NuSTAR Observations of Abell 665 and 2146: Constraints on Nonthermal Emission

Observations from past missions such as RXTE and Beppo-SAX suggested the presence of inverse Compton (IC) scattering at hard X-ray energies within the intracluster medium of some massive galaxy clusters. In subsequent years, observations by, e.g., Suzaku, and now NuSTAR, have not been able to confirm these detections. We report on NuSTAR hard X-ray searches for IC emission in two massive galaxy clusters, A665 and A2146. To constrain the global IC flux in these two clusters, we fit global NuSTAR spectra with three models: single (1T) and two-temperature (2T) models, and a 1T plus power-law component (T+IC). The temperature components are meant to characterize the thermal ICM emission, while the power law represents the IC emission. We find that the 3-30 keV A665 and 3-20 keV A2146 spectra are best described by thermal emission alone, with average global temperatures of kT = (9.15 +/- 0.1) keV for A665 and kT = (8.29 +/- 0.1) keV for A 2146. We constrain the IC flux to F-NT <0.60 x 10(-12) erg s(-1) cm(-2) and F-NT < 0.85 x 10(-12) erg s(-1) cm(-2) (20-80 keV) for A665 and A2146, respectively both at the 90% confidence level. When we couple the IC flux limits with 1.4 GHz diffuse radio data from the VLA, we set lower limits on the average magnetic field strengths of >0.14 mu G and >0.011 mu G for A665 and A2146, respectively.