INTEGRAL Picture of the Month
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The reawakening of a sleeping pulsar
XTE J1946+274 belongs to the class of transient Be-X-ray pulsars, which consist of a neutron star orbiting a Be-type companion. After its first detection in the late 1990s during a giant outburst, the source went into a state of quasi-periodic flaring, with several fainter outbursts separated by about 80 days, i.e., half of the orbital period. In 2001, XTE J1946+274 went into a nearly one decade long state of quiescence. In mid 2010, the source reawakened showing the same outburst behaviour as before (see left panel; P and A indicate the epochs of periastron and apastron passages, respectively). The outburst mechanisms causing such an enigmatic behavior are until today not understood. Possible explanations include a misalignment between the neutron star's orbit and the equatorial plane of the Be star, as well as tidal deformations of the Be disk caused by the neutron star's gravitational field.
However, also the spectroscopic characteristics of XTE J1946+274 are highly interesting: In the outburst series of the late 1990s, a cyclotron resonance scattering feature (CRSF) at ~35 keV has been detected. Such features are caused by the quantisation of the motion of electrons along the B-field lines near the surface of the neutron star, where magnetic field strengths of the order of 1012 G are reached. For the 2010 outburst series, we find only weak evidence for the presence of such a feature and with a significantly different centroid energy at about 20 keV. The CRSF therefore probably originates in a different line-forming region than in the previous outbursts. Panel a of the figure on the right hand side shows the quasi simultaneous data set with Swift/XRT, RXTE/PCA, and INTEGRAL/ISGRI as well as the best model fit. Panels b-e show the residuals when adding the single model components to the broadband model continuum, i.e., a mysterious absorption feature around 10 keV, the emission of the Galactic ridge, and the source intrinsic iron K alpha fluorescence line at 6.4 keV. This fit demonstrates the importance of having many simultaneous instruments available to cover a broad energy range to be able to constrain the spectral parameters.
- "The reawakening of the sleeping X-ray pulsar XTE J1946+274"
S. Müller et al.