More INTEGRAL observations of H1743-322/IGR J17464-3213: evolution of the source into outburst

ATel #1745; L. Prat, J. Rodriguez (CEA Saclay, France), M. Cadolle Bel (ESA/ESAC, Spain), on behalf of a larger collaboration
on 26 Sep 2008; 12:58 UT
Distributed as an Instant Email Notice Request For Observations
Credential Certification: Jerome Rodriguez (jrodriguez@cea.fr)

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INTEGRAL observations from the Galactic Center Key Program performed between UT 2008 Sep 23 12:36 and 2008 Sep 25 05:09 confirm the renewed activity of H1743-322/IGR J17464-3213 (See Kuulkers et al. 2008 Atel #1739). Previous observations of the field by INTEGRAL between 2008-09-20UTC02:44:28 and 2008-09-22UTC12:20:45 did not yield detections with 3-sigma upper limits of 1.5, 2.5, and 6.1 mCrab in the 20-40, 40-80 and 80-150 keV respectively.

The source entered the field of view of IBIS/ISGRI on 2008-09-23 16:36:45.130 UTC and is clearly detected afterward. Our analysis shows a clear evolution of the 20-40 keV flux from 23 mCrab at the first detection to 55.3 mCrab on Sep 25 04:50 UTC. Although less significant a similar evolution of the flux is also seen at higher energies.

We then divided the observations into two main intervals from which mosaics and spectra were accumulated.
Between Sep 23 12:33 and Sep 24 12:59, the source is detected with fluxes of ~26 mCrab (20-40 keV), 36 mCrab (40-80 keV), and 50 mCrab (80-150 keV), with S/N ratio ranging from ~15 to 30 sigma for these ranges.
The IBIS/ISGRI spectrum can be well fitted with a simple power-law in the 20-300 keV range. The photon index is 1.66 (±0.08) (reduced chi-squared =1.40 for 10 dof). We remark, however, that a power law with a cutoff (cutoffpl in XSPEC) provides a better fit (reduced chi square=1.11 for 9 dof) with a photon index of 1.3 (-0.3, +0.2), and a cutoff energy Ecut=151 (-66, +54) keV. The 20-200 keV flux is 8.9 e-10 erg/cm2/s.
Between Sep 24 13:01 and Sep 25 04:47, the source is detected with fluxes of ~47 mCrab (20-40 keV), 66 mCrab (40-80 keV), 89 mCrab (80-150 keV), with S/N ratio ranging from 20 to 40 sigma for these ranges.
Here a simple power law with a photon index 1.64 (±0.05) provides an acceptable fit to the spectrum (reduced chi square=1.75 for 10 dof). Adding a cut-off does not improve the fit. The 20-200 keV flux is 1.6 e-9 erg/cm2/s.

Our results indicate that H1743-322/IGR J17464-3213 is in the Hard State seen at the beginning of outbursts of black holes binaries. This state is usually associated to a powerful steady outflow. We note that the detection of the source by INTEGRAL occurs very early in the outburst, a phase that is still rather unknown, and very rarely followed. The rapid evolution in the flux over the course of our monitoring may indicate that it could soon transit to softer states.

Multiwavelength observations of this renewed outburst of H 1743-322 are strongly encouraged.