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INTEGRAL detection of bright hard X-ray emission from the RS CVn star GT Mus

ATel #9255; V. Sguera (INAF-IASF Bologna, Italy), A. Bazzano, M. Fiocchi, P. Ubertini, L. Natalucci (INAF-IAPS Roma, Italy), L. Sidoli (INAF-IASF Milano, Italy), A. J. Bird (Univ. of Southampton, UK), E. Kuulkers (ESA/ESAC, Spain), L. Bassani (INAF-IASF Bologna, Italy)
on 18 Jul 2016; 21:34 UT
Credential Certification: Eleonora Torresi (torresi@iasfbo.inaf.it)

Subjects: X-ray, Star, Transient

The RS CVn star GT Mus is currently undergoing X-ray activity as detected by MAXI/GSC (Atel#9251) starting on 2016-07-16 13:08:30 UT (trigger time). The 4-10 keV flux (about 150 mCrab) is among the highest ones measured with the MAXI/GSC.

INTEGRAL has recently performed Galactic Plane scan (GPS) observations of the Musca region during revolution 1702, the source GT Mus was detected at about 8.7 sigma level (22-60 kev) with an average flux of 22.4+/-2.6 mCrab. The effective exposure time on-source is about 4 ks (from 17 Jul 05:30 UTC to 17 Jul 13:55 UTC). No significant emission was detected in the band 60-100 keV, with a 3 sigma upper limit of about 20 mCrab. The source was never in the field of view of the JEM-X monitors during this observation. The IBIS/ISGRI spectrum is well fitted by a soft power law with photon index 4.1+/-0.6 and average 22-60 keV (20-40 keV) flux of 3.3 x 10-10 erg cm-2 s-1 (3.6 x 10-10 erg cm-2 s-1). The corresponding 22-60 keV (20-40 keV) luminosity (assuming a distance of 172 pc) is 1.2 x 1033 erg s-1 (1.3 x 1033 erg s-1).

During the previous GPS observations in revolution 1701 (13-14 Jul 2016) GT Mus was also in the field of view of IBIS/ISGRI with an effective exposure time of about 11 ks. It was not significantly detected, we infer a 3 sigma upper limit of about 4.5 mCrab (22-60 keV)

This is the second outburst of GT Mus reported as detected by IBIS/ISGRI above 20 keV. The previous one was detected on Jul 2003, it lasted about 0.8 days and reached at its peak a 20-40 keV flux of about 24 mCrab (Bird et al. 2016, ApJS, 223, 15; Bird et al. 2010, ApJS, 186, 1).