Cyclotron Resonance Energies at a Low X-Ray Luminosity: A0535+262 Observed with Suzaku
Mukai, K.; Yonetoku, D.; Mihara, T.; Takahashi, H.; Tanaka, T.; Sugita, S.; Angelini, L.; Enoto, T.; Kitaguchi, T.; Terada, Y.; Isobe, N.; Makishima, K.; Suzuki, M.; Nakajima, M.; Yoshida, A.; Soong, Y.; Watanabe, S.; Kitamoto, S.; Dotani, T.; Nakagawa, Y.; Naik, S.; Cottam, J.; Kotani, T.; Nagase, F.; Bautz, M.; Kohmura, T.; Kokubun, M.; Sudoh, K.; Doty, J.; Kelley, R.; Kissel, S.
Japan, United States
Abstract
The binary X-ray pulsar A0535+262 was observed with the Suzaku X-ray observatory on 2005 September 14 for a net exposure of 22 ks. The source was in the declining phase of a minor outburst, exhibiting 3-50 keV luminosity of ~3.7×1035 ergs s-1 at an assumed distance of 2 kpc. In spite of the very low source intensity (about 30 mcrab at 20 keV), its electron cyclotron resonance was detected clearly with the Suzaku Hard X-Ray Detector, in absorption at about 45 keV. The resonance energy is found to be essentially the same as that measured when the source is almost 2 orders of magnitude more luminous. These results are compared with the luminosity-dependent changes in the cyclotron resonance energy, observed from 4U 0115+63 and X0331+53.