Detection of a cyclotron line and its second harmonic in 4U1907+09

TL;DR

This paper reports the detection of a cyclotron line and its harmonic in the X-ray spectrum of 4U1907+09, revealing a neutron star magnetic field of approximately 2.1 x 10^12 G.

Contribution

First detection of a cyclotron line and its harmonic in 4U1907+09, providing direct measurement of the neutron star's magnetic field strength.

Findings

Detected cyclotron absorption line at ~19 keV

Detected second harmonic at ~39 keV

Estimated magnetic field strength of 2.1 x 10^12 G

Abstract

We report the detection of a cyclotron absorption line and its second harmonic in the average spectrum of the high mass X-ray binary 4U1907+09 observed by the BeppoSAX satellite on 1997 September 27 and 28. The broad band spectral capability of BeppoSAX allowed a good determination of the continuum against which the two absorption features are evident at ~ 19 and ~ 39 keV. Correcting for the gravitational redshift of a ~ 1.4 solar mass neutron star, the inferred surface magnetic field strength is B = 2.1 10E12 G that is typical for this kind of systems. We also detected an iron emission line at ~ 6.4 keV, with an equivalent width of ~ 60 eV.