547 Raubenheimer B.C
- I Kreykenbohm, P Kretschmar, J Wilms
- 547 Raubenheimer B.C
We present results from a 20 h RXTE observation of Vela X-1, including a peculiar low state of a few hours duration, during which the pulsation of the X-ray emission ceased, while significant non-pulsed emission remained. This “quiescent state” was preceded by a “normal state” without any unusual signs and followed by a “high state” of several hours of increased activity with strong, flaring pulsations. While there is clear spectral evolution from the normal state to the low state, the spectra of the following high state are surprisingly similar to those of the low state. INTRODUCTION Vela X-1 (4U 0900–40) is an eclipsing high mass X-ray binary consisting of the 23 M⊙ B0.5Ib supergiant HD 77581 and a neutron star with an orbital period of 8.964 d and a spin period of about 283 s (van Kerkwijk et al., 1995, and references therein). The persistent X-ray flux from the neutron star is known to be very variable exhibiting strong flares and low states. Inoue et al. (1984) and Kreykenbohm et al. (1999) have observed low states of near quiescence where no pulsations were seen for a short amount of time. Before or after these low states normal pulsations were observed. During an observation of Vela X-1 for 12 consecutive orbits in January 1998 by the Rossi X-ray Timing Explorer (RXTE ), we have by chance observed such a quiescent state for the first time from the beginning to the end, preceded and followed by the usual pulsations. LIGHTCURVES AND PULSE PROFILES As Fig. 1 demonstrates, the source flux suddenly decreased between orbits 2 and 3, reaching its minimum during orbit 4. At the same time the source pulsations decreased strongly, while significant non-pulsed source flux remained. This is shown in detail in Fig. 2. The pulsed fraction decreased from 30%–50%, depending on the energy band, to 7%–9%. Note that even at the lowest state, the overall source flux was >5 times the predicted background level in the energy range used.