[ Previous | Next | ADS ]

Vela X-1 in a highly active state

ATel #711; I. Kreykenbohm (ISDC, Geneva & IAA, Tuebingen), R. Staubert (IAA, Tuebingen), A. von Kienlin (MPE, Garching), S. Martinez-Nunez (Valencia), M. Mas-Hesse (LAEFF, Madrid), C. Sanchez (ESAC, Madrid), G. G.C. Palumbo (Bologna University), R. A. Sunyaev (IKI, Moscow & MPA, Garching)
on 26 Jan 2006; 14:54 UT
Credential Certification: Nami Mowlavi (Nami.Mowlavi@obs.unige.ch)

Subjects: X-ray, Gamma Ray, Request for Observations, Binary, Neutron Star, Pulsar

Recent routine INTEGRAL Galactic Plane Scan (GPS) observations including the Vela region have shown that Vela X-1 has entered a highly active state, similar to the December 2003 flaring state (Staubert et al., 2004, Conf. Proc. 5th Integral Workshop).
During the GPS observations on 2006 January 16 and 2006 January 22, Vela X-1 was in the field of view of ISGRI for 9 ksec and 12 ksec, respectively. The averaged flux in both observations was approximately 0.7 Crab (20-40 keV). On top of the already high average flux level, flaring behavior with a flux increase by a factor of 3 was observed: in some pointings, the average flux was higher than 1 Crab with a maximum of almost 1.5 Crab (20-40 keV).
More detailed analysis of the ISGRI lightcurve in the 20-40 keV band with a time resolution of 20 seconds showed that the source was strongly pulsating with a pulsed fraction around 75%. During the pulse maxima, count rates of more than 200 cts/sec were frequently observed, the peak count rate being 305 cts/sec which corresponds to more than 2.5 Crab.
Above 40keV, the source was still strongly detected, but the average count rate was only 10% of the 20-40 keV rate and no flares were evident.
Unfortunately the source was neither in the field of view of JEM-X nor the optical monitor OMC during both observations. Since the spectrometer SPI was annealing, also no high resolution spectral data are available.
Follow-up observations are strongly encouraged.