HARD X-RAY EMISSION FROM THE NGC 5044 GROUP

Abstract

Observations made with the Rossi X-ray Timing Explorer Proportional Counter Array (PCA) to constrain the hard X-ray emission in the NGC 5044 group are reported here. Modeling a combined PCA and ROSAT position sensitive proportional counter spectrum with a 0.5–15 keV energy range shows excess hard emission above 4 keV. Addition of a power-law component with a spectral index of 2.6–2.8 and a luminosity of 2.6 × 1042 erg s−1 within 700 kpc in the observed energy band removes these residuals. Thus, there is a detection of a significant non-thermal component that is 32% of the total X-ray emission. Point-source emission makes up at most 14% of the non-thermal emission from the NGC 5044 group. Therefore, the diffuse, point-source-subtracted, non-thermal component is (2.2–3.0) × 1042 erg s−1. The cosmic-ray electron energy density is 3.6 × 10−12 erg cm−3 and the average magnetic field is 0.034 μG in the largest radio emitting region. The ratio of cosmic-ray electron energy density to magnetic field energy density, ∼2.5 × 104, is significantly out of equipartition and is therefore atypical of radio lobes. In addition, the group's small size and low non-thermal energy density strongly contradicts the size–energy relationship found for radio lobes. Thus, it is unlikely related to the active galaxy and is most likely a relic of the merger. The energy in cosmic rays and magnetic field is consistent with simulations of cosmic-ray acceleration by merger shocks.