A very intense X-ray outburst of Vela X-1 detected with INTEGRAL
ATel #211; R. Krivonos (IKI/RSDC, Moscow), N. Produit (ISDC, Versoix), I. Kreykenbohm (IAA, Tubingen & ISDC), R. Staubert (IAA, Tubingen), A. von Kienlin (MPE, Garching), C. Winkler (ESTEC, Noordwijk), N. Gehrels (GSFC, NASA)
on 2 Dec 2003; 14:00 UT
Distributed as an Instant Email Notice Request For Observations
Credential Certification: S.A.Grebenev (sergei@hea.iki.rssi.ru)
Subjects: Radio, Far-Infra-Red, Infra-Red, Optical, X-ray, Gamma Ray, Request for Observations, Binary, Neutron Star, Pulsar
We report a very intense hard X-ray outburst detected from Vela X-1 with INTEGRAL/IBIS in the course of deep Core Program observations of the Vela region.
The outburst began on November 28, 2003 at 02:32:52 UT and lasted 15.2 ksec. The S/N ratio measured from the source at the image reached ~270 standard deviations. From a preliminary analysis the photon flux in the 15-40 keV band of ISGRI was seen to raise by an order of magnitude from less then 500 mCrab to about 7 Crab during the first 5400 s (three pointings). In the 40-60 keV band, the flux increased from ~125 mCrab to more than 1 Crab. Then the flux steadily decreased. We saw obvious oscillations in the count rate with the Vela X-1 spin period during the whole duration of the outburst.
The source was also detected with SPI (at the S/N ratio of ~170 sigma in the energy band 20-40 keV). The spectrum measured by SPI did not show any significant changes, except the spectrum being harder than usual, during the outburst. Unfortunately, the source was outside the JEM-X field of view at this epoch.
It seems it is the first time that the source demonstrates such an outburst during the whole history of its observations. The current monitoring shows that its flux is still above the persistent level. We encourage follow up observations at other wavelengths.