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INTEGRAL detection of a strong outburst in 4U 1954+31

ATel #796; S. Paltani (ISDC, Geneva), D. Attie (CEA, Saclay), A. Domingo Garau (LAEFF, Madrid), S. Larsson (Stockholm Observatory), J. Paul (CEA, Saclay), P. Shtykovsky (SRI, Moscow), A. Strong (MPI, Garching), O. R.Williams (ISOC/ESAC, Madrid)
on 16 Apr 2006; 12:17 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Nami Mowlavi (Nami.Mowlavi@obs.unige.ch)

Subjects: X-ray, Gamma Ray, Binary, Transient

We report the dectection of a ~150 mCrab outburst in the transient high mass X-ray binary 4U 1954+31, observed in the Galactic Plane Survey (GPS) by INTEGRAL. The flux in the IBIS/ISGRI instrument reached 17.7 counts/s (165 mCrab, 32-sigma detection) in the 20-40 keV band. The observations started at 00:14 UTC on Apr 15, 2006. The flux was already ~15 counts/s (140 mcrab). It peaked 40 min later, and then fell to about 10 counts/s (95 mcrab). The source was not detected anymore in the pointing starting two hours after the first detection; as the source was well off-axis, this gives an upper limit of about 4 counts/s (35 mCrab). 4U 1954+31 is also detected in the 40-80 keV band, with a flux of 3.4 counts/s (55 mCrab). The significance is about 6 sigmas. Unfortunately, 4U 1954+31 does not cross the field of view of Jem-X during this GPS scan.
This is the first detection of this source in ISGRI, although its location in the Galactic Plane and close to Cyg X-1 makes that it has been observed a large number of times. The source is also not present in the 2nd IBIS/ISGRI catalogue (Bird et al., 2006, ApJ 636, 765). Inspection of the R-XTE ASM light curve shows that the source is regularly detected and that bursts of the order of 50 mCrab in the 2-10 keV band occur 2-3 times per year, lasting a few days. These bursts usually occur during long periods (~a year) of increased activity.
4U 1954+31 will be reobserved in the GPS on Apr 24.