NuSTAR DISCOVERY OF A CYCLOTRON LINE IN KS 1947+300

Abstract

We present a spectral analysis of three simultaneous Nuclear Spectroscopy Telescope Array and Swift/XRT observations of the transient Be-neutron star binary KS 1947+300 taken during its outburst in 2013/2014. These broadband observations were supported by Swift/XRT monitoring snapshots every three days, which we use to study the evolution of the spectrum over the outburst. We find strong changes of the power-law photon index, which shows a weak trend of softening with increasing X-ray flux. The neutron star shows very strong pulsations with a period of P ≈ 18.8 s. The 0.8-79 keV broadband spectrum can be described by a power law with an exponential cutoff and a blackbody component at low energies. During the second observation we detect a cyclotron resonant scattering feature at 12.5 keV, which is absent in the phase-averaged spectra of observations 1 and 3. Pulse phase-resolved spectroscopy reveals that the strength of the feature changes strongly with pulse phase and is most prominent during the broad minimum of the pulse profile. At the same phases the line also becomes visible in the first and third observation at the same energy. This discovery implies that KS 1947+300 has a magnetic field strength of B ≈ 1.1 × 10 12 (1 + z) G, which is at the lower end of known cyclotron line sources.

Suzakuobservations of the 2013 outburst of KS 1947+300

“…We caution that this is by no means a detection or a confirmation of a CRSF in the Suzaku data. However, there is a qualitative agreement of the Suzaku data and the results by Fürst et al (2014) regarding the CRSF. Although observed in a number of other sources (e.g., 4U 0115+634 or V 0332+53, Heindl et al 1999;Pottschmidt et al 2005, respectively), we find no indications of a second harmonic, which we would expect around 24 keV, although PIN and NuSTAR are both very sensitive in this energy The best-fit parameters are given in Table 2, and Fig.…”

“…As mentioned above, Fürst et al (2014) discovered a cyclotron line at 12.2 keV in the NuSTAR data. Unfortunately, this is exactly in the gap between the XIS and the HXD spectra.…”

“…These observations were quasi-simultaneous with Nuclear Spectroscopic Telescope Array (NuSTAR) observations in which Fürst et al (2014) present evidence for the presence of a Cyclotron Resonant Scattering Feature (CRSF or cyclotron line) at 12.2 keV. The detection of CRSFs allows us to directly infer the magnetic field strength in the emission region, which is a fundamental parameter of the neutron star (see, e.g., Harding & Lai 2006, for a review).…”

“…We fixed the line energy and width to the values from Fürst et al (2014) and only fitted the line strength. Fits that include the CRSF describe the data better, especially for Obs.…”

High-mass X-ray binaries in the Milky Way

“…Spectral parameters are slightly dependent on the source luminosity and in average consistent with a photon index of Γ ∼ 1.1, E cut ∼ 10 keV and E f old ∼ 25 keV (Tsygankov and Lutovinov, 2005b). According to the NuSTAR observations an emission continuum is modified by the pulse phase dependent cyclotron scattering feature at ∼ 12.5 keV Fürst et al (2014a). SWIFT J2000.6+3210 was recently discovered by the Swift/BAT telescope (Tueller et al, 2005) and optically identified with an early BV or mid BIII star (Halpern, 2006;Burenin et al, 2006;Masetti et al, 2008a).…”