546 (= MAXI J1910-057), which was reported to be transitioning to the hard state (ATels #4273, #4295), was in the field of view of INTEGRAL's hard X-ray imager ISGRI during observations of the Scutum/Sagittarius region performed from 2012 August 20 at 23:28:03 to August 21 at 03:09:59 UTC (INTEGRAL revolution 1203). A mosaic image combining 12.6 ks of data reveals that Swift J1910.2-0546 is detected by ISGRI at a significance level of 13.7 sigma in the 18--40 keV energy band and at 9.4 sigma in the 40--100 keV energy band. Source count rates in these bands are 15.3+/-1.1 cps (74+/-5 mCrab) and 7.2+/-0.8 cps (70+/-8 mCrab), respectively. The former is consistent with the most recent daily flux measurement (MJD 56159) by Swift-BAT in a similar energy range (courtesy of H.A. Krimm, GSFC-USRA). The source spectrum measured by ISGRI shows that Swift J1910.2-0546 is detected up to ~200 keV. Fitting this spectrum (9.1 ks of effective exposure time) with a power-law model yields a photon index of 1.8+/-0.3 (at 90% confidence) for a reduced chi^2/d.o.f.=1.0/9; adding an exponential cutoff does not improve the fit. The observed 20--100-keV flux is 1.5e-9 ergs/cm^2/s. The detection of Swift J1910.2-0546 at these energies confirms that the source is in the hard state. Although it was outside the field of view of INTEGRAL's X-ray monitor JEM-X during this observation, further INTEGRAL observations in the direction of Swift J1910.2-0546 are planned for the next few weeks. Light curves and images of this source (and others in the field) can be found at the ISA project home page: http://sprg.ssl.berkeley.edu/~bodaghee/isa We thank the ISDC shift team for alerting us to the presence of Swift J1910.2-0546 in our observations and for their help in analyzing the data.