RXTE observations of the 1A 1118-61 in an outburst, and the discovery of a cyclotron line

摘要

We present the analysis of RXTE monitoring data obtained during the January2009 outburst of the hard X-ray transient 1A 1118-61. Using these observationsthe broadband (3.5-120 keV) spectrum of the source was measured for the firsttime ever. We have found that the broadband continuum spectrum of the source issimilar to other accreting pulsars and is well described by severalconventionally used phenomenological models. We have discovered that regardlessof the applied continuum model, a prominent broad absorption feature at ~55 keVis observed. We interpret this feature as a cyclotron resonance scatteringfeature (CRSF). The observed CRSF energy is one of the highest known andcorresponds to a magnetic field of B~4.8 x 10^12 G in the scattering region.Our data also indicate the presence of an iron emission line presence that hasnot been previously reported for 1A 1118-61. Timing properties of the source,including a strong spin-up, were found to be similar to those observed byCGRO/BATSE during the previous outburst, but the broadband capabilities of RXTEreveal a more complicated energy dependency of the pulse-profile.