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INTEGRAL-detection of the ongoing outburst of the BeXRB KS 1947+300

ATel #6129; M. Kuehnel, (Remeis-Observatory & ECAP/Univ. Erlangen, Germany), C. Ferrigno, V. Esposito (ISDC/Univ. Geneva, Switzerland), V. Grinberg (MIT, USA), F. Fuerst (Caltech-SRL, USA), K. Pottschmidt (CRESST/UMBC/NASA-GSFC, USA), R. Ballhausen, I. Kreykenbohm, and J. Wilms (Remeis-Observatory & ECAP/Univ. Erlangen, Germany)
on 8 May 2014; 21:21 UT
Credential Certification: Matthias Kuehnel (matthias.kuehnel@sternwarte.uni-erlangen.de)

Subjects: X-ray, Binary, Neutron Star, Transient, Pulsar

Referred to by ATel #: 6265, 6439

The Be X-ray binary pulsar KS 1947+300 is currently showing activity as reported by Finger et al. (ATel #6123). The source was visible in the INTEGRAL field-of-view (MJD 56781.0 to 56783.6) during the Cyg X-1 observation.

NuSTAR observations of the previous outburst of this source in 2013 (Fuerst et al., 2014, ApJ 784, L40) showed that its spectrum could be described by a cut-off power-law and a cyclotron resonance scattering feature centered at 12 keV. We used the same spectral model and found the following best-fit parameters with their 90% confidence intervals (chi square=31.8 with 19 dof, setting a 3% systematic uncertainty in the data):
Gamma = 1.06(±0.12)
Efold = 31.0(±4) keV
Ecentroid = 16.7(+3.4)(-2.5) keV
Width = 2.5 keV (fixed to the NuSTAR result)
Depth = 0.191(+0.197)(-0.185).
The source flux (3-60 keV) is 1.29(+0.16)(-0.14) × 10-9 erg/s/cm2 and absorption could not be detected due to the limited spectral range. We notice that the cyclotron line detection is marginal in this observation, characterized by a low flux.

The time averaged folding energy Efold found in the NuSTAR data (the source flux was between 6.49 and 9.08 × 10-9 erg/s/cm2 in the 1 to 60 keV regime) was between 22 and 24 keV, while the photon index varied between 0.93 and 0.98. The variation of these parameters over pulse phase was much stronger. Shortly after pulse phase 0.2, the folding energy reached 35(±5) keV, while the photon index increased to 1.1(±0.05). These pulse phase specific values match the time averaged parameters we have presented above at a much lower luminosity level.

The current activity of KS 1947+300, which seems to be weaker than the previous outburst, might be a unique opportunity to study any luminosity dependencies of its X-ray spectrum.