Rapid Spin‐up Episodes in the Wind‐fed Accreting Pulsar GX 301−2

Abstract

The accreting pulsar GX 301[2 (P \ 680 s) has been observed continuously by the large-area detectors of the Burst and Transient Source Experiment (BATSE) instrument on the Compton Gamma Ray Observatory since 1991 April 5. Orbital parameters determined from these data are consistent with previous measurements, with improved accuracy in the current orbital epoch. The most striking features in the pulsar frequency history are two steady and rapid spin-up episodes, with Hz s~1, l5 B (3È5) ] 10~12 each lasting for about 30 days. They probably represent the formation of transient accretion disks in this wind-fed pulsar. Except for these spin-up episodes, there are virtually no net changes in the neutron star spin frequency on long timescales. We suggest that the long-term spin-up trend observed since 1984 Hz s~1) may be due entirely to brief (B20 days) spin-up episodes similar to those we (l5 B 2 ] 10~13 have discovered. We assess di †erent accretion models and their ability to explain the orbital phase dependence of the observed Ñux. In addition to the previously observed preperiastron peak at orbital phase 0.956^0.022, we also Ðnd a smaller peak close to apastron at orbital phase 0.498^0.057. We show that if the companion starÏs e †ective temperature is less than 22,000 K, then it must have a mass and a M c \ 70 M _ radius so as not to overÐll the tidal lobe at periastron. In order not to overÑow the Roche R c \ 85 R _ lobe at periastron, the corresponding values are and These constraints are M c \ 55 M _ R c \ 68 R _. nearly at odds with the reclassiÐcation by Kaper et al. of the companion as a B1 Ia] hypergiant.

The variable cyclotron line of GX 301–2

“…To obtain pulse profiles, the bin (PCA) or photon arrival (HEXTE) times were first barycentered and then corrected for the orbital motion of GX 301−2 using the ephemeris of Koh et al (1997). We used epoch folding (Leahy et al 1983) to determine the pulse period during the periastron passage to be 684.2±0.2 sec, thus confirming that GX 301−2 is continuing its spin down.…”

“…Since the first measurement of the period with Ariel-5 (White et al 1976), it has undergone dramatic variations: the period was stable at ∼700 sec until 1984, then the pulsar was spun up between 1985 and 1990 (Koh et al 1997;Bildsten et al 1997) to a period of ∼675 sec. In 1993, the spin behavior reversed (Pravdo & Ghosh 2001) and the pulsar has since been spinning down almost continously: we determined the pulse period of our observation (JD 2451830.8) to be 684.2 sec (see below).…”

“…de The neutron star is in an eccentric (e = 0.462) orbit with a period of 41.498 ± 0.002 days (Koh et al 1997;Sato et al 1986). Due to the violent wind accretion the source is extremely variable -X-ray luminosity changes of a factor of five within one hour being common (Rothschild & Soong 1987).…”

“…We used the same procedures as in our earlier analysis of RXTE data from Vela X- (Koh et al 1997). The periastron passage has been extrapolated based on the ephemeris of Koh et al (1997). The flare shortly before the periastron passage is very evident.…”

“…The time of the observation is marked by dashes and stronger line thickness, covering a significant part of the orbit due to the high velocity of the neutron star during periastron passage. The grey inner circle represents the size of Wray 977 when using the old values of Parkes et al (1980) which are also used by Koh et al (1997 Fig. 3.…”