tokens: the same procedure as used for generating the LAXPC PDS. NuSTAR observations of 2015 October 22 show a single broad peak at ∼1 mHz with 1024 bins per interval, whereas two clearly resolved peaks at ∼1 and 2 mHz are visible in the PDS with 512 bins per time interval. In the NuSTAR PDS of 2015 October 30, QPO peaks at ∼1 and 2 mHz are detected using 1024 bins as well as 512 bins per interval. We have, therefore, used 512 bins per interval for both of the NuSTAR observations to generate PDSs, as shown in Figure 5 . The NuSTAR PDS of 2015 October 30 also shows the presence of 12 and 27 mHz QPOs with a coherence factor and rms (%) of 8 and 3% and 4 and 3%, respectively. A summary of the characteristics of the ∼1 mHz and ∼2 mHz QPOs from LAXPC and NuSTAR are presented in Table 3 . Figure 5. PDSs (in the units of (rms mean −1 ) 2 Hz −1 ) of 4U 0115+63 obtained for the outburst from NuSTAR in the 3–79 keV energy band ((a), (e)). PDSs in the 3–80 keV energy band of LAXPC 10 (b), 20 (c), and 30 (d). Arrows indicate ∼1 mHz and ∼2 mHz QPOs in the PDSs of NuSTAR and LAXPC detectors. Table 3 Summary of the Characteristics of the ∼1 mHz and ∼2 mHz QPOs Observed from LAXPC and NuSTAR To investigate the energy dependence of the QPO characteristics, PDSs were extracted in the 3–15, 15–30, 30–50, and 50–80 keV bands for each of the LAXPC units. This was achieved by extracting data from Layer 1 for 3–15 keV, Layers 1 and 2 for 15–30 keV, and Layers 1–5 for the 30–50 and 50–80 keV energy bands. PDSs in the units of (rms mean −1 ) 2 Hz −1 are generated with 10 s binning and by subtracting Poissonian noise. Prominent QPOs are observed at ∼1 mHz and ∼2 mHz in the energy bands up to 50 keV. Above 50 keV the 1 mHz and 2 mHz QPOs are insignificant. Details of QPO characteristics like frequency, rms amplitude, and quality factor are summarized in Table 3 . The coherence factor ( ν / δν ) lies in the range 7.6–11.1 for the ∼1 mHz QPO and 2.8–6.0 for the ∼2 mHz QPO, respectively. It can be observed from Table 3 that the coherence factor of the ∼1 mHz QPO decreases from 3–15 keV to 15–30 keV and then increases at higher energies. rms amplitude (%) of the ∼1 mHz QPO increases from 3–15 keV to 15–30 keV and then decreases at higher energies. It may be noticed that the coherence factor increases with energy for the ∼2 mHz QPO and rms amplitude (%) of the ∼2 mHz QPO increases until 30 keV and then decreases at higher energy. 3.1.1. ∼1 mHz and ∼2 mHz QPOs from <italic>RXTE</italic>/PCA Observations PCA/ RXTE Standard 1 mode light curves (10 s binned) in 2–60 keV, generated using the FTOOLS task “lcurve” in HEASOFT for of all of the observations showing ∼mHz QPOs, are presented in Figures 6 (a), (b), and (c). It may be noted that during the 2004 observation, three PCUs were on, while in 2008 observations two PCUs were on, and the count rates in the figures reflect this. In the 2011 light curve in Figure 6 (c), the jump in the count rate in the middle of the light curve is not
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