NuSTAR observation at the peak of the 2023 outburst of 4U 0115+63

ATel #15989; K. Berger (Remeis-Observatory and ECAP, FAU Erlangen-Nuernberg), J. Wilms (Remeis-Observatory and ECAP, FAU Erlangen-Nuernberg), R. E. Rothschild (CASS, University of California San Diego), K. Pottschmidt (UMBC, NASA GSFC, CRESST II), C. Malacaria (ISSI), J. B. Coley (Howard University, NASA GSFC, CRESST II), P. Pradhan (ERAU, Prescott)
on 14 Apr 2023; 17:10 UT
Credential Certification: Katja Pottschmidt (katja@umbc.edu)

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We report on a NuSTAR observation taken shortly after the peak of the latest outburst of the Be High Mass X-ray Binary 4U 0115+63 (ATel #15967, #15975, #15978 ). The outburst started around 2023 March 28, reached its peak around 2023 April 7 as observed by MAXI and has been declining since. We triggered a NuSTAR observation close to the peak of the outburst. NuSTAR observed the source on 2023 April 10 (MJD 60044.00) with a total exposure of 12.8 ks. The observation was performed at a luminosity of 8.06x10^37 erg s^-1, assuming a distance of 5.7 kpc (Gaia Collaboration 2022).

We describe the spectrum in the energy range from 4.5 keV to 60 keV using a cut-off power law with a photon index of 0.34+/-0.05 and a cut-off energy of 9.1+/-0.2 keV. In total, the fit required four cyclotron resonant scattering features, with the fundamental line centered at 11.87+/-0.06 keV, with a strength of 0.38+/-0.06 and a width of 1.61+/-0.06 keV. The energies of the harmonic lines were fixed to be integer multiples of the fundamental line. We modeled the lines using Gaussian optical depth profiles (gabs). In addition, a broad Gaussian emission feature, with a width of 3.3+/-0.2 keV centered at 6.7+/-0.3 keV, was used to model the 10 keV feature (see, e.g., Mueller et al. 2013). We also included three narrow Gaussian lines at 6.4, 6.7 and 6.97 keV to model the Fe K-alpha, Fe XXV, Fe XXVI lines (see also ATel #15978). This model describes the data well, with chi^2= 532.22 for 327 degrees of freedom, resulting in a red. chi^2 of 1.63.

Similar to the NuSTAR observations of October 2015, the light curve of the source shows strong, regularly occurring peaks. This variability is caused by the system's ~2 mHz QPO (e.g., Heindl et al. 1999), which is a typical occurrence close to the peak of the outbursts of this system. In addition pulsations at the 3.6 s spin period of the system's neutron star can be detected.

We thank the NuSTAR team for accepting the TOO observations and for the quick execution of the observing program.

References:
Gaia Collaboration. 2022, VizieR Online Data Catalog, I/355
Heindl, W. A., Coburn, W., Gruber, D. E., et al. 1999, ApJ, 521, L49
Mueller, S., Ferrigno, C., Kuehnel, M., et al. 2013, A&A, 551, A6