Hard X-ray Emission from Cassiopeia A SNR

We report the results of extracting the hard X-ray continuum spectrum of Cas A SNR from RXTE/PCA Target of Opportunity observations (TOO) and CGRO/OSSE observations. The data can rule out the single thermal bremsstrahlung model for Cas A continuum between 2 and 150 keV. The single power law model gives a mediocre fit (∼5%) to the data with a power-law index, Γ = 2.94±0.02. A model with two component (bremsstrahlung + bremsstrahlung or bremsstrahlung + power law) gives a good fit. The power law index is quite constrained suggesting that this continuum might not be the X-ray thermal bremmstrahlung from accelerated MeV electrons at shock fronts [1] which would have Γ ≃2.26. With several SNRs detected by ASCA showing a hard power-law nonthermal X-ray continuum, we expect a similar situation for Cas A SNR which has Γ=2.98±0.09. We discuss the implication of the hardest nonthermal X-rays detected from Cas A to the synchrotron radiation model. INTRODUCTION X-ray observations of supernova remnants have been stimulated by recent reports of nonthermal power-law X-ray detections suggesting supernovae as sites of charged particle accelerations and sources of cosmic rays. The first strong evidence for charged particle acceleration near supernova shock fronts in the X-ray energy band is demonstrated by the morphological and spectral correlation between X-ray and radio emission from the bright NE and SW rims of SN 1006 [2]. The brightest radio emission regions show almost featureless power law X-ray spectra when compared with SN 1006 central region which is dominated by emission lines of highly ionized elements in a non-equilibrium ionization thermal plasma. The nonthermal X-ray component has been modeled as a synchrotron emission from electrons acFIGURE 1. The thermal bremsstrahlung (kT=7.93 keV; dashed line) + power-law (Γ=2.98; dashed-dotted line) + 6 K X-ray line model produces a good fit (solid line) to the PCA & OSSE data (crosses) with a χ/dof=1.077, dof=62. celerated to ∼100 TeV within shock fronts [3–5]. In this model, the radio spectrum is produced by synchrotron radiation of electrons accelerated to ∼GeV energies with the radio power-law index being less steep than the X-ray power-law index. Several similar evidences also have been demonstrated by ASCA measurement of RX J1713.7-3946 [6] and IC 443 [7], and RXTE and OSSE measurements of Cas A [8–10]. OSSE with a total accumulation time of 15×10 detector-seconds, detected a hard continuum between 40-150 keV from Cas A SNR at a 4σ confidence with a flux of ∼9×10 γ cm s keV at 100 keV [10]. The detection is the hardest X-ray detection from a SNR without plerionic source. However, the shape of the continuum has not been strongly constrained. The continuum can either be a bremsstrahlung with kT≃35 keV or a power law with Γ ≃3.06. In this paper, we use the 2-30 keV RXTE/PCA TOO and the 40-150 keV CGRO/OSSE Cas A data to better determine the shape of Cas A hard X-ray continuum.