AAS 205th Meeting, 9-13 January 2005
Session 102 Neutron Stars
Poster, Wednesday, January 12, 2005, 9:20am-6:30pm, Exhibit Hall

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[102.09] INTEGRAL Observations of Vela X-1: Hard-X-Ray Flaring and Cyclotron Line Determinations

C. Shrader (NASA GSFC)

The wind-fed, accretion powered X-ray pulsar Vela X-1 was observed extensively during the first two years of the INTEGRAL mission. In the latter part of 2003, the source exhibited intense flaring, and a cyclotron line feature at ~50-55 keV was reported based on spectra obtained as part of the INTEGRAL Core Program collaboration (Kretschmar et al 2004, astro-ph/0406052). A putative accompanying feature at ~1/2 of this energy could not be definitively established by those authors. We have performed our own analysis of the publicly available data, particularly those obtained with the high-resolution germanium spectrometer (SPI), where we emphasize a distinct approach. Specifically, we have utilized the full (diagonal + off-diagonal terms) SPI detector response matrix in our analysis, unlike the standard coded-mask analysis which invokes a diagonal approximation. Additionally, in our method, the source and background are simultaneously obtained in a single chi-square de-convolution, with a goal of minimizing possible systematics. We have also made use of the reprocessed INTEGRAL events data to accurately barycenter and phase-bin the SPI, IBIS and JEM-X data, and access energy and intensity dependencies of the pulse profile. We address possible effects of the low-energy SPI calibration on measurement of the fundamental cyclotron feature in Vela X-1. Finally, we consider several additional sources, and assess the overall prospects of the instrument for contributions to this field.

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