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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 ∼3.614 30 ± 0.000 03 s, 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 ∼10–15 keV, second harmonic at ∼22 keV, third at ∼34 keV and fourth at ∼45 keV) 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 3–100 keV. 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 4–50 keV. The 5–50 keV luminosities at an assumed distance of 7 kpc are determined to be in the range (1.5–12) × 10 37  erg s −1  during the outburst. The fundamental absorption‐line energy varies during the outburst: around 15 keV during the rising phase, transiting to ∼10 keV during the peak of the outburst, and further coming back to ∼15 keV 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.

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