Suzaku view of the neutron star in the dipping source 4U 1822-37

Abstract

The dipping X-ray source 4U 1822-37 was observed by Suzaku on 2006 October 20 for a net exposure of 37 ks. The source was detected with the XIS at a 1-10 keV flux of 5.5 × 10^-10 erg cm^-2 s^-1, and with the HXD (HXD-PIN) at a 10-50 keV flux of 8.9 × 10^-10 erg cm^-2 s^-1. With HXD-PIN, the pulsation was detected at a barycentric period of 0.592437 s, and its change rate was reconfirmed as -2.43 × 10^-12 s s^-1. The 1-50 keV spectra of 4U 1822-37 were found to be very similar to those of Her X-1 in the slopes, cutoff, and iron lines. Three iron lines (Fe Kα, Fe XXV, and Fe XXVI) were detected, on top of a 1-50 keV continuum that is described by an NPEX model plus a soft blackbody. In addition, a cyclotron resonance scattering feature was detected significantly (> 99% confidence), at an energy of 33 ± 2 keV with a depth of 0.4^{ + 0.6}_{-0.3}. Therefore, the neutron star in this source is concluded to have a strong magnetic field of 2.8 × 10¹² G. Further assuming that the source has a relatively high intrinsic luminosity of several times 10³⁷ erg s^-1, its spectral and timing properties are consistently explained.