Suzaku View of the Neutron Star in the Dipping Source 4U 1822$-$37

Abstract

The dipping X-ray source 4U 1822$-$37 was observed by Suzaku on 2006 Octrober 20 for a net exposure of 37 ks. The source was detected with the XIS at a 1-10 keV flux of 5.5$\times10^{-10}$ erg cm$^{-2}$ s$^{-1}$, and with the HXD (HXD-PIN) at a 10-50 keV flux of 8.9$\times10^{-10}$ 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\times$10$^{-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 K$\alpha$, 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$\pm$2 keV with a depth of 0.4$^{+0.6}_{-0.3}$. Therefore, the neutron star in this source is concluded to have a strong magnetic field of 2.8$\times10^{12}$ G. Further assuming that the source has a relatively high intrinsic luminosity of several times 10$^{37}$ erg s$^{-1}$, its spectral and timing properties are consistently explained.