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Session 74 - Pulsars.
Oral session, Thursday, June 11
DeAnza/Mesa,
During January 1997, we observed the Vela Pulsar for 100 ksec with the instruments on board the Rossi X-ray Timing Explorer (RXTE). Until recently, Vela had been an elusive source of pulsed emission in the X-ray and hard X-ray band, an ironic outcome since it is the brightest gamma-ray source in the sky above 100 MeV. The X-ray picture began to be filled in by ROSAT (Ögelman et al. 1993) with the detection of soft X-ray pulsations, then by an OSSE detection of soft gamma-rays (Strickman et al. 1996). Finally, we reported at the 4th Compton Symposium in 1997 on the initial results of our RXTE observation. These included 1) the first detection of pulsations from 2-30 keV; 2) a mean pulse profile similar to that at gamma-ray energies; 3) a rather hard, phase dependent spectrum described by a power law model with photon index from 0.5 to 1.5 depending on phase; and 4) evidence of variation of the peak separation with energy.
More recent analysis has indicated that we may be seeing variation in the pulsed flux with time as well. In particular, we see evidence that some pulse profile components may turn on and off from day to day. These phenomena cannot be described in detail due to the relatively poor statistics associated with their detection. We anticipate that a much longer observation scheduled for spring 1998 may help resolve the issue.
Ögelman et al. 1993, Nature, 361, 136
Strickman et al. 1996, ApJ., 460, 735
