A state change of H1743-322 confirmed by INTEGRAL observations

ATel #1808; L. Prat, J. Rodriguez (CEA, France), M. Cadolle Bel, E. Kuulkers (ESAC, Spain), J. A. Tomsick (SSL Berkeley, USA), S. Corbel, M. Coriat (CEA & Univ. Paris 7, France), A. Goldwurm (CEA & APC, France), J. Wilms (ECAP, Germany)
on 24 Oct 2008; 08:41 UT
Credential Certification: Jerome Rodriguez (jrodriguez@cea.fr)

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The last INTEGRAL observations from the Galactic Center (GC) and Galactic Bulge (GB) programs confirm the state transition undergone by H1743-322 into a softer state (Atel #1804).

Since its renewed activity spotted on UT 2008 Sep 23 (See Atel #1739), H1743-322 was followed every ~2 days by INTEGRAL as part of these two programs (Atel #1745), and as part of ToO observations (Atel #1779, #1798). Since the onset on the outburst, the ISGRI spectra can be well fitted by a power-law with a high-energy cutoff (reduced chi-squared between 0.8 and 1.5 depending on the observation). Between Sep 23 and Oct 19, the source stayed in a Low-Hard State (LHS) with spectral indexes in the range 1.4--1.7. The 20-200 keV flux, after a quick initial increase, slowly decayed from 4.54 e-9 erg/cm2/s at the apex on Oct 4 to 3.53 e-9 erg/cm2/s on Oct 16.

However, the last 3 observations showed drastic changes in the spectral characteristics of the source.
On Oct 19, during a 12.6 ks GB observation, the photon index was 1.84 (±0.05), and the 20-200 keV flux was 3.03 e-9 erg/cm2/s.
On Oct 21, during a 42 ks GC observation, the photon index increased to 2.3 (±0.1), while the 20-200 keV flux dropped to 1.67 e-9 erg/cm2/s. This transition was further confirmed by the third observation that occurred on Oct 22, during a 12.6 ks GB observation. The photon index was then 2.2 (±0.1), and the 20-200 keV flux was 1.75 e-9 erg/cm2/s. Therefore, the 20-200 keV flux decreased by ~45% within two days and, at the same time, a softening of the spectra occurred.

RXTE observations made on 2008 October 23 03:18-04:39 UT (Atel #1804) allowed the identification of this new spectral state with a Hard-Intermediate State (HIMS). Our INTEGRAL observations therefore confirm the state transition of H 1743-322. In addition, the monitoring of this source allows us to locate the time of the transition between 2008 Oct 19 and Oct 21, hence at least two days before the RXTE observations. The rapid evolution in the hard X-ray range may indicate an acceleration of the source evolution, probably towards softer states.

We'll continue the monitoring of the source through GC and GB, as soon as new observations will be scheduled. Further multiwavelength observations are strongly encouraged.