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INTEGRAL detection of XTE J2012+381 during the earliest stages of its new outburst

ATel #15847; J. Rodriguez (CEA/AIM, Fr), J. Chenevez (DTU, Dk), J. Tomsick (U. Berkeley/SSL, USA), J. Wilms (ECAP, Ge), M. Fiocchi, A. Bazzano (INAF-IAPS, It), , K. Pottschmidt (GSFC, USA), J. Steiner (CfA, USA), T. Bouchet (CEA/AIM, Fr), F. Cangemi, A. Coleiro (APC, Fr), E. Egron (INAF/OAC, It), V. Grinberg (ESA, NL), P.-O. Petrucci, M. Clavel (IPAG, Fr)
on 5 Jan 2023; 08:53 UT
Credential Certification: Jerome Rodriguez (jrodriguez@cea.fr)

Subjects: X-ray, Gamma Ray, Black Hole, Transient

Referred to by ATel #: 15855

The X-ray binary XTE J2012+381 is currently undergoing a new outburst first detected by MAXI/GSC ((ATels #15826) on December 25, and later followed with Swift, NICER, and INTEGRAL (ATels #15827 #15829, #15844).

This black hole candidate was in the field of view of INTEGRAL/IBIS and partly in INTEGRAL/JEM-X during the the long term monitoring of Cygnus X-1 (PI J. Wilms). The latest two observations respectively took place from December 19 17h52 UTC to December 20 05h51 UTC (INTEGRAL revolution 2585) and between December 23 17h47 UTC to December 24 14h47 UTC (INTEGRAL revolution 2586)

During the former observation XTE J2012+381 is not detected by IBIS with 3sigma upper limits of about 7 mCrab and 12 mCrab in the 30-50 keV and 50-100 keV energy ranges. It is, however, clearly visible in revolution 2586 with 3-10 keV, 10-25 keV, 30-50 keV, 50-100 keV, and 100-150 keV fluxes of about 28, 30, 34, 35 and 42 mCrab, showing the source was in a rather hard state during this observation. This detection, prior to the MAXI one, shows the source outburst had already started at least a day before the MAXI detection.

The preliminary IBIS/ISGRI 30-150 keV spectrum is not well fitted by a simple power law, with a reduced chi square of 2.4. A broken power law provides a good fit with Γ1=1.2 (-0.5 +0.4), Ebreak~50+/-7 keV and Γ2=3.30 (-0.5 +0.9 ; chi2 = 13.3 for 15dof). A cut-off power law also provides a good description of the spectrum (reduced chi2 ~1), but when all parameters are left free to vary, the photon index is extremely hard (Gamma<0) and the cut-off energy is below the low energy threshold of the spectral fit. Fixing the photon index to a more classical value for a BHC, Γ=1.5, leads to a poorer but still acceptable chi2 (27.6 for 17 dof). These are obtained using near real time, therefore incomplete data, and thus should be taken with some caution.

However, our results indicate that XTE J2012+381 was in a hard spectral state during this observation. The report of a state transition with NICER (ATel #15829) and the significant evolution seen with the INTEGRAL data the following day (ATel #15844) indicates a short-lived hard state at the beginning of XTE J2012+381 outburst.

The source will unfortunately be out of reach from the INTEGRAL instruments until March 20, 2023.

JR and TB acknowledge partial funding from the French space agency (CNES)