Cyclotron resonance energies and orbital elements of accretion pulsar 4U 0115+63 during the giant outburst in 2008

Abstract

In this paper, we present both timing and spectral analyses of the outburst of 4U 0115+63 in 2008 April-May with International Gamma-Ray Astrophysics Laboratory (INTEGRAL) and Rossi X-ray Timing Explorer (RXTE) observations. We have determined the spin period of the neutron star at ~3.61430 ± 0.00003s, a spin-up rate during the outburst of , the angle of periapsis ω= 48°67 ± 0°04 in 2008 and its variation (apsidal motion) when comparing with previous measured values of ω. We also confirm the relation of spin-up torque versus luminosity in this source during the giant outburst. The hard X-ray spectral properties of 4U 0115+63 during the outburst are studied with INTEGRAL and RXTE. Four cyclotron absorption lines (fundamental at ~10-15keV, second harmonic at ~22keV, third at ~34keV and fourth at ~45keV) are detected using the spectra from combined data of the Imager On-board the INTEGRAL Satellite (IBIS) and Joint European X-ray Monitor (JEM-X) on-board INTEGRAL in the energy range 3-100keV. Two cyclotron absorption lines are measured from the spectra combining Proportional Counter Array (PCA) and High Energy X-ray Timing Experiment (HEXTE) data in the energy band 4-50keV. The 5-50keV luminosities at an assumed distance of 7kpc are determined to be in the range (1.5-12) × 1037ergs-1 during the outburst. The fundamental absorption-line energy varies during the outburst: around 15keV during the rising phase, transiting to ~10keV during the peak of the outburst, and further coming back to ~15keV during the decreasing phase. The variations of photon index show the correlation with the fundamental line energy changes: the source becomes harder around the peak of the outburst and softer in both rising and decreasing phases. This correlation and the transition processes during the outburst need further study in both observations and theoretical work. The known relation of the fundamental line energy and X-ray luminosity is confirmed by our results; however, our discoveries suggest that some other factors besides luminosity play an important role in fundamental line energy variations and spectral transitions. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.