INTEGRAL catches state transition of Black Hole Candidate 4U1630-47

ATel #15988; T. Bouchet (CEA-Saclay/DAP-AIM), T. Belloni (INAF-OA Brera), J. Wilms (Remeis-Observatory, FAU Erlangen-Nuernberg), F. Cangemi (U. Paris Cite/APC), J. Rodriguez (CEA-Saclay/DAP-AIM), V. Grinberg (ESA/ESTEC), P. Laurent (CEA-Saclay/DAP-AIM), P. Thalhammer (Remeis-Observatory, FAU Erlangen-Nuernberg)
on 13 Apr 2023; 16:58 UT
Credential Certification: Jerome Rodriguez (jrodriguez@cea.fr)

Tweet

The black hole candidate 4U 1630-47 has been in outburst for about 9 months (ATel #15575). It is monitored daily by the Swift/BAT and MAXI instruments and has regularly been in the field of view of various instruments onboard the INTEGRAL satellite, especially during monitoring programs of the Galactic Centre (ATel #15924).

A first public Target of Opportunity (ToO) observation of 4U1630-47 was made by INTEGRAL between 2023 March 9-12 where it was detected by both INTEGRAL/JEM-X (3-30 keV) and INTEGRAL/IBIS (30-1000 keV) at fluxes of 250, 164, 97 mCrabs in the 30-50, 50-100 and 100-200 keV bands, respectively. The source spectrum of both instruments can be well modeled by a power law with a photon index of 2.94 +/- 0.02 (chi2= 55.75 for 45 degrees of freedom) with no apparent high-energy cut-off. We also detect an unexpected signal at high-energy with a flux of 34 mCrabs between 300-1000 keV.

A new INTEGRAL-ToO observation was performed between 2023 April 7-9. The detected fluxes by IBIS are significantly lower, with 82, 77, 65 mCrabs in the 30-50, 50-100 and 100-200 keV bands. The JEM-X and IBIS combined spectrum is well described (chi2= 50.87 for 48 dof) by a disk black body with an inner disk temperature of 1.2 keV and a cut-off power law with a photon index of 2.2 +/- 0.2 but no constraint on the cut-off energy. This state is quite similar to the one observed with INTEGRAL and Swift in 2023 January (ATel#15924).

This transition into a dimmer state seems to be confirmed by the MAXI instrument, where the flux in the 2-20 keV band dropped from 0.6 Crabs to around 0.2 Crabs on April 6. Swift/BAT also recorded a drop from 0.4 to 0.1 Crabs in the 15-50 keV band on April 5.

This spectral change could be associated with possible ejections, therefore radio follow-up observations are encouraged. As of now the source has still not gone back to quiescence, so it is possible that it will make a transition into the hard state in the near future.

The latest X-ray light curves can be found on the following pages:

- Swift/BAT

- MAXI

- INTEGRAL

The authors thank the INTEGRAL project scientist E. Kuulkers and the INTEGRAL staff for accepting and planning the INTEGRAL ToOs.