Spectral evolution of IGR J17098-3628 observed by INTEGRAL and RXTE

ATel #447; S. A. Grebenev (IKI, Moscow), S. V. Molkov (IKI, Moscow), M. G. Revnivtsev (IKI, Moscow; MPA, Garching), R. A. Sunyaev (IKI, Moscow; MPA, Garching)
on 29 Mar 2005; 22:51 UT
Distributed as an Instant Email Notice Request For Observations
Credential Certification: S.A.Grebenev (sergei@hea.iki.rssi.ru)

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Two next sets of INTEGRAL deep Galactic center field observations (on March 26.104-26.778 and 28.054-28.464, 2005 UT) showed that IGR J17098-3628 (ATEL #444) is strongly evolving in both brightness and spectral shape. The flux measured during these sets was equal to 57.1+/-1.5 and 40.1+/-2.5 mCrab in the 18-45 keV band and 55.4+/-3.1 and 16.6+/-4.6 mCrab in the 45-80 keV band. Comparison with the flux values given in ATEL #444 points on the steady softening of the source X-ray spectrum.

The spectra measured on March 24-25 and March 26 could be satisfactory described by a simple power law in the broad 18-200 keV band without any signs of a high energy cut-off. The photon index was equal to 1.81+/-0.09 and 2.20+/- 0.06, respectively. The spectrum measured on March 28 was significant only below 70 keV. Its approximation with a power law led to the photon index 3.00+/-0.25.

The light curves of IGR J17098-3628 obtained with ISGRI as well as the related X-ray image can be found at www-site: http://hea.iki.rssi.ru/~sergei/igrj17098-3628.html.

The RXTE carried out the cross-scanning of this region on March 29.179-29.227 (UT) and detected the source with the 3-20 keV flux of about 80 mCrab (we thank the RXTE team for organizing such a prompt observation). The best-fit position (R.A.=17h09m38, Decl.=-36o27'41'', epoch 2000.0, error radius 5' ) is generally consistent (2.1' off) with the ISGRI position reported in ATEL #444. The spectrum was complex consisting of a soft black body component with kT~1.0 keV and a hard power law tail with the photon index of about 2.5. The hydrogen column density did not exceed 10^22 cm^-2. Assuming that the source is located at the Galactic center distance (8 kpc) its broad band X-ray luminosity can be estimated to be ~2x10^37 erg/s.

The observed behaviour of IGR J17098-3628 indicates that it is a new reliable black hole candidate and a possible X-ray nova at the initial stage of its outburst (during the transition to the very high state).

We encourage further observations of this source at all wavelengths.