Observations of X-Ray Pulsars

We report on OSSE observations of the Be binary pulsars A0535+26, GRO J1008{57, and 4U 0115+63 at energies above 50 keV. The spectra are generally thermal, with exponential folding energies of 15{25 keV, and no evidence for hard power-law tails. We have clearly detected a strong cyclotron absorption line at 110 keV from A0535+26 [1], and we nd a suggestion of an absorption line near 90 keV from GRO J1008{57. Introduction We report on GRO/OSSE observations of three X-ray pulsars with Betype stellar companions, namely A0535+26, GRO J1008{57, and 4U 0115+63. Be binary systems are known to be transient hard X-ray emitters, with the increased emission usually interpreted as episodic mass transfer to the neutron star from the Be companion. Outbursts may vary dramatically in intensity from orbit to orbit, and they can last for a signi cant fraction of the binary period. A0535+26 The Be binary A0535+26 is a recurrent transient pulsar with pulse period 104 s and recurrence period 111 d [2]. OSSE observed A0535+26 over 1994 Feb 8{17, during the rising portion of a giant outburst [1]. We found that the phase-averaged spectrum showed unambiguous evidence for an absorption line at 110 3 keV with optical depth = 1:8 0:5. Because of OSSE's 45 keV threshold, we were unable to make a de nitive statement on the presence of a 55 keV absorption line; however we could conclude that its optical depth must be signi cantly smaller than at 110 keV ( 55 < 0:3 110 at 95% conf.). If the 110 keV feature is due to resonant cyclotron scattering, it implies a magnetic eld strength of 9:5 10 (1+ z) G (half this value if the rst harmonic is at 55 keV), the highest magnetic eld measured by cyclotron absorption [1]. Fig. 1 shows background-subtracted pulse pro les in four energy bands, obtained by folding 4-s rates according to the BATSE ephemeris [3]. At the lowest energies, the pulse pro les show a clear double-peaked structure, with an asymmetric main pulse and a broad secondary pulse. It is apparent from the four pulse pro les, particularly in the secondary peak and the trailing edge of the main peak, that the spectrum is phase dependent. Furthermore, the emission above the 110 keV feature contains a substantial pulsed component (Fig. 1d; Fig. 2). Phase-resolved spectroscopic analysis is in progress. The energy dependence of the modulated portion of the total emission is shown in Fig. 2. We de ne the \pulsed fraction" fp as the ratio of the Report Documentation Page Form Approved OMB No. 0704-0188 Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. 1. REPORT DATE 1995 2. REPORT TYPE 3. DATES COVERED 00-00-1995 to 00-00-1995 4. TITLE AND SUBTITLE OSSE Observations of X-Ray Pulsars 5a. CONTRACT NUMBER