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Possible transition of Aquila X-1 to a hard state seen with INTEGRAL

ATEL # 1229; J. Rodriguez (CEA/SAp & AIM, Saclay, France), S.E. Shaw (U. Southampton, UK), L. Prat (CEA/SAp & AIM, Saclay, France), D.C. Hannikainen (Univ. of Helsinki, Finland)
on 5 Oct 2007; 12:34 UT
Password Certification: Jerome Rodriguez (

Subjects: Radio, Millimeter, Infra-Red, X-ray, Gamma Ray, Binaries, Neutron Stars, Transients

Observations of the field around GRS 1915+105 with INTEGRAL performed from UT 2007 October 4 4:03:00 to 9:49:00 UTC, revealed the presence of a bright source whose position is consistent with the neutron star X-ray binary Aql X-1.
The source is detected with IBIS/ISGRI at a significance of 7.9 sigma with a flux of ~23.2 mCrab between 20-40 keV.

Although it was seen to undergo a bright outburst in soft X-rays since 2007 Sept. 14 (ATel #1216) and in the optical (ATel #1218, #1219), our previous INTEGRAL observations of this region made on 2007 Sept. 09, 15, and 21, had failed to detect it with 20-40 keV upper limits of ~9 mCrab.

A preliminary ISGRI spectral analysis with version 7.0 of the INTEGRAL software indicates that the 15-300 keV spectrum of Aql X-1 is well represented by a power law of spectral index Gamma=2.0 0.2 (1 sigma error, reduced chi2= 1.08 for 11 dof). The 20-200 keV flux is 5.7e-10 erg/s/cm-2. The spectrum can be seen here

The RXTE/ASM daily monitoring shows that soft X-ray flux from the source is currently decaying. Given the detection at high energy, the photon index of the power law and the decrease at soft X-rays, it is very likely that the source is entering a hard X-ray state. In X-ray binaries, this state is usually associated with the presence of a compact jet detected at radio wavelengths.

Follow up observations are encouraged.

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R. E. Rutledge , Editor
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