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X-ray brightening and softening of SWIFT J174510.8-262411

ATel #4401; S. A. Grebenev (IKI, Moscow), R. A. Sunyaev (IKI, Moscow; MPA, Garching)
on 20 Sep 2012; 06:30 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Sergei Grebenev (sergei@hea.iki.rssi.ru)

Subjects: X-ray, Black Hole, Transient

Referred to by ATel #: 4450, 4456

The sky field around the hard X-ray transient Swift J174510.8-26241 (GCN #13774, #13775) was intensively observed with INTEGRAL on Sept. 12-15 - a couple of days before its discovery. The observations were carried out under our program of studying the Galactic hard X-ray background. The 3-sigma upper limit for the flux from this source was estimated to be 0.75 mCrab in the 20-60 keV band. No any hard X-ray flares or other transient phenomena were detected.

INTEGRAL TOO observations of this transient (PI T. Belloni), started on Sept. 18, 20 12, show that the 20-60 keV flux from the source increased by 60% during one day (since ATEL #4381, #4383) and approaches now to the Crab level. It is now more than 1000 times brighter than during our observations on Sept 12-15. The source has a hard spectrum that can be described in the 3-400 keV band in the first approximation with a single model of an absorbed power-law with a high energy cut-off. At the end of Sept. 18 its photon index was 1.86+/-0.02 and the cut-off energy close to 147+/-10 keV. Being compared with the INTEGRAL Galactic bulge observation (PI E. Kuulkers) on Sept. 17 (photon index 1.35+/-0.04 and cut-off energy 120+/-10 keV, see also ATEL #4381) these values indicate fast softening of the emission that is typical for the raising phase of X-ray novae and suggests the coming transition to the soft spectral state. The spectra can be found at the www-page given below. The luminosity measured in the 3-400 keV band under the assumption of the source location at the Galactic center distance (8 kpc) exceeds now 3x10^38 erg/s (it was 1.4x10^38 erg/s on Sept. 17 and less than 2x10^35 erg/sec on Sept. 12-15). The current value corresponds to the Eddington luminosity for accretion onto a compact object of 2 solar masses.

Text of the second part of this telegram is based on public observational data of the INTEGRAL TOO and Galactic bulge observations.

Softening of the broad band X-ray spectrum of Swift J174510.8-26241 according to the INTEGRAL IBIS/ISGRI and JEM-X observations on Sept. 17 -18, 2012