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INTEGRAL observations of the new X-ray transient Swift J1658.2-4242

ATel #11306; S. A. Grebenev, I. A. Mereminskiy, A. V. Prosvetov (Space Research Institute, Moscow), L. Ducci (IAAT Tuebingen and ISDC, Versoix), E. Bozzo, V. Savchenko, C. Ferrigno (ISDC, Versoix)
on 16 Feb 2018; 15:13 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Sergei Grebenev (sergei@hea.iki.rssi.ru)

Subjects: X-ray, Transient

Referred to by ATel #: 11307, 11311, 11318, 11321, 11322, 11336, 11358, 11375

During the on-going INTEGRAL observations of the Galactic center field performed from 2018 February 13 at 08:06 to February 16 at 13:01 (UTC) IBIS/ISGRI instrument on board INTEGRAL detected the new X-ray transient Swift J1658.2-4242 (GCN #22416 and #22417).

Due to the large off-axis angle of Swift J1658.2-4242 with respect to the INTEGRAL pointing direction, the source was only sporadically within the IBIS/ISGRI field of view. Its 20-60 keV flux measured at different epochs was:

       Time (MJD)         | Flux (20-60 keV)
58162.47 - 58162.67 | <17 (1 sigma)
58162.93 - 58163.14 | 10+/-8 mCrab
58163.31 - 58163.52 | 32+/-5 mCrab
58164.39 - 58164.56 | 99+/-9 mCrab
58164.81 - 58164.89 | 89+/-21 mCrab
58165.29 - 58165.41 | 41+/-24 mCrab

During the fourth interval (MJD 58164.39-58164.56), we measured the highest fluxes in two energy bands: 84+/-6 mCrab (20-40 keV) and 97+/-9 mCrab (40-80 keV). This indicates that the source spectrum had a hardness compatible with the one of the Crab Nebula (photon index 2.1) or slightly harder, as typical of black-hole binaries in the hard state.

While measured with large errors the last points in the table let us to suggest that the hard flux has stabilized or even begins to decrease. This may be a result of the source transition to another spectral state or the beginning of its complete switch-off. The source was always outside the INTEGRAL/JEM-X field-of-view, thus preventing us from verifying this hypothesis through simultaneous flux measurements in the softer X-ray band.

Further observations of the source in the soft and hard X-rays, as well as in other energy domains, are encouraged.