Fermi Gamma-ray Space Telescope

NuSTAR Detection of Nonthermal Emission from the Supernova Remnant W49B and Its Implications for Gamma-ray Emission Mechanism

Takaaki Tanaka
(Hiroya Yamaguchi, Daniel R. Wik, Hiroyuki Uchida, Yasunobu Uchiyama, Felix A. Aharonian, Aya Bamba, Daniel Castro, Adam R. Foster, Robert Petre, Jeonghee Rho, Randall K. Smith, Brian J. Williams)


The supernova remnant W49B is one of the well studied sources that are known to be interacting with dense molecular gas. Gamma-ray detections by the Fermi LAT (Abdo et al. 2010; H.E.S.S. Collaboration 2018) and also by H.E.S.S. (H.E.S.S. Collaboration 2018) made the SNR one of the interesting objects to study particle acceleration. The observed gamma-ray luminosity of 10^35 erg/s can be translated into huge energy density of emitting nonthermal particles of 10^4 eV/cm3 (Abdo et al. 2010). A break-like spectral feature is detected with the Fermi LAT at the energy of 300 MeV, suggesting that pion decay is the physical process responsible for the gamma-ray emission (H.E.S.S. Collaboration 2018). However, electron bremsstrahlung cannot be ruled out, and thus the emission mechanism still is left as an open question. We observed W49B with the NuSTAR with an exposure time of 120 ks. Although W49B has been known as a pure thermal emission source in the X-ray band, the unprecedented hard X-ray sensitivity of NuSTAR allowed us to detect power-law-like component extending up to ~ 20 keV. The newly found component has a hard spectrum with a photon index of 1.4+/-1.0. We found that the NuSTAR emission is best interpreted as nonthermal electron bremsstrahlung. Adding our NuSTAR data to the multi-wavelength spectra in the literature, we can constrain the gamma-ray emission mechanism as detailed in the presentation. After discussing particle acceleration in W49B, we also will talk about possible synergy between NuSTAR and the Fermi LAT that we plan in the near future to study similar supernova remnants.